Shrinkage is the volume change of concrete without influence of external forces. Drying shrinkage, which occurs after the concrete hardening, is one of the most important causes of the cracking in concrete members. In addition to adverse effects on the appearance of concrete, cracks may reduce the strength and durability by increasing the permeability of concrete and facilitate the entry of aggressive agents into it. Free and restrained drying shrinkage are the most important types of shrinkage. The present study investigated the effect of two admixtures—including damp proofing and expansive materials— on compressive strength, tensile strength, electrical resistivity, modulus of elasticity, unrestrained and restrained shrinkage and water absorption of concrete specimens. In addition, the effect of admixtures on crack properties was investigated. The cracks properties, which occurred in unrestrained shrinkage test, such as average and maximum crack width and total cracks area, were computed by image processing. The results showed that the use of damp proofing additive, (i.e. calcium stearate) reduced maximum free drying shrinkage by 42.4%, maximum restrained drying shrinkage by 22.8%, maximum crack width by 51% and final crack area by 21%. Moreover, the use of expansive additive (i.e. aluminum powder) almost eliminated the free shrinkage, but on average, it increased the restrained drying shrinkage by 18% and reduced the maximum crack width and the final crack area by 2.67 and 27%, respectively.

收缩是混凝土在不受外力影响的情况下的体积变化。混凝土硬化后发生的干缩是混凝土构件开裂的最重要原因之一。除了对混凝土外观产生不利影响之外，裂缝还可能通过增加混凝土的渗透性并促进侵蚀性物质进入其中而降低强度和耐久性。自由干缩和受限制干缩是最重要的缩水类型。本研究调查了两种外加剂——包括防潮材料和膨胀材料——对混凝土试件的抗压强度、抗拉强度、电阻率、弹性模量、无约束和有约束的收缩和吸水率的影响。此外，还研究了外加剂对裂纹性能的影响。通过图像处理计算了在无约束收缩试验中出现的裂纹特性，如平均和最大裂纹宽度和总裂纹面积。结果表明，防潮添加剂（即硬脂酸钙）的使用使最大自由干燥收缩率降低了42.4%，最大限制干燥收缩率降低了22.8%，最大裂缝宽度降低了51%，最终裂缝面积降低了21%。此外，膨胀剂（即铝粉）的使用几乎消除了自由收缩，但平均而言，它使约束干燥收缩率增加了18%，最大裂缝宽度和最终裂缝面积分别减少了2.67%和27%。结果表明，防潮添加剂（即硬脂酸钙）的使用使最大自由干燥收缩率降低了42.4%，最大限制干燥收缩率降低了22.8%，最大裂缝宽度降低了51%，最终裂缝面积降低了21%。此外，膨胀剂（即铝粉）的使用几乎消除了自由收缩，但平均而言，它使约束干燥收缩率增加了18%，最大裂缝宽度和最终裂缝面积分别减少了2.67%和27%。结果表明，防潮添加剂（即硬脂酸钙）的使用使最大自由干燥收缩率降低了42.4%，最大限制干燥收缩率降低了22.8%，最大裂缝宽度降低了51%，最终裂缝面积降低了21%。此外，膨胀剂（即铝粉）的使用几乎消除了自由收缩，但平均而言，它使约束干燥收缩率增加了18%，最大裂缝宽度和最终裂缝面积分别减少了2.67%和27%。