Jaren Knighton and Dr. W. Spencer Guthrie, Civil and Environmental Engineering
The pavement engineering industry needs a quick, inexpensive laboratory testing method for evaluating the structural benefit that can be expected in the field when geogrid is incorporated in a pavement. Geogrid is typically made of rigid polypropylene with ribs that form openings, called apertures, as shown in Figure 1. Larger aggregate in the base material penetrates the apertures to interlock with the geogrid and provide support in the pavement structure. The added support provided by geogrid has been shown to decrease rutting and cracking in asphalt pavements. The purpose of this project was to explore two widely accepted testing methods, California bearing ratio (CBR) and quick shear testing, to determine if these methods can be used to evaluate the effectiveness of geogrid reinforcement in aggregate base materials.
Testing was performed on aggregate base material taken from Trenton gravel pit #3 and the Point of the Mountain pit, both in northern Utah. Multiple unreinforced and geogrid-reinforced specimens were tested to allow for statistical analysis of the results. CBR testing was performed in general accordance with American Society for Testing and Materials (ASTM) D1883 (Standard Test Method for CBR (California Bearing Ratio) of Laboratory-Compacted Soils). The testing consists of driving a 1.95-in.-diameter loading piston into the top of a specimen at a strain rate of 0.05 in./min. Load measurements are recorded for every 0.1 in. of penetration up to 0.5 in. and used to calculate a CBR value. This testing was performed on four unreinforced and eight reinforced specimens that were 6 in. in diameter and 4.6 in. in height using an Instron material testing machine available at BYU. For reinforced specimens, the geogrid was compacted in place approximately 1.85 in. from the top of the specimen in general accordance with ASTM D1557 (Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort (56,000 ft-lbf/ft3 (2,700 kN-m/m3).
Quick shear testing was performed as described in American Association of State Highway and Transportation Officials (AASHTO) T 307 (Determining the Resilient Modulus of Soils and Aggregate Materials). The testing consists of applying a constant strain rate of 0.12 in./min in compression with a constant confining pressure of 5 psi and continuously measuring the load until specimen failure. The test results were used to calculate the ratio of stress to strain, known as the modulus of elasticity, when the specimen reached 2% strain. This testing was performed on four unreinforced and 12 reinforced specimens that were 6 in. in diameter and 12 in. in height using the computer-controlled, servo-hydraulic UTM-100 equipment also available at BYU. For reinforced specimens, the geogrid was compacted in place approximately 3 or 6 in. from the top of the specimen in general accordance with ASTM D1557.
Results and Discussion
To evaluate the results, a t-test was performed on each data set using Minitab software. In the test, the null hypothesis was that the structural capacity of the geogrid-reinforced material was less than or equal to that of the unreinforced material, while the alternative hypothesis was that the structural capacity of the geogrid-reinforced material was greater than that of the unreinforced material. For this research, a p-value less than or equal to 0.05 permitted rejection of the null hypothesis and acceptance of the alternative hypothesis and indicated that the given laboratory test was able to differentiate between unreinforced and reinforced specimens.
The average CBR and modulus of elasticity values, standard deviations, and p-values calculated from the data obtained in the CBR and quick shear testing are shown in Tables 1 and 2. The lowest p-values occurred in the analyses of the quick shear test data, and both were less than or equal to the stated threshold of 0.05, while neither of the p-values associated with the CBR test data were less than or equal to 0.05. Therefore, the quick shear test holds promise for evaluating the structural benefit that can be expected in the field when geogrid is incorporated in a pavement.
Unreinforced and geogrid-reinforced aggregate base materials were tested using CBR and quick shear testing protocols. The data were then evaluated using t-tests to determine if either laboratory test is able to evaluate the effectiveness of geogrid reinforcement. The results of the t-tests show that the effect of geogrid reinforcement is readily apparent using the quick shear test, which is recommended for use in this application. The CBR test is not recommended for evaluating the effectiveness of geogrid reinforcement. These results are applicable only to materials similar to those evaluated in this research.