Evaluation of mechanical properties of polymer modified mortars and their bond strength to steel

Today, the use of different kinds of polymer, as the modifier of some repair mortars properties, is growing. Given the damages to concrete structures, it is necessary to use appropriate repair layers. In concrete structures, concrete and steel are connected, and in most cases, repair layers are applied in direct connections with steel. Therefore, in this research, the shear and tensile bond strength between steel and styrene-butadiene rubber polymer modified mortars was measured using semi-destructive friction-transfer and pull-off tests. In the "pull-off" test, to determine the bond between the mortar and the steel, a core with a 50mm diameter and is first mounted on the test surface using a diamond drill bit and a metal cylinder with a diameter of 50 mm and a thickness of 20 mm is attached to the partial core. Then, the tensile force is applied to the cylinder by means of a "pull-off" device to make the partial core fail. To measure adhesion with friction transfer method, first a small core was created from the mortar surface to the steel substrate surface using the coring machine. Thereafter, the friction transfer metal device was fixed onto the core and the torsional moment was applied using a typical torque wrench in order to cause failure in the core. Moreover, the effect of polymer on the shrinkage of mortar was evaluated. Shrinkage is one of the important problems that negatively affects the adhesion of repair mortar and steel. Due to the fact that hydrated cement paste has capillary pores that contain some water, shrinkage occurs after this moisture leaves the pores. The effect of polymer on mortars was investigated by taking images with a scanning electron microscope and using the “Image-J” and “Origin” software programs. Afterward, in order to evaluate the mechanical properties of mortars, the in-situ compressive and flexural strengths of the mortars were determined, and the calibration curves were plotted by comparing them with standard laboratory tests. Then, relationships were proposed to convert the results of in-situ tests to the compressive and flexural strength of the polymer modified mortars. Eventually, the cracks and stresses that appeared in the mortars were provided using ABAQUS software. The obtained results indicated the effect of polymer in reducing the shrinkage of mortars and increasing the shear and tensile bond strength between steel and mortar, along with a high correlation coefficient between the measurements in the in-situ and laboratory tests. Comparing the modified mortars with polymer and ordinary mortar, it is observed that at the age of 90 days, adding 10, 15 and 20% of SBR reduced the amount of shrinkage to 35.3%, 4.2% and 45.4%, respectively. Addition of styrene butadiene rubber to the repair mortar increased the shear bond strength obtained from the "friction transfer" test between the mortar and steel at the ages of 7, 42 and 90 days by 44.4, 178.2 and 303.1%, respectively. Adding SBR to the repair mortar increased the tensile strength of the "pull-off" test between the mortar and the steel at the ages of 7, 42 and 90 days by 58.7, 183.4 and 291.2%, respectively. A good agreement was also observed between the numerical and experimental results.