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Shotcrete FAQs behind the new plane. We would assume that this work would be crews. Thus, ACI 506R was updated to reflect the successful done from the top down installing new soil nails and shotcrete industry practice. facing in lifts of approximately 4 to 5 ft (1.2 to 1.5 m). Question: We have a school project in California. It is for a Question: My question is not directly on shotcrete/gunite, but structure with shotcrete walls and a shotcrete dome roof shot a similar material—cementitious fireproofing material: a mix- with an inflatable form. There is disagreement on the nozzlemen ture of portland cement, vermiculite, and other additives, qualification panels. One group says that a panel should be sprayed on to steel structures like a shotcrete. Because there shot for each nozzleman for each position (three panels: one is not much published information/data on this product, I am vertical, and two for different slopes of the dome) in a single trying to find my answers based on shotcrete: layer with the most congested reinforcing bar in any single 1. How do you check the compressive strength of an applied layer to simulate job conditions. A second group maintains that shotcrete? I know typically it is tested from test cubes casted the same three panels should be shot, but they should be built at the time of application. The test cube may be used when up over a period of 6 days in gradual layers to represent the testing for compressive strength, and one makes only one layering of the actual shooting. or two test cubes. If you have a doubt on applied shotcrete I think that the first group is correct and complies with the (that is, the mixture including water, prior to application), intent of ACI 506. Shooting one-layer panels with the most how can one check if everything is correct? Alternatively, congested reinforcing bar to be placed in any one layer would if the cubes were not taken, is there any other testing method best simulate the job placement conditions. I don’t see any to check for compressive strength of the material installed? added advantage in shooting qualification panels over a period 2. What other inspections/tests are done during spraying or of days in layers and seems to be reading too much into after installation on shotcrete to make sure that it is “simulating” jobsite conditions. installed properly? Answer: In construction of shotcrete dome roofs with inflatable Answer: We do not have any direct input on cementitious forms, the structural thickness of the dome is built out in layers fireproofing, as it is likely applied at much lower velocities to prevent overloading the support offered by the inflatable with lower impact forces than shotcrete. Shotcrete is generally form and foam. Thus, your nozzleman qualification panels tested by cores taken from shotcreted test panels (ASTM should be representative of the dome construction methods. C1140/C1140M), but according to ASTM, can also be estab- This would include shooting orientation (vertical and varying lished from sawed cubes. If trying to establish compressive slopes), shooting procedures (layers), and with the most con- strengths from in-place shotcreted materials, ASTM C1604/ gested reinforcing. When shotcrete is applied in layers, all you C1604M would be applicable. need to do is wait for the first layer to stiffen sufficiently (usu- ally called initial set), before applying the next layer. It is not Question: We are considering sealing off the openings of some necessary to wait for days before applying the next lift. existing water tunnels by constructing reinforced concrete walls within the openings. One wall, for instance, is 9.25 ft wide by Question: I’ve been testing shotcrete cores for compression 12 ft high (3 x 4 m) and is to be 22 in. (559 mm) thick. Other strength according to ACI 506, ASTM C1604, and ASTM C1385. walls will be in the range of 18 to 26 in. (457 to 660 mm) thick. The only thing that we have been doing out of specifications is What are the limits as to the maximum thickness/size wall that the panel. Our panels are 18 x 18 in. (457 x 457 mm). We have shotcrete can be used to construct? Also, I noticed that in the been coring at the center of the panel 2 days after it has been latest revision of ACI 506R-05 that the previous recommended cast. We test these cores at 7 and 28 days, and the strength of limits on the reinforcing bar size have been removed. I believe cores reflects passing at 7 days but failing at 28 days. in the past, the reinforcing bars were limited to the smaller-size Can you please tell me what could be the cause of this? bars to reduce the development of sand pockets behind the bars. Is there a recommended limit on the size bars that can be used? Answer: ASTM C1140/C1140M-11, “Standard Practice for Perhaps it was in the ACI standard but I just overlooked it. Preparing and Testing Specimens from Shotcrete Test Panels,” is the appropriate ASTM standard for producing and coring Answer: Experienced shotcrete contractors with qualified test panels. ASTM C1140 specifies panel size as a minimum crews have often shot structural concrete sections 24 to 30 in. of 24 x 24 in. (610 x 610 mm) with a minimum 3.5 in. (89 mm) (610 to 762 mm) thick with reinforcing bars up to No. 11 in depth. Without more information on the materials used in the size. There is no real limit to how thick you can build a shotcrete shotcrete and the type of shotcrete, it is impossible to identify “wall.” Though earlier versions of ACI 506R, “Guide to what may be causing the lower compressive strength tests. Shotcrete,” did recommend limiting reinforcing bars sizes, The compressive strength should increase between 7 and modern equipment, concrete mixtures, and shooting techniques 28 days on a curve equivalent to cast concrete. Strength deg- have proven that large-diameter steel reinforcing bars can be radation between 7 and 28 days may be a result of poor shotcrete properly encased on a routine basis by experienced shotcrete application or problems with coring or curing of the samples. 76 Shotcrete • Spring 2015


2015SpringShotcreteEMag
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