Efflorescence and the Discoloration of Concrete

EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE by

Peter Russell BSc FICE FIStructE FIHE FIOB formerly Chief Engineer in Scotland of the Advisory Division of the Cement and Concrete Association

A VIEWPOINT PUBLICATION

VIEWPOINT PUBLICATIONS Books published in the Viewpoint Publications series deal with all practical aspects of concrete, concrete technology and allied subjects in relation to civil and structural engineering, building and architecture. 13.026 First published 1983 This edition published in the Taylor & Francis e-Library, 2005. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.” ISBN 0-203-97502-2 Master e-book ISBN

ISBN 0 86310 011 2 (Print Edition) Viewpoint Publications are designed and published by EYRE & SPOTTISWOODE PUBLICATIONS LTD Swan House, 32 Swan Court, Leatherhead, Surrey KT22 8AH © Eyre & Spottiswoode Publications Ltd Any recommendations made and opinions expressed in this book are the author’s, based on his own personal experience. No liability or responsibility of any kind (including liability for negligence) is accepted by the Publisher, its servants or agents.

THE AUTHOR

Peter Russell was Chief Engineer in Scotland for the Cement & Concrete Association from 1948 to 1978. He is now engaged in the preparation of advisory documents for the building industry and is writing a book covering those thirty productive years from hydro-electric schemes to oil platforms, for which he received the Silver Jubilee Medal. He has contributed to professional journals and is currently talking at some universities on the theme of construction and the environment. Peter Russell has been Chairman of the Institutions of Civil, Structural and Highway Engineers, and of the Concrete Society, serving also on the Board of the Edinburgh College of Art. He played a part in the attainment of aesthetic as well as structural quality over the past generation in a northern climate which lent itself to a special study of the subject now under review. His thanks are due to the Cement & Concrete Association of whose excellent work he has intimate knowledge, and valuable assistance was further given by the staff of the Building Research Establishment.

PREFACE

Disfigurement of any concrete surface is at least a source of annoyance, and this publication attempts to mitigate the problem in its chemical and physical aspects. It deals with the broad scope of elemental attack by the vagaries of weather and by salts as they move to the face of a building to result in discoloration and, at times, disruption. Much research has in particular been devoted to the cause and nature of efflorescence, an expressive yet ambiguous word which suggests flowering and fruition but describes an effect that almost vies with vandalism. While elusive, however, it is not an intractable phenomenon and can be cured in more senses than one. Its stalactites or bloom can be alleviated by common sense and scientific forethought, given all the rules of quality control. This largely aesthetic problem is conditioned by factors such as temperature, absorption and lack of imagination, with cement and aggregates of lesser importance. The text covers staining of many kinds, superficial but unsightly, and due to faulty workmanship or detailing. Concrete is too readily assumed to have strength and durability only, with an acceptable countenance underestimated or ignored. This work should help towards an appreciation of its potential in visual as well as structural terms. Peter Russell September 1982

v

Plate 1: A retaining wall of engineering brickwork with disfigurement at the mortar joints as the result of salts finding their way from the soil behind. These should have been isolated by a waterproofing barrier over the full height of the wall, and it is possible that the mortar was too rich, with consequent shrinkage and fine cracking.

CONTENTS

THE AUTHOR

iii

PREFACE

iv

CONTENTS

vi

1

INTRODUCTION

1

2

CAUSES

3

2.1

Basic factors

3

2.1.1

Mechanism

5

2.1.2

Hydration

6

2.1.3

Formwork

6

2.1.4

Rainwater

7

2.1.5

Weathering

7

2.2

Blockwork

8

2.3

Brickwork

9

2.4

Mortar

9

SOLUBLE SALTS

13

3.1

Carbonation

14

3.2

Contamination

15

4

PREVENTION

17

4.1

Concrete quality

18

4.2

Admixtures

18

4.3

Blockwork

19

4.4

Cladding

19

4.5

Mortar

20

3

vii

4.6

Joints

21

4.7

Pigments

22

4.8

Detailing

22

4.9

Damp-proofing

23

4.10

Formwork

23

4.11

Rendering

24

4.12

Urban environments

24

4.13

Surface treatment

25

REMOVAL

29

5.1

Washing and brushing

29

5.2

Acids

30

6

CONCLUSION

33

7

BIBLIOGRAPHY

35

8

APPENDIX: CHECKLIST TABLE

41

5

viii

1 INTRODUCTION

Efflorescence is a white coating appearing on the external face of a wall as a result of the migration of salts in solution to the surface where they crystallize unevenly. These blemishes may be derived either from the constituents of building materials based on Portland cement or from alkalis in the ground, and are salt deposits caused by the evaporation of their saturated solution. In normal atmospheric conditions they leave a harmless, superficial and often temporary discoloration which is nevertheless disturbing, and whose irregular configuration may persist for a number of years. Disruptive spalling of the surface can result if the salts are deep seated. Discoloration of concrete has many manifestations and the common cause is excessive ingress of rainwater due to faulty detailing. An effect similar to efflorescence may be caused by rain cleaning part of the wall, but this pattern is generally more extensive and is dependent on the extent of glazed areas and projections on the face. Chemistry plays an important part in that calcium hydroxide, liberated by the hydration of cement, is carried in solution to the surface of concrete or mortar where carbonation takes place. The location and extent of this relatively insoluble substance is largely dependent on the porosity of the external layers. Materials with an open texture allow air to penetrate some distance below the surface, and a saturated solution moving from the interior can be carbonated before it reaches the outer skin. Concrete of intermediate porosity having a close texture, permeable to salt solutions but not permitting air to enter, may suffer a visible deposit on the outer surface. Efflorescence is the result of a combination of events occurring inside and outside a wall. The presence of even small amounts of salt, with attendant moisture, initiates evaporation or hydrostatic pressure causing movement of the solution to the exterior. The resulting quantity and effect of leaching of the concentration depends on its composition and solubility. The extent of efflorescence is also determined by temperature and humidity, and it is more likely to occur in seasons with a slower rate of drying. Bloom is another phenomenon, similar in appearance to efflorescence, which is caused by carbon dioxide in the atmosphere reacting with free lime produced during the setting of cement and forming insoluble calcium carbonate deposits. Sometimes this is of appreciable thickness, so that encrustation may be a

2 INTRODUCTION

better description. It is naturally more noticeable on a dark surface than light, and exemplified by streaking down the face of engineering bricks from the mortar joint or a paint-like deposit on concrete. While the causes of efflorescence are relatively easy to explain without the complexity of chemical equations, some knowledge is required of the nature of soluble salts and their aesthetic and physical results. Almost any salt can appear, but each may react differently with atmospheric gases which are themselves variable. Another source is from the earth behind retaining walls or under solid floors, so that this should be carefully isolated. Above all, imaginative detailing of a structure is essential to shed water from the face or to dissipate its flow. At best, efflorescence is an irritation and at worst an agent of disruption, but while the layer is annoying and generally unacceptable, it is barely noticeable on concrete made with white cement and in some cases can be advantageous in reducing porosity, and sealing fine cracks or crazing on the surface. Efflorescence is more apparent in the spring, although remedial conditions are less severe during this season and showery weather is likely to aid the washing process. Because concrete as a structural medium is relatively new, it suffers by man’s inability to forestall elemental attack; he does not make the same allowance for discoloration as he would with other materials long subject to the most blatant of blemishes. Sporadic cracking and faulty jointing are of importance, and in anticipating the effects of weather every opening must be considered suspect, whether glazed or not. All building materials have to meet a number of requirements when exposed to a variety of polluting and destructive elements which stem from the atmosphere, the soil or the materials themselves. There must, therefore, be a clear understanding not only of the properties of the constituent parts of the structure but also of the factors which influence their performance both integrally and superficially. Uninterrupted areas are especially vulnerable. Concrete has indeed prospered in an infinite range of function and form, but in its wake has come the problem of lack of attention to the facade and ignorance of the effects of salts and their deposits. It is an indispensable medium, but suffers variability at the hands of planner and builder, and by climatic conditions in the long term. More attention must be given to arresting or redirecting moisture by effective barriers, and to forestalling cracking by due provision for expansion and contraction.

