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The quantities of water retained for the different conditions of test are expressed in percentages of the cement by weight. The quantities of combined water assumed are 12.6, 14.0, 15.0, 16.2 and 17.4 per cent of the weight of the cement for the five mixes from left to right. This increase in combined water with an increase in water- cement ratio is in accordance with the data quoted from Wilson's tests. It can be seen by a study of Tables I, II and III that these allowances for Fig. 1 represent about 7 days moist curing if the basis for fixing the combined water is assumed as dry air at 120 deg. Combined water and water tightness is studied in the light of the foregoing discussion, it will be easy to see how the water tightness of the concrete is so closely related to the quantity of mixing water and the extent of the curing. This is made particularly apparent through the upper diagram, in which each ordinate represents a unit volume of the paste from the corresponding mix in the lower diagram. Comparing the rich mix at the left with the lean mix at the right, it is seen that in the former 36 per cent of the volume of the paste is represented by uncombined water, while in the lean mix the uncombined water is 60 per cent of the volume.

The effect of this difference on the water tightness of the paste is even greater than the percentages indicate. The size of the individual pore spaces in which this uncombined water is held also affects the permeability, for the flow of water under pressure is materially impeded by a reduction in size of the channel. The rich mix, therefore, with its 36 per cent of pores distributed through 64 per cent solid paste, will have a decided advantage over the lean mix, with 60 per cent pores distributed through 40 per cent solid paste volume. The preceding passage is concerned only with the spaces occupied by uncombined water. Air voids and honeycomb, where they occur, would obviously alter the leakage relations. This shows the need for such consistencies and concrete aggregate proportions as will prevent air voids and honeycomb if water tightness is to be achieved. The vital influence of curing on water tightness can also be visualized from the upper diagram of Fig. 1. Suppose, for example, that, by more thorough curing, the quantity of combined water be made twice as large as that indicated. This would give a proportion of pore space to total paste volume of 19 per cent for the rich mix and 46 per cent for the lean. Using the method of comparison of the preceding section it is seen that 19 per cent pores in 81 per cent solid paste will be a much more resistant paste even than the one with 36 per cent pores in 64 per cent solid paste. In the case of the lean mixture also such curing is seen to make an important difference.

The 60 per cent pores and 40 per cent solid matter now become 46 per cent pores and 54 per cent solids. Considerable experimental data are presented showing that this building up of the internal structure of the concrete through the continued hydration of the cement is one of the most effective means of increasing the water tightness. Figure 1 will also be found to show that there is a lower limit to the quantity of cement below which watertight concrete cannot be expected. The diagrams reveal at once that, as the cement quantity decreases from the rich toward the lean mixes, the water-cement ratio necessarily increases so that the proportion of pore space in the paste increases. Since the amount of combined water is evidently related to the quantity of cement, no amount of curing can add enough water to the solid volume of the very lean mixes to overcome this deficiency in cement. Even though a certain concrete aggregate combination may be highly favorable (by requiring less paste than another), the concrete cannot develop a high degree of water tightness unless it contains enough cement to permit an effective building up of the internal structure through curing.

Are You in Winchendon Massachusetts? Do You Need Concrete Cutting?

We Are Your Local Concrete Cutting Company

Call 508-283-3135

We Service all surrounding Cities & Towns.