Issue 41

L. Zhang et alii, Frattura ed Integrità Strutturale, 41 (2017) 356-368; DOI: 10.3221/IGF-ESIS.41.47 357 water glass series, low-alkali or alkali-free series [3-4]. Currently, shotcrete mainly adopts powdered accelerators in China, mainly applied to dry-mix shotcrete, has a large amount of resilience and dust, cannot be uniformly mixed with aggregates, and causes large dosage, therefore seriously affecting the later strength, increasing the construction cost, and retarding the construction progress. The liquid accelerator mainly applied to wet-mix shotcrete, has instable quality, generally large dosage (6% to 10%), and immature supporting construction technology. Therefore, to resolve the preceding problems, a new liquid accelerator for shotcrete, with the dosage about 2%, has properties in all aspects superior to first-class goods stipulated in the JC477-2005 Flash setting admixtures for shotcrete , to better meet construction needs. The shotcrete containing the accelerator can be set within 2 to 10 minutes after spraying so that it can provide the support resistance (radial force) for surrounding rock in a timely manner, change the biaxial stress status without support to triaxial stress status of the surface rock for the surrounding rock, and improve the strength of surrounding rock. The shotcrete layer is a flexible support and can gradually coordinate with deformations of surrounding rock by adjusting deformations, therefore improving the stress status, and giving full play to the self-bearing capacity of surrounding rock [5-6]. This paper adopted the numerical simulation method to study the growth of early strength of shotcrete added with the new liquid accelerator with the passage of time as well as interaction between the growth and the stress release of surrounding rock recently excavated, and compare shotcrete added with the traditional and new liquid accelerator on the supporting role. In this paper, chapter 2 introduces the development thinking, material composition, and functional mechanism of this liquid accelerator. Chapter 3 mainly studies the impact of its ingredients on the properties of cement and concrete through laboratory tests. Chapter 4 mainly studies the effects of support for surrounding rock of shotcrete added with this liquid accelerator, adopts the numerical simulation method to study the impact on surrounding rock supporting effects of shotcrete with varied ages and strength, and considers the interaction between the stress release rate of surrounding rock and shotcrete support. Chapter 5 mainly verifies the performance of this liquid accelerator in actual work via field tests. D EVELOPMENT OF LIQUID ACCELERATOR Development thinking of liquid accelerator ccording to a series of studies on specialty chemicals, it can be found that the shotcrete accelerator has a large room for improvement and that the accelerator in the current market has many problems. For example, the powdered accelerator has a serious dust problem and cannot be dissolved and display effects in a just 0.01-second mixing time. Therefore, the surface of shotcrete is hard and most coarse aggregates are rebounded. Moreover, the compressive strength of 28-day (or other ages) shotcrete is not ideal and far from meeting design specifications, which brings potential risks to engineering safety, causes large usage of cement and high construction cost of shotcrete. Currently, the liquid accelerator used onsite is easy to deteriorate and produce precipitate, needs large dosage (generally 6% to 10% of the cement quantity), causes serious later strength loss for shotcrete, cannot adapt itself well to cement, and causes a large amount of resilience and slurry shedding in actual field applications. Due to these reasons, the new liquid accelerator is developed. Composition of liquid accelerator (1) Mother liquor preparation This liquid accelerator is mainly prepared by KOH and Al(OH) 3 in a certain mole ratio, with the mole ratio of potassium to aluminum below 1.2. A certain amount of KOH and Al(OH) 3 solid powder is weighed, poured into a three-necked flask, and reacted with added quantitative water. The temperature is controlled above 120 ℃ , and the electric stirring rod is used to constantly stir the liquid during the reaction. The reaction is carried out for two hours, and the solution is filtered to obtain the mother liquor. (2) Mother liquor compounding To improve its performance, other components are added with compound the mother liquor, including the polyacrylamide, triethanolamine, water reducing admixture, lithium salt, and stabilizer. This mother liquor is a kind of aluminate accelerator featuring small dosage and better coagulating effects. However, the single solution cannot meet various requirements of shotcrete and therefore the mother liquor is compounded. Polyacrylamide is a kind of thickener used mainly to increase the cohesiveness of shotcrete and reduce resilience during construction, triethanolamine is a kind of early strength agent that can shorten the setting time, water reducing admixture is mainly to reduce water consumption and improve shotcrete strength, lithium salt can inhibit the alkali-aggregate reaction of shotcrete, stabilizer can effectively improve the stability of the solution. A