International Research Journal on Engineering Vol. 2(2), pp. 028-035, February, 2014 Available online at http://www.apexjournal.org ©2014 Apex Journal International Full Length Research Impact of sudden temperature change of fresh concrete on its flexural strength Twubahimana Joseph Désiré 1 and Mbereyaho Léopold 2 * 1 Saint Joseph Integrated Technical College, P.o. Box 1330 Kigali. 2 University of Rwanda, College of Science and Technology (COSTECH) BP 3900 Kigali Accepted 9 January, 2014 The concrete structures are submitted to different actions during their exploitation time. There are static or dynamic loads, which act on different elements of the structures; there are also atmospheric actions capable to change properties of the materials composing the concrete. Under normal conditions in Rwanda, fresh concrete is exposed to temperatures ranging from 10 to 40°C. Concrete has a much better temperature variation resistance than certain other materials such as wood or steel. As the temperature sometimes changes suddenly, the precise impact of sudden temperature of fresh concrete on its strength needs to be studied carefully to solve problems such as evaluating the degree of damage to concrete structures. This study focuses on quantifying the strength degradation of the concrete. This study investigates the impact of impact of sudden temperature change of fresh concrete on its flexural strength. The tests have been conducted on the most used concrete constituents in Rwanda. The properties of concrete were assessed in civil engineering laboratory and the specimens used were prismatic and all test specimens were prepared in accordance with ASTM 192-00. After the moulding of concrete specimens, the temperature of the fresh concrete was immediately changed, and the new temperature of concrete was kept constant for a period of 24hours. After 24 hours the specimens were cured continuously in limewater after the initial 24-hr moist cure period before testing. The variations of the initial temperature (20°C) of concrete which have were considered in this study are -20°C, -15°C , -10°C, +10°C and 20°C. For all cases studied in this research the specimens were tested at the age of 3, 7, 14 and 28 days. It was found that the sudden temperature change of fresh concrete leads to the change of the concrete flexural strength. Key words: Concrete flexural strength, aggregates, fresh concrete, temperature change. INTRODUCTION Concrete curing Proper curing practice is one of the important steps in making high quality concrete. A good mix design with low water-cement ratio alone cannot ensure good concrete. The favourable conditions to be set up at early hardening periods for best results are: i. Adequate moisture within concrete to ensure sufficient water for continuing hydration process, and ii. Warm temperature to help the chemical reaction. *Corresponding author. Email: mbermalu@yahoo.fr In addition, the length of curing is also important. The first three days are most critical in the life of Portland cement concrete (Simeonov, Building Materials. Sofia, 1991). In this period the hardening concrete is susceptible to permanent damage. On an average, the one-year strength of continuously moist cured concrete is 40 percent (Darakchiev, Building materials testing. Sofia, 1990) higher than that of 28-days moist cured concrete, while no moist-curing can lower the strength to about 40 per cent. Moist curing for the first 7 to 14 days may result in a compressive strength of 70 to 85 per cent of that of 28 day moist-curing (Manoilov, Design of Reinforced concrete structures, Sofia, 1991). The hydration of a concrete mixture is a process that liberates heat and the rate of heat generation is accelerated with an increase in