2 CAUSES

2.1 BASIC FACTORS Portland cement combines with water in concrete mixes to produce highly alkaline products and this chemical reaction is known as hydration. The make and content of cement has little effect on efflorescence, except that a very rich mix lends itself to crazing and, if too weak, is less than durable. Larger quantities of lime than usual need not cause a serious problem, but it is always desirable to use minimum free water consistent with full compaction. Dry air can increase the risk of blemish whereas humid air reduces it. A low temperature retards the build-up of a protective carbonate layer. Organic impurities in loam and clay may lower the strength, and estuarine aggregates should be well washed. The essential requirements of good concrete are impermeability, continuity of placing and uniformity of quality, with any protective coatings firmly bonded. All faces exposed to sea-spray are vulnerable and many industrial processes disgorge salts which are transferred to porous materials by the polluted atmosphere. The nature of stored materials such as fertilizers must be taken into account, as should the likelihood of leakage from sinks and services in general. Although concrete is versatile with a wide range of functions, the action of weather can be complimentary or otherwise, depending on presentation to the environment and the experience of the designer. Documentation abounds in printed and photographic form on the general theme of discoloration, but efflorescence as the end-product of climate and chemistry is rarely discussed. The location of a building relative to its neighbours is a crucial factor in the planning analysis and in predicting the intensity of wind and rain. Today’s pollution, albeit linked with technical progress, enhances discoloration by its contribution to amorphous staining, made more prominent by its contrast with the normal colour of concrete. When all the accepted rules of mixing and placing are strictly obeyed, discoloration is still possible, but it is axiomatic that the denser the concrete the less chance there will be for extraneous water to be absorbed into it and, on evaporation, to bring out the defiling chemicals. Uniformity of concrete quality is

4 EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE

crucial and the constituents should if possible be taken from constant sources. Admix tures do not adversely affect staining, and the use of integral pigments may give better colour control, minimize surface variations and aid workability. Consideration of appearance and of overall durability impose even stricter limits on workmanship, whilst curing, as later amplified, is of paramount importance. Apart from isolated areas of formwork leakage or irregular absorption by the lining, other causes of discoloration include concreting in cold weather, casting in excessively thick layers and, above all, release of water to the surface during placing and compaction. Another factor is mixing time, and longer periods may reduce the extent to which water and cement separate, as may extended vibration. The atmosphere may also contribute industrial effluents and traffic fumes to produce chaotic patterns around corbels and ledges, which in any case concentrate rainwater. The outer skin of concrete, however skilfully moulded, takes the full force of attack and must of necessity be durable, whether airentrained or otherwise. Appearance is largely determined by the properties of the cement paste, which in practice is regularly wetted so that dirt lodges in its pores; the rich laitance is etched to change its colour towards that of the fine aggregate. As the grime builds up it will give a measure of protection against further etching, but may show more contrasting efflorescence if the salt source is still operative. A more encrusted growth is produced by water percolating through cracks in a wall bringing lime to the surface, but this to some extent protects reinforcement. Efflorescence can be confused with streaking caused by rainwater which cleans along irregular lines, and here again experience will indicate profiling or at least diffusion. Concrete, in the same way as any other material, becomes dirty, and with tall buildings care must be taken to counteract the effects of wind turbulence. Painting may have to be considered but this should always be seen as a second line of defence. What is unforgiveable is the recurrent blemishing of an otherwise welldesigned structure simply because of a careless attitude to long term performance. Efflorescence is liable to occur on hardened concrete if, during the pouring of successive lifts, the joint is not perfectly sealed, as free water can carry a concentration of salts along myriad paths. Weather patterns are so often determined by the configuration and efficiency of movement joints, so that their location must be carefully established. Daywork joints must also be positioned economically and intelligently. Cracking of in situ walls or across a building unit allows moisture to concentrate along channels which feed the fissures and highlight those already evident at the end of sills and at badly formed joints between precast units. Structural elements, such as the abutments of bridges, are prone to the migration of groundwater into them, and retaining walls are targets for outpourings from mortar.

CAUSES 5

2.1.1 Mechanism The formation of efflorescence depends upon a number of chemical and physical processes involving the nature and solubility of salts and their transfer through porous concrete. Local conditions dictate how these evolve, so that the deposits can be unpredictable in extent and location. Once a solution has become saturated, cooling will cause most salts to separate out in solid form, but loss of water by evaporation will also result in deposition, even without change in temperature. There may be a time lapse between the completion of a building and the appearance of the effects of salts, and their immediate location is no certain guide to provenance, but simply a convenient drying surface. The structure of concrete will dictate its drying rate and the place of disfiguration. With a fine-pored surface, capillary forces will draw free water through the wall, crystals will appear on the exposed face and variable porosity will result in sporadic staining. With an open-textured surface, water will evaporate short of the face and the salts remain just within the wall, possibly causing disruption. Salt-laden moisture may leave precipitations taking the form of localized striation corresponding with joints or architectural features. It is essential to keep track of such movement and its degree of replenishment, to study the origin and nature of the salts, and to have prior knowledge of spillage or defects in drainage. The unpredictable festoon can be seen in buildings soon after completion but will gradually fade away with the aid of wind and rain if its source has been stemmed. As concrete is a mixture of natural constituents, attention must be paid to selection of raw materials and their proportioning, compaction and curing. The last of these does not imply a remedial process but ensures adequate hydration of the immediate surface for consistency in colour and durability. The containing shutter must itself be treated with due deference, as should rigidity, striking time and release agent. It has been said that there are nearly a hundred kinds of concrete blemish, many of which have a common cause, such as variable shrinkage and absorption, with the catalyst so often the entry of soluble salts. Other forms are sand discoloration, blowholes, aggregate transparency, mottling, scouring and scaling. Water in the concrete mix is normally distributed across the wall and can be adversely augmented if protective measures are not taken against absorption from the ground or rainfall. Efflorescence is more likely to occur if the building is erected during winter and if drying through the external wall is rapid in the early part of the year. When salts disappear from a surface, they have not necessarily been washed away, but have sometimes been absorbed back into the wall and will reappear at progressively lower concentrations. If, after precautions such as covering the works have been taken, they recur on drying, then excessive water is entering the wall through faulty detailing or waterproofing.

6 EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE

2.1.2 Hydration The whole subject of salts is discussed later but it is worth noting that in addition to the alkalis of potash and soda, cements contain sulphate as gypsum added at grinding to control the set and on mixing with water these go into solution. After a few hours the sulphate is fixed insolubly, leaving the alkalis as hydroxides which are gradually converted into carbonates by the atmosphere to leave a surface deposit. If the sulphate is transferred with the mixing water by capillary action to an adjoining building unit, the quantity of salt in that unit will also be increased. In other cases staining is caused by the movement of soluble calcium sulphate to the concrete surface, this generally emanating from soil in contact with it. The reaction of hydrating cement with carbon dioxide in the air reduces alkalinity. Such carbonation also increases shrinkage of the concrete and can lead to cracking, although not penetrating too deeply. Aggregates near the surface which are not well graded may also cause cracking, distinguishable from that caused by drying shrinkage in that it takes longer to develop. Only in very porous concrete is there a problem, and here an assurance of adequate cover to reinforcement is essential. Cracking through which salts find ingress can also arise from differential settlement. 2.1.3 Formwork The time of stripping formwork is not necessarily significant, but blemishes may be caused by warping or deflection of the form which leave an air gap allowing rapid localized drying and a finish inconsistent with the rest of the surface. Careful selection of form face can inhibit most types of staining although some linings are unsuitable and easily damaged. A dark coloration is common if casting temperatures are low but this can be offset by insulation. Variable surface appearance is a difficult problem, but in broad terms quality depends on good placing and an evenly applied release agent. Efflorescence is not so likely to occur on surfaces cast against absorptive forms, which encourage the water in the mix to carry cement particles to the surface where they are deposited as dense skin of low permeability, so restricting the migration of lime and reducing carbonation. Impervious forms, on the other hand, trap excess water at the face. In some cases light coloured areas tend to occur near the top of a lift and not lower down where the pressure of the plastic concrete brings a relatively greater flow of water into the receptive lining. Exposure of concrete to the air leads to evaporation of the pore water, and if this is rapid with only vapour reaching the surface, the carbonate will be deposited below the surface with possible disruption. There is also an intermediate state causing localized effect by gross variations in porosity due to poor compaction and

