Shrinkage of Natural Aggregates in Concrete
T
THIS
Digest, which is based on the results of work carried out at the Scottish Laboratory of the British Building Research Station, discusses the effects on concrete of using shrinkable natural aggregate; it gives a method of determining their shrinkage, with recommendations as to the most suitable uses for concrete made with particular aggregates.
HE drying shrinkage of concrete made in accordance with good practice is usually fairly small-of the order of 0.03%-and is normal. ly taken into account in design. This movement is generally attributed to the properties of the cement, since most natural aggregates for concrete used in the United Kingdom, whe ther of gravel or crushed rock, do not expand or shrink on wetting and drying. Certain aggregates, how- ever. e.g. some dolerites, do exhibit high volume changes on wetting and drying, leading to drying shrinkages of concretes made with them which may be as much as four times those obtained with non-shrinkable aggre- gates. In fact, it has been found that when a shrinkable aggregate is used, the shrinkage of the concrete is increased by an amount at least equal to the shrinkage of the aggre- gate itself (see Appendix I).
Examination of aggregates from many sources, particularly in Scot- land, has shown that drying shrink- age and moisture expansion is not uncommon or confined to dolerites. This movement inevitably gives rise to an increase in the shrinkage of the concrete even when otherwise good practice is followed, though it does not lead necessarily to undue cracking or poor weathering proper. ties. For example. two aggregate; widely used in Scotland, one a crushed whinstone, the other a pit gravel, repeatedly produced con- crete with a shrinkage of 0.1% mak- ing it impossible to manufacture precast units complying with the shrinkage limits of the relevant Bri- tish Standards. The shrinkage of concrete in asymmetrically reinforc ed concrete slabs causes deflection. Laboratory work has shown that the greater the shrinkage of the aggre- gate used for the slabs, the greater their deflection.
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The level of shrinkage resulting from the use of shrinkable aggre. gate can be reduced to some extent by taking the normal precautions, such as control of the mix propor tions the leaner a concrete is in cement, the lower in general will be its shrinkage and control of the quantity of mixing water the lower the water content, the lower will be the shrinkage of the concrete. Even when such precautions are taken, however, the use of shrink- able aggregates may still have im- portant practical consequences.
UNREINFORCED CONCRETE
In mass concrete, e.g. gravity dams, the overall shrinkage of the concrete may not be of prime im- portance since complete drying out is unlikely ever to take place. Any increase in movement between ad- joining monoliths will be taken care of by the normal system of water bars. Nevertheless, during drought conditions it is possible that exposed parts of a dam could dry out at the surface and give rise to a network of cracks which could lead
lead to spalling of the concrete due to frost action.
Precast concrete products, also, may shrink by
an unacceptable amount; where shrinkable aggre- gates are used, the limits for shrink- age laid down in British Standards 2028, 1217 and 1180 are sometimes unattainable.
It has been found that concrete with a high shrinkage is more liable to deteriorate when exposed to the weather in precast concrete products such as cladding units, cills, copes and kerbs.
There is a disproportionate in- crease in the shrinkage of no-finės concrete when shrinkable aggregate is used, see Table 2, and this,
coupled with its low tensile strength, appreciably increases the risk of cracking.
REINFORCED CONCRETE
The effect of high shrinkage in reinforced concrete is now being studied at the Scottish Laboratory. Information from abroad indicates the importance of shrinkage in this context. Excessive deflection of a reinforced concrete bridge in South Africa has been attributed to highly shrinkable aggregate: because the main members were asymmetrically reinforced, a shrinkage differential was set up across the section which resulted in warping. Drying out to this extent is unlikely in Britain. but similar effects might occur in furnace blocks and electrically heated floors, for example. It has been demonstrated that simply-sup- ported slabs made with high-shrink- age concrete, without imposed load- those ing, deflect far more than made with concrete of normal shrinkage.
In storey-height cladding units, consisting of panels stiffened by robust ribs, the risk of cracking is always high because of differential shrinkage, and the position is ag- gravated if a shrinkable aggregate is used.
RECOMMENDATIONS
In considering for what purpose concrete made with a particular shrinkable aggregate is suitable, it is necessary first to determine the shrinkage of the concrete accord- ing to a standardised procedure. A test method has been developed at the Scottish Laboratory, the actual measurement of shrinkage being based on the test procedure of B.S.1881, and is given in detail in Appendix II.
THE HONG KONG & FAR EAST BUILDER VOLUME 18, NUMBER 2
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