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PoolWhat is the minimum spacing between reinforcing bar recommended in a shotcrete swimming pool deep-end wall. The engineer is calling out for 12 in. (300 mm) walls with a double mat of No. 5 bars at 4 in. (100 mm) on center each way. In my opinion, this does not give enough space to properly encase the reinforcing bar with shotcrete without creating voids and trapping uncontrollable rebound.

The engineer designs the wall based on the loads anticipated on it throughout its service life. The minimum spacing for noncontact lap splices as indicated in ACI 506.2, “Specification for Shotcrete,” is: “Clearance of at least three times the diameter of the largest reinforcing bar;

RepairStructuralI have a project with a segmental retaining wall. The project is located in Maryland. The CMUs of the retaining wall have significantly deteriorated in several areas. The geo-grid fabric appears to be in good condition in the areas where it could be observed. I suspect the deterioration is caused by saltwater runoff during snow events combined with the freeze/thaw cycles. There are some localized areas where the masonry units have completely disintegrated. The largest area being approximately 4 ft2 (0.37 m2). Is it feasible to remove the deteriorated masonry material, down to a sound surface, and shotcrete the face to restore (or exceed) the structural integrity of masonry units? If so, will this restore the structural integrity of the segmental retaining wall?

Shotcrete is routinely used for the repair of deteriorated concrete masonry and brick. Shotcrete should easily have a minimum compressive strength of 4000 psi (28 MPa) when properly shot and cured. According to the National Concrete Masonry Association, current CMU units have a minimum 2000

ArchitecturalI am an architect working on a project in which a masonry building was “coated” with a shotcrete or gunite material. It is reported that this was troweled on approximately 20 years ago. We have made test holes and it is adhered very well, except where moisture penetration through the parapet has compromised adhesion. This is installed with five layers approximately 1 in. (25 mm) thick. We are considering leaving the material in place, patching as required, and removing the existing nonpermeable paints (which are peeling). Our thought is to coat the surface with a variegated colored stain, allowing for permeability of vapor. Can the material be patched as necessary? What is the recommended finish? Do we need to be concerned that the material could permit water penetration? Are there certified contractors in the Cleveland, OH, area?

From what you have described, the original work may have been done by the shotcrete process or hand-applied. Typically in the shotcrete process, over a large area at the thickness of 5 in. (127 mm), you would expect to see reinforcing mesh or reinforcing bars.

Infrastructure

August 29, 2024

2024

UndergroundI am currently involved in the design of a large retaining wall for a job in Boston. One option under consid­eration is the use of soil nails with shotcrete lagging. The design anticipates a 100-year service life. What can I tell my client to realistically expect from the shotcrete option? Is shotcrete durable in the freezing-and-thawing conditions in this area? What is the best way to improve the longevity of the product?

The simplest way to clarify things is to advise your client that shotcrete is not a product but a process. Shotcreting is a process of installing concrete at a high velocity. Because the concrete is installed at a high velocity, it will have a higher

GeneralCan shotcrete be painted like other concrete? Can an elastomeric paint, 100% acrylic latex house paint or solvent acrylic be used? I have a customer who wants to paint a tank which uses shotcrete. With normal concrete the surface must be 30 days or older, pH is approximately 7-8 and moisture content is low, remove efflorescence or laitance, etc., then it is ready to paint or coat. Do the same restrictions for shotcrete?

Shotcrete is pneumatically applied concrete. All surface prep work for concrete will be the same for shotcrete applications. Before a recommendation can be made, is this tank going to be painted on the outside or the inside? Second if this tank is to be painted

RepairWe’re looking at adding approximately 4 in. (100 mm) of shotcrete to an existing 8 in. (200 mm) wall to meet new load requirements. What’s the minimum cover between the rein­forcement and existing wall for proper encapsulation of the reinforcement?

A minimum clearance for the reinforcment off the existing surface should be 0.75 in. (19 mm) or one bar diameter, whichever is greater, to allow a good flow of material around the reinforcing steel.