CAUSES 7

segregation. In this case water reaches the surface through relatively large channels and leaves an erratic deposit, although much less common on precast elements. 2.1.4 Rainwater Rainwater absorbed into any porous material is released by natural drying, the significant factors being solar radiation, wind speed, atmospheric humidity and the aspect of a building. The nature of concrete itself plays a large part in determining loss of water from a surface and it is difficult to measure the overall affect with such a variety of influences. Penetration is obviously greater in regions where the rain index is high and the walls have less chance of drying out between downpours. The criterion by which the degree of exposure can be judged is described in BRE Digest 127. Sites may be graded between sheltered and severe but, in areas of moderate exposure, high buildings which stand above their surroundings or on hilltops should be regarded as being one grade more exposed than indicated. Although incidence of rain conforms to a rough pattern for any given locality, short intense periods may be experienced from any direction and much of the penetration of walls occurs during a few prolonged storms accompanied by strong wind. The annual mean index gives a fair indication of the total amount to be expected, but while helpful in gauging the likely frequency and intensity, is less useful in assessing run-off from surfaces or entry through joints. Experience of the behaviour of concrete in any situation is of critical value, as is guidance from the Meteorological Office. The resistance of a solid wall is appreciably increased by the application of rendering in accordance with BS 5262, while blockwork may also benefit in areas of severe exposure by the prevention of ingress through joints, cracks or the unit itself. Walls with a cavity afford a more effective barrier than a single leaf, provided the quality of materials and workmanship is equally high. 2.1.5 Weathering Concrete does not normally weather well over long periods in terms of discoloration; a comparison should, therefore, be made of the design of similar projects with a vew to controlled variation and intentional highlighting of detailing to distract the eye. It exhibits its age by symptoms such as dirt accumulation, growth of algae and efflorescence, some being superficial but others deep seated. The surface obviously changes from its initial freshness, with properties that alter in porosity and frost susceptibility. This outside layer varies, absorption determining the ability to hold the grime or resist aggression from the air, so that exposure may be uniform but the result patchy. Weathering should ideally enhance the appearance of a building if only by traditional mellowing, but the very lightness in tone of concrete in plain walling

8 EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE

is a disadvantage in combatting darker stains, while the modular patterns of precast units should render these less prominent. Mortar is another matter in that clay brickwork, and even blockwork, can be disfigured to almost paint-pot degree if a balance is not reached as to its constituents. Curing is the final act of surface attention, by water spray or otherwise, to ensure that the susceptible external layer is not dried too quickly, by hydrating the cement particles at the surface and minimizing vulnerable laitance. 2.2 BLOCKWORK Two forms of efflorescence may be found on concrete blocks—sodium/ potassium carbonates and calcium carbonate. Each is derived from corresponding free hydroxides brought to the surface during hardening. Sodium and potassium carbonates appear as a soft deposit which is easily washed or brushed off, particularly if the unit has been exposed to soaking and slow drying in a stockyard. Calcium carbonate is not so common and its harder crust is more difficult to remove. Masonry can bring together a wet mortar and a block containing variable moisture such that the water content of the wall is relatively high and deposit more likely to form on completion of the work. Concrete blocks, lightweight or otherwise, should never be set in unnecessarily strong mortar as this can lead to cracking in or near the joints, whereas a weaker mix yields enough to accept small movements in a wall, and hairline cracking will be distributed or less permeable. Strong cement/sand mixes are appropriate for heavy engineering works in units of comparable strength or in foundations and below damp-proof courses, but may display more efflorescence than a mix containing lime. While the latter contributes more salt, the denser joints offer more resistance to the passage of moisture and the precipitate sometimes spreads over the blockwork on evaporation from the wall as a whole. In the same way a white deposit round the edges of blocks suggests that the units have absorbed water from a mortar containing alkaline sulphates. Even if there was no absorption from the joints during laying, these sulphates may still be transferred to the units if an unfinished wall is not covered in heavy rain. The answer lies in matching the qualities of blocks and mortar by using cement/lime/sand or masonry mixes which are not stronger than necessary. It is worthy of note that a technical survey was undertaken by the International Concrete Block Commission to collect data on the improvement of long term colour integrity of masonry. Efflorescence on precast products in general may appear as white patches or as an overall lightening of colour. Where the units are exposed, acidic rainwater will slowly dissolve the film, and in the case of paving slabs, abrasion by foot traffic will remove it more rapidly.

CAUSES 9

2.3 BRICKWORK Efflorescence on brickwork can appear as a loose white powder or a hard glossy substance, partly covering and penetrating the face. It can be a seasonal occurrence and become less marked in ensuing years, but often affects retaining and freestanding walls and is removed only slowly by rain. Magnesium sulphate is particularly disruptive and its deposition not readily washed from either the whole face or under copings from which lime has leached. The only satisfactory treatment may be to render the wall after removing all loose material and raking out the mortar which may itself be impregnated. Certain brickwork has very low porosity and little sulphate content, whereas a highly porous mortar will allow even a minute quantity of salt diffused along its courses to be shown clearly against the background. Flettons may be highly absorptive with appreciable sulphates, and within themselves may result in salts appearing on the surface over a wide area of the wall. A white deposit is brought to the surface by the dry winds of April, sucking salts into its external face either through the joints or the units depending on relative percolation. The greatest risk is from wind-blown rain finding its way through cracks in joints which are as strong as the bricks but on shrinking permit water movement. Many tarnished multi-storey buildings are victims of this lack of knowledge as to the interaction of various parts of a wall at different shrinkage and drying rates. Clay products are prone to a legacy of lime, with deposits more prominent on their relatively dark surface and exhibiting the same crystallization of salts which have percolated in solution to the face. Again the extent depends on the attendant amounts of both salt and water, and on their chemical nature. Salts of calcium, sodium and potassium are, in the main, superficial but spoiling, some of these already being present in the clay or formed at firing, with calcium sulphate of low solubility constituting the major source. Salts in brickwork can have an influence on adjacent concrete in transferring from one material to another. 2.4 MORTAR Cement-based mortars may be attacked by sulphates derived from clay bricks as well as from external sources such as flue gases. The onset is gradual and only occurs when the brickwork is consistently damp. The most common effect of soluble salts is to produce a deposit which, although unsightly, is usually temporary and harmless. A high percentage of calcium sulphate may cause little efflorescence, while a lower porportion of sodium or magnesium sulphate can give rise to heavy impregnation at the end of a long wet spell. Crystallization of the salts within the brickwork may, however, cause spalling of the face similar to frost attack. While keeping winter working in mind, it must never be assumed

10 EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE

that the higher the strength of the mortar the better, because when it is too strong it simply concentrates movement into fewer but wider cracks. The use of low alkali masonry cement will reduce the staining capacity of mortar but the overall design of a building must ensure that the walls are kept as dry as possible by, say, an overhanging roof and the shielding of surfaces from rainwater generally. Drains should be of ample capacity and maintained in serviceable condition. Present day mortars are fortunately much more durable and likely to resist erosion in highly contaminated air. Guidance is given in BS 3921 as to discoloration of brickwork. Although cement contains less salts than hydraulic lime, care should be taken to prevent migration of mixing water from mortar to bricks or blocks before the cement has hardened, as salts in solution switch easily from one part to another by capillary action. It is quite possible for a wall which is initially free of salts to be contaminated by their movement from the mortar, as indicated by the predominance of sodium or potassium compounds at the surface. There is little evidence to suggest that mortar plasticizers contribute significantly to efflorescence because they are added in such small amounts.

