IMPROVING CONCRETE BY WETTING AGENTS
TH
Reprinted from "THE RHODESIAN BUILDING AND ALLIED TRADES JOURNAL"
HE apparent simplicity of making concrete may easily lead to a lowering of the standard of performance and to unsatisfactory results unless certain fundamental principles be fully appreciated.
Of the three essential ingredients of cement - aggregate, cement and water-the most important, and yet probably the one most frequently disregarded, is undoubtedly the water. The qualities of stone, sand and cement are naturally of con- siderable importance for good con- crete, as also are their ratios one to
the other in so far as they determine to a large extent the mechanical properties of the wet mix, but it is the quantity of water present in the mix which determines more than anything the quality of the finished
concrete.
It is convenient to express water- cement ratios as the quotient obtain- ed by dividing the weight of water present (including that present in the sand and stone) by the weight of cement in the mix. Thus, a mix consisting of 4 cwt. stone (containing 2 per cent. moisture), 2 cwt, sand (containing 5 per cent. moisture), and 1 cwt. cement, to which is added 4 gallons water during mixing, would have a water content of ap- proximately 9-lb. (in the stone) plus 11-lb. (in the sand) plus 45-lb. (added), a total of 65-lb. and a cement content of 112-lb. The
water-cement ratio is therefore
65/112 or 0.58. It may be noted that the amount of water contained in the aggregate is an appreciable fraction of the whole amount, and any disregard of this contained water may result in a serious increase in waler-cement ratio and a correspon- ding loss in strength. The moisture contents of the aggregate quoted above are reasonable average figures for temperate climates, but when freshly washed or following heavy rainfall, the water contents of the sand may rise to 10 per cent. or even higher; that of the stone will not increase so much (to 3-4 per cent. perhaps).
The importance of maintaining the correct water-cement ratio may be judged from the following typical crushing tests carried out on concrete cubes made from mixes differing only in water-cement ratio.
Water Cement.
Ratio
Compressive Strength (lb. per sq. in.)
0.50
5,850
0.54
5,000
0.58
4,100
0.62
3,200
0.66
2,300
It will be seen that if in the exam- ple quoted previously, the mixture been 3 per cent. and 7 per cent. contents of the stone and sand had
respectively, the total water content would have been 13-lb. plus 15-lb. plus 45-lb., or 74-lb, and the water- cement ratio 0.66. From the above table it will be seen that such a
relatively small alteration in the water content of the aggregate would have had the effect of reducing the compressive strength of the resultant concrete from 4,100 p.s.i. to 2,300 p.s.i., a loss of 44 per cent.
The measurement of all materials used in concrete making calls for more care than is usually given, and the most satisfactory method is un- doubtedly that of weighing all components, including water.
If the method of measuring by volume has to be used, allowance must be made for the considerable increase in volume which sand undergoes when passing from the dry to the moist state, or "bulking as it is termed. The increase may amount to as much as 33 per cent. in volume when the sand contains only 6 per cent. water, so that a cubic foot of such sand weighs only about 80-lb. instead of about 100-lb. (the weight when dry. Hence, if
it be intended to add 100-lb. of sand to a mix, and the measurements be done by volume, 1 cu. ft. of dry sand will suffice, but 1-1/3 cu. ft. of moist sand must be added to give the desired weight.
The significance of the proportions of aggregates to cement may now be considered. As already stated above, these proportions do not enter into the question of the strength of the concrete (assuming, of course, that the aggregates are sound) pro- vided that the latter is fully compact- ed, that is, consolidated so that it is free from voids. It is the fulfilment of this last condition which has a direct bearing on the amount of aggregate which may be incor- porated into the mix. A mix containing little cement may thus yield on complete compaction a con-
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crete having the same compressive strength as a rich one if the water- cement ratios be the same, but it will be obviously much more difficult to achieve complete compaction in the case of the former mix through there being less total water present, and hence a reduced fluiding. If the total water be actually insufficient to permit complete compaction, the compressive strength will be reduced and the concrete will contain voids. Complete compaction may then be attained by the addition of more water to the mix, but this will also lower the strength to a value cor- responding with the increased water-
cement ratio.
The properties of aggregates in the mix thus determine its work- ability, namely, the amount of energy required to compact the wet concrete. Mixes containing high proportions of aggregates are less workable than richer mixes, and require more tam- ping in order to consolidate them, or in
in order to achieve efficient packing round reinforcement.
Workability is a most important property of concrete, since no matter how dry a mix is designed in the interests of high compressive strength, if it cannot be placed and consolidated efficiently the advantage of the potentially higher strength is completely lost. On the other hand, the use of an excessively rich mix in order to achieve good workability will only result in a wastage of
cement unless the water-cement ratio
is also reduced.
Portland cement is a complex mixture of various calcium silicates in admixture with compounds of calcium, aluminium and iron, which is capable of reacting with water to form a hard mass. This inter-action between the solid cement particles and the water is the essential feature of the setting and development of strength of concrete and it is most important that every particle of cement should have ample op- portunity of reacting as completely as possible with the water present. There are, however, several factors which tend to make this inter-action more difficult. First, the cement particles may be clumped together, so that the water cannot easily reach those in the centre of the mass. Secondly, the reaction itself results in the formation of a jelly-like