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2014SumShotcreteEMag

Shotcrete FAQs Question: We are shooting 5000 psi (34.5 MPa) shotcrete. Answer: Efflorescence is common on many exposed concrete Because of rising temperatures, the mixture is getting too stiff and cement mortar applications. Generally it is seen when cracks to pump, and the inspector will not let us add water. What in concrete or mortar are exposed to water rather than accumu- should be done in this situation? lating within the crack. The basic mechanism creating efflores- cence is when concrete is exposed to water for a long time; Answer: At the point at which concrete/shotcrete temperature is excess free lime (calcium hydroxide) in the cement paste goes starting to rise and the mixture is stiffening up, adding water into solution with water (leaches). Then when that water even- should not be allowed. Water should only be added when the tually leaves the crack and dries on the surface, the white residue mixture is stable and only up to the water specified in the approved of calcium hydroxide creates what is termed “efflorescence.” mixture design. In warm or hot conditions, retarders, set stabi- It is very common to see efflorescence on brick structures lizing admixtures, or ice may be needed to keep the mixture stable where the mortar joints are exposed to rainwater that leaches for the period of time to transport and pump the load. out the calcium hydroxide and the resulting white efflorescence is highlighted on the dark-colored face of the brick. In concrete Question: Is there any documentation showing that it is okay tanks, it is often found in cracks that can accumulate water for to tie off to a man-lift basket? I have never found any. a sufficient time to leach the calcium hydroxide. The bottoms How do other contractors deal with ACI requirements of an of vertical cracks or low areas in horizontally oriented cracks air lance, knowing that OSHA has contradicting standards of often show the greatest buildup of efflorescence. These can be air wand pressure? surface cracks that are exposed to rainwater or through wall cracks that are exposed to water contained within the tank. Answer: OSHA requires that the personnel in aerial man bas- Although the tank was cured properly to help deal with kets be tied off with the appropriate harness and lanyard. When long-term drying shrinkage, surface cracking on shotcrete often you are in a JLG or other type of man lift, the only place to tie results from early-age plastic shrinkage cracks. These are off to is to the basket or boom bracket. This question may be shallow cracks that form within hours (or minutes, in extreme better answered by studying current OSHA documents. conditions) of placement due to rapid evaporation of water We cannot recall any of our members being cited for using from the exposed surface of fresh concrete (common in exposed an air lance or blow pipe. floor slabs or in your case the fresh shotcrete wall surface). To answer your question regarding when it will stop, the Question: Can you refer me to the standards for adding water answer is it won’t unless the cracks are sealed, or water is to ready mixed shotcrete? prevented from getting into the cracks. Cement-rich shotcrete has more than enough free lime to continue the leaching for Answer: Wet-mix shotcrete is a placement method for concrete. decades. Although surface-applied coatings may initially span Ready mixed concrete used for wet-mix shotcrete needs to small cracks, as the walls of tanks expand and contract due to meet the requirements of ACI 506.2-13, “Specification for filling and emptying, and undergoing daily and seasonal thermal Shotcrete.” ACI 506.2 specifies concrete materials shall meet changes, the surface cracks will open and close slightly and ASTM C94/C94M, “Standard Specification for Ready-Mixed eventually mirror through the coating. Coatings designed to Concrete.” ACI 506.2 also requires you shall batch, mix, and tolerate moving cracks would likely be much thicker than the deliver wet-mixture shotcrete in accordance with Specification 7 mils used on your project. If the cracks are through-wall C94/C94M, or Specification C1116/C1116M if fiber-reinforced. cracks that are seeping from the contained water, the crack will Further guidance may be found in ACI 506R-05, “Guide to need to be sealed, most commonly by injection of polyurethane Shotcrete,” and ACI 304R-00, “Guide for Measuring, Mixing, grout or interior surface coatings. Transporting, and Placing Concrete.” To answer your question on how to prevent this in the future, early-age plastic shrinkage cracks can be reduced by fogging Question: Could you provide information regarding the the fresh shotcrete surface to keep the surface humidity high appearance of efflorescence on a newly constructed 2 million and reduce evaporation of the water at the surface of the gal. (7.57 million L) holding tank? The tank was constructed concrete. Also, using fibers in the shotcrete can help reduce correctly and has held water for over 6 months. A leak test plastic shrinkage cracking. In hot or windy climates, placing shows no water loss over a 72-hour period, and no moisture the final layer of shotcrete during the coolest or calmest time has been seen on the surface, but efflorescence has been noted. of the day may help, too. The tank was painted after the shotcrete was properly To answer the question if additional paint would seal the cured. (The applied paint was inspected by a NACE inspector cracks, simply coating with an additional 7 mil (0.2 mm) and found to be approximately 7 mils 0.2 mm and meets coating would provide a temporary seal, but more than likely the specification.) At what point will this stop and what is the the crack will mirror through after some period of exposure. A best practice to prevent it from happening again? Would coating designer would need to evaluate the crack widths and covering the areas where it has occurred with additional paint potential movement to design a coating system that would seal the cracks? provide a long-term seal. 68 Shotcrete • Summer 2014


2014SumShotcreteEMag
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