Essay On Research Significance
VARIABILITY OF MECHANICAL PROPERTIES OF BASALT FIBER REINFORCED POLYMER BARS MANUFACTURED BY WET-LAYUP METHOD
The civil engineering sector has benefited from the development of different materials that offer different benefits such as reliability and high tensile strength. Fiber reinforced polymers (FRP) are one of the materials that have proved their usefulness in the construction of bridges and buildings. FPR has been researched extensively and has proven to be a reliable construction material. There are two types of FPR bars commonly used as reinforcement material; these are glass (GFRP) and carbon ( CFRP).The third type is aramid (AFRP) that is used to a less extent compared to the other two. The research studies carried out on FRP shown positive results making them a preferred choice compared to the traditional choices of metallic materials that engineers have been offered.
FPR is preferred for its low weight density and high tensile strength. The three types of FRP have been used as reinforcing bars in concrete structures. Their performance has been exemplary as they provide the needed strength, durability, and stiffness. A new type of FRP has been developed by using basalt fibers. The basalt fibers used in the manufacture of the BFRP are produced from solidified lava. The advantage of the BFRP is that since it is made from igneous rock, the ingredients cannot pose any health hazards. This makes it environmentally friendly and ecologically harmless. In addition, the process of extruding basalt fibers is energy efficient when compared to other fiber types.The basalt fibers are superior to other fibers in that they provide a thermal range of performance of between -259 to 960 degrees Celsius. They also have inertness, high resistance to acids, and resistance to corrosion, radiation, and UV light. In addition, the BFRP has a high tensile strength of about 305 ksi and significant resistance to impact and vibration.
The basalt fiber is cheaper than the carbon fiber. The research carried out previously shows that the basalt fibers are good for both external and internal reinforcement of concrete structures. Basalt fibers have also been used to make hybrid composites with nylon and aramid fibers to achieve improved impact resistance. The basalt fiber has gained wide acceptability in the sector and is used in different forms such as cages, bars, spirals, mesh, chopped fiber and textile fabric for structural reinforcements. Pultrusion is the extensively used method of making basalt fibers. However, the more economical automated wet lay method has also been used to make the BFRP bars with success. Verification of the mechanical properties of the bars made using the automated wet-lay process is necessary so that the bars can be used in practical applications. There is, therefore, the need to conduct experimental tests that will be used to determine guaranteed properties and their variations which are required for quality control and design purposes.
The automatic wet lay-up process in a single stage produces bars from bobbins. The process is different from the conventional pultrusion process as used in the production of commercial FRP bars. The process supports flexibility in the manufacturing of different sizes and shapes of FRP bars. In the wet lay process, the bars are wrapped with a helix that gives the desired unevenness producing the superior bond strength. The process can be used to produce BFRP bars of between 0.66-32 mm. The method is cost effective because production is maintained when the same process is repeated with the utilization of the same fixtures and same computer program without any modifications.
The wet lay-up method has been used extensively in fabrication of composite parts. However, the method is new in the production of BFRP and promises to be cost effective. The study, therefore, focused on establishing the variations of the mechanical properties of the BFRP bars produced using the wet lay-up process. The reliability of the results for carbon, aramid and glass FRP bars is assured through the use of the minimum number of test specimens as directed in the ACI 440_3R-04. However, the equations provided cannot be applied with certainty to the wet lay-up DFRP because it is a different fiber and a different process of manufacturing. The study, therefore, aimed at investigating the distribution of obtained experimental test data for BFRP bars from the wet lay-up process using statistical techniques. The study also examined the minimum number of tests necessary in order to meet the required confidence level. Finally, the statistical distribution of the data was used to validate the applicability of the ACI 440.1R equations in the calculation of the guaranteed properties of BFRP bars.
Mechanical properties of FRP bars are tested using the methods developed by several national committees. For example, the ACI 440_3R-04 test method 2 where the variables are type and size of FRP bar. The tensile modulus of elasticity, the tensile strength, and the rupture strain of the FRP bars are among the properties tested. The CSA 5806 Annex C is also another method used in determining the mechanical properties of FRP bars. The tensile strength of a specimen is determined by dividing the tensile load at failure over the nominal cross-sectional area. The extensometer is the laboratory device that is commonly used to record strain data. The elastic modulus of the bar is derived from the line connecting the 20 and 50 percent of the ultimate tensile strength. The suddenness of the failure of the bars does not allow the extensometer to record reliable results on the rupture strain; it is calculated using the equations in the ACI 440_3R-04.
