Cold joint is one of the common construction challenges in concrete structures that can significantly reduce the mechanical strength of concrete, particularly under compressive and tensile stresses. The aim of this study is to investigate the effect of cold joints and to evaluate the performance of strengthening techniques, including shear keys and surface grooves, in improving the strength of specimens containing cold joints. In this study, a series of cubic specimens for compressive tests and cylindrical specimens for tensile tests were cast under controlled laboratory conditions. The specimens were categorized into four groups: monolithic concrete (without cold joint), concrete with a cold joint, and concrete with a cold joint strengthened by shear keys and surface grooves. The results of the tensile tests showed that the presence of a cold joint led to a 25% reduction in tensile strength compared with the control specimens. The use of shear keys reduced this loss to 10%, while surface grooves limited it to 17%. In the compressive tests, the specimens with a cold joint exhibited a 12% reduction in compressive strength compared to the monolithic specimens, whereas this reduction for the specimens with shear keys and surface grooves was 7% and 10%, respectively. Based on the obtained results, both strengthening methods demonstrated a significant improvement in mitigating the adverse effects of cold joints. Furthermore, comparison of the results indicates that cold joints have a smaller influence on compressive strength than on tensile strength, which highlights the greater sensitivity of the joint region to tensile stresses and underscores the need for more detailed investigation.