Summary A certain amount of work already published has indicated that the transmission length in pre-tensioned concrete increases very considerably with time. Evans reports a transmission length increase for 0·08 in. diameter high-tensile steel wire from 58 diameters to 125 diameters in two and a half years, with most of the increase taking place in the first month after transfer. A Swiss report also quotes figures showing an eventual doubling of the transmission length at transfer for wire of 2 and 3 mm diameter. Recent work by the Cement and Concrete Association on 0·2 in. diameter smooth wire indicates no such increase in transmission length, there being little movement, over several months, of the point at which maximum concrete strains were reached in the units tested.
Summary This paper describes the principle, design and use of a form of demountable mechanical strain gauge which has been found particularly useful for determining strains in concrete structures. The instrument which uses a lever and dial gauge is cheap to make and use, and is located on 8 in. gauge lengths which are formed by pairs of punched and drilled steel discs glued to the concrete surface. With practice, 200 strains per hour may be measured with an accuracy of about ±5 × 10−6 under most laboratory test conditions. Even greater accuracy has been obtained using the gauge in its ideal horizontal position and with the readings lying within a small range on the dial. In development tests of this kind 90 per cent of measured strains were within ±3 × 10−6 of the mean. Tests on various locating discs show that normal, drilled mild steel discs give the best results.
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