DURABILITY OF PAVEMENT CONCRETE WITH REPLACEMENT OF CEMENT BY FLYASH AND PORTLAND LIMESTONE CEMENT
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Abstract
The durability of concrete can be directly related to the permeability of the concrete. While the permeability is not the only characteristic that can relate to durability, it is amongst the most important predictors of service life. Lower permeability is related to increased durability. The less permeable a concrete mixture is the less likely the concrete will deteriorate from freezing action and ingress of deleterious chemicals. Durability measured through the use of permeability testing was used on different pavement concrete mixtures for this study. The North Carolina Department of Transportation provided mixtures that are best representative of the actual mixtures used in the state for pavement. The pavement mixtures varied in composition of cementitious materials. Different cements and cementitious materials, including a portland limestone cement and cement replacement by fly ash, were used in the mixtures to provide a durability comparison. The w/cm ratio, slump, and air content were kept constant to provide a better comparison for the effects of the cement and replacements of cement. To give a control mix for better comparison, an ordinary portland cement mixture (OPC) was compared against OPC mixtures with fly ash and portland limestone cement mixtures (PLC) both with and without fly ash. The permeability of the concrete can be measured by a variety of permeability tests. The tests used in this study are the rapid chloride penetration test, the surface resistivity test, the sorptivity test and the air permeability test. Some of these tests are very well established, such as the rapid chloride penetration test (RCPT), and others like the surface resistivity test, and were compared to the more established tests to determine if it was a viable test to be used. The surface resistivity test and RCPT were found to be very well correlated, which agreed with the literature. When trying to analyze the results, it was determined that a better way to categorize the permeability of the concrete was needed. To try to better categorize the mixtures, the major permeability tests mentioned were used to create a permeability index.When analyzing the results from the permeability test, it is obvious that the mixtures containing both fly ash and limestone have a lower permeability; this means these mixtures will have higher durability. The fly ash addition had the greatest reduction effect on the permeability of the mixtures. The mixtures containing portland limestone cement did not have as significant of a reduction in permeability when compared to when fly ash was added, but the reduction was present. The limestone only aided in reduction of permeability when fly ash was present in the mixture. These results were expected based on the literature. One main reason for the increase in durability is from the increase in particle packing that reduces the size of the pores and decreases the permeability. The results also show that some test methods are much more representative of the actual permeability than others. The results proved that the addition of fly ash and limestone gave significant reductions in permeability. This significant reduction would mean an increase in durability of the concrete that would be very useful for the NCDOT to increase the service life of their pavements used around the state. The permeability index used to help prove this significant reduction had some very good correlations to the test methods used to develop the index, but a wider range of mixtures would help validate the index.