Heat was delt with as a factor of natural environment which makes asphalt concretes harden. This effect to asphalt concretes was clarified by means of laboratory tests. And physical properties of some construction joints were also discussed.wo types of aggregates and two types of asphalts and cured in an oven at 70°C for eight weeks in maximum. Next, the stresses induced by thermal cycling (10-20°C,100cycles) were observed and flexural tests were also carried out. At the same time, asphalts extracted from heat-cured asphalt concretes were analyzed.
　The main conclusions are as follows:
(1) Asphalt concrete becomes hard and brittle during the process of heat curing at 70°C.
(2) The thermally induced stresses of asphalt concrete increase with curing time and they may be estimated by linear viscoelastic theory with noticeable accuracy.
(3) Cold joint fails more easily than others.
(4) One method of preventing the hardening of asphalt concrete is to raise the asphalt content.