Portal: General

The short version of this discussion is that performance specifications provide a list of desired results. The contractor takes this list and selects materials and methods to produce the desired results. The contractor assumes responsibility for results. Prescriptive specifications are very specific as to what materials, proportions, and methods of installation are to be used. The specifier assumes responsibility for the results. The contractor must be able to demonstrate compliance with the specification. Which method is better? The answer to this question is highly dependent on the nature of the project. However, in general, performance specifications produce a higher probability of achieving the desired results as the contractor is better able to use his expertise as it applies to project conditions.

There are four basic reasons to require preconstruction qualification testing:

1. To prove the suitability of the fresh shotcrete mixture design for the intended use;
2. To verify the proposed mixture will produce the required strength and any other specified hardened shotcrete properties;
3. To prove the ability of the nozzleman (and blowpipe oper­ator, if required) to place dense, homogeneous shotcrete completely encasing the reinforcing steel under field conditions; and
4. To prove the desired surface finish can be achieved.

This testing must be discussed in detail with the shotcrete contractor in advance with a clear understanding of the expected outcomes and the process for any required adjustments. Requiring ACI Nozzleman certification is an important requirement in screening for qualified nozzle operators. However, it is not a guarantee that the nozzleman has applied shotcrete under the same conditions to be encountered on your project. Therefore, a preconstruction plan is an important part of critical projects. Other prequalification testing may be necessary depending on the nature of the work.

There are a number of ways to treat these stains. Successful treatment will depend on the specific material in the stain and the depth of the stain. The first step would be to try to draw out the material from the surface by applying a poultice of finely ground kitty litter, cement powder, or talc and allow the surface to dry. Repeat this application if necessary.

Next, try a scrubbing a nominally dry detergent powder into the surface. Allow the powder to dry and rinse off the surface. Follow this treatment with a liquid detergent scrubbed with a bristle brush into the surface. Allow the liquid to remain in the surface for 1 to 2 days, then rinse thoroughly. Should the staining persist, you may want to try a proprietary stain remover specifically intended for use on concrete.

Muratic acid is also an option. However, muratic acid can have deleterious affects on the concrete if not thoroughly removed. Because of its potential to attack concrete aggregates and mortar, along with the hazards inherent with applying and removing acid, muratic acid should only be used with the guidance of an experienced consultant. Following a thorough power washing, the surface should be mechanically roughened to ensure proper bond with the shotcrete.

ASTM C 33 contains a variety of aggregate gradations. Gradations recommended for shotcrete applications can be found in ASTM C 1436, Standard Specification for Materials for Shotcrete, or ACI 506, Guide to Shotcrete. Note that ACI 506 includes the caveat that “aggregates failing to comply with gradations shown in Table 2.1 may be used if preconstruction testing proves that they give satisfactory results or if acceptable service records are available.”

Shotcrete is a method of concrete placement, not a product. Therefore, concrete placed by the shotcrete method will exhibit the same characteristics as concrete placed by other methods. Mixture designs and proportions for shotcrete are modified for high-velocity placement. The high velocity provides some performance improvements over conventional cast-in-place methods when properly placed.

Shotcrete containing silica fume will tend to be more adhesive (sticking to substrate surfaces) and cohesive (adhesion to itself). This will result in quicker build-up (greater thicknesses per pass) and possibly reduced need for acceler­ators. Silica fume additions also result in dramatic reductions in rebound, particularly with the dry-mix process.

No. Shotcrete is a method of placing concrete. Therefore, any applicable certifications would apply to concrete regardless of the method of placement.

ACI 506.2, “Specification for Shotcrete,” recommends that a test panel be produced for every 50 yd3 (38 m3) of shotcrete placed or one per day, whichever is less. A minimum of three cores are to be cut from the test panel for compressive strength testing in accordance with ASTM C 42, “Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete.” Testing must be performed in accordance with ASTM C 1140, “Standard Practice for Preparing and Testing Specimens from Shotcrete Panels.” The average of the strength results from the cores must be at least 85% of the specified strength with no individual core less that 75% of the specified strength.

The application of shotcrete can be done successfully with either method. The dry-mix shotcrete process tends to be more favorable for lower volume placements. It is also a more flexible method, allowing for more frequent relocations of equipment. Equipment is more easily cleaned at the end of the placement. The nozzleman must exercise great care in adding the necessary amount of water while shooting.

The wet-mix shotcrete method is more favorable for larger volume placements. Rebound is substantially less than in the dry-mix shotcrete process. The nozzleman does not have to be concerned with controlling the water addition. This method is less efficient when there is a requirement for frequently starting and stopping placements. The wet shotcrete mixture has a limited “pot-life.”

Remember, shotcrete is not a special product. It is a method of placing concrete. All the recommended practices for concrete placed by any other method, such as curing and protection, also apply to shotcrete.

See ACI 506R, Sec. 1.7 (ACI document). Typical shrinkage varies in the range of 0.06 to 0.10 percent after 28 days drying. It is typically slightly higher than similar strength concrete, mostly due to less and/or smaller coarse aggregate in the shotcrete mix.