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Concrete Cutting One
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The curves for all the other combinations in both the limestone and gravel concretes fall within the area between the high and low curves shown. It is of interest to note that in both cases the grading showing the lowest position for the water-cement ratio curve is the one with the largest gap in the size distribution. The total range in position of the curves is quite small considering the actual differences in grading represented. The real significance of grading of the concrete aggregate is in relation to the workability and economy of concrete mixtures. This is considered in detail in Chapter V. Differences in the water- cement ratio strength relation for concrete aggregates of different types have frequently been brought out. In tests of this factor it is important to take into consideration the differences in absorption between the different materials or else use concrete aggregates which have first been saturated and then brought to the surface-dry condition before using. With this correction the differences between different concrete aggregates ordinarily become of much less importance for the usual range of water-cement ratios. Figure 6 shows the results of a group of tests in which concrete aggregates of different types were used. For these concrete aggregates the greatest difference is seen to be quite small, and for most of the materials the difference is negligible.

Such differences as do exist in the harder materials are more likely to be due to differences in shape and surface characteristics than to strength or mineral structure. The bond between the concrete paste and the concrete aggregate surface is probably one of the most important causes of differences between different types of concrete aggregate. This is affected by the shape and surface characteristics, particularly the angularity and roughness and the amount and character of dust adhering to the surface.  Naturally, materials which are structurally weak or friable cannot be expected to give the same strength as the structurally sound materials. In all the discussions in this book involving strength of concrete it is presumed that only materials satisfactory in these regards will be used. As will be brought out later, the differences between different types of materials are more pronounced in their effect on transverse and tensile strength than on compressive strength, the largest variations occurring for those cases where the bond or tensile strength is low. Grading variable; moist cured; age at test, 28 days. Figure 7 shows another group of tests using coarse concrete aggregate of different types, in which a single curve interprets quite well the relation between strength and water-cement ratio for the different materials. In this group of tests there was also some difference in grading, which may be responsible for some of the spread in the points, though probably not a great amount. Figure 5 which illustrates the effect of grading also provides a comparison of two different coarse concrete aggregates. In this figure it will be noted that the position of the water- cement ratio curve for the gravel is quite close to that for the limestone for the higher water ratios. For the lower water ratios, however, the strength shown by the limestone for any water ratio exceeds that for the gravel by an amount varying up to a maximum of nearly 1000 lb. per square inch. This may be the result, either of a better bond, afforded by the rough angular particles, or of the greater amount of paste in the unit volume of concrete. Other groups of tests with these same materials have shown the same divergence in curves for the lower water ratios, that is, for the richer mixes. Attention was given to the possibility of building up the internal structure of concrete through continued curing, and some data were presented showing the amount of water that could become a permanent part of the solid mass.

The fact that curing increases the strength of concrete is generally recognized but it is not likely that the full possibilities for improvement through additional curing are widely realized. The data presented here will show something of these possibilities and bring out the minor significance of the other variables discussed when considered in comparison with this major factor. Figure 8 shows the water-cement ratio strength relation for concrete at the age of 4 months for different periods of moist curing. Comparing the curves for 3 and 21 days moist curing, it will be seen that the extra 18 days in moist sand adds from 1200 to 1800 lb. per square inch to the strength of the concrete at 4 months. The comparison between the 21 days moist curing and the curing concrete damp for the entire period cannot be fairly made from this figure, as the difference in moisture condition at time of test is involved. The curve representing concrete damp sand storage for the entire 4 months would be still higher if the specimen had been dried before testing. This difference in strength due to moisture condition at time of test is generally recognized. Figure 9 gives another comparison of the effect of curing. As in Fig. 8, if the moist-cured specimens had been dried before testing, the comparison would have shown a still greater advantage for the moist curing. This would be true for both the 28-day and 1-year tests. In these curves it is seen also how the two factors of water content and degree of chemical combination are interrelated in the development of strength in the cement paste. In shape, the curves are generally similar to those from the other tests, each degree of curing providing its In Fig. 8, mix 1:4 by volume; gravel concrete; age at test, 4 months. In Fig. 9, mix variable, ranging from 1: 31 to 1: 9; consistency constant; slump, 3 to 4 in. own curve. Thus, changing either the water ratio or degree of curing does not alter the importance of the other. From a study of the data in Figs. 8 and 9 it is seen how important is the matter of curing as compared with the small differences shown for the other variables which have been considered. As will be brought out at other points in this text, these data show the futility of too fine distinctions in these lesser variables when such important differences are possible through additional moist curing. It was suggested in Chapter I that age was one of the factors in the curing of concrete. This has also been recognized in the discussions of Figs. 8 and 9. Thus, age and curing cannot be separated; an increase in age merely providing for further chemical combinations if the conditions are favorable for continued reaction. This is illustrated in Fig. 10, which shows the effect of age on the water-cement ratio strength relation for moist- cured concrete. There are two points of special interest concerning the curves in this figure. First, the similarity in the curves representing the water-cement ratio strength relation for the various ages; and second, the significance of variations in mix, consistency, size of aggregate, and grading. On this latter point it will be noted that for each age the water-cement ratio fixes the strength, regardless of the cement content, consistency, or aggregate characteristics. This point, which has been brought out in the discussions of the other figures, emphasizes the fundamental character of the water-cement ratio strength law. The similarity of the curves for different ages is significant in reference to the comment frequently made that the mixes of high water ratio eventually gain strength  more rapidly than the mixes with the lower water content. That this is not the case can be seen from the fact that these curves continue to diverge as the water- cement ratio is reduced. Of an importance comparable with differences in curing is the effect of the characteristics of the cement on the water-cement ratio strength relation. While all cements behave similarly, they do not all gain their strength at the same rate. This is clearly brought out in Fig. 11, which shows tests of concrete made from 32 brands of cement.

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

We Are Your Local Concrete Cutting Company

Call 508-283-3135

We Service all surrounding Cities & Towns.