Shotcreting

Strengthening concrete masonry walls by reinforcing and grouting, or adding shotcrete layer on the outside walls. 

Another option would be to add steel or fiber reinforced shotcrctc layer to the exterior surface of a CMU wall. The shotcrete adds significant ductility to the wall, increases the inertia mass and bonds the existing blocks This prevents the blocks from becoming dislodged when the wall undergoes defied ions. This type of exterior construction can be accomplished with minimal interruptions to the building functions. Foundation modifications may be needed to accommodate additional dead load from shotcrete application. 

Addition of a new section on miscellaneous admixtures including shotcrete admixtures, corrosion inhibitors, and admixtures for recycling wash water and plastic concrete. 

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

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

The aluminate-based admixtures undoubtedly provide the best wet-mix shotcrete, making it possible to build thick linings even in overhead work. The high alkali content and the consequent health hazard are the main constraints to their more widespread use. Furthermore, there is concern that the admixture may promote the alkali-aggregate reaction in concretes containing reactive aggregates, as well as sulfate attack [117]. 

Table 6.16 Shotcrete strength obtained at different ages (Prudencio)...

Fig. 6.21 Comparison between the performance of shotcrete with alkali-free and sodium-aluminate-based accelerators (Prudencio [108]).

Tensile and flexural strengths values are usually lower than what would be expected from compressive strength values. This probably reflects the effect of laminations and other defects in the shotcrete on tensile strength. Flexural strength values follow a similar relationship observed for regular concrete. 

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. 

Shrinkage higher values are obtained for mixes containing the admixture in comparison with the corresponding mixes with no admixture. Shrinkage increases with higher admixture and water contents of the mix, for both wet and dry shotcrete. 

The addition of silica fume to dry-mix shotcrete in proportions of 10-15% by mass of Portland cement substantially improves the adhesive and cohesive properties of the freshly applied shotcrete. Silica fume creates a very dense and sticky mix with an almost complete lack of bleeding. These characteristics of the material are attributed to the extreme densification achieved by packing of the ultrafine silica fume particles between the cement particles in the plastic (fresh) shotcrete. Practical ramifications of these characteristics include the observations which follow [107, 117-119]. 

Washout resistance Silica fiime shotcrete is very resistant to washout by water. It is thus well suited to operations where freshly applied shotcrete will be exposed to running water or tidal flushing, e.g. rehabilitation of marine structures in the intertidal zone, construction of dikes, and creek channelization. 

Application thickness It imparts the ability to apply the material overhead in a single pass at thicknesses that previously had been unattainable with conventional shotcrete. Vertical surfaces can be built out to any desired thickness in a single pass, without the use of accelerators and without sloughing or tearing of the plastic shotcrete. 

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. 

Rebound Because of marked improvements of adhesion and cohesion, the amount of rebound is significantly reduced. In controlled tests, rebound of a conventional dry-mix shotcrete, applied to a thickness of 50 mm (2 in) to the smooth overhead surface of concrete deck slabs, was measured at 40% by mass (the rebound was caught in tarpaulins and weighed). Rebound under the same circumstances was reduced to 25% by mass in dry-mix silica fume shotcrete. 

Compressive strength Tests conducted on cores extracted from test panels shot during routine quality control show that silica fume shotcrete produce consistently higher compressive strengths and lower permeability in the hardened shotcrete than shotcrete where accelerators were used. 

At present the only reliable method of dispensing powder shotcrete admixtures directly into the wet mix at a uniform rate at the nozzle is by the use of a log feeder. In this method the powder is compressed to a log which, when placed in the feeder, is gradually ground off and carried away in the air stream to the nozzle. The accelerator mixes with the other shotcrete ingredients at the nozzle and in transit to the sprayed surface. The amount of admixture added is controlled by the rate of advance of the log in the feeder. The method of dispensing liquid admixtures in a wet mix is essentially the same as that used in the dry-mix operation, except that admixture is injected into the air stream instead of the water line. 

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]. 

Caustic materials contained in the admixtures can cause reactions on exposed skin, ranging from mild irritation to severe bums. Most of the ingredients are classed as primary irritants. With liquid admixtures, bums can occur immediately. Because of the serious hazard to exposed skin and eyes, personal equipment is required for all those in the vicinity of the shotcrete machine and nozzle. These include long-sleeved shirt or coveralls. In addition, prior to shooting, protective cream or lotion should be applied to skin areas likely to be exposed to the caustic chemicals. A suggested treatment is to wash exposed skin with water and rinse with vinegar followed by treatment with a protective cream. All exposures to the eye require the immediate attention of a physician immediate action would be to flush the eye with copious amounts of water [120, 121]. 

In general the required performance criteria and product uniformity parameters stipulated in shotcrete specifications are as follows ... 

Mahar, J.W., Parker, H.W. and Weller, W.W. (1975). Shotcrete Practice in Underground Construction, Report No. FRA-OR D 75-90, Department of Civil Engineering, University of Illinois,... 

Silica fume when used in conjunction with superplasticizers plays an important role in improving the cohesion of the shotcrete mix, allowing for the build-up of the sprayed concrete on vertical surfaces, within a short period, without sloughing off. In addition the combination of these admixtures has provided ease of pumping, better compaction and greatly reduced the rebound, thus minimizing waste and cleaning time. Typical mixture proportions and properties for wet-mix superplasticized shotcrete are presented in references 69 and 72. 

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