Basalt FRP bars made of vinyl ester matrix, and basalt fibers were used. All the bars had a fiber volume fraction of about fifty percent. The study employed three bar sizes with nominal diameters of 0.39(R10), 0.28(R7) and 0.17(R4) and corresponding fiber diameters of 0.28, 0.2 and 0.12in. To prevent crushing failure at the ends steel tube anchors were used at the ends of the test specimens. The steel tubes that were used were made of general purpose two part epoxy mixed with dried sand. The anchors were cured at room temperature for over 24 hours. The uniaxial tensile testing was done using a universal testing machine. Each test was completed within 1-10 minutes that was achieved by adopting a constant rate of loading. The equations provided in the ACI 440_1R-06 were used to calculate the guaranteed tensile strength, guaranteed tensile modulus and the guaranteed rupture strain. The ACI 440_1R equations are based on the assumption that the strengths and the strains frequency distributions from the test specimens satisfy normal distribution.
Cost prohibitions may cause the test specimens to be fewer than 25, but the equations in the CSA 5807-10 facilitate calculations as long as the data points are normally distributed. The analysis of variance (ANOVA) is used to obtain the minimum guaranteed test values when the test results for each set are less than 25. The ANOVA techniques are used to analyze the validity of data derived from a combination of different populations. The k-s test is performed when verifying whether the data is normally distributed.
Test results and Analysis
The results in the paper were based on three sizes of BFRP and their gross diameters. Regression points were used to draw a straight line between 20-50% of the ultimate tensile strength whose slope was used to determine the elastic modulus for each of the specimen. The R4 bars that were tested were 11 and had an average tensile strength of 167 ksi, average rupture strain of 0.0295 and a modulus elasticity of 6488ksi. One bar was an outlier as it failed prematurely by splitting at one end. The average tensile strength of the ten R7 bars was 159ksi; average rupture strain was 0.0272 and a modulus elasticity of 1694ksi. The R7 showed a lower tensile strength compared to the R4 bars. The R10 bars failed prematurely due to slippage at the ends that were overcome by increasing the length of the end anchors. The R10 bars had a lower bound tensile strength of 157ksi. The majority of the R10 bars did not experience tensile failure rather the test specimens split at the middle while the rest failed at one end. The R10 test specimen had a rupture strain of 0.0245 and a modulus elasticity of 6488ksi. The results were used to calculate the minimum guaranteed mechanical properties for the R10, R7, and R4 bars.
Statistical analysis done to verify normal distribution of the data included the histogram, but the reliability of the results questioned because the sample was not large, a requirement for using the histogram. The probability plot a graphical approach was also used to determine if the population was normally distributed. In this method, the data is described as having a normal distribution if the points fall along a line and not the case if the points have significant fluctuations. The method is highly subjective, and the results are highly unreliable. In the study, only data from the R7 bars reflected normal distribution under this method. R4and R10 showed reasonable normal distribution. The K-S test was also employed in checking the normality of the data. The study had used less than the required minimum of 25 tests for each of the bar sets. Therefore, the tests were merged into one group of 34 data points. The validation for combining the data was done based on the results of the ANOVA. At 95% significance level, the technique verified that the F-value was less than the F-critical confirming that there was no significant difference between the mean tensile strengths of the three sets. The result of the K-S test was a p value of 0.791. The p value was greater than the required value of 0.05 which meant that the assumption that the data points had a normal distribution was valid. The resulting overall mean average mean tensile strength from the data was 160 ksi with a standard deviation of 11.3 Ksi. The results of the ANOVA allowed for the combined data to be used to make comparisons of the average and guaranteed tensile strength based on the CSA 5807 and ACI 440.1R
The variability of the data is within the required limits which allowed for the successful combination of the three data sets into a large one of 34 points. This was supported by the results of the ANOVA that showed no significant difference between the mean tensile strength of the 10-14 BFRP bars that were tested. The results achieved by using the automated wet lay-up BFRP bars as test specimens were found have a normal distribution under the K-S test. The study met the required 99.875 % of confidence for test specimens with FRP bars as proven through the statistical analysis. The study determined acceptable variability meaning that the equations provided in the ACI 440.1R and CSA 5807 guidelines can be applied to the determination of guaranteed tensile strength, rupture strain, and modulus elasticity. The guaranteed tensile strength results from ACI 440.1R represented about 8% smaller values as compared to the results from CSA 5807.
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