International Journal of Engineering and Technical Research (IJETR) ISSN: 2321-0869, Volume-2, Issue-9, September 2014 368 www.erpublication.org Abstract— The concrete preparation works and its curing takes place under variable air temperatures. This variation depending on the weather/climate conditions is very sensitive for the South- Saharan region, especially in areas between tropics with climate varying from warm to hot. The 28-day concrete strength considered as the basic strength in design of reinforced concrete structures is calculated from tests results of samples prepared under above weather conditions under which the concreting is done. This study on “Influence of air temperature on mechanical properties of concrete” was conducted with objective of investigating on the effect of temperature variation on mechanical properties of concrete while changing the proportion of its components. The compressive strength, tensile strength and flexural strength were analyzed for concrete samples prepared from three different mix ratios (1:2:4; 1:1.5:3 and 1:1:2), and placed under three different temperatures (9, 20 and 36 0 C) at their early stage of curing during 24 hours. The results from testing after 28days proved that bellow the normal temperature of curing condition (around 20 0 C), the mechanical properties of concrete slightly decrease, while when same samples are placed above the normal temperature, these properties considerably decrease. Therefore special measures should be taken when the concrete works are to be conducted under varying air temperature in order to avoid the negative impact. Index Terms— Concrete, air temperature, mechanical properties, concrete components, concrete mix ratio, Water and Cement Ratio I. INTRODUCTION Today, concrete structures are dominating in the building construction industry, not only in Rwanda but also in the great part of the world. Among other many advantages of concrete structures, the most important merit resides in its mechanical properties as a building material [9] Concrete is a composite material composed of water, coarse granular material (the fine and coarse aggregate or filler) embedded in a hard matrix of material (the cement or binder) that fills the space among the aggregate particles and glues them together [2] and [10]. Concrete has a great variety of applications because it meets not only structural demands but also lends itself readily to architectural treatment. In buildings, concrete is used for Manuscript received September 23, 2014. Dr Mbereyaho Leopold, University of Rwanda, College of Science and Technology (CST), Postal address: BP 3900 Kigali Mr Harerimana Charles, Graduates, College of Science and Technology (Former Institute of Science and Technology - KIST), Postal address: BP 3900 Kigali Mr Kagambira Eric, Graduates, College of Science and Technology (Former Institute of Science and Technology - KIST), Postal address: BP 3900 Kigali footings, columns, beams, girders, wall, slabs, and roof units; in short, all important building elements. Other important concrete applications are in road pavements, airport runways, bridges, dams, irrigation canals, water-diversion structures, sewage-treatment plants, and water-distribution pipelines. A great deal of concrete is used in manufacturing masonry units, such as concrete bricks and blocs [5]. As far as the temperature effect concerns, there are two positions to pay attention. On one hand, the normal procedure adopted for concrete in fair weather will not be valid for concreting when the temperature is lower or below the freezing point. The production of concrete in cold weather introduces special and peculiar problems, such as delay in setting and hardening, damage to concrete in plastic condition when exposed to below freezing point owing to the formation of ice lenses. Therefore it is essential to maintain the temperature of the concrete positively above 0 0 C, possibly at much higher temperature. On the other hand, normal methods of mixing, transporting and placing of concrete will not be exactly applicable to extreme weather conditions. Special problems are faced in making, placing and compacting concrete in hot weather and in cold weather. Even if it is difficult to define what hot weather condition is, just for convenience, it is regarded that the concrete placed at an atmospheric temperature above 40 0 C is considered as hot weather concreting [4]. Many authors from all sides of the world have worked on effects of change of temperature on different mechanical properties with consideration of different factors, such as admixtures, water cement ratios, concrete age, etc. [1], [7],[8], etc. It was established that the temperature change may cause negative as well as positive impact. II. METHODOLOGY FOR THE STUDY Under different selected temperature conditions, this study was focused on three important mechanical properties of concrete which are Compressive strength, tensile strength and flexural strength. Temperature changes were selected based on Rwandan Conditions, where the minimum temperature (with a high probability) was 9 0 C and the maximum of 36 0 C. 20 0 C is considered as a normal curing temperature for concrete. Concrete was tasted at 28days age. The laboratory testing methods of concrete are in accordance with ASTM International. The study started with checking on the variation of temperatures in Rwanda, what established considered temperatures above mentioned. Influence of Air Temperature on Mechanical Properties of Concrete Dr Mbereyaho Leopold, Mr Harerimana Charles, Mr Kagambira Eric