Successful cold weather placements require more than just modifying a mixture. The mixture temperature, condition of the substrate, and the placing and curing environment are also important considerations. Generally, one is discouraged from trying to apply shotcrete if substrate temperatures are too cold and the ambient temperature is at 40 °F (5 °C) and falling. There are, however, exceptions for extreme situations such as shotcreting in permafrost ground conditions, where it is not possible (or advisable) to heat up the substrate. In such conditions, special accelerated dry-mix shotcretes (in conjunction with the use of heated materials) have been successfully used. This type of work is highly specialized and not recommended for the novice.
Accelerators can be added to shotcrete mixtures to help overcome cold weather conditions. The accelerator can be either a liquid accelerator added with the mixing water at the nozzle or a dry-powdered accelerator in prebagged dry-mix shotcrete. Caution is advised when using accelerators containing calcium chloride, as the use of these materials may accelerate corrosion of reinforcing steel. More information can be found in ACI 306R, “Cold Weather Concreting,” available from the American Concrete Institute, www.concrete.org.
A limestone coarse aggregate will generally be harder and more angular than what you are used to seeing in shotcrete mixtures. It really shouldn’t be a problem to use. In dry-process gunning, it is considerably more abrasive so there is more wear and tear on equipment, such as hoses, bowls, and wear plates, but it generally guns fine. In wet-process gunning, a sharper aggregate may not flow as easily through the hoses as smoother sand and pea gravel aggregates would. These are issues that the shotcrete contractor will have to address. They should have no effect on the quality of the in-place shotcrete.
There are several different finishes that are specified for shotcrete. One is a natural gun finish, which is the natural finish as sprayed (often used in slope protection). Another is a cut-down finish, which is cut-to-grade with the edge of a trowel or cutting rod (this finish is often flashed and sealed with a light gun finish to seal and texture the surface). Often in concrete repair, a trowel finish is specified where the shotcrete is cut down with the edge of a trowel or cutting rod to grade after the initial set of the material, and the surface is lightly flashed and toweled. Several different finishes can be achieved with shotcrete, but it should not be pushed or floated with the flat part of the trowel, as is done with poured concrete. It is important to wait for the initial set of the material and to use the edge of the trowel to cut the high points or shave the surface to achieve the grade or effect desired. Several excellent articles describing shotcrete finishes and finishing techniques are available as free downloads from the ASA website: shotcrete.org. One article, Technical Tip: Technical Tips on Shotcrete Finishes, written by Denis Beaupre, describes the different finishes that can be applied to shotcrete. Another article of interest would be Finishes for Retaining Walls by Marcus H. von der Hofen. Go the Publications section of the ASA website, click on “Click here to search the archive of Shotcrete Publications” and type “Shotcrete Finish” in the search window.
ASTM C 1604, Standard Test Method for Obtaining and Testing Drilled Cores, covers cores that are obtained for determination of length, compressive strength, or split tensile strength. In addition to discovering the thickness of the applied shotcrete and its strength, a visual assessment can be made to evaluate the shotcrete quality, workmanship, shotcrete-to-substrate bond, and condition of the reinforcement. Shotcrete core strength is affected by core orientation relative to the direction of the shotcrete application. Therefore vertical, sub-horizontal, and overhead application of the same shotcrete may show variability. If obtaining cores for determination of compressive strength, cores containing wire mesh or reinforcing bars may not be used. Also, if a sample has been damaged in the process of removal, it cannot be used for strength determination. Cores must have a diameter of at least 3.0 in. unless otherwise permitted by the specifier. Cores with diameters less than 3.0 may demonstrate somewhat lower strengths and have greater variability. They may also be more sensitive to length-diameter ratio. Cores with length-diameter (L/D) ratios greater than 2.1 must be sawed to produce a capped or ground specimen with a L/D ratio between 1.9 and 2.1. Strength results from cores with L/D ratios less than 1.75 must be corrected as detailed in ASTM C42. A core having a length of less than 95% of its diameter before capping or a length less than its diameter after capping or grinding shall not be tested unless otherwise directed by the specifier. To avoid introducing the effects of moisture gradients of wetting and drying, extracted cores are to be stored in a sealed plastic bag at all times except during end preparation and a maximum of 2 hours prior to capping. Prior to capping, it is a good idea to determine the density of each core. Reported results should include the following: length of the core as drilled reported to the nearest ¼” (5 mm); length of the test specimen before capping or grinding reported to the nearest 0.1 in. (2 mm) and average diameter to the nearest 0.01 in. (0.2 mm); compressive strength reported to the nearest 10 psi (0.1 mpa) if the diameter is reported to the nearest 0.01 in. (0.2 mm) or nearest 50 psi (0.5 mpa) if the diameter is reported to the nearest 0.1 in. (2 mm); direction of the application of the load with respect to the horizontal plane of the shotcrete as placed; moisture conditioning history; date and time of test; nominal maximum size of the shotcrete aggregate; if determined, the estimated density; and any deviation from the stated test method and the reason for the deviation.
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:
- To prove the suitability of the fresh shotcrete mixture design for the intended use;
- To verify the proposed mixture will produce the required strength and any other specified hardened shotcrete properties;
- 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
- 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.
