1.1 BACKGROUND OF THE STUDY
The mechanical properties of concrete cannot be overemphasized, these properties vary based on the different types of materials that constituted such concrete which includes water cement ratio; types of aggregate used (fine and coarse), admixtures etc. On the other hand, quality control of every part of the concrete-making, that is, the environmental and climatic condition, negligence of the operator in mixing, placing and curing of concrete contributes to the good performance of concrete. Therefore, concrete reactions to these factors may result in failure of structures made from them when the required strength to support the design load cannot bemobilized.
The failure of a concrete can sometimes be mild with visible cracks and deflection or severe, leading to partial or total collapse of the structure either during the construction or post-construction stage. Incidences of failures of structures linked to bad concrete practice are abounding in Nigeria particularly in major cities such as Lagos, Port-Harcourt, and Abuja, among others. The NIDB building in Broad Street, Lagos which collapsed in November 2006, and killed about five (5) workers and injured several other staff and costumers of the Bank is a typical example that implicated bad concrete mechanical properties among others. In August 2009, a shopping mall collapsed in Abuja, claiming life, leaving many maimed and destroyed properties worth millions of Naira, as reported in the Punch Newspaper of Feb.26, 2010.
1.2 STATEMENT OF THE PROBLEM
It is imperative that concrete will fit as a quality material for construction when the ingredients are properly sourced and selected. Aggregate, coarse (crushed rock and gravel) and fine (river sand) aggregate as the case may be, must be properly sourced and appropriately selected for an intended project. Furthermore, when crushed rock is manufactured under a regulated standard and practice procedure to obtain the desired mechanical properties, gravel is obtainable from weathered parent rock hence there is inconsistency in its mechanical properties.
Gravel also must remain sound in all kinds of weather conditions. Moreover, Gravel in the absence of clay and fine particles will adhere well to cement or other mixing agents but if clay is left in the mix, they quickly breakdown and weaken the structure.
In building construction, crushed rock is required in large amount than gravel (if at all will be used) for reinforced concrete. Hence crushed rock, which will require time and modern facilities for production is not readily available in under-developed and rural places for construction. The cost of transportation to the site becomes one of the reasons why modern projects are being undermined in these places.
Buildings collapse more frequently during the construction period than in service because at the construction or infant stage concrete are more vulnerable to stresses imposed on its members. These stresses are due to construction load, load due to moving object, earth material and other sources of live load. Some other failures, where for example, the available skills do not match the present technology, most times the minimum standard specifications are being undermined. It is the degree to which the mechanical properties of the concrete is determined, that measure the quality of the concrete. Hence, this project will conduct a comparative analysis of the combination of coarse aggregate size fractions on the mechanical properties of concrete.
1.3 AIM AND OBJECTIVES OF THE STUDY
The general aim of this study is to determine the effect of various coarse aggregate size fractions on the mechanical properties of concrete.
The specific objectives of this research are:
- To achieve an optimum proportion of different natural coarse aggregate samples used in the concrete mix
- To investigate the influence of natural aggregate on the workability and strength of concrete
- To compare the results of the objective in (i) and (ii) above with a control concrete made with crushed granite aggregate.
- To assess the degree of effect of impact load on different natural gravel aggregates.
1.4 SCOPE OF THE STUDY
The scope of this study shall include the followings:
- To obtain relevant available literature for review to understand what has already been known on the topic.
- To obtain samples of natural gravel aggregates from Ogbomoso central only for the purpose of the experiments.
- To conduct standard experiments using the procedures in British standard to investigate the mechanical properties of the concrete.
- To analyze the result obtained from the laboratory tests and compare results with control tests and standard specifications, in other to make certain conclusions.
- To make recommendations for future researchers on the topic based on the results of the tests.
1.5 JUSTIFICATION FOR THE STUDY
With the established fact that gravel belongs to the same class as granite, both being an aggregate, it can be used independent of granite, whereas on many occasions the latter would give higher strength than the former at the expense of high cost. Hitherto, many contractors and builders cast reinforced concrete with crushed rock, while local gravel has been underuse to only mass concrete casting and other lower load bearing structures. Hence, this research work is built on various combination of different local gravel aggregate to achieve an optimum strength at the expense of the granite aggregate. This would be required in concrete production industry to minimize cost of using crushed stones which is costlier than the gravel and to produce concrete with an optimum strength.
Finally, impurities in form of clay and silt present in the aggregate reduce the quality of concrete, hence there is need to check the effect of impurity in the aggregate.