pozzolanic material and filler effect in the concrete . The filler effect is also caused by two important factors which are the nucleation effect that enhances the cement hydration and the packing effect where the small particles fill the void inside the concrete . On the contrary , Yuksel and Kurama [ 22 ] and [ 23 ] reported a decrease in compressive strength when using coal bottom ash as fine aggregates replacement . This is probably due to the low bonding between the fine coal bottom ash and granite in the cement paste .
Ultrasonic pulse velocity ( UPV ) Table 5 shows the ultrasonic pulse velocity for different mix proportions after seven days , 14 days and 28 days of curing . As shown in Figure 5 , the UPV value of control concrete is 4535 m / s while the UPV value for coal concrete is 4498 m / s at 28 days . The UPV value of coal concrete has comparable results with the UPV value of control concrete . This is due to the coal concrete being as dense as the control concrete .
Concrete Mixes
Table 5 . UPV values for control and coal concretes
UPV value ( m / s )
7 days 14 days 28 days Control 4214 4283 4535 Coal 4216 4348 4590
Figure 5 shows that a strong and positive linear relationship exists between the compressive strength of control concrete and coal and the UPV value . An accurate estimation of the correlation is illustrated by the coefficient of determination ( R 2 ) value of 99 % for control concrete and 97 % for coal concrete . The relationship between the UPV value is in the range of 4214m / s to 4590m / s and the compressive strength values in the range of 21.88MPa to 37.28 . The high strength selfconcretes with fly ash and blast furnace slag as a substitute for cement depicted a similar relationship [ 24 ].
Splitting Tensile Strength Figure 6 shows the splitting tensile strength of different mix designs of concrete at 28 days after casting . As shown in Figure 6 , the splitting tensile strength of coal concrete was 3.42MPa whereas the splitting tensile strength of control concrete was 3.39MPa . Obviously , the splitting tensile strength of coal concrete is comparable with conventional concrete . Concrete always has minor cracks on its surface because it is made with aggregate . These small cracks are enlarged when the concrete is subjected to tensile forces . This could ultimately result in the concrete breaking apart . However , when the concrete is subjected to compressive forces , the small cracks do not pose any problems .
Figure 5 . Relationship between compressive strength and UPV value of concrete mixes
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