Effect of autogenous healing of concrete subjected to periodic freeze-thaw cycles

Abstract

It is shown that if concrete that was subjected to a load (not to failure) is place under favorable temperature and moisture conditions, partial or complete recovery of its original solidity will occur [1,2]. Such a phenomenon is called autogenous healing of concrete. During cyclic freezing and thawing of concrete internal stresses occur in it, leading to the formation of microcracks and to a change in the structure of the material similar to that which occurs under the effect of an external load. As a result of the formation of microcracks unhydrated cement particleszare exposed, which leads to the development of hydrolysis and hydration processes. In the case of a small opening of the cracks under conditions of a damp environment the latter are closed by newly formed products. During har~ening of concrete in air carbonization of calcium hydroxide occurs and the microcracks are filled with calcite. Equilibrium of the saturated solution with the solid phase is dynamic. Therefore with the appearance of new active surfaces equilibrium is disturbed and the process of precipitation of the solid phase with gradual filling of the microcracks begins anew.Am increase of the strength and frost resistance of concrete that was kept under favorable conditions in the breaks between freeze-thaw cycles was noted in the works of a number of investigators [i, 3, 4, 5]. However, a quantitative evaluation of this effect has not been made.The effect of autogenous healing of concrete on its frost resistance was studied under laboratory conditions on mortar and concrete specimens. We investigated cement mortar of Composition i : 4 with w/c -0.6 and concrete of composition: cement 260 kg/m s, sand 670 kg/m s , gravel stone of fraction5-20mm1490kg/cm s,w/c-0.6withoutadditives. In both cases Portland cement M400 of the BelgOrod plant and natural sand Mf = 2.1 were used.Preparation of the mortar and concrete mixes and molding of the specimens were carried out in conformity with the requirements of the standards. After making the specimens they were held for 24 days in a normal storage chamber and then were saturated with water at atmospheric pressure for 4 days.The mortar specimens were divided into three groups: to determine the kinetics of the grain in compressive strength, to test frost resistance according to the standard, and to test frost resistance under the periodic effect of freeze--thaw cycles. Frost resistance was tested in conformity with GOST 10060-76 by the main and accelerated methods.The change in the structure of concrete was evaluated from the results of determining the compressive strength and on the basis of the dynamic modulus of elasticity (Ed). The mortar specimens were removed from the test after every 25 freeze--thaw cycles and then held for 30 days in water at a temperature of 18 • 2~The strength and E d were determined before the start and at the end of each break. The concrete specimens tested by the main method (at a freezing temperature of -17 ~ were removed after 200, 400, 470-500, and 633 cycles and were al...

Regional study on structural dynamic property of buildings in China

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