Accelerating Admixtures for Shotcrete

摘要

A variety of additives and admixtures are added to shotcrete to improve strength, adhesiveness, cohesiveness, freezing/thawing and abrasion resistance characteristics, and reduce rebound. Accelerators are being used increasingly in both dry- and wet-process applications. Accelerators are common in the dry process to increase early strength and reduce dust and rebound and in the wet process are used to achieve rapid set and early strength. The choice of a particular accelerator and its dosage is largely governed by the setting time required for the shotcrete application. Various watersoluble salts of the alkali metals can be used to accelerate the setting of cement. Most of the set accelerators used today are based on alkali aluminates in combination with carbonates and hydroxides and produced in both liquid and powder form. The performance of these accelerators depends on the cement chemical composition and fineness, and the presence of mineral additions such as flyash, and blastfurnace slag. This performance is generally evaluated using setting tests on cement/accelerator pastes despite the belief of some researchers that this procedure can produce misleading results. This paper describes the main accelerators used in shotcrete and presents some results of field tests performed on shocrete panels evaluating the behavior of different accelerators. Particular attention is given to a new liquid alkali-free shotcrete accelerator that showed a very interesting behavior at very early ages and no strength loss at later ages. The early strengths were evaluated using non-destructive tests (Constant Depth Penetrometer and Constant Energy Penetrometer).