Working With Shotcrete, continued

Shooting the Shotcrete

In our area, all shotcrete subs are unionized, but they don't have a problem working on nonunion residential jobs. Our regular sub is a company called Dees-Hennessey, which also specializes in earth-retention shoring. The crew shows up on the site with a 16- to 20-foot flatbed truck that has a high-capacity concrete pump in tow. The truck bed contains a big 350-cfm compressor and a couple of huge reels of 2-inch pump hose (Figure 4).

Figure 4.In addition to the ready-mix truck visible at rear (top), a shotcrete job requires a high-powered compressor mounted on a flatbed truck and a tow-behind concrete pump. The author's shotcrete sub brings a second concrete pump to the site as a backup. The batched concrete is discharged directly into the hopper of the pump and is then forced through a 2-inch hose to the nozzle, where compressed air is added (bottom).

The equipment is positioned so that ready-mix trucks can back up and discharge concrete directly into the hopper of the concrete pump. The concrete itself is mixed at the batch plant to the engineer's specifications. These typically call for a 7- or 8-sack mix — substantially richer than the 5- or 6-sack mix used in poured applications — and pea-gravel aggregate. We try to park the equipment as close to the work as possible, but the concrete pump has the power to move the material a long way if necessary. Our sub has successfully pumped concrete up to 600 feet horizontally and 300 feet vertically, although not both at the same time.

The nozzleman. The quality of a shotcrete job is only as good as the nozzleman, as the person at the business end of the hose is called (Figure 5). The nozzle has to be oscillated continuously to prevent a defect called "shadowing," in which the concrete builds up on the front of the strands of rebar without filling the area behind. A good nozzleman can keep an eye on the slump of the material and build its thickness in a series of 3- to 4-foot lifts without sagging or other defects (an accelerator is often added at the batch plant to allow the material to stiffen soon after it's blown on).

Figure 5.The shotcrete nozzleman guns a steady stream of concrete against a below-grade foundation wall while the blowpipe operator prepares to blow away unconsolidated debris, known as rebound. The area to the right has already been screeded to the level of the guide wires.

Doing all this well takes a lot of practice. To become certified as a nozzleman by the American Concrete Institute, a worker must accumulate at least 500 hours of work experience or training and pass both a written exam and a field performance exam.

Dealing with rebound. The nozzleman is assisted by a blowpipe operator, who is responsible for blowing away the rebound, or material that bounces off the fresh surface rather than sticking. This is essential because rebound material is mostly aggregate with little cement paste. If it's allowed to build up and is covered over, the result will be a permanent weak spot. The blowpipe is simply a length of 3/4-inch pipe connected to the compressor with a jackhammer-type hose and controlled with a gate valve. The operator — who is usually also a nozzleman or a nozzleman trainee — moves in step with the nozzleman to maintain a clean leading edge.

Something like 10% or 15% of the pumped-in material typically ends up as rebound. On a big pour, that's enough to keep several laborers busy with shovels, sometimes with the assistance of a material conveyor (Figure 6). The laborers also help the nozzleman drag the heavy concrete hose from place to place.

Figure 6.Up to 15% of the concrete used on a given job ends up as rebound, and several laborers may be kept busy removing this waste material. A material conveyor is an efficient way of moving rebound from a basement to ground level.

Test panels and inspection. The nozzleman's skill is particularly tested in areas where the reinforcing steel is tightly spaced (Figure 7), which can make it difficult to fill around the rebar without leaving voids and other defects. When this "congested steel" condition is unavoidable, the engineer will usually require the shotcrete sub to make a preconstruction test panel. This 4x4-foot mockup is shot by the same nozzleman who will later do the actual work, using the specified concrete mix. This test panel is then cured for five to ten days, cored and inspected for shadowing around the steel, and crushed for a strength reading.

Figure 7.Heavily reinforced structural elements such as this pilaster are susceptible to shadowing and other defects if the shotcrete is applied improperly. To ensure quality, a mocked-up test panel is often shot, allowed to cure, and tested before construction begins.

In our area, we're also required to have an inspector on the site whenever we're applying shotcrete with engineering specs of 3,000 psi or more. (Practically speaking, this includes every shotcrete job we do.) This "special inspector," who works for an independent engineering firm, watches over the entire process to ensure that nothing is done that could leave hidden defects in the work.

Screeding and Finishing

After each 3- to 4-foot lift, the nozzleman moves his operation to allow the finishers to screed the slightly set-up material to the level of the guide wires (Figure 8).

Figure 8.As each successive 3- to 4-foot lift of shotcrete is built to the required depth, it's screeded to the guide wires. The lift below has already been screeded and troweled; when the current lift has been finished the same way, the nozzleman will spray another lift over the exposed rebar visible at the top of the photo.

Once screeding is complete, the guide wires are clipped and removed. Screeding leaves the surface flat but very rough, so it's troweled to provide a more acceptable finish. For utility installations, we usually want a wood or rubber-float finish (Figure 9). Quite often we apply a steel trowel finish to our walls if they will be in an area that's exposed to view.

Figure 9.Shotcrete can be finished and ornamented in various ways, but a simple rubber-float finish is a common choice for utility applications.

Other more ornamental finishes are also possible. People have attempted some light stamping, although this has to be done carefully because it can cause vibration that may lead to slumping. The concrete can also be finished with stucco. One of my favorite non-stucco finishes was on a pool structure in Atherton, where we shot from the inside of the structure, stripped the forms after curing, and bush-hammered the formed exterior face, resulting in a woven cloth-like effect. The inside of the vault was rubber floated, which was fine for the pool equipment room.

Curing. Once the shotcrete has been screeded and troweled to the required finish, it's cured about like ordinary poured concrete. To limit cracking and prevent shrinkage and other defects, we spray the finished shotcrete with a chemical curing compound that dries to form a plastic moisture barrier. The manufacturer of the product we use specifies that it shouldn't be applied until excess bleed water has disappeared, but because there's little bleed from the low-slump shotcrete mix, we don't have to wait long to apply it to the exposed face. We coat the remaining face after we strip the forms, usually about five days later.

The only time we don't use the curing compound is when the concrete will be covered with tile or faced with stone, because the sealant film can compromise adhesion. In that case, we'll cure the concrete the old-fashioned way, by covering it with blankets and keeping them misted with water.