My firm is a general contracting entity that frequently uses shotcrete subcontractors. When project specifications are not clear on testing, I have been relying on the advice of my shotcrete subcontractors on the frequency of taking tests for compliance with strength requirements. We always shoot a test panel prior to starting construction. How much testing should we be doing during construction?

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.

As a specifier, should I specify which process—dry or wet—should be used on my projects? What are the significant differences?

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.

I’ve been a pool builder all my life and I use your magazine as a technical source and I really enjoy it. I found a conflict: In Shotcrete Summer 2004, page 30, the answer to the second question suggests the use of 8% as batched air content with max sized coarse aggregate of 3/8 inch. The conflict I have is that a) won’t 8% as batched drop to 1-2% after wet gunning? and b) previous articles suggested the use of 15-22% air as batched to help get it through the hose and to achieve 8% in place. Can you clarify?

For over 30 years in Canada we have been designing wet mix shotcrete for exterior exposure (rock-slope stabilization, tunnel portals, canals and beams, infrastructure rehabilitation, etc.) to have air content at the point of discharge into the pump to be in the 7 to 10% range. Pumping and the impact on shooting reduces the air content in the in-place shotcrete by about half. i.e. we find the in-place air content in the shotcrete to consistently be in about the 3.5 to 5.0% range. (Only about 1 to 2% air content is lost in pumping; the rest is lost in impacting on the receiving surface).

The air content is measured either by digging out the in-place shotcrete (or dig it out of a shot test panel) and reconsolidating it in the base of the air pressure meter in the ASTM C231 test and conducting the test. Alternatively the shotcrete can be shot directly into the air pressure meter base. It provides virtually the same value as obtained with dug-out shotcrete (as described above), provided the nozzle is held perpendicular to the air pressure meter base, and at the appropriate distance for proper consolidation of the shotcrete.

Testing on numerous projects has demonstrated that shotcrete with 3.5 to 5% in-place air content has a good air voids system ( air content, spacing factor and specific surface), when analyzed in the ASTM C457 test. Such shotcrete has been demonstrated to have good freeze/thaw durability in the ASTM C666 test and deicing salt scaling resistance in the ASTM C672 test. More importantly, feedback from the field demonstrates that such air entrained shotcrete with many thousands of cycles of freezing and thawing in the field over several decades display good durability. There are many research and case-history examples in the published shotcrete literature to support these observations. (See references 1 and 2 below)

With respect to the use of very high air contents at the pump (15-22%), this has been more of a research initiative, used on only a few projects in Quebec, and is not common practice, nor in this writer’s opinion, necessary.

There is another benefit which accrues from the use of air entraining admixtures to get 7-10% air content in the shotcrete discharged at the pump. As any concrete user knows, as the air content increases, the slump goes up. For shotcrete mixes (which have high cementitious contents and low rock contents compared to concretes) this makes the mix easier to pump and shoot. Thus it is common to shoot air entrained wet mix shotcrete at 100 to125mm (4 to 5 inch) slump. On impacting on the receiving surface, as the air content is reduced by about half, the slump of the in-place shotcrete is also instantaneously reduced by about half. (This can be demonstrated by digging the shotcrete out of the in-place material, or a test panel and conducting a slump test on it). We refer to this phenomenon as the “slump killing “process and have used it to advantage on many shotcrete projects. With a good air entrained shotcrete mix design (particularly when silica fume is used) we commonly shoot vertical sections as much as 500mm (20in) thick at 100 to 125mm (4 to 5 inch) slump in a single pass with no problems of sagging or sloughing (fall-out), without having to resort to the use of accelerators.

Finally, there are a few situations where 7 to 10% air content in the shotcrete at discharge into the pump may not work. These are situations where excess air content reduction could occur during shotcrete conveyance, such as dropping shotcrete down a pipe from the surface in an underground mine and catching it in a kettle or remixer unit. In this case, air, if needed, is best added underground in the remixer. Also, pumping shotcrete long distances (particularly pumping shotcrete downhill) may result in excessive loss of air content in the line, which could cause a slump reduction in the line and possible pumping problems. Other than for situations such as these, we always use 7-10% air content in the shotcrete at the point of discharge into the pump (even if it is not needed for frost resistance reasons) because of its enhanced pumping and “slump killer effects”.

Reference 1: Morgan, D.R., “Freeze-Thaw Durability of Shotcrete”, Concrete International, Vol. 11, No. 8, August, 1989, pp 86-93

Reference 2: Morgan, D.R., Kirkness, A.J., McAskill, N. and Duke, N., “Freeze-Thaw Durability of Wet-Mix and Dry-Mix Shotcretes with Silica Fume and Steel Fibers”, ASTM Cement, Concrete Aggregates, Vol. 10, No. 2, Winter 1988, pp 96-102.