CAUSES 11

Plate 2: A marked contrast between the upper vertically diverting fluting and the plain rendered base which should have been similarly treated. Alternatively, the lower wall might have been covered with exposed aggregate panels.

12

3 SOLUBLE SALTS

Salt is defined as a compound of molecules which in the solid state are geometrically packed as crystals, but which lead an independent existence in aqueous solution. The action of salts is in part associated with cement-based materials as the source of alkalis according to the location and porosity of the medium. The resulting deposit is the product of their migration through concrete of poor quality, and of subsequent evaporation which can build up an appreciable crust on or within the surface. The readiness with which this forms depends on the nature of the salts, and if the first fine crystals in the pores are such that still smaller capillary passages are left, further crystals may surmount these. Salts derived from external sources, or moving from one part of a wall to another, are of fundamental importance, and their composition is to a great extent determined by their origin. Portland cement and hydraulic lime in mortars provide the sulphates and carbonates of sodium and potassium, whereas the presence of nitrates or chlorides points to groundwater as the source. Chlorides may also come from rubble fill and polluted atmospheres. Sulphates are found in building materials, industrial waste and some clay soils although the top few feet are generally free. All these salts, together with calcium oxide, are very soluble in water, and even calcium sulphate dissolves to some extent. The two main constituents of cement are dicalcium and tricalcium silicates, which react with water in concrete to give products which are highly alkaline. Carbon dioxide in the air reacts with the surface layer of the concrete to reduce alkalinity in the process known as carbonation. This often intractable film is the result of the crystallization or precipitation of salts in solution, replenished from inside the wall and found also within the pores in company with excess water. The chemical action takes place more rapidly when the concrete is at an intermediate stage between saturated and completely dry, but less so if it is well cured. In the initial stage, calcium hydroxide or free lime is released from the cement during the setting process and migrates to the surface where it is converted into calcium carbonate or chalk which is not readily removed because of its low solubility. In the early stages of hydration, lime remains in solution until the concrete hardens, and efflorescence can be caused by very small concentrations. As carbonation proceeds in the normal course of weathering, the remaining

14 EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE

hydration products, including calcium silicates and aluminates, are slowly decomposed by sulphur dioxide from industrial airborne sources, leading to calcium sulphate. In good quality concrete this produces little change in appearance, as the sulphate acts as a binder and, while retaining dirt in sheltered places, helps to keep washed areas clean because of its slight solubility. Magnesium sulphate is responsible for many of the failures resulting from soluble salts, although it is not present in great quantities and is readily washed away except in sheltered parts. Potassium sulphate forms a hard film, whereas sodium sulphate produces a fluffy deposit when it crystallizes. Surface failure is often most severe when the salts are in more than one state of hydration, and solar heat may convert these to a form occupying several times the earlier volume, with excessive pore pressure and powdering. Depending on their origin, nature, distribution and quantity, salts may result in the following effects: (a) temporary but unsightly fluffy encrustation; (b) a glossy skin that causes blistering of the surface; (c) a reaction with certain compounds in the cement which gives rise to softening and possible sulphate attack; (d) crystallization within the surface pores resulting in pitting and disruption and known as crypto-efflorescence. The source and solubility of salts vary, their path is devious and their movement dictated by moisture and humidity. Ability to dissolve increases with temperature, and calcium hydroxide is among the few salts which act more freely with cold water than hot. 3.1 CARBONATION Air in permeating concrete decomposes the accessible hydrated compounds to reduce alkalinity in the process known as carbonation. This is normally limited to the exposed surface and, if the concrete is well compacted and of low permeability, the resulting white deposit is built up very slowly in the presence of moisture. Its occurrence also depends on the aggregates, cement and admixture content, water/cement ratio, curing and exposure. The proportion of carbon dioxide in the air is not usually significant, but may be influenced by pollution. On the other hand a degree of carbonation and corresponding reduction in alkalinity has a bearing on the corrosion of reinforcement. Normal mixing water has only a small percentage of carbonic acid dissolved in it and has little aggressive effect. Since partly hydrated cement paste is more permeable, factors which delay hydration such as cold working conditions may increase the surface deposit. Although there is minimal transfer of calcium hydroxide from the interior of a wellbuilt concrete wall, there are always localized effects caused by variations in

SOLUBLE SALTS 15

porosity, in which case water is channelled into the surface, bringing with it dissolved hydroxide and leading to heavy isolated excrescence. Moreover, rain can dissolve calcium carbonate or calcium sulphate from concrete copings to leave salts lower down if the wall is not designed to eject rainwater clear of the surface or at least dissipate it. 3.2 CONTAMINATION Concrete can be contaminated by colliery shale used as hardcore in foundations and under floor slabs, while the soil itself provides a possible reservoir which is continually renewed. Some aggregates may contain sulphur compounds and soluble alkalis, but crushed stone, gravel and sand from well-established quarries are normally free of deleterious chemicals. Unwashed sea-sand should not be used, for apart from its salt content, it will absorb moisture from the air to aggravate the situation. Precast products are only prone to attack from outside sources. Trouble is not usually experienced with cladding unless it is located against salt-bearing brickwork and it should in any case be fixed when both surfaces are dry.

Plate 3: A good example of striated walling bush-hammered to expose aggregate and diverting the flow of rainwater within its channels. The facade is eyecatching, with efflorescence submerged with the fluting and masked by the bold exposure of the aggregate between the windows.

16 EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE

Plate 4: This fault is common in structures such as car parks where the joint detail is badly designed and the workmanship poor. The edge beam could have been effectively covered by exposed aggregate panels or fluted to mask the junction between road slab and supporting cantilevered beam. An uncontrolled flow of water is finding a ready exit for salts in solution within the concrete or from outside.

4 PREVENTION

Designer, operative and occupier all have a part to play in avoiding disfigurement which may become evident sooner or later. Control of migrating salts is essential and water must be denied entry to a wall. These are the enemies of visual acceptance and conservation, but can be subdued by imaginative direction of rain, even in temperamental weather. Precautions should be taken at all times against the transfer of soluble chemicals into the fabric of a building, and it must be remembered that atmospheric gases adversely affect calcerous materials. When deciding on mix proportions it may be better to have a slightly lower fines content and a richer mix, but too little sand will allow segregation. Placing should be as rapid as possible consistent with full compaction, and the head of concrete should normally be less than 300 mm above the level of vibration. The designer must attempt to predict weather conditions over many years, and his aim should be to let each section of the building be cleaned by rain along lines which may actually improve its appearance. Vertical ribbing and fluting will control the flow and discourage concentration, and a finish can profitably be specified by reference to an existing structure in similar location and time. The water course must be predicted to its ultimate destination, and strategies should include intense modelling to overwhelm staining. This channelling will accumulate dirt in sheltered unwashed positions, but more evenly. The effect of weather on concrete may take the form of crazing or frost scaling, and the most vulnerable parts are those with laitance on the surface. Long term appearance can be improved by exposing new rough-textured surfaces containing a high proportion of inert particles, and the alternative approach is to constrain the flow along pre-determined paths which will be visually rewarding. CP 121 for walling provides a driving rain index for each of eight wind directions at 20 stations, while diagrams include details of copings to cavity walls and recommendations for bonding, jointing and pointing. Tables offer suggestions for mortar mixes, methods of reducing cracking, exclusion of rain and damp-proof courses. Rainwater must be evenly organized with attention to string course and cope; the minimum of run-off from flat roofs should be ensured by upstands at least 200 mm high. Below windows it may be expedient to inisist on hidden gutters behind panels so that these weather evenly, while a system of concealed flow may be successfully conceived by the use of neoprene tubing.

