Experimental Study of Concrete Mixes with Nano-Silica and Silica Fume

Abstract

During the manufacturing of cement, when limestone and clay are crushed and heated at high temperature, there is emission of global warming gasses including carbon dioxide (CO2) into the atmosphere. As the current atmospheric concentration of CO2 has reached an alarming high value to the tune of 410 ppm (April, 2017), it has become obligatory to develop ‘Green concretes’ in order to decrease the CO2 emission from cement industry. The aim of the study undertaken herein is to highlight the utilization of nano silica in high performance concrete in order to reduce the environmental pollution and to also see if the strength as well as the durability properties of the concrete can be altered positively. In the experimental work undertaken, cement was partially replaced by nano silica at 2%, 3% & 4% replacement levels and replacement level of silica fume was kept constant at 8% for four different water binder ratios (w/b) of 0.30, 0.34, 0.38 and 0.42. The concrete samples were casted and tested for strength (compressive and split tensile) and durability properties (abrasion resistance and rapid chloride ion penetration resistance) after 7, 28 and 56 days of curing ages. Scanning Electron Microscopy, X-Ray diffraction and Energy Dispersive Spectroscopy tests were also conducted to analyze changes in the concrete microstructure. The results were statistically analyzed as well. The test results after 56 days curing period show 34.54 % and 36.52 % increase in compressive and split tensile strength for w/b ratios 0.38 and 0.30, respectively whereas a 56.31% reduction in average loss of thickness and 38% increase in resistance of chloride ions penetration was observed for w/b ratio of 0.30. Although, the experimental results also show that the maximum increase in compressive and split tensile strength, maximum reduction in abrasion and maximum decrease in rapid chloride ion penetration value were found at 0.30 w/b ratios at the replacement level of 4% of nano silica and 8% silica fume for 56 days of curing. It is also observed that average loss of thickness decreases with the increase in compressive strength of concrete. The percentage decrease in average loss of thickness was doubled or near to double with the percentage increase in compressive strength of concrete after 28 days of curing. The Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) test results showing the level of Ca (OH)2 in plain concrete and consumption level of Ca (OH)2 along with level of CSH in concrete containing nano silica & silica fume have also been presented. The relation between compressive strength and other properties i.e. split tensile strength, abrasion resistance and rapid chloride penetration of high-performance concrete has been developed and the results are found significant. The verification of actual compressive and rebound hammer compressive strength has been done by determining the percentage of difference between these two and found the result well within specified limit. The correlation among split tensile and compressive strength ratios has been developed to calculate the Integral Absolute Error (IAE) and it has been found that the maximum values are within specified limit. The comparison of relationship of compressive and split tensile strength of the current study with the study of other three authors has been done and found the result well within line of the study of Oluokun et al. (1991). The reliability analysis has been done by considering the results of compressive strength of concrete in terms of mean, standard deviation and with-in-test coefficient of variation corresponding to the variation in the designated parameters which have already been generated. The compressive strength data generated experimentally has been analyzed using normal-probability distribution and partial safety factors of composite concretes have been evaluated. The experimental results show that the properties of concrete having nano silica and silica fume in combination were better than that of a plain concrete. The with-in-test coefficient of variation (WCV) being less than the acceptance value of 3.0 percent (ACI-214-77) indicates that the testing condition is uniform and under very good control. It is known that the compressive strength that is to be advised at site can be done so with greater degree of confidence if the partial safety is on lower side. There is little variation in the PSF values, for mixes with and without nano-silica and silica fume, for reliability indices of 1, 1.3 and 2.0, whereas the change is significant for β = 3.0, indicating that to achieve higher reliability at site, a strict control on water-binder ratio plays a significant role in strength development of concrete. For concrete mixes having 2% nano-silica, for a reliability index of β = 3.0, the lowest values of partial safety factors are achieved for w/b ratios of 0.38 at all ages, indicating that higher quality control is required while replicating mixes, specifically at the w/b ratio of 0.38. For all other reliability indices at all the w/b ratios the variation in the partial safety factor values is almost negligible. It can be concluded that for mixes where early age strength is the criteria, the optimum replacement percentage of cement by nano-silica is 2 to 3%, whereas, for concrete mixes cured for normal or higher ages, it is easy to replicate mixes, containing 3 to 4% nano-silica, at the site. The partial safety factor decreases with the decrease in with-in-test coefficient of variation leading to the conclusion that a better-quality control of the concrete mixes would lead to optimal design of reinforcement for mixes containing optimal dosages of nano-silica and silica fume. The replacement of cement with nano-silica and silica fume leads to lower values of WCV and partial safety factor (PSF) indicating that it is advisable to use these supplementary cementitious materials for producing efficient RCC designs. The results confirmed that the joint use of nano silica and silica fume has better impact on strength and durability properties than conventional concrete. The results show that the use of nano silica and silica fume as replacement of cement not only makes the concrete more durable but also reduces the emission of CO2 during the production of cement. It also solves the waste disposal problem along with saving of the natural resources.