Use of Set Accelerating Admixtures in Dry-Mix Shotcrete

Tests were carried out to study the influence of set accelerating admixtures on the properties of dry-mix shotcrete, especially its durability. A total of 27 mixtures were shot using two different types of cement (ASTM Type I with silica fume as partial cement replacement and ASTM Type III) and five different accelerators at various dosages. Half of these mixtures were shot with an air-entraining admixture. In addition to the determination of the setting time, the following properties were measured (on the hardened shotcrete): Air void characteristics, compressive strength, drying shrinkage, rapid freezing and thawing resistance and deicer salt scaling resistance. Results indicate that although all accelerators can reduce the initial and final setting times, some of them considerably decrease the durability, particularly the deicer salt scaling resistance. As a rule, better results were obtained with the mixtures containing the Type III cement than with those containing the Type I and silica fume. The tests confirm the importance of the use of an air-entraining admixture to obtain an adequate frost and deicer salt scaling resistance.