Shotcrete admixtures

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

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]. 

Miscellaneous admixtures Those other than what have already been described and are known to impart many benefits to concrete. Examples are latexes, corrosion inhibiting admixtures, alkali-aggregate expansion reducing admixtures, pigments, pumping aids, shotcreting admixtures, etc. 

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]. 

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. 

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]. 

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. 

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. 

Shotcrete or gunned mortar can be used on spalled areas by an experienced contractor who has the necessary equipment. The mortar or cement plaster is usnally formnlated to a 1 4.5 ratio of cement to sand, although richer 1 3 mixtures are often used. Ik as in some cases, the shotcrete does not have the necessary adhesive qualities, a bonding agent (a latex-cement slurry or an epoxy compound) is used as a prime coat. Shotcrete applied in very thin layers may have to be reinforced with a latex emulsion admixture. When the necessary equipment can be obtained, this application can be made by plant maintenance people. Care shonld be taken in the application of shotcrete on vertical walls and ceilings. Eqnipment that prodnces vibration may cause the shotcrete to spall before it cures. 

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