The term “spacing factor” refers to the distance between air bubbles in hardened concrete. All concrete has some air bubbles, usually in the range of 1 or 2%, referred to as “entrapped air”. These bubbles provide no freeze/thaw protection. Where freeze/thaw protection is desired, air bubbles are intentionally introduced, or entrained, into the plastic concrete mixture. These microscopic bubbles protect the mortar portion of the concrete by providing space for water in the concrete to expand during the freezing process. If these bubbles were not available for this purpose, the expansion of the water would damage the mortar. An important characteristic of a good air-void system is the spacing factor. Bubbles need to be in close proximity so the water migrating through the concrete does not have to travel far to find a bubble in which the water can expand. Ideally the spacing factor will be less than 0.008 in. This analysis is performed on hardened concrete by a trained petrographer using test method ASTM C 457. There usually is some slight variance between petrographers evaluating the same concrete sample.
I am an architecture student and would like any information you could provide in regard to the proper and typical mix ratios of cement to sand.
The best reference for shotcrete Questions in general is ACI 506 – Specification for Shotcrete. It is available from the American Concrete Institute.
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 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 density than conventional concrete in most cases. The increased density will provide reduced permeability and higher durability.
A shotcrete mixture can be designed and proportioned to meet virtually any job requirement. In this case, air entrainment must be specified. Whenever any concrete mixture (shotcrete mixtures included) will be exposed to freezing and thawing while critically saturated, air entrainment must be part of the mixture. The amount of air entrainment required depends on the maximum size of the coarse aggregate used. In general, for a mixture with a maximum-sized coarse aggregate of 3/8 in. (10 mm), the air content should be about 8% as-batched for a severe exposure condition.
Another key to longevity is reduction of permeability. As a mixture becomes denser, the transmission of fluids through the mixture becomes more difficult. This is especially critical when trying to protect reinforcing steel. When chloride ions and oxygen reach reinforcing steel, corrosion is initiated. Increasing the density by using products like silica fume, slag cement, and fly ash dramatically decreases permeability.
Discuss the curing and protection plan with the contractor prior to the start of shotcreting. Failure to cure and protect properly is the most common reason for poor concrete or shotcrete performance.
Another often overlooked element in obtaining an extended type of service life is maintenance of the concrete structure. By periodically cleaning the concrete and applying an appropriate surface sealer, materials that may lead to deterioration are removed from the surface and not allowed to penetrate the pore structure of the concrete.
We are going to be using shotcrete for repairs in a parking structure. We have no experience performing this work and will be subcontracting this portion of the job. What should we be watching for when the shotcrete is being applied?
Surface preparation is a critical operation. The substrate must be prepared properly. All deteriorated concrete must be removed. This is generally accomplished with light-duty chipping hammers, scarifiers, or scabblers. The remaining concrete is then sandblasted or waterblasted to remove the concrete “bruised” by the initial removal operation. The objective is to create a clean, sound surface with the proper surface roughness to receive the shotcrete.
After the surface preparation, the substrate must be saturated with clean water and then allowed to dry to a saturated, surface-dry condition immediately prior to shotcreting. Shotcrete should not be applied to a bone-dry surface as the substrate will absorb water in the shotcrete mixture intended for hydration of the cement. Also, a bone-dry surface will tend to allow plastic and drying shrinkage cracks to form. Conversely, a surface that is wet at the time of shotcreting will result in a high water-cement ratio (w/c) at the interface between the substrate and the shotcrete. High w/c at the interface will result in significantly lower bond strengths.
As with all concrete, proper curing and protection is critical. Failure to cure properly will result in lower shotcrete strengths and may cause some delaminations if drying shrinkage causes stresses that exceed early bond strength. Plastic shrinkage cracking and “crazing” may also result from failure to cure and protect properly. Moist curing is the preferred method of curing. If moist curing is not feasible, membrane curing compounds may be used.
Finally, be sure the nozzleman who will be applying shotcrete on your project is certified by the American Concrete Institute (ACI). Certified nozzlemen have been trained and tested on the requirements for proper shotcrete application. Insisting on this certification dramatically increases the probability that you will get the desired results.
I know air entrainment is required in concrete exposed to cycles of freezing and thawing while saturated. However, the shotcrete I am going to be applying on a project in Chicago is on a vertical surface where the water will essentially run off the surface. Do I still need to worry about air content?
You are correct in stating that entrained air is necessary in concrete that is exposed to freezing and thawing while critically saturated. Even vertical walls can get critically saturated in places. Because you are working in a part of the country that experiences significant freezing and thawing, it is imperative that you maintain sufficient air content in the shotcrete. Remember, you are going to lose some air content in the placement process so the air content of the shotcrete mixture going into your pump must be higher than the desired in-place air content. It is a wise idea to do some testing in advance of the actual shotcreting to determine how much air content you will lose.
Our firm has no experience designing for shotcrete applications. We have been investigating the process and would like to know what we should be looking for as the shotcrete is placed. Are there special features or problems in shotcreting?
Proper placement is the most important element in achieving good shotcrete results. Most defects that occur in shotcrete are due to poor placement. Shotcrete success depends largely on the skill and actions of the nozzleman. The nozzleman’s goal is to achieve adequate compaction and good encasement of the reinforcement (if present) with no entrapped rebound or hardened overspray. For this reason, it is important to require that the nozzleman be ACI certified for the application. There are specific certifications for both wet and dry processes as well as vertical and overhead applications. If the nozzleman is certified, the probability that you will get the desired results is significantly increased. For more information on certification, visit the ASA website, shotcrete.org, and click on Certification.
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 will be using shotcrete to repair a concrete box culvert that has some minor spalling. Do we need to apply a bonding agent before applying the shotcrete? How should we prepare the surface?
No bonding agent is required. A key to a successful repair is proper surface preparation. The surface receiving the shotcrete must have the deteriorated material completely removed, be thoroughly cleaned, and in a saturated surface-dry condition (SSD) at the time of shotcrete application. Another key item is proper curing and protection following shotcreting. Details can be found in the Task Force 37 Report “Guide Specification for Shotcrete Repair of Highway Bridges.” The document is available from the American Association of State Highway and Transportation Officials (AASHTO), Washington, DC.
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.