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Composition of concrete.
2. There are, however, some points concerning which some modification or reconsideration appears to be desirable. One of those concerns the composition of the Concrete.
Three qualities of Cement Concrete are specified, for different portions of the work, composed as follows:-
Parts by Measure.
1st Quality. 2nd Quality, 3rd Quality.
Determina- tion of voils in
Cement.... Sand
Broken stones
1
1
୬
5
1
21 6
Now, I am of opinion that in making concrete, care should be taken to make the resultant mass thoroughly homogeneous and free from voids. To fulfil this condition, it is essential that the voids between the broken stone forming the matrix shall be entirely filled with cement mortar, and secondly that the interstices between the sand-grains shall be completely filled with cement. Most of the troubles which have occasionally been experienced in connection with concrete, are traceable to porosity. Portland cement, though practically insoluble when in solid mass, is not so when finely-divided. If water be caused to percolate through a porous mass of Portland cement and sand, the interstices between the grains not being fully filled with cement, the mass will disintegrate in time.
It has, therefore, been my practice, in case of important works like the present, not to resort to any arbitrary prescription, as to the composition of concrete; but to determine, experimentally, the percentage of voids in the matrix and in the sand. and then to fix the proportions in a scientific manner.
3. The proportion of voids in the broken stone may be determined as follows. Take a sample of the broken-stone, and soak it in water. Then drain off all water, broken stone. not actually absorbed. Take any water-tight vessel, such as a bucket or cask. Weigh it empty. Let its weight empty, or tare, be T, then fill with water, and let the weight of bucket and water be W, so that the nett weight of the contained water will be W-T. Empty out the water, and fill the bucket with stones. Weigh again, let the weight of the bucket and stones be S. Now add water, so as to fill the interstices, between the stones. Weigh again.
Weigh again. Let the final weight (bucket, water
Voidsin sand.
and stones) be P. Then
P-S
W-T
Percentage of Voids.
The percentage of voids in stones, broken to approximately uniform cubes, like road-metal, is about 50%. This may however be materially reduced by using assorted sizes, the smaller particles inserting themselves between the larger masses. Having determined the actual percentage of voids, a sufficient proportion of cement mortar must be provided, to fill them, when the cement and sand are mixed and moistened ready for use, in a plastic condition.
4. The voids in the sand which is to be mixed with the cement may be deter- mined in like manner. Usually they amount to about 33%, so that three of sand to one of cement, is about the poorest mixture that can be expected to be non-porous, It usually suffices however to determine, experimentally, the volume of plastic mortar which is formed by some given proportion of cement, sand and water. For example, take say one cubic foot of cement and two of sand. Mix with water, into a paste of proper consistency, and measure the resulting mortar in a cubic foot box. Usually two and one will give 2 to 22 of finished mortar. Suppose that the latter is the case, and that the percentage of voids, in the matrix, is 50%. Then the proper proportions will be 42: 2: 1.
The proposed mixtures may or may not fulfil the conditions of the case. This will depend upon the percentage of voids in the matrix. The first quality mortar seems likely to be homogeneous. One of cement and two of sand makes, usually, about two parts of mortar, which, with the usual voids, will suffice for 4
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