18 EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE

Concrete is unpredictable in surface coloration even when every care is taken in its casting. An understanding of the value of curing is essential in preventing too rapid drying which can lead to dusting and a friable finish. One approach is certainly the provision of vertical fluting at constant or variable spacings, with boldly expressed panels separated by distinct joints; the accent must be on pattern and profile to control rain dispersal and hide its hallmarks. It is sometimes beneficial to provide a separate, narrow feature at returns or corners near the top of a building, one side of which will be washed relative to the other depending on the prevailing wind, but eye-catching by its contrast with the adjoining main walls. Key words are detailing and diffusion, bearing in mind the consequential loss of face in more senses than one if salts are allowed to have their way. Weather in such cases is not the discourse of fools; its graceless outcome must be sidetracked, while rain and the natural ingredients of the ground as the arch enemies must be sensibly directed. Efflorescence is the end product of design and execution which are less than adequate, but can be controlled if not altogether prevented. Our graffitic generation should at least try to eliminate it, for concrete so afflicted is natures own vandalism; fortunately it can yield to scientific if not to psychological treatment. 4.1 CONCRETE QUALITY It is crucial that the concrete should be as dense and impenetrable as possible as the rate of salt attack is dependent upon the ease with which water can enter its surface or move within its mass. Particular attention should be given to optimum cement content, proportioning of aggregates and workability consistent with full compaction. It must be remembered that concrete is quantifiable on paper but not on site; the age at which it is put into use is another factor, as the material is vulnerable when still green, and external protective measures may be required in exceptional cases. To ensure minimum porosity and a hard, durable surface, particular care is required from the very beginning. A new understanding of concrete rheology is needed for a reduction in segregation and bleeding during placing and in its defacement from whatever cause. Quality control begins with the selection of its constituents and ends with thorough external treatment. 4.2 ADMIXTURES Well-dispensed admixtures can reduce permeability with increased durability so that rain penetration is reduced and colour retention achieved; good mix design and constant care are the basic guarantees against such contingencies. Airentraining or water-reducing admixtures and plasticizers will improve resistance to frost and to sulphates; workability aids assist in compaction and allow

PREVENTION 19

lower water/cement ratios. Repellents reduce the passage of moisture by countering capillary action, butyl stearate permits it to be better distributed through the matrix as an emulsion, while integral waterproofers can inhibit the migration of salts and lessen the need for cleaning. The subject is further covered in the section on mortar. 4.3 BLOCKWORK A building should be dimensioned to suit the precast module aesthetically and practically. Blocks should be carefully unloaded before laying, preferably above 4° C, and have joints of appropriate strength. Even units designed for facing work can be penetrated by driving rain, but effective membranes and cavities deal satisfactorily with excessive surface flow. Movement joints should be provided at changes of height and wall thickness to anticipate expansion of floor slabs, and at all openings. Common defects include unfilled perpends, bonding patterns not maintained, faulty lintel bearings and blocks built into a wall when excessively wet. Flexible damp-proof materials should be stored away from heat, and protected from damage by squashing. Blockwork should be divided into rectangular panels, bed-joint reinforcement inserted in courses above and below windows, and slip-planes inserted between the wall and a concrete roof. The units must be kept dry to minimize shrinkage quite apart from discoloration, and covers should not restrict the circulation of air, as otherwise condensation may form. The blocks should be used in order of delivery, permit inspection and sampling, and must be protected from sulphatebearing ground to avoid chemicals being drawn into them. Precast elements may be contaminated on site or in the makers yard, and different kinds should be kept apart. It is wise to stack them on a platform above the ground and to guard against corrosion of metals, frost action and change in moisture content. Lintels should have matured and dried before being built into the wall to prevent cracking at the ends due to shrinkage, while those cast in situ should be propped and allowed time to develop strength before carrying a load. Long units should be supported on damp-proof courses to permit movement. Care should be taken in the accurate setting out of the first course to avoid subsequent inaccuracy of the superstructure, and all units laid on a full bed of mortar with joints adequately finished. A careful study should be made of overall design concepts and the effective damp-proofing of walls subject to excessive surface flow and rain ingress. 4.4 CLADDING The choice of cladding will be dictated by a combination of practical, economic and aesthetic factors. Concrete can vary considerably in its weathering

20 EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE

characteristics and the designer should be aware of the likely changes in colour of both the units and their jointing. Conspicuous staining can be caused by uneven rain washing, particularly in association with glazed areas, and it should be anticipated that there will be some rain penetration and condensation within the cavity. Adequate provision for drainage and damp-proofing over openings should, therefore, be ensured, as any trapped water will find a way out, bringing salts with it. Cladding should normally be left to weather naturally, as cleaning systems sometimes adversely affect appearance. The migration of salts may be prevented by coating adjoining brickwork with a waterproofing agent before the cladding is fixed, providing a cavity of width recommended in CP 297 and CP 298. In certain cases efflorescence can be removed by a dilute acid followed by copious washing, and there is evidence that it may initially be reduced or eliminated by the application of a silicone-based waterproofer. BS 3826 recommends that waterrepellents should be applied to completed walls rather than to individual building units to avoid inadvertent treatment of beds which will affect mortar bonding, and to ensure that the faces of joints are also treated. The surface of precast units covered with silicone will give protection for some years. 4.5 MORTAR The main functions of mortar are to provide even load distribution, resist external forces, release stresses that might otherwise crack the wall and control alignment. Without admixtures, the best compromise is 1/1/6 of cement/lime/sand, although a 1/2/9 mix may be used for lower strength lightweight blocks when there is no risk of freezing during construction. A mortar should be no stronger than necessary for structural and durability purposes. Other properties such as workability and rate of hardening are also important, although mortar need only support the given load and have adequate weather resistance. Movements during construction can be taken up by a slow-setting mortar with the minimum of cracking and drying shrinkage, further reduced by the use of air-entraining plasticizers. A mix which is too rich may result in porosity of the jointing by hairline cracks which encourage penetration, whilst one which is too wet may soak the building units, unnecessarily promoting efflorescence by increasing moisture movement and dangerous if frost is expected. Most plasticizers contain an additive which improves the bond to high suction blocks. Masonry cements consist of Portland clinker and a small proportion of airentraining agent, together with a mineral filler which increases cohesion without undue water demand. These cements are usually lighter in colour than normal and have lower shrinkage and a better resistance to freeze/thaw conditions. Movement of excess sulphate-laden water from the mortar into blocks or bricks will occur if the units are very dry at the time of laying, but on the other hand

PREVENTION 21

prior soaking may lead to loss of adhesion, so that the right balance must be sought. Mortars containing admixtures can be more durable, assist in pigmented colour retention and have a water-retaining capacity which provides a tighter bond and prevents passage of moisture. By this means re-wetting of the mortar to keep it workable is unnecessary, and greater plasticity allows a longer spread. Some admixtures are supplied in liquid form ready for use, with less water needed for gauging, no difference in colour and the absence of chemical action. A mix of 1 part cement to 6 parts sand, plus a plasticizer, will improve resistance to rain penetration and frost attack, but in exposed conditions the cement content should be increased by richening to 4½ parts sand. Efflorescence forming round the edges of building units suggests that these have absorbed water from the mortar before it hardened and that its constituents have been converted into alkaline sulphates. There must be a happy balance between the porosities, but always consistent with overall durability. Even when no moisture has been absorbed from mortar during laying, these compounds may still be transferred to the blocks if the partly finished wall is left unprotected during rain, resulting in the formation of white bands. A similar effect is seen if the unit itself is impermeable but the mortar is less so. The chief danger to brickwork from the presence of salts is that of sulphate attack, showing itself as expansion of the mortar or horizontal cracking of the rendering which becomes hollow and falls away. If the wall has to be rebuilt and salts are likely to persist, the mortar should be made with sulphate-resisting cement, but the basic aim is to prevent further attack by the provision of a damp-proof course at all levels. Unnecessarily strong mortars can lead to wide cracks, while a weaker but still durable mix yields sufficiently to accommodate small movements in a wall, with distribution into cracking which is less permeable. Mortar made on site should preferably be mixed by a machine which is regularly cleaned to avoid contamination, or if by hand in small quantities on watertight platforms. Pointing is also significant as a drying surface, for when it is denser than the mortar, evaporation at the joints will be restricted. Calcium chloride is not effective in protecting from frost by heat evolution, and may lead to dampness and corrosion of wall ties. 4.6 JOINTS The greater use of thin easily assembled concrete units, the more the need to consider the dangers of water ingress. Wall thickness no longer overcomes the natural absorbency of the material or its constituent parts, so that jointing needs very special attention. Ideally a feature should be made of all joints and, although grooves may give rise to streaking, the division of large areas into panels of appropriate size will distract sufficiently by their composite appeal. When precast panels are being fixed, accurate alignment to agreed tolerance is essential so that

