Effects of Sand Quality on Strength Properties of Concrete: A Case Study on Nairobi City County and Its Environs

Abstract

Collapse of buildings in Kenya and other nations has been on the increase and has resulted to loss of lives, inflicted injuries and led to huge loss of investment for developers. In a span of 8 years, fourteen buildings collapsed in Kenya, half of them occurring in Nairobi City County and its environs. Past researches identify the major causes of buildings failure as dependent on the quality of building materials used, workmanship employed in the concrete mix proportioning, construction methodology, defective designs and non-compliance with specifications. Information on the quality of commercial building sands being used in concrete making in Nairobi and their effect on strength properties of concrete was lacking. Non-destructive testing (NDT) methods such Schmidt rebound hammer as present a convenient way of assessing the quality of concrete both in the field and in laboratory thus promoting sustainable development.

The objective of this research was to establish the level of silt and clay content and organic impurities found in building sand being used in Nairobi City County and its environs and the effect of these impurities on the compressive and bond strength of concrete. 27 sand samples were collected from 8 main supply points in Nairobi City County and its environs. Laboratory tests carried out included examination for particle texture and shapes, sieve analysis, specific gravity, chemical tests, testing for silt and clay content and organic impurities, cube testing for compressive strength using universal testing machine and rebound hammer and pull out test.

The results of the investigation showed that majority of building sand being supplied in Nairobi City County and its environs exceed the allowable limits for silt and clay content and organic impurities as set out in BS 882, IS and ASTM standards. With regard to compressive strength, 38% of the concrete cubes made from sand with varying sand impurities failed to meet the expected design strength of 25N/mm2 at the age of 28 days. Linear regression and correlation showed that presence of silt and clay content and organic impurities contribute to reduction in cube strength and bond strength between concrete and reinforcement by 44% and 56.8% respectively. It is concluded that the allowable maximum amount of silt and clay content organic impurities are 4.8% and 0.106 ohms respectively beyond which concrete is likely to fail. Increase in workability led to significant decrease in cube strength. Particles shapes and texture had implication on the resulting compressive strength. Testing of quality of sand for every construction project is recommended as well as policy formulation to this effect. There was close correlation between NDT and DT (destructive testing) results implying use of rebound hammer for monitoring of concrete is convenient, cheaper and the testing element is not damaged during testing.