PRE-STRESSING OF CONCRETE

Press, Volume LXXXIX, Issue 27088, 10 July 1953, Page 6

PRE-STRESSING OF CONCRETE

SCALE MODEL RESEARCH AT UNIVERSITY Research on pre-stressed concrete for bridge construction has reached a test stage in the civil engineering department of Canterbury University College with the completion of a model simulating a decking slab five times larger. An honours student has made the model, and the effects of controlled loads, applied by jacks, are being measured by electric gauges. Generally speaking, pre-stressing is carried out by inserting high-strength steel rods ~ through the concrete, stretching them, and then allowing the reflex action, after they are locked at the ends, to compress the material.

In the model at the National School of Engineering, regular longitudinal pre-stressing (over the length of the span) has been incorporated, and the trials are being made with variable transverse pre-stressing to determine the amount required to give all the benefits of monolithic construction. The electric gauges, with terminals sealed all over the surface, show exactly any changes in stress through a given loading.

Engineers are aware that a great deal of the cost of a conventional concrete bridge is absorbed in the preparation of boxing and other work on the site. Pre-casting of standard members at the factory, where concrete of higher quality could give lighter units, has been advocated. Prestressing affords even greater economy, because units can be increased in area and strength without adding to weight in transportation.