22 EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE

there is minimum interruption to the flow of rainwater. The performance of sealants is important, as the failure of mastics in expansion joints by loss of adhesion or by restraint must be avoided. If, in spite of precautions, a light deposit does appear, there is obviously faulty damp-proofing or drainage. No remedial action will hide a poorly made junction between lifts, and this is best attained by casting against a slightly roughened face which is dry and not treated with a priming layer of mortar. Contraction joints may also present a visual problem in that they will appear as strongly marked lines and must be regularly spaced and bear some relation to aspects of the elevation such as sills and lintels. Sometimes a small lip is left which, if not buffed off, can divert the flow of water to result in undesirable staining. 4.7 PIGMENTS Pigments minimize surface differences, contribute visual interest and should be regarded as inert fines. Sufficient intensity of colour is obtainable by a small addition, although consistency can be affected by differing vibration, leading to local concentration. Moreover, it will in time be masked by soot deposits and should, therefore, be secondary to form, texture and three-dimensional features capable of dictating water paths. In coastal areas it may be advisable to specify a shade lighter than would otherwise be chosen, as sea salts play a role in the disfiguration of darker surfaces. 4.8 DETAILING New concrete has a lightness of tone which emphasizes differential grime, to which efflorescence will make its own contribution if the surface is ill-conceived. Detailing cannot be dismissed as irrelevant and there has justifiably been a longstanding practice of capping which, for good measure, gives a strong outline. Adequate throating will throw the water clear and although it will probably blow back at a lower level, it will at least be scattered and less pronounced. Consideration must be given to the relationship of all the materials and features making up the structure, with particular emphasis on openings and projections; although forming a pattern themselves, their individual treatment is crucial, as even a small crack can cause discoloration if it is fed by rain on a large area of glazing. Care must also be taken to anticipate the impact of contaminated salts, air pollution and the general susceptibility of wallings, rendering and backings, before deciding on suitable protective measures. Traditional units were smaller than the present day concrete monoliths and, while each element varied slightly, the overall effect was often strong enough to subdue weather marking. The modern preference for cement in mortar has also upset this balance. Lack of attention to the disciplines of practical construction

PREVENTION 23

using sensitive detailing has led to some criticism of concrete, especially in the current quest for new shapes and finishes which are not always proof against the rigours of weathering. For example, a mastic joint would allow for slight movement to prevent ingress and it may be advisable to specify sulphate-resisting cement where chemically vulnerable in wetter districts. There is often a finely balanced combination of direction and duration of rain with variable evaporation and in all prior assessment of this movement, salt-laden or otherwise, previous experience of any locality is invaluable and salutary. Such a study should be implemented by a consideration of all surface projections and by the provision of membranes below ground floors and around foundations. BS 4315 lists methods of measuring resistance to rain penetration by recording the increase in area of dampness and amount of leakage. Transfer of dirt from a horizontal to a vertical surface can be avoided by raking backwards, while reveals may be recessed and sills inclined to grooves discharging into a drainage pipe. Rain splashings from pavements can often contribute to the disruption of plinths, and there is a case for polished granite in this situation. 4.9 DAMP-PROOFING Damp-proof courses should be inserted just above ground level and below overhangs, with flashings of impervious sheeting firmly bedded and provided with adequate lapping to cover any intersection or vulnerable joint. Copings should be specified for walls to throw rainwater clear of the faces, well-dripped and prevented from displacement. The aim is to keep any form of moisture out of the structure and to reduce concrete shrinkage; it is necessary to recognize the effect of leakage other than that due to cracking, and to foresee all points of ingress. 4.10 FORMWORK The performance of a concrete face is largely conditioned by formwork. Its effect on discoloration arises from variations such as lining absorption, displacement of release agent and workmanship during and after compaction. The form should be well supported to ensure rigidity in countering excessive vibration; lack of ability is sometimes evident along bridge parapets where a regular pattern coincides with vertical supports, and this can be aggravated by carelessness at the erection and removal stages. After striking, a carbonation layer is established on exposure to air; the more porous the concrete the thicker the layer will be. The surface of structures to be backfilled should, therefore, be left for a few days to develop this protective skin. Free water on a newly cast, non-carbonated surface is undesirable and it is often a disadvantage to strip the shutter at an early age, quite apart from mechanical damage caused by premature removal. Recommendations as to winter concreting

24 EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE

are given in CP 110 where a strength of 5 N/mm2 is suggested at a concrete temperature not less than 5°C, and striking times are tabulated in CIRIA Report 36. For high quality fairfaced concrete the goal is a surface as struck without the need to make good. The number of uses should be restricted, all fixings applied from behind and the holes set out to an agreeable design. Joints in the shutter should be well taped to avoid grout runs spoiling the lift below, while release agents should be carefully selected and consistently applied. 4.11 RENDERING Rendering mixes should be weaker than the surfaces to which they are applied and each successive coat no stronger nor thicker than its predecessor. New work should be kept damp for the first three days and the second coat delayed until the first has hardened. Although slight watering may be necessary to reduced suction, free moisture should never be left on the surface, and suitable admixtures may be used to reduce loss of water and improve workability. Dense coatings such as 1/3 cement/sand should not be applied to walls with appreciable sulphate content and in danger of shrinkage cracking. If so damaged, the facing should be stripped off and the brickwork allowed to dry before re-application with a more porous mix of 1/1/6 cement/lime/sand which permits readier evaporation, and using a sulphate-resisting cement for both rendering and jointing mortar. 4.12 URBAN ENVIRONMENTS The designer in an urban area should regard pollution as inevitable. The extent of sulphur dioxide in the air can be regrettably high and to offset this, coloured concrete or exposed aggregates should be specified. The cement content of all facings must, of course, be consistent with durability. The other key factors in assessment of any kind are location, aspect, severity of climatic conditions and the effects of aggressive salts. While one approach may lie in cleaning the building regularly, this will accentuate the dirt but mask a light deposit. A hopeful maxim is that the rougher a wall the less the signs of staining, so that it may be better to specify a coarse-textured dissipating surface. Discoloration of concrete can be caused by chemical changes in adjacent materials or emerging deposits washed by rainwater from windows above, however much these openings divert from its overall pattern. Backward sloping surfaces have a tendency to streak because of partial water flow, while forward slopes may collect silt at their bases. Ideally the elevation should be modelled to attract this in areas of greater shadow, if by so doing the other parts are kept cleaner. The art of concealment and prevention should be practised, with attention given to extreme weather attack on exposed faces or relatively unwashed alcoves and returns.

