A Comparative Study On Mitigation Of The Effect Of Silt And Clay Content Of Sand So As To Maintain The Fresh And Hardened Properties Of Concrete Arround Jimma

Abstract

Aggregate supply from local sources play a fundamental role in cost effective concrete production. But locally available Natural fine aggregate consist of some impurities like silt and clay which causes problems in fresh and hardened property of concrete, such as decreasing workability and compressive strength of concrete. The general objective of the research was to mitigate the effect of silt and clay content of sand so as to maintain the fresh and hardened properties of concrete. In these research three methods was used to mitigate the effect of silt and clay. These are washing the sand to make free of silt and clay, adding extra cement on predetermined cement and blending with quality sand having silt and clay below Ethiopian standard. In washing the sand, the time and the amount of water needed to wash specific volume of sand was also determined. For this study the sand was collected from six supply point to Jimma such as, Asendabo, Nada, Yedi, Chewak, Werabe and Gambela and their level of silt and clay content was determined. Then Five samples of sand with varying silt and clay was taken (i. e 0.94%, 6%, 10%, 14%, and 18%) and three cubes (of 150 x 150 x 150 mm3) of 1:2:3 mixture with water cement ratio of 0.5 for compressive strength at 7 and 28 days of curing time was made for each percentage of silt and clay and the percentage strength reduction has been determined. Then another four sample with silt and clay content having the same value to the above five sample except 0.94% was prepared and for each percent of silt and clay, a three trial of cement increment was applied and three cubes for compressive strength at 7 and 28 days of curing time was made for each trial of cement increment. As a result, it has been found that the sand being supplied to Jimma town contained silt and clay content that exceeded the allowable limit specified on Ethiopian standard. It ranges from 2.5% to 20%. As the silt and clay content of sand increase from 0.94% to 18%, the percentage reduction of the compressive strength is 46.12%. The addition of cement improves the workability and the compressive strength of concrete. As the silt and clay increase from 0.94% to 18%, 27% of additional cement was required to eliminates the negative effects of silt and clay in aggregates and to maintain the required workability and strength of concrete. And also the extra cost incurred in the production of 1m3 of concrete in terms of washing the sand was between 4.73-11.5% whereas cement increment was between 2.5-15.4% depending on the amount of silt and clay. And also it has been found that blending of 30% Asendabo sand with 70% Gambela sand and 20% Yedi sand with 80% Chewaka sand reduce the level of silt and clay to 6% . Finally it is concluded that the compressive strength and the workability of the concrete are indirectly proportional to the amount of silt and clay in the fine aggregate. Any increment of this finer material decreases the strength and workability of the concrete. The workability and compressive strength of concrete can be maintained by washing the sand or by adding extra cement and by blending with quality sand so that for sand containing silt and clay below 10%, it is more economical to use cement increment method and for sand containing silt and clay above 10% ,it is more economical to use washing method.