The Structural Engineer > Archive > Volume 9 (1931) > Issues > Issue 12 > Modern Research on the Nature of Materials of Construction (Steel and Concrete) Discussion on Mr. Ja
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Modern Research on the Nature of Materials of Construction (Steel and Concrete) Discussion on Mr. Jackson's Paper (Concluded)

In a written contribution Major C.G. Lymm states:- In this very interesting paper, one of the most interssting parts is that dealing with the allied phenomena of shrinkage and plastic flow in concrete. There seem to be good reasons for thinking that what is commonly called plastic flow is really accelerated shrinkage. It is difficult to conceive real plastic flow in concrete. In a metal such as steel, the whole m4terial is plastic, even though it may form different crystals of pearlite, cementite, etc. By far the greater part of concrete is mineral aggregate, and Prof. J. R. Shanks’ experiments show that a concrete may take a permanent set, though the rock of which it is composed has no plastic flow under even heavier stresses. Plastic flow in metals takes place at stresses above the yield point, but concrete has no definite yield point, and takes a permanent set under very low stresses. Real plastic flow is always accompanied by a transverse plastic deformation. There seems to be little literature on transverse plastic flow in concrete, but Prof. Davis, of California, states definitely that there is evidence of considerable transverse shrinkage, but no evidence of transverse expansion under load. Prof. Davis is carrying out extensive experiments on creep in concrete, and communicated a paper to the American Concrete Institute last March. In the discussion on this paper, Prof. Maney described an experiment he had carried out. He cast two cylinders, 8 in. by 32 in., one solid, one hollow, with walls 2 in. thick, and measured the axial shrinkage on the walls of the hollow cylinder, and on the solid cylinder at the surface and centre. In 20 days the hollow cylinder shrank 650 millionths, the skin of the solid cylinder shrank 250 miIlionths, and the centre shrank 100 millionths, The lag between the free shrinkage of the hollow cylinder and the restrained skin shrinkage of the solid one was 400 millionths, -which is approximately twice the creep reported by .Prof. Davis and Dr. Glanville, as resulting from onemonth’s loading of concrete loaded at the age of one month. It is permissible to suspect that the creep of the solid specimen under load is an attempt to arrive at the same condition which the hollow cylinder reached by free shrinkage.