风化和混凝土彼得罗素变色.pdf

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 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. 2INTRODUCTION 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. Todays 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 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 air- entrained 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. 4EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE 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. CAUSES5 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 6EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE 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