PREVENTION 25

Concrete is generally less permeable than traditional materials, but its absorbency is not uniform and it colours unevenly. No building is self-cleansing and smooth surfaces tend to weather in the manner dictated by rainfall along random paths. The remedy is to influence this movement by regulating its sequence or to scatter by the use of say abrasive blasting. The striations of a ribbed profile hide imperfections, and when the edges are hammered off they assume the appearance of hewn rock. Streaking may only occasionally be acceptable as, for example, on the soffit of bridges, but even this can be objectionable where it is obviously the result of poor design. When considering surface permeability it should be remembered that calcined flints are themselves minutely crazed in baking and absorb water in hairline cracks, while cement paste between fine aggregate particles will also absorb unless conscientiously cured. A good concrete can be marred by dark patches with a lower water/cement ratio than the body of the wall, and it is difficult to ensure complete uniformity over large smooth areas. It may be advisable to emphasize shadow effects and even the lifts themselves without excessive obtrusion. The effect of disfiguration can also be reduced by planning contrast between adjacent faces using panels alternating in colour and profile. If the concrete is at risk from chemical or corrosive attack at the rear face it would be sensible to coat with a waterproof backing, well-bonded and flexible enough to inhibit cracks or faulty joints through which salts might filter. 4.13 SURFACE TREATMENT The specified requirements for concrete surfaces to suit extreme weathering are stringent, but must be offset against the cost of cleaning or repair. Efflorescence is only one form of tarnish in a range from scaffolding rust to grout spillage but, unlike others, is often an in-built irritation. It may be necessary to paint the surface after careful preparation, as has profitably been done on motorways, where discoloration is offensive even at speed, but concrete can be restored to something approaching its original appearance. Painting may be considered for aesthetic reasons or for easier run-off. The colour scheme should be wellbalanced using the many shades available, but their number kept to a minimum for any given project. Another approach is to use transparent coatings such as silicones which fill the pores as water-repellents but can result in subdued evaporation and the possible build-up of disruptive crystalline growth under the surface. Such coatings should not be used to improve the durability of the concrete if already suspect by virtue of salt action. A soluble silicate in powder form may give reduced permeability and protection against the creation of further crystals in the pores, but this assumes good mix design and quality control. All walls containing cementitious products are vulnerable to alkali attack and either non-saponifiable paints or alkali-resistant agents may be employed.

26 EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE

Plate 5: The result of not extending the exposed aggregate panels to cover the edge beam. Efflorescence in the lower part has been dissipated by the bold rounded aggregate. There has been failure to ensure a satisfactory bond between slab and beam, and to prevent ingress of water. Another approach would have been vertical fluting instead of concrete off a plain shutter.

Emulsions are also valuable in that they are formulated with polymers of similar properties; when used for rendering they should be applied thinly and should penetrate the surface rather than create a glossy film to which paint may not adhere. Efflorescence can be difficult to scrape off and may disrupt an impervious decorative layer, but can be covered over if a suitable primer is applied.

PREVENTION 27

Plate 6: This illustrates the failure to prevent efflorescence at the junction of otherwise attractive panels by a suitable mortar mix, with the resulting entry of water from the deck.

28

5 REMOVAL

5.1 WASHING AND BRUSHING Feathery or fluffy efflorescence should be allowed to dry out before removal with a bristle brush; even if glossy and encrusted it is often possible to eradicate by scraping when it is not strongly keyed, first softening the deposits and then discarding them before washing the surface with water. This should preferably be carried out by men skilled in the procedure and with a knowledge of local conditions. If some of the salts soak back into the wall and re-appear, the same techniques should be adopted until their movement ceases or they at least become acceptable. Continued physical treatment, vigorous or otherwise, demands sensitive workmanship to avoid damage, and restoration of important buildings should only be undertaken by specialist firms. Before dealing with an extensive outbreak of efflorescence, it is advisable to consider the effect of cleaning a small section which may show up another. It is better to leave certain areas until the concrete is, say, a month old to let nature take its course. In any case, little can be done to remove hydration discoloration which is a more deep-seated contrast in shade. While efflorescence can often be removed by water only, and possibly with jetting, any treatment aimed at suppressing evaporation may lead to crystallization below the surface, entrapment in the pores and spalling. In sheltered spots, deposits not reached by rain can sometimes be removed by repeated washing. It is important to remember that dirt at any point not only affects appearance but acts as a reservoir for harmful chemicals and hides evidence of decay. All may be dissolved in due course by natural means, but if it is too tenacious and the time factor important, other remedies must be applied. Particular care should be taken with structures of architectural or historical importance when cleaning by mechanical means such as grit-blasting and grinding or buffing discs, always guarding against dust if the surface contains free silica. If in doubt it is better to use hand tools, non-ferrous brushes or abrasive blocks rather than rotary power unless the staining is deep-seated and widespread. Wire brushing should only be applied in extreme cases and never to sand-faced surfaces

30 EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE

likely to be damaged by attention of this kind. If cleaning by water spray working from the top of the building, it should be directed only in sufficient quantity to keep the deposits moist until they soften, generally in a few minutes. They may be amenable to pressure hosing, supplemented by careful and timely brushing, a disadvantage being possible leakage into the actual building, although modern techniques have made this unlikely. Operations should be restricted to those areas giving most offence, but at the risk of blotching, as attempts to fade out are rarely successful. 5.2 ACIDS If brushing is unsuccessful, treatment with a 5–10% solution of hydrochloric acid is normally effective. The surface would first be wetted down and the acid applied with a soft brush or broom. Then, as soon as the deposit has dissolved and chemical action ceased, the area should be washed thoroughly, starting from above and ensuring that galvanized and aluminium metalwork is protected. For integrally coloured concrete a more dilute 2% solution will prevent undue penetration which may otherwise expose the aggregate in places and radically change the texture. Again the face should be dampened beforehand to prevent absorption of the acid. In all cases a trial should be carried out on an inconspicuous part, applying the solution to areas about a 1 m2 or less, waiting a few minutes before removing the salt with a stiff brush and cleansing copiously. In the main operation, entire walls should, if necessary, be treated to avoid aggravating the situation. Among other forms of agent are those containing hydrofluoric acid, leaving behind no soluble chemicals but running the risk of unsightly etching of windows. Also recommended for cement-based surfaces is the application of a 10% solution of phosphoric acid which converts lime to an insoluble compound before suitable priming and painting. If the wall cannot be obtained in a dry condition it must be covered with a porous alkali-resistant coating, ensuring that conditions of heat and ventilation are good. Although proprietary materials are available for the removal of mortar stains, there is no chemical which will completely inhibit the effects of a saturated salt solution, even if the wall is sealed by silicone-based repellents. A typical cleaner, stored in plastic or rubber containers, is applied by brush and immediately washed. Coverage of this mild acid is about 20 m2 per 5 litres depending on surface porosity and the thickness and extent of deposits. It will also deal with algae growth and normal atmospheric staining, and there is no residual chemical reaction when it is used either neat or in equal parts with water. However, in the first place the use of a softer brush and dilute solution over a restricted area is recommended and the surface should be quickly washed down, as indeed is good practice after every application of this kind, whatever the concentration. At joints, while stains caused by leaching from fresh mortar or soaked blocks can also be removed by dilute acid, it may be

REMOVAL 31

advisable to paint the surface of rendering rather than attempt to clean it if the deposit is difficult to eradicate or likely to re-appear. Corrosive substances should be handled with rubber gloves, and goggles and other protective clothing worn. Care must also be taken to wipe splashes off the skin with bicarbonate of soda or at least soap and water. Above all, it is best to seek expert advice and to engage specialist operatives, as acids can permanently scarify and must be treated with respect. The cleaning of a building should be sensibly planned, including safety measures, and a study made of all available methods relative to the extent and origin of the stains.

32

6 CONCLUSION

Discoloration of concrete surfaces, however transient, can present a complex problem to designer and contractor alike in its cause and prevention. The degree of disfigurement is determined by vagaries in weather and workmanship but initially results from lack of knowledge on how best to disperse rainwater over a structure. Many blemishes arise from defective detailing, damp-proofing and construction joints as the crucial factors, whilst localized staining can be caused by variation in porosity. It is often difficult to prevent the movement of salts which pass into solution and on evaporation become more concentrated into solids which are superficial but unsightly; while porous materials are more likely to bring these to the surface, there can be disruption if evaporation is inhibited. All elements of a building should be selected as to minimum salt content, and saturation prevented by sensible stacking and protection of work as it proceeds. When rendering a wall, too much sand or excessive trowelling should be avoided. Unduly strong jointing mortar can lead to fine cracks, through which the alkaline solution may filter and leave its mark. The answer also lies in breaking up the surface by attractive profiling or the imaginative use of panels and exposed aggregate. Above all, care should be taken to ensure that the concrete is of high quality and reliable durability, with attention given to grading of the mix, optimum cement content and effective curing.

