Effect of elevated temparature on compressive strength, pulse velocity and conversion of high alumina cement concrete
Authors: Quon D H H , Malhotra V M 
Published: April 2018

High alumina cement has achieved considerable attention in recent years due to structural failures associated with the use of this cement in England. Canadian Standards Association Committee A23.1, Concrete Materials and Methods of Concrete Construction, is
currently debating the use of high alumina cement in structural concrete for reference in the standard. This investigation was therefore undertaken to obtain data on the performance of test specimens cured at temperatures from 21 to 66 °C under both humid and dry conditions. A series of 2-ft3
(0.056-m3 ) concrete mixes was made in the laboratory using crushed gravel and natural sand as coarse and fine aggregate respectively. Thirty 4 x 8-in. (102 x 203 mm) cylinders were cast rom each mix. Following the initial moist curing period of 24 hours at 18°C, one third of the cylinders were
subjected to standard moist curing at 21 0C, one third were cured at 30 and 38 0C in water and the remaining one third were cured at 30, 38 and 66 0C under dry heat conditions. The curing period varied from 1 day to 180 days for each condition of curing. At each age, weight and pulse velocity of
cylinders were determined before testing the specimens in compression. Small samples of mortar from the tested specimens were subjected to X-ray diffraction (XRD) and differential thermal analysis (DTA) studies to determine the nature of hydrates and the degree of conversion. The change in
compressive strength of high alumina cement concrete with age followed the established pattern. The turn was followed by a slow increase in strength with age. However, there was on exception: the cylinders exposed to dry heat at 66 0C, after following the usual pattern of strength development with
age as mentioned above, indicated a trend towards further loss in compressive strength. The rate of conversion of high alumina cement concrete, i.e., the conversion of hexagonal hydrates (CaO.Al203.10H20) to cubic hydrates (3CaO.Al203.6H20), is a function of the temperature to which test cylinders
are exposed, irrespective of the water/cement ratio of the concrete. The rate of conversion also appears to be independent of the type of exposure, i.e., whether it is dry heat or immersion in water. There is no correlation between the degree of conversion and compressive strength of high alumina
cement concrete so that the former cannot be used as a measure of compressive strength. The pulse velocity technique appears to be an excellent means of monitoring the long-term changes in compressive strength due to conversion in high alumina cement concrete. This is especially so for test
specimens exposed to dry heat, in which case this technique is superior to the determination of the degree of conversion by DTA. However its use to determine absolute values of strength cannot be recommended. X-ray diffraction studies and scanning electron photomicrographs were shown to provide
useful supporting data in the investigation of conversion phenomena but these studies are not considered essential for this purpose.