Abstract
From 1989, we have carried out experimental studies on the estimation of compressive strength of concrete by non-destructive methods. Among the non-destructive methods, the ultrasonic pulse velocity method was used to estimate the strength development in massive concrete test pieces. Three types of massive concrete test pieces were prepared, that is, the first was 20 cm (height) ×100cm (width) ×100cm (length), the second 20×40×70cm, the third 70×20×40 cm. At the same time, we also prepared cylindrical and prismatic test pieces. We measured the inside temperature of the massive concrete test pieces three or five days earlier which were needed to strip the form. After the required curing duration the cores with a 10cm diameter were removed from the massive concrete test pieces and, their density, ultrasonic pulse velocity and compressive strength were measured.
The results obtained are as follows.
(1) The more massive the concrete becomes, the higher the maximum inside temperature also becomes and the time which is required to reach to a stable ultimate inside temperature, namely, the same as that of room becomes longer.
(2) At the same age, the more massive the concrete becomes, the lower the compressive strength becomes.
(3) For massive concrete, the difference in the ultrasonic pulse velocity between the inside and outside disappears after water curing for about one year.
(4) The tall concrete element such as for massive concrete test pieces with a height 70cm has a lower strength in the upper part with a depth of about 10cm, because this part has many capillary pores and water paths due to bleeding.