34 EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE

Plate 7: A close-up view of a parapet where fluting has certainly helped to break up the surface and direct the flow of rainwater down the grooves at varied centres, but the feature has not been taken down to the foot of the wall and has failed to prevent corresponding stains on the lowest quarter of the face.

7 BIBLIOGRAPHY

VIEWPOINT PUBLICATIONS The following are available from Eyre & Spottiswoode Publications Limited, Distribution Centre, George Philip Services Limited, Arndale Road, Wick, Littlehampton, West Sussex BN17 7EN. TATTERSHALL, G.H. The workability of concrete. 1976, 138 pages, A5. Order No: 11.008 RICHARDSON, J.G. Concrete notebook. 1974, 92 pages, A4. Order No: 12.063 RICHARDSON, J.G. Formwork construction and practice. 1977, 275 pages, A4. Order No: 13.019 RICHARDSON, J.G. Formwork notebook. 1982, 2nd edition, 120 pages, A4. Order No: 12.082

CEMENT & CONCRETE ASSOCIATION The following are available from the Publications Sales Unit, Cement & Concrete Association, Wexham Springs, Slough SL3 6PL, Berkshire. 42.313 42.380 42.384 42.414 42.467 42.480 42.493 42.505 45.007 45.012

The effect of weather on the formation of efflorescence Concrete surface blemishes The influence of concrete mix proportions and type of form face on the appearance of concrete Construction joints in concrete A survey of literature relating to the properties and use of concrete blocks The crazing of concrete An investigation into the incidence of colour variation in formed concrete surfaces Mechanical damage to concrete by early removal of formwork Winter concreting The determination of proportions of aggregates

36 BIBLIOGRAPHY

45.013 Concreting in hot weather 45.015 Chemical methods of removing stains from concrete 45.016 Impurities in aggregates for concrete 45.020 Controlling algae and other growths on concrete 45.022 Air-entrained concrete 45.030 Superplasticizing admixtures in concrete 45.031 Concrete mixes for general purposes 45.034 Concrete in sulphate-bearing ground

Man on the job leaflets 45.101 45.102 45.103 45.104 45.105 45.106 45.107 45.108 45.109 45.110 45.111 45.112 45.113 45.114 45.115 45.116 45.117 45.118 46.001 46.019 46.504 47.008 47.010 47.018 47.020 47.101

Cements Aggregates—delivery and storage Testing aggregates Concrete admixtures Reinforcement Batching and mixing concrete on site Transporting and pumping concrete Placing and compacting concrete Curing concrete Making good and finishing Tooling concrete Testing for workability Concrete test cubes Construction joints Formwork Ready-mixed concrete Dry lean concrete Concreting in cold weather Concrete finishes for highway structures The appearance of concrete highway structures Watertight concrete construction Abrasive blasting of concrete surfaces Specification for high quality finishes Striated finishes for in situ concrete The curing of concrete Visual concrete—design and production

47.102 47.103

External rendering The control of blemishes in concrete

BIBLIOGRAPHY 37

47.104 Efflorescence on concrete 48.010 White concrete 48.037 Concrete practice 48.041 Precast concrete cladding 48.043 Model specification for concrete blockwork 94.017 The brighter face of concrete REPRINT 1/80 Structural concrete finishes: A guide to selection and production CATALOGUE 1982: Part 1—Publications, slide sets and films Part 2—Research, development and technical reports

BUILDING RESEARCH ESTABLISHMENT The following are produced by the Building Research Establishment of the Department of the Environment, and are published by H M Stationery Office. BRE Digest No 45: Design and appearance—1 No 46: Design and appearance—2 No 54: Damp-proofing solid floors No 77: Damp-proof courses No 85: Joints between concrete wall panels: open-drained joints No 113: Cleaning external surfaces of buildings No 125: Colourless treatments for masonry No 127: An index of exposure to driving rain No 137: Principles of joint design No 160: Mortars for bricklaying No 176: Failure patterns and implications No 177: Decay and conservation of stone masonry No 196: External rendered finishes No 237: Materials for concrete No 244: Concrete mixes No 245: Rising damp in walls No 250: Concrete in sulphate-bearing soils No 255: Index of digests

Current Paper 23/77: Chemical resistance of concrete Information Paper 6/81: Carbonation of concrete made with natural aggregates BRE Information Directory 1982 HMSO Sectional list No 61: Construction 1982

38 BIBLIOGRAPHY

BRITISH STANDARDS INSTITUTION The following British Standards and Codes of Practice are published by the British Standards Institution, 101 Pentonville Road, London N1 9ND. BS 12: 1978 BS 743: 1970 BS 882: 1975 BS 890: 1972 BS 1014: 1975 BS 1180: 1972 BS 1198: 1976 BS 1199: 1976 BS 1200: 1976 BS 2028, 1364: 1968 BS 3826: 1969 BS 3921: 1974 BS 4315: Part 2: 1970 BS 4551: 1980 BS 4887: 1973 BS 5075: 1982 BS 5262: 1976 CP 110: 1972 CP 111: 1970 CP 121: Part 1: 1973 CP 297: 1972 CP 298: 1972 YEARBOOK 1983

Ordinary and rapid-hardening Portland cement Materials for damp-proof courses Aggregates from natural sources for concrete Building limes Pigments for Portland cement and Portland cement products Concrete bricks and fixing bricks Building sands from natural sources Sands for external renderings Sands for mortars Precast concrete blocks Silicone-based water repellents for masonry Clay bricks and blocks Methods of test for resistance to air and water penetration Methods of testing mortars and specification for mortar sand Mortar plasticizers Concrete admixtures Code of practice for external rendered finishes The structural use of concrete Structural recommendations for loadbearing walls Brick and block masonry Precast concrete cladding (non-loadbearing) Natural stone cladding (non-loadbearing) Summaries of British Standards

ADDITIONAL BIBLIOGRAPHY PERKINS, P.H. Concrete structures: repair, waterproofing and protection. Applied Science Publishers Ltd., London, 1977. GAGE, M. & KIRKBRIDE, T. Design in blockwork. Architectural Press Ltd., London, 1980 WILSON, J.G. Exposed concrete finishes. Vol 1—Finishes in in situ concrete and Vol 2— Finishes to precast concrete. G R Books Ltd., London, 1962. GAGE, M. Guide to exposed concrete finishes. Architectural Press Ltd., London, 1970 NEVILLE, A.M.and CHATTERTON, M. New concrete technologies and building design. Pitman Publishing Ltd., London, 1979.

BIBLIOGRAPHY 39

ADDLESON, L. Materials for building. Vol 3—Water and effects. Iliffe, London, 1972. BUILDING RESEARCH ESTABLISHMENT. Principles of modern building. Vol 1—The building and the wall, 1975 and Vol 2—Floors and roofs, 1977. HMSO, London. CONCRETE SOCIETY. The weathering of concrete. Symposium, London, 1977. CONCRETE SOCIETY. Guide to chemical admixtures. London, 1980. SPECIFICATION. Building materials and products. Architectural Press Ltd., London, 1982. SEALANT MANUFACTURERS CONFERENCE. Manual of good sealant practice. London, 1976. MARSH, P. Air and rain penetration of buildings. Construction Press Ltd., Lancaster, 1977.

40 BIBLIOGRAPHY

Plate 8: Good example of fluting and tiling on a self-cleansing facade.

8 APPENDIX: CHECKLIST TABLE

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