INFLUENCE OF COARSE AGGREGATE TYPES ON FRACTURE PROPERTIES OF CONCRETE SUBJECTED TO HIGH TEMPERATURE HEATING

Abstract

 It has been reported that strength properties are changed by increasing heat, decreasing to a greater extent as the temperature rises. Since concrete subjected to high temperature heating can suffer strength loss, it can also be prone to cracking. However, there have been few reports on cracking in concrete under the effect of high temperature heating. When investigating the cracking behavior, it is considered important to evaluate not only the crack propagation properties but also crack initiation. Fracture mechanics was therefore employed in this study to investigate the fracture properties related to crack initiation and propagation in concrete. This paper reports on the influence of coarse aggregate types on the fracture properties of concrete subjected to high temperatures up to 800°C.
 The fracture properties were evaluated based on tension-softening curves, determined by polylinear approximation through inverse analysis of load versus crack mouth opening displacement (CMOD) curves. Specifically, the evaluation was carried out using the initial cohesive stress and fracture energy, the meanings of crack initiation and crack propagation.  Four different types of rock, sandstone, granodiorite, limestone and chert, were used as coarse aggregate. Concrete was proportioned with a water-cement ratio, target slump, and target air content of 57%, 18 cm, and 4.5%, respectively. The coarse aggregate content was kept constant at 356 liters/m³, while the slump and air content were adjusted by the admixture dosage. Specimens were demolded 2 days after placing, water-cured at 20°C up to an age of 26 weeks, and then subjected to heating followed by testing.
 Two programmable muffle furnaces (inside W310 by D610 by H310 mm) with heaters on both side walls were used for heating. The target temperatures in the furnace were 100, 200, 300, 400, 500, 600, 700 and 800°C. Unheated (20°C) specimens were also tested for comparison. The heating rate was 0.5°C/min. After attaining the target, the temperature was kept for 168 h and specimens were allowed to cool naturally to the level of outdoor air temperature.
 Notched specimens of size of 100 × 100 × 120 mm were used for the wedge-splitting tests. A servo-hydraulic testing machine of closed-loop type was used. The rate of CMOD at the opening mouth was set to 0.02 mm/min.
 The follow conclusions could be drawn on the basis of this study:
 (1) The fracture surface areas of both concrete and mortar affected by high temperature heating linearly increase as the temperature rises. The fracture surfaces of concrete tend to be larger than those of mortar.
 (2) The maximum load of sandstone on the L-CMOD curve scarcely changes up to 300°C, whereas those of other specimens are highest at 100°C. When comparing the maximum loads at the same heating temperatures, that of mortar is higher and that of limestone is lower than the others.
 (3) The tension softening curves express generally decreasing cohesive stress with the increase in the COD. Amid these trends, the cohesive stress of mortar, whose initial cohesive stress is greater than those of concretes, decreases at a slower rate than those of concretes. As to concrete specimens, the reductions in the cohesive stress of sandstone, granodiorite, and chert take on similar trends, but that of limestone is faster than the other concrete specimens.
 (4) At a heating temperature of 100°C, the initial cohesive stress of mortar significantly increases, while those of sandstone, granodiorite, and chert slightly increase and that of limestone slightly decreases. At higher temperatures, the initial cohesive stress decrease.
 (5) Up to a heating temperature of 300°C, the fracture energy of all specimens increases but then tends to decrease as the heating temperature increases.