Laboratory Evaluation Of Cement Reinforced And Cement-Flyash-Gravel (CFG) Reinforced Concrete Pavement Sections

Roads have developed from ordinary gravel roads to asphalt roads to concrete roads, each with increasing improvement in structural properties. Most of the time the natural subgrade is not strong enough to carry or support loads from vehicles and other traffic, so it is normally required to build a pavement on top of the subgrade (the in-situ soil). Instead of building a road exclusively made of concrete, which can be very expensive, pavement layers can be reinforced or stabilized. Reinforcement or stabilization is aimed at improving the engineering properties of the soil and subsequently that of the pavement. Reinforcement or stabilization can be in the form of geosynthetics (geotextiles or geogrids), mechanical (compaction) or chemical (addition of certain chemicals or additives). This research concentrates mainly on chemical stabilization, specifically cement and fly ash stabilization of pavement layers of a road. Cement stabilization of bases and subbases is the most common form of chemical stabilization; this is because cement when compared to other additives provides considerable strength to the mixture and has excellent engineering properties. However cement treated bases have shown poor performances in the past. Cement treated bases and subbases exhibit excessive pumping, faulting, and shrinkage cracking. These are thought to be more pronounced as the cement content is increased. A lot of research has been done over the years to help combat these problems. Supplementary cementitious materials (SCM's) like lime, granulated blast furnace slag (BFS), flyash (FA), and amorphous silica (AS) have been added to soil-cement mixtures to help reduce or eliminate the above mentioned undesirable effects on soil - cement mixes. In addition to reducing and eliminating such undesirable effects, addition of SCM's improves the mechanical and structural properties of the concrete and subsequently cut material costs (as some cement is replaced by SCM's, which are cheaper than cement). These supplementary cementitious materials however perform differently with different soils or rocks and also under different curing conditions and durations, so the type and amount of supplementary cementitious material used, type of soil or rock used, conditions under which the road (insitu) or the sample (laboratory) is cured and duration of curing are thought to grossly affect strength, structural and mechanical properties of the road or laboratory sample.This thesis presents research on the benefits of adding flyash to cement mixes for stabilizing or reinforcing pavement layers and how factors like mix design, temperature, humidity and time affect the structural and mechanical properties of stabilized pavement layers. A cohesionless-frictional soil, ordinary Portland cement and Class F flyash was used. Soil classification, density and compaction and unconfined compressive strength tests were performed as these properties identify or relate to other engineering properties such as stiffness, durability etc. Samples were fabricated in the laboratory to simulate insitu stabilization process. Samples made of different proportions of cement and flyash where made, compacted and cured at different curing conditions and durations. Fourteen different mixes were used and cementitious material content was between 0% and 20% of dry weight of gravel. Curing conditions were 20°c and 100% humidity, room temperature and humidity and 60°c and 100% humidity. Curing durations were 7 days and 28 days. For each mix, six samples were made and cured at the three different curing conditions and the two different curing durations. After the specified curing durations, unconfined compressive tests (UCS) were done on the cured samples and strength results in Psi (pounds per square inch) were recorded for subsequent analysis, to see which one is the most economic (in terms of least cement or flyash content) but with good mechanical and structural properties.The strength results got were analyzed theoretically and numerically (using Statistical Package for Social Scientists, SPSS). In the theoretical analysis, simple excel plots were drawn in the form of histograms to show how strength varies with respect to mix, temperature, humidity and number of days. In the numerical analyses, linear regression analyses were done using SPSS software. The statistical outputs showed how strength, mix, temperature, humidity and number of days are correlated and the extent of their correlation. The theoretical and numerical analyses agree with each other in every way.From the two analyses, it was concluded that flyash enhances the mechanical and structural properties of cement mixes used in stabilizing or reinforcing pavement layers. For the different mixes, cement samples with flyash incorporated in them were less brittle and showed an increase in strength than their cement only counterparts. Also it can be concluded that factors like, mix design, temperature, humidity and time affect the structural and mechanical properties of cement-flyash-gravel (CFG) stabilized pavement layers greatly. For both cement and flyash, as cement and flyash content is increased, strength increases, but starts to decrease after a certain percentage is exceeded. Within the specified curing conditions and durations, as temperature, humidity and number of days increases, strength of samples increased. Cement only and more cement than flyash samples gain strength at a faster rate within the first 7 days but continue to gain strength at a slower rate afterwards. It is the opposite for flyash only and more flyash than cement samples, their rate of strength gain is slow within the first 7 days but increases afterwards.