Portal: General

According to ACI 506R-05, the core grading method in ACI CP-60(02) is only to be used for nozzleman evaluation. (This is typically done in ACI Shotcrete Nozzleman Certification sessions and/or in preconstruction testing.) The core grading method should not be used to evaluate structures.

Regarding sample size for compressive strength, the core length-to-diameter ratio should be in the range of 1:1 to 2:1, with length-to-diameter core strength correction factors applied as per the requirements in ASTM C 42, Clause 7.9.1. Shotcrete test panels are typically between 3.5 to 5 in. (89 to 127 mm) deep. Thus, either 3 or 4 in. (76 to 102 mm) diameter cores should be drilled for compressive strength testing, depending on test panel thickness. We would also suggest referring to ASTM C 1604/C 1604M for securing and testing cores of shotcrete. This new test method allows smaller core diameters for shotcrete in an effort to provide for increased length-to-diameter ratios. Care should be taken when interpreting the compressive strengths using smaller-diameter cores because of the possible presence of voids, which may result in compressive strengths that are not representative of the actual in-place shotcrete.

A bonding agent is not required or recommended. A properly prepared substrate in a saturated surface-dry condition (SSD) is the optimum condition for application of shotcrete. Bonding agents may act as a bond breaker in some circumstances.

ASTM C 94, “Standard Specification for Ready Mixed Concrete” states that concrete must be unloaded within 90 minutes of contact between water, cement, and aggregates, or before the mixer drum has revolved 300 revolutions—whichever comes first. This limit, however, may be waived by the purchaser if the concrete has sufficient workability that it can be placed without the addition of water. In hot weather, the 90-minute limit may be reduced by the purchaser.

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.