EFFECTIVE APPLICATIONS OF RETARDING ADMIXTURE TO IMPROVE THE
PERFORMANCE OF CONCRETE IN HOT WEATHER
Syed Khaleeq Ahmad
DISCUSSION OF RESULTS
Effect of Pozzolith on initial setting time
The test results obtained to study the effect of Pozzolith on initial setting time of cement are presented in Table 2 which shows an increase in the initial setting time of cement from 85 minutes without Pozzolith to 112 minutes with 1.2% Pozzolith. This retardation of initial setting time will be very advantageous in the case of concreting in hot weather. In case of transporting concrete from the ready-mix plant to the construction site, it will be advantageous to have a high initial setting time. It will also be helpful in placing concrete with a good bond in different layers.
Effect on workability and water-cement ratio
To study the effect of Pozzolith on workability of cement, concrete mixes of 1:2:4 were prepared with different percentages of Pozzolith. Slump test was used to determine the workability and the results presented in Table 3 and Fig. 1 show an increase in slump value with an increase in the percentage of Pozzolith. This improvement in workability enhances the chance of getting a concrete mix of high workability with less water-cement ratio. In normal conditions it was observed that to achieve a slump value of 67 a water-cement ratio of 0.6 is required. Hence there is a saving of 9.4% water by adding only 0.4% of Pozzolith.
Effect on water absorption
Concrete cubes were weighed before and after curing for 28 days. The results obtained for water absorption as a percentage of concrete are shown in Table 4 and Fig. 2. These results clearly indicate that water absorption is inversely proportional to percentage of Pozzolith. A decrease in water absorption indicates a decrease in porosity, which in turn increases the density of concrete. This will definitely help to reduce the permeability of concrete and hence improvement in water-tightness of concrete.
Effect on compressive strength
The compressive strength of concrete was tested after curing the cubes for 28 days. The average value of compressive strength as shown in Table 5 and Fig. 3 is increased from 46.5 N/mm2 to 53.4 N/mm2 with an increase in percentage of Pozzolith associated with high workability. This will help to design a concrete mix of high strength with less water-cement ratio.
Effect on flexural strength
Table 6 and Fig. 4 represent the variations in flexural strength of concrete as a result of adding different percentages of Pozzolith. These results also show an increase in the value of flexural strength from 3.32 N/mm2 to 3.72 N/mm2 with an increase in the percentage of Pozzolith from 0% to 1.2% by weight of cement.
Effect on performance of fine aggregate
Different types of fine aggregates (<600 mm, well graded and >600 mm) were used to study the effect of Pozzolith on the performance of fine aggregate. The results given in Table 7 show an increase in the compressive strength by adding Pozzolith. The fine aggregate <600 mm requires more water in normal conditions whereas by the addition of Pozzolith the water requirement is reduced. The use of fine aggregate will produce concrete of high density and less porosity, which will greatly enhance the related properties of concrete.
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Last updated on Thursday November 29, 2012