I am looking for any information regarding the use of construction joints for permanent shotcrete wall facing. I have found information on placing shotcrete over existing construction joints but none regarding the use of construction joints for the shotcrete wall facing itself.

In many experiences, the spacing and design of the joints are the same as you would expect for a cast in place wall. Walls have been constructed with no joint, with contraction and expansion joints, with a joint that is caulked, with joints containing waterstop, and just about anything else you might see in a cast in place wall. In short, it is suggested to look to the direction given for cast in place concrete. The construction joint should be designed similar to the needs of any cast in place wall.

I am a civil engineer working on the rehabilitation of a low fixed crest concrete dam of 6 foot height. After stitching of cracks and patch repairs, we want to specify a 2.5″ shotcrete facing on the down-stream side to protect from high velocity-induced erosion. The up-stream side will be sealed with a betonite-clay liner to save costs. To get a very dense concrete, we are thinking of 8000 psi airentrained, fiber-reinforced mixture. Should we use a WWF reinforcement? Should this be a wet or dry application?

Whether to use the wet or dry process depends primarily on your production schedule. With wet you will get much higher production; it will be easier to entrain air; and rebound and dust will be less. It is suggested that you use a wet-mix, steel fiber reinforced, air entrained, silica fume shotcrete, mechanically connected with L-bar anchors and small diameter bars (not mesh) spanning between the anchors. For precedence with this type of retrofit of the face of a dam, see the publication on “Seismic Retrofit of Littlerock Dam, by Forrest, Morgan in ACI, Concrete International, November, 1995, pp. 30-36, or an abbreviated version of the paper in the ASA Shotcrete Magazine, May,1999, pp. 46-55. If you must specify the shotcrete you can use ASTM C 1436, “Specification for Materials for Shotcrete”, which will cover all the materials mentioned, including fibers. For a general shotcrete specification you should review ACI 506.2. You should not use welded wire fabric and fibers together. Fibers will hang up on the mesh causing voids behind the mesh. I recommend a steel fiber meeting ASTM C 1436, Type I, Deformed at approximately 85 lbs/c.y. (50 kgs/c.m.). The steel fibers will tend to lie in the plain of the shotcrete surface; however, you should be aware that some fibers may protrude from the surface, and over time will corrode. Thirty years of experience shows corrosion is only to carbonation depth (2-3 mm), and corrosion of one fiber does not effect other fibers nor disrupt the shotcrete. Staining of the shotcrete surface is a possibility. Some spray a thin ( ½ in.) layer of non-fibrous shotcrete as a final finish to cover fibers.

I am a structural engineer working on a project in Southern California. We are creating specifications for the use of shotcrete for basement walls. However, I cannot find any information on compressive strength requirements for shotcrete in the building code. We are basing our design on compressive strengths ranging from 3500 to 4500 psi. Are there minimum and maximum allowable compressive strengths for shotcrete?

To the best of our knowledge, there is no maximum compressive strength limitation. The minimum compressive strength would be dictated by your structural calculations as it would be with any structural concrete design. The most common compressive strength specifically encountered by ASA members in your area is a minimum of 4000 psi at 28 days.

We are building a new home. Foundation contractors who place traditional basement walls tell us they would never go into a house built with walls constructed using shotcrete. When used for walls, can shotcrete be of equivalent strength as placed concrete?

Shotcrete is a method of building a structure using a concrete mixture. A shotcrete mixture likely would exceed the compressive strength of most mixtures used for placed walls because the application of shotcrete requires a much lower water-cementitious material ratio than commonly found in residential wall mixtures. A shotcrete mixture will have a water-cementitious material ratio of approximately 0.50, yielding a compressive strength of about 4000 psi at 28 days. Poured wall mixtures have ratios of approximately 0.70 and compressive strengths of 2500 to 3000 psi. The lower water-cementitious material ratios of shotcrete mixtures produce other benefits such as reduced shrinkage and lower permeability. Additionally, the greater compaction of shotcrete achieved through the velocity of placement improves compressive strength and durability.

We have a project that calls for new 6 in. concrete shearwalls formed and placed against the existing structure from the basement up to the fourth floor to enable an additional seven floors to be added to the structure. Our engineer has suggested that the new shearwalls be constructed using shotcrete. We are not familiar with using this system for structural applications. Most of the information we have gotten relates to using shotcrete for swimming pools and cosmetic applications. What advice can you provide?

The use of shotcrete for structural applications has been documented in numerous articles in Shotcrete magazine, Concrete International, and other publications. The key is to find a shotcrete contractor experienced in structural applications. Investigate the contractor’s project history to determine his/her experience. A contractor experienced in this type of structural enhancement will be most helpful in achieving the desired result in an economical and timely manner.

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