Currently, shear transfer models have diverse assumptions about the contribution of shear reinforcements and concrete to the shear capacity of interfaces between concretes cast at different times (that is, cold joints). Furthermore, the knowledge about the effect of high-strength shear reinforcements (HSSR) having a nominal yield strength exceeding 414 MPa (60 ksi), is still limited. This study aims to determine the extent to which shear reinforcements and concrete contribute to the shear capacity of cold joints and explore the effect of HSSR on shear transfer behavior. To this end, nine cold joint push-off specimens were tested to investigate the effects of yield strength of shear reinforcement (YSSR) and concrete compressive strength (CCS) on the interface shear strength and stiffness. In addition, code provisions evaluation was used to identify the degree of concrete contribution, based on a database of 108 cold joint push-off test results established in this study. Test results indicate that the increase of YSSR (rising from 400 MPa to 600 MPa) has no evident effect on the shear strength and stiffness of cold joints, whereas the increase of CCS can significantly increase them. Moreover, when CCS rises from 20 MPa to 50 MPa, the interface shear capacity of push-off specimens increases by approximately 110%, i.e., from 103.5 kN to 217.5 kN. Hence, concrete has a considerable contribution, whereas the contribution of shear reinforcements is not as great as expected. Code provisions evaluation results also indicate that concrete considerably contributes to the shear capacity in the form of cohesion. On the basis of these findings and current design equations, a shear capacity equation, which accounts for the effect of CCS and the limited participation of shear reinforcements, is proposed. This equation is found to provide reasonable prediction of cold joint shear capacity, especially for the cold joint composed of high-strength concrete.

This study aimed to improve the mechanical properties and water resistance of calcined gypsum from phosphogypsum (CGP) by incorporating organic additives and inorganic admixtures. The effects of the dosage of these additives—including kaolin, nano-SiO₂, polycarboxylic acid superplasticizer, and sodium methyl silicate—on the properties (flexural strength, compressive strength, water absorption, and softening coefficient) of CGP composites (CGPCs) were investigated. A high water resistance of the CGPCs was achieved using nano-SiO₂ and sodium methyl silicate modification, superplasticizer addition, and the partial replacement of gypsum with mineral admixtures. The results showed that the flexural and compressive strength of the composites hit 4.61 MPa and 19.54 MPa, respectively, while the softening coefficient was 0.70 and the water absorption rate was 19.85%. Microstructural investigation confirmed that the combination of nano-SiO₂ and kaolin led to the formation of calcium silicate hydrate. Additionally, the superplasticizer played a crucial role in reducing the water-to-cement ratio, while unhydrated mineral particles had a filling effect, thereby enhancing the density of the hardened paste. The sodium methyl silicate formed a hydrophobic film on the surface of the hardened paste, increasing the contact angle to 109.01° and improving the water resistance of the CGPCs.