We are currently in the process of doing a seismic upgrade to one of our parking structures using shotcrete. During this process, the murals that are painted on the interior walls are being removed and will be repainted at a later date. How long do I wait before it is cured enough to begin painting?

The easy answer is that shotcrete material is the same as concrete material and that the same rules or guidelines would apply to shotcrete as to concrete. We usually tell our customers to present this question to the painters. The curing process and chemical reactions are greatest in the first 28 days. Generally a paint or coating is not applied until after the curing of the shotcrete is complete, or mostly so, and the moisture content of the shotcrete is below a point specified by the coating manufacturer.

Our development has 8 recirculating water ponds of various sizes. All are vinyl liner under concrete construction. Some ponds have developed leaks due to cracking of the concrete. Will shotcrete provide an adequate seal to stop the leaks for an appreciable time?

When trying to find a contractor in your area, please visit the Corporate Member page of this website. When constructing water ponds, the liner is always under the concrete just in case the concrete cracks not on top. Master Builders makes a product called Master Seal 345 which is designed to waterproof the concrete before the shotcrete is placed. Using a macro synthetic fiber for strength, flexural and to control shrinkage cracking will help. It comes down to proper prep work prior to placement and curing of the concrete (7 days of water) to control cracks. Bentonite shotcrete could be a possibility or perhaps plastic shotcrete (cement and bentonite shotcrete).

I’m looking for information as to the thickness design of shotcrete for ditch slope lining purposes. Can you direct me?

Typically, the thickness is a minimum of 3 inches and slope lining in the 6 to 8 inch range is often installed. The reinforcing is also variable with the lightest sections with no reinforcing or a low dosage of polyfibers or light welded wire fabric and the heavier sections with rebar. Basically, a lot of different designs can be used. We are not aware of any widely used standards.

We are shotcreting our first wall and the contractor tells us that in shotcrete, the lapping of the bars is not done by putting the bars alongside each other as in conventional pouring of concrete but rather a gap is left between the bars in order to avoid voids behind bars bundles. A two-inch gap is being used on our job. Is there a publication that deals with reinforcing steel placement in shotcrete in general and one that deals with bar laps in particular?

The ACI 506R-90 Guide to Shotcrete, Section 5.4.2 is the publication you are looking for. Amongst other things it states: “If the design allows, lapping of the reinforcing splices should be avoided. Lapped bars should be spaced apart at least three times the diameter of the largest bar at the splice”. If laps are not permitted by the design, then it is best to lap the bars one on top of the other (relative to the shooting orientation), rather than side-by-side, to facilitates proper encapsulation with shotcrete.

I have a special request for a shotcrete mix design. My company has been using shotcrete for about three years, here in Alaska. I have recently had a request to shotcrete a 60’x50′ duck pond to make it waterproof. The problems I am running into are that moose keep walking into the pond, and the pond is on the side of a hill with built up edges around the outside. The mix design I am looking for needs to have an epoxy or some kind of adhesive to help stop the water from running out the cracks. Last, are there any fabric or plastic materials that I could lay down and spray the wet shotcrete on to put on the sides of the pond?

This inquiry involves a lot more than just mix design. First, additives to the mix by themselves will not keep the shotcrete from cracking. To minimize leakage for the proposed application, he will have to use either a waterproofing membrane on top of the shotcrete, or plaster like would be used on a swimming pool. Putting a membrane behind the shotcrete would only serve to keep ground water from entering the pond through the back side. The other aspect to be addressed is the fact that all concrete shrinks, and that is what causes the cracks. So anything that can be done to minimize shrinkage should help. To name just a few items: avoid shooting on a windy and or low humidity day; use aggregates in the mix that have a good record regarding shrinkage; avoid excessive cement content in the mix; use reinforcing steel (mesh or rebar); synthetic fibers help reduce early plastic shrinkage; proper curing is absolutely essential!

I wish to request expert advice from ASA in regard to the Gunite Contractor’s Association method that we are using to make test cylinders (i.e. 6″ diameter and 12″ high shot into a form of 3/4″ square mesh hardware cloth). Since we are currently in the process of guniting a silo and have today received 3,250 psi rather than the mix designed 4,000 psi 7-day strengths, we would appreciate your prompt response.

The method of using 6″ diameter by 12″ long wire mesh cylinders has not been used regularly in several years. The most accepted means of taking samples is as specified in ACI 506 documents which generally require a sample panel of approximately 18″X18″ by 4″ thick from which cores are taken. The cores should be taken at a minimum distance from the edge of the thickness of the panel to yield fair test results. ACI 506.4R-94 references under testing of shotcrete, ASTM C 1140-03 (Standard Practice for Preparing and Testing Specimens from Shotcrete Test Panels. Also ASTM C42/C 42M-03 (Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete. Standard 18″X18″X4″ panels are typically made. ASTM C 1140-03 states a 24″X24″X4″, cores are to be taken 1 core diameter plus one inch from any side of the test panel.