Determination of Compressive and Flexural Strengths of In-situ Pozzolanic Concrete Containing Polypropylene and Glass Fibers Using "Twist-off" Method

Fibers incorporated in concrete, have different shapes and sizes. The main reasons for incorporating fibers in concrete, are to prevent cracking, disintegration of concrete and to increase its resistance to dynamic loading and explosion. As for ordinary concrete, pozzolanic materials can be used along with the fibers. In this paper, the effect of the incorporation of polypropylene and glass fibers in concrete is investigated using the newly developed “Twist-off” method. The “Twist-off” method developed by Naderi is a partially destructive method with high accuracy and very low cost and negligible surface damage. In this method, a metallic disc with 25mm height and 40mm diameter is attached to the surface of concrete under test, using epoxy adhesive. After the setting and hardening of the glue, an ordinary torque-meter is used to apply a torque until the metallic disc is separated from the surface of the concrete. Since the adhesional strength of the epoxy resin to concrete surface is much higher than the strength of the concrete, the failure is bound to happen at the concrete surface. Therefore, the failure measured torque is used to estimate the concrete strength. In order to estimate the compressive strength of the concrete by the method of “Twist-off”, a calibration graph is prepared by employing concrete cubes with different strength. The “Twist-off” strength of these cubes were measured and they are related to their compressive strengths, using ordinary compressive testing machine. Therefore, compressive strength of pozzolanic concrete containing glass and polypropylene fibers were also measured, using 150 mm concrete cubes. The surface strength of the fiber concrete cubes was measured using the “Twistoff” method. The results obtained in this research, show that, addition of glass and polypropylene fibers to pozzolanic concrete increases the surface strength, compressive strength, modulus of rapture of concrete and the load bearing capacity of concrete beams. The random distribution of fibers in concrete beams, increases its ductility and at the failure load, pull out of the fibers are predominant compared to their breakage. The results tend to show that the addition of two percent fibers tend to increase the strength, ductility and the modulus of the elasticity of the pozzolanic concrete. It was also observed that while the addition of glass fibers increases the surface strength of the low strength concrete (40 MPa or less), its addition to the concrete reduces its strength. Compared with the effect of the polypropylene fibers, it was seen that, the increase in the surface strength of concrete with glass fibers is more pronounced. Examination of the results presented in this paper tend to indicate that the “Twist-off “ method can be used for the determination of the surface strength of concrete as well as its compressive strength, with acceptable accuracy and very little surface damage to tested area. Compared with other in-situe methods for concrete strength assessment, the ”Twist-off” method is much cheaper and needs no expert operators. Statistical analysis of the comparative results of the “Twist-off” and the ordinary compressive testing method indicates that the torque obtained in the “Twist-off” method, can be directly related to the compressive strength of the concrete without the need to calculate stress intensities. The relation between the “Twist-off” and compressive strength tests is seen to be linear.