Crushing Strength of Cinder Concrete

Different experimenters have

obtained very different results for the modulus, owing partly to variations in the concrete, partly to the different stresses between which the modulus is taken, partly to the difficulties of measuring the very small deformations, and partly to whether gross or net deformations are used. The net deforma tions give the larger values for the modulus. Values have been obtained ranging from 1,500,000 to 5,000,000 lb. per sq. in.

Table 41 gives the modulus for different proportions and different ages. These values were deduced in connection with the crushing tests described in * 398, which see. In computing the results in Table 41 net deformations (the total deformations minus the set) were used.

Concrete shows a permanent set under ;mall loads, and hence concrete can hardly besaid to have an elastic limit in the usual sense. However, there appears to be a limit to the stress which can be repeated indefinitely without continuing to add to the deformation, and for practical purposes this may be taken as the elastic limit. This limit is from 50 to 60 per cent of the

ultimate compressive strength.

The weight of concrete varies with the unit weight of the ingredients, the proportions, the maximum size and the grad ing of the aggregate, the amount the water, the amount of ramming, and the age. The maximum difference between portland and natural cement concrete, due to the greater weight of portland cement, is 4 or 5 lb. per cu. ft. The best grade of 1 : 2 : 4 concrete when dry weighs about as follows; trap 155 lb. per cu. ft., conglomerate or gravel 152 lb. per cu. ft., limestone 150 lb. per cu. ft., sandstone 145 lb. per cu. ft., cinder 110 lb. per cu. ft.; and a 1 : 3 : 6 mixture when dry weighs as follows: trap 150 lb. per cu. ft., conglomerate or gravel 145 lb. per cu. ft., cinder 105 lb. per cu. ft.* Concrete made of blast-furnace slag weighs from 110 to 120 lb. per cu. ft.; and that made of coke from 80 to 90 lb. per cu. ft.