Assessment Of Quality Control Practices For In-Situ Concrete: Case Study Of Nairobi City County

Abstract

Site production of in-situ concrete by small contractors (registration category NCA5 to NCA8), face numerous challenges ranging from site safety and organisation, poor conditions of equipment, understanding of design information, lack of experienced supervisory staff and other field obstacles. These challenges compromise the quality of concrete manufactured and the constructed facility. This study therefore investigated key site practices under which in-situ concrete is being produced by this category of contractors. This was done with a view to establishing if these practices have effect on the quality of concrete so manufactured. The study objectives were to compare achieved strengths with target mean strengths (measured in compressive strength units) with additional exploration of the effect of some of these key practices on the strength of concrete produced. One hundred and Thirteen (113) building contractors registered under NCA5 to NCA8 categories were sampled through multilevel sampling and stratified random sampling for concrete cubes for testing. Performance ratings were carried out based on the selected key site practices. When rated as good, fair or poor, the findings showed that only 11% of contractors attained good rating on site safety; only 15.5% attained good rating on site organisation; and 18% recorded good rating with regards to condition of equipment. On key site quality control practices; 97% of contractors did not record water-cement ratio; 99% did not carry out slump tests; and 100% did not have weighing instruments on site for accurate batching. Although singularly these poor site practices had only small impacts on concrete quality as indicated by the achieved compressive strength, together they led to significant variations in the concrete strength (as per hypothesis testing results) – minimum 12.5 N/mm2 and maximum 36.5 N/mm2 - as further exemplified by the average mean strength achieved from the samples was 24.22 N/mm2 as compared to the targeted standard strength of 25 N/mm2. Improvement of quality of in-situ concrete production must therefore focus on enhancement of appropriate site practices. To improve in-situ concrete production, the study recommends more proactive supervision by consultants, capacity building amongst contractors via training and availing of appropriate equipment and technology; and strict quality control of the in-situ concrete production process. These recommendations have the potential to improve the country’s construction industry through enhanced quality management, safety standards, and the achieved value for clients’ finances.