STUDY OF THE CHARACTERISTICS OF CONCRETE SUBJECTED TO HIGH TEMPERATURES

THE STUDY DESCRIBED IS AN ESSENTIAL STEP IN THE PRELIMINARY MATHEMATICAL EVALUATION OF THE RESISTANCE OF CONCRETE TO FIRE. THREE TYPES OF CONCRETE MIX-DESIGNS DIFFERING BY THE NATURE OF THE INERT AGGREGATES WERE USED. IN ORDER TO OBSERVE THE EVOLUTION IN THE MECHANICAL CHARACTERISTICS, EG, STRENGTH AND MODULUS OF ELASTICITY IN COMPRESSION, A SPECIAL OVEN WAS DESIGNED, IN WHICH A THERMAL STRESS SIMILAR TO THE STRESS CAUSED BY A STANDARD FIRE COULD BE APPLIED TO 15 X 15 X 30 CM PRISMS THROUGH THE BASES OF THE COMPRESSION MACHINE. THE SAMPLES WERE SUBJECTED TO TEMPERATURES BETWEEN 200 DEG C AND 900 DEG C. IT WAS OBSERVED THAT THE COMPRESSIVE STRENGTH OF CONCRETE DECREASES WHEN THE TEMPERATURE INCREASES AND THAT LIMESTONE CONCRETE AND LIGHTWEIGHT STRUCTURAL CONCRETE HAVE A BETTER BEHAVIOUR AT 400 DEG C THAN SILICEOUS GRAVEL CONCRETE. AS REGARDS THERMAL PROPERTIES, THE THERMAL CONDUCTIVITY (L) OF CONCRETE DECREASES WHEN THE TEMPERATURE RISES AND IS SHOWN TO BE DIRECTLY LINKED TO THE THERMAL CONDUCTIVITY OF THE AGGREGATES. THE MASS HEAT (C) OF CONCRETE DOES NOT APPEAR TO DEPEND ON THE TYPE OF CONCRETE WHILE THERMAL EXPANSION (A) BETWEEN 20 DEG C AND 200 DEG C PROVES TO BE IMPORTANT WHEN THE FIRST THERMAL STRESS OCCURS AND TENDS SOON AFTER TO COME NEAR THE THERMAL EXPANSION COEFFICIENT AT 20 DEG C.