This thesis is an investigation of the effect of welding heat on the properties of hardened concrete. In precast concrete structures, where the bearing plates are embedded in the concrete, the welding heat on these plates, generated by the electric arc, penetrates into the concrete. It has been known for many years that high temperatures will lower the modulus of elasticity and the ultimate strength. This thesis is concerned with the “critical” temperatures at which losses begin and the arc energy required to develop this temperature at a reasonable depth into the concrete. In the investigation, the “critical” temperature, using limestone aggregate concrete that might be typical for precast construction, was found to be between 900°F and 1,000°F. The investigation, to determine the arc energy required to develop this temperature, was halted when under severe heating, where the arc burned through both plates, the maximum temperature at a point one half inch from the bearing plate was 710°F. Although cracking was severe in the immediate vicinity of the plate, the moduli of elasticity were lowered only slightly and the welding heat was not high enough to affect the ultimate strength. / Master of Science