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Shotcrete FAQs rial with air velocity of 60 to 80 mph (97 to 129 km/h) from Because shotcrete is a method for placing concrete, many, if the nozzle to consolidate the concrete material in place. It not most, of the properties of fiber-reinforced concrete, as found cannot be hand-applied. You may consider hand-applied pre- in ACI 544 committee documents, are applicable to shotcrete. packaged mortar mixtures to achieve your results, although strength and durability may be less than a similar section with Question: We have a concrete tunnel repair project, where shotcrete because the hand-applied material is not fully com- expansion joints in a 10 x 10 ft (3 x 3 m) tunnel need to be pacted. Another alternative is to create an inner cylinder and repaired. The joints are on the top, bottom, and both sides of cast concrete in the space between the box and the form. The the tunnel; therefore, we have vertical and overhead applica- concrete could then be vibrated for consolidation. tions. The detail calls for deteriorated concrete to be removed to a depth of about 6 in. (152 mm), existing reinforcing steel Question: Lately, I’ve been testing shotcrete cores for compres- to be cleaned and preserved, and old waterstop removed and sion strength according ACI 506, ASTM 1604, and ASTM 1385. new PVC waterstop installed. How far can shotcrete be pumped The only thing that we have been doing out of specifications is for an application in a tunnel? We are looking at about 200 ft the panel. Our panels are done: 18 x 18 in. (457 x 457 mm). (61 m) for the shotcrete to be pumped from the supply to the We have been coring at the center of the panel 2 days after repairs. Is this constructible? casting. We test the cores at 7 and 28 days and the strength of cores reflects passing at 7 days but failing at 28 days. Answer: Yes, this is definitely a great application for shotcrete. Can you please tell me what could be the cause of this? Either wet- or dry-mix shotcrete can easily be used in tunnels with hose lengths of 200 ft (61 m). An experienced shotcrete Answer: ASTM 1140/1140M-11, “Standard Practice for Pre- contractor will be able to select the appropriate process based paring and Testing Specimens from Shotcrete Test Panels,” on the site, availability of material, and their particular equip- is the appropriate ASTM standard for producing and coring ment and trained shotcrete crews. test panels. ASTM C1140 specifies panel size as a minimum of 24 x 24 in. (610 x 610 mm) with a minimum 3.5 in. (89 mm) depth. Without more information on the materials used in the shotcrete, and the type of shotcrete, it is impossible to iden- tify what may be causing the lower com- pressive strength tests. The compressive strength should in - crease between 7 and 28 days on a curve equivalent to cast concrete. A strength degradation between 7 and 28 days may be a result of poor shotcrete application or problems with coring or curing of the samples. Question: Is there a reference where I can obtain some guide regarding the tensile strength of fiber-reinforced shot- crete (steel and poly)? Answer: ACI 506.1R-08, “Guide to Fiber- Reinforced Shotcrete,” is a good reference for general use of fibers in shotcrete. If looking at underground applications for shotcrete, ACI 506.5R-09, “Guide to Specifying Underground Shotcrete,” will also offer guidance. The ACI 506 docu- ments are available in hard copy format on our bookstore website with ASA member discounts (www.shotcrete.org) or on the ACI website (www.concrete. org) in hard copy or PDF formats. Shotcrete • Summer 2015 63


2015SummerShotcreteEMag
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