Shotcrete in Concrete Repair
Shotcrete is defined as “mortar or concrete pneumatically projected at high speed onto a surface” (American Concrete Institute, 1990). There are two basic types of shotcrete—dry mix and wet mix. In dry mix shotcrete, the dry cement, sand, and coarse aggregate, if used, are premixed with only sufficient water to reduce dusting.
This mixture is then forced through the delivery line to the nozzle by compressed air (figure ). At the nozzle, sufficient water is added to the moving stream to meet the requirements of cement hydration. Figure shows the nozzle and water ring of a dry mix shotcrete nozzle. For wet mix shotcrete, the cement, sand, and coarse aggregate are first conventionally mixed with water (figure ), and the resulting concrete is then pumped to the nozzle where compressed air propels the wet mixture onto the desired surface (figure ).
The two types of shotcrete produce mixes with different water contents and different application characteristics as a result of the distinctly different mixing processes. Dry mix shotcrete suffers high dust generation and rebound losses varying from about 15 percent to up to 50 percent. Wet mix shotcrete must contain enough water to permit pumping through the delivery line.
Wet mix shotcrete, as a result, may experience significantly more cracking problems due to the excess water and drying shrinkage. Advances in the development of the high range water reducing admixtures, pumping aids, and concrete pumping equipment since about 1960 have greatly reduced these problems, and wet mix shotcrete is now being used more frequently in repair construction.
Shotcrete is a very versatile construction material that can be readily placed and successfully used for a variety of concrete repair applications. The necessity of form work can be eliminated in many repair applications.
It has been used to repair canal and spillway linings and walls, the faces of dams, tunnel linings, highway bridges and tunnels, deteriorating natural rock walls and earthen slopes, and to thicken and strengthen existing concrete structures. Provided the proper materials, equipment, and procedures are employed, such shotcrete repairs can be accomplished quickly and economically.
This apparent ease of application should not cause one to believe that shotcrete repair is a simple procedure or one that can be haphazardly or improperly applied with impunity. The following two paragraphs contain a very descriptive warning of such practices:
“Regardless of the considerable ad-vantages of the shotcrete process and its ability to provide finished work of the highest quality, a large amount of poor and sometimes unacceptable work has unfortunately occurred in the past, with the result that many design and construction professionals are hesitant to employ the process.As with all construction methods, failure to employ proper procedures will result in inferior work. In the case of shotcrete the deficiencies can be severe, requiring complete removal and replacement.
“Deficiencies in shotcrete applications usually fall into one of four categories: failure to bond to the receiving substrate, delamination at construction joints or faces of the application layers, incomplete filing of the material behind the reinforcing, and embedment of rebound or other unsatisfactory material.” (Warner, 1995). Each of the above-listed deficiencies has occurred on Reclamation repair projects.
Perhaps more important with shotcrete than with any other standard concrete repair method, if highly qualified, well trained, and competent workmen cannot be employed, it is advisable to consider using some other repair procedure. The quality closely depends upon the skill and experience of one person, the nozzleman.
Reclamation specifications require employment of only formally certified nozzlemen for shotcrete repairs. The on-the-job training necessary to develop the experience and skill needed to achieve such certification for Reclamation work should occur prior to the
nozzleman’s arrival at the job.
Preparation
Concrete to be repaired with shotcrete should be prepared in a manner identical to the preparation required for replacement concrete, section 29.(a).
Experience indicates, however, that surface preparation for shotcrete repair is more critical than for replacement concrete. It is essential with shotcrete repairs that the shotcrete have a clean, sound concrete base for bond.
Wet mix shot crete equipment. The premixed shotcrete is delivered to the shot crete pump by a transit truck.
however, type I-II, low alkali cement is adequate. Water, sand, and coarse aggregate used in shotcrete should also meet the requirements for replacement concrete, except that the maximum size coarse aggregate should not exceed 3/8 of an inch. Additives for shotcrete should meet the requirements of ASTM designation C 494, Chemical Admixtures for Concrete.
It is normally not possible to accomplish air entrainment with dry mix shotcrete. Lack of air entrainment may lead to dry mix shotcrete having lower than desired freeze-thaw resistance. Wet mix shotcrete should be proportioned to contain 6 to 8 percent entrained air.
It is sometimes desirable to use accelerating admixtures in shotcrete where rapid setting or rapid strength development is required. Calcium chloride accelerators have long been used, but there are now sufficient non-chloride
containing accelerators in the market- place to make the use of calcium chloride inadvisable. The use of calcium chloride accelerators is particularly unadvisable in shotcrete applications containing reinforcing steel or steel fibers.
Fiber reinforcement has been used in shotcrete since the early 1970s, and the M-47 specifications in appendix A contain specifications for steel fiber reinforcement. The American Concrete Institute has published a state of the art report on fiber reinforced shotcrete (American Concrete Institute, 1984), and this document should be consulted if the use of fiber reinforced shotcrete is being considered. It should be recognized that application of fiber reinforced shotcrete is more difficult and requires more experienced nozzlemen.
Application of Shotcrete
The detailed discussion of shot-crete application techniques and technology is beyond the scope of this guide. The American Concrete Institute has published a recommended practice and a specification for materials, proportioning, and application of shot-crete (American Concrete Institute, 1966; 1977). These documents should be studied before attempting shot crete repairs.
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Curing
Proper curing of shotcrete is essential if high strength properties, durability, and long service life are to be obtained. The M- 47 specifications permit water curing or curing of shot crete by application of curing compounds.
It is important to begin curing by applying approved curing compounds or water spray before there has been evaporative water loss from the shot crete, particularly during periods of high temperatures, low humidity, or high wind conditions. Improvements in bond strength will be obtained by continuing curing for periods of up to a month.