Shotcrete process

Admixtures used either in the wet or dry shotcrete process are required to comply with the following specifications [109] ... 

Deflocculated Castables As the name suggests, deflocculated castables contain deflocculating agents. They are used in the shotcreting process. 

Almost the same improvements are obtained by applying the repair mortar through a dry shotcrete process. Within this process the water is mixed with the dry mortar only in the jet. After that the mixed mortar is immediately sprayed onto the surface. Despite this extremely short mixing and almost no slake time, the redispersible powder redisperses quickly and completely enough in order to improve the tensile adhesion strength and the flexural strength in almost the same magnitude compared to a conventional application by hand. 

Pneumatically applied concrete containing a maximum coarse aggregate size of 10 mm is usually termed shotcrete. The process is particularly suited to... 

Bond strength high bond strength can be attained between layers of shotcrete and the rock surface for both wet and dry mix processes when admixtures are used. Bond strength increases initially with dosages up to 2% and thereafter decreases with further addition. 

Adhesion Adhesion of the plastic dry-mix silica fume shotcrete, particularly in wet areas, is substantially improved. For example, adhesion in areas such as locks, dry docks, tunnels, and even leaking structures, can only be achieved through the use of high concentrations of shotcrete accelerators, to create a flash-setting condition, a process that is detrimental to the long-term durability of the hardened shotcrete. Silica fume has been found to promote excellent adhesion in such conditions with minimal or no accelerator addition. 

In order to minimize rebound, thorough blending of the ingredients should be done. The aggregate should be well coated with the cement in the dry-mix process. Since shotcrete admixtures promote shrinkage, it is essential that the sprayed surfaces be kept moist for at least 7 days. Ambient temperature conditions should be maintained at a level so that air in contact with the shotcrete surface is above freezing - at least for 7 days [106, 115]. 

Among several production methods the most commonly used were the so-called Hatschek, Magnani and Manville processes. The asbestos fibre cement elements were also produced by shotcreting. More details on the production methods may be found, among other sources, in Hannant (1978) and in Bentur and Mindess (1990). 

Another kind of admixture is used when an acceleration of normal processes of cement hydration is not needed, but a very rapid set in a few minutes is necessary to execute particular technological operations, such as repair or shotcrete of wet surfaces. Such admixtures are based on aluminium chloride, potassium carbonate, ferric salts and others. 

There are two types of gunning processes—dry gunning and shotcreting. Table 20.4 shows a comparison between the two. 

Shotcreting Using a modification of normal shotcreting techniques, it has been found possible to produce steel and polypropylene fibre shotcretes, for use particularly for lining of tunnels, and for stabilization of rock slopes. With this method, too, relatively high volumes of fibres can be added to the mix. Pulp type processes For asbestos cement replacements (cellulose or other fibres are used as a replacement for the asbestos), the fibres are dispersed in a cement slurry, which is then dewatered to produce thin sheet materials. These can be built up to the required thickness by layering. This process yields fibre contents of typically from 9% to over 20% by volume. 

PVA FRC can be produced in a number of ways, including premixing and shotcreting. Thin sheets, pipes and other shapes have also been produced using an extrusion process [33,135]. 

Fibre shotcrete has much higher in situ cement contents, approaching 600-700 kg/m in some dry process applications. 

The process of shotcreting results in a considerable loss of material due to rebound. In dry-mix shotcrete, the total loss of material is generally in the range of 30-40% [68], and even in wet-mix shotcrete the losses are generally of the order of... 

---