I work for a Commercial pool builder. We are doing a Jr. Olympic pool (competition pool) 40 ft by 70 ft (12 m by 21 m) with a 20 ft by 10 ft (6 m by 3 m) swim out area with steps. I would rather have the entire pool shot with shotcrete. The office wants the floor poured first then shoot the walls the next day. Can you talk about any advantages or disadvantages to either? Normally we are in a remote location with no power or water. We’ve had issues with shotcrete cracking on the floors and I understand that can happen because of many factors but poured concrete can as well.

Casting the floor first is recommended. Note that rebound and trimmings from vertical wall placement are not fully consolidated concrete and must not be used as part of a structural concrete section. Thus, casting first allows:

  • No collection of rebound and trimmings from wall construction to get trapped below floor reinforcing steel.
  • Provides a stable surface to collect and remove rebound and trimmings.
  • Is generally faster placement and allows full consolidation of concrete throughout the floor thickness by immersion vibrators.
  • If using wet-mix shotcrete the shotcrete pump can serve as a line pump for pumping concrete into the floor thus requiring no extra equipment.
  • Allows proper joint preparation at the floor/wall joint to provide a watertight joint.

 

 

Our pool company just completed the shotcrete today and temperatures are expected to reach near freezing tonight. Should the shotcrete still cure properly? Should we be concerned?

You say near freezing, so we assume it did not reach freezing. Though the newly place concrete will not gain strength quickly it should not be damaged. As temperatures rise during the day the warmer temperatures should allow more strength gain. Concrete has a chemical reaction to build strength, that generates internal heat. If freezing weather was expected, we would recommend use of a protective blanket on the concrete surface to help hold the heat in and promote quicker strength gain. Generally, in concrete we like to see the concrete surface temperature at 50 °F (10 °C) or higher in cold weather to encourage strength gain.

 

 

I am working on a fire re-build project near Napa, CA. The existing building has 12 in. (300 mm) thick shotcrete basement walls. There are many areas where the concrete cover spalled off from the rebar. The rebar appears to be ingood condition with no visible buckling. Is it possible to repair these walls with a 1 1/2 to 2 in. (38 to 50 mm) layer of shotcrete to restore the concrete cover? If so, what sort of surface prep is required? Is a concrete bonding agent required?

Yes, this is definitely a good application for shotcrete. To restore the cover:

  • Chip or hydrodemo back to sound concrete. If a reinforcing bar is more than half exposed, chip at least 1 in. (25 mm) back behind the bar.
  • Make sure the surface is roughened and clean.
  • Bring the concrete surface to saturated surface dry (SSD) condition. This means the surface feels damp but water is not picked up on a hand.
  • Make sure the shotcrete placement is properly executed. Use of an ACI-certified shotcrete nozzleman is recommended.
  • No bonding agent should be used. It will interfere with the natural bonding characteristics of shotcrete placement.
  • If chipping out a section do not feather edge. Provide a ¾ to 1 in. (19 to 25 mm)) depth of cut at the edge to provide adequate thickness for the integrity of the shotcreted material at the edge.

 

This article on the excellent bond between shotcrete provides more detail: https://shotcrete.org/wp-content/uploads/2020/05/2014Spr_TechnicalTip.pdf

 

I’m interested in any information you can provide about recommended expansion/contraction joint spacing for lazy river concrete flumes.

Shotcrete is a placement method for concrete. As the lazy river is a long concrete trough that is intended to be functionally watertight, the best guidance is ACI, 350-06 Code Requirements for Environmental Engineering Concrete Structures. As the predominate stresses in the horizontal direction result from shrinkage and temperature, Section 7.12.2.1 provides guidance on joint spacing based on the provided reinforcement ratio in the concrete section. The closer the movement joints, the less reinforcement required. Chapter 5 of ACI 350.4R-04, Design Considerations for Environmental Engineering Concrete Structures, has guidance on design of joints for water containing structures to help maintain water tightness.

We are working on a large infrastructure project in New York state. We have access and approval issues for a new concrete mixture design. Our understanding is we can specify a Pre-Bagged or Pre-Packaged Shotcrete mix for a wet-mix application. Is this the correct terminology? Are there companies that produce this type of product?

We see both Pre-Bagged and Pre-Packaged terminology used. ASTM C1480 / C1480M – 07(2012) uses the rather unwieldy “Standard Specification for Packaged, Pre-Blended, Dry, Combined Materials for Use in Wet or Dry Shotcrete Application.”  Most suppliers of packaged dry concrete materials for shotcrete have formulations designed for wet-mix applications. You can find our corporate members who supply pack-aged materials on our website in the Buyers Guide (shotcrete.org/BuyersGuide), select the Category, “Shotcrete Materials-Mixture Sales” and the Subcategory, “Wet Mix.” 

We are working on a repair/renovation project in Boston. A long-concealed wall next to an adjacent property is now visible as the adjacent property is being renovated. We have been told that our wall must have a 2-hour fire rating. Our wall is composed of CMU masonry and exposed structural steel members. Applying shotcrete to the CMU and steel is a good solution for several reasons. Would you provide shotcrete specifications that will have a 2-hour fire rating on CMU and structural steel?

Shotcrete is a placement method for concrete. Thus, the fire resistance for shotcrete placement is the same as concrete. The primary reference for fire resistance of concrete is ACI 216.1-14(19) Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies. 

would like to ask if there are any articles, references, etc, which reference procedures to determine the maturity of the concrete applied via shotcrete? Basically, how to generate the validation curves?

Shotcrete is a placement method for concrete. Thus, generating the maturity curves would be based on the concrete mixtures. There are several online resources about the maturity method. One that discusses production of the curves is from the Minnesota DOT and available in PDF format at  www.dot.state.mn.us/materials/concretedocs/MaturityMethodProcedure.pdf. 

I have a 24 in (600 mm) thick shotcrete wall that will have to be scanned for voids. The project has been struggling to locate a local expert who has the capability to scan this thick of a wall. In addition, this new 24 in thick shotcrete wall was dowelled and made “as one unit” together with an existing 24 inches thick cast in place wall. What technique should I use to get the right proportion?

Shotcrete is a placement method for concrete. Thus, all non-destructive testing (NDT) applicable to concrete walls would be usable on your wall. However, it is difficult to get good results with a scanning system for heavily reinforced concrete walls of your thickness. Impact Echo and Impulse Response are two one-sided techniques that can provide good results for a portion of the 24 in thickness though would likely not be able to scan the entire depth. Ultrasonic Pulse Velocity is a potential if you can access both sides of the wall. For one-sided investigation at greater depth you may be able to use a MIRA system. It is sophisticated tomographic system that says it can test from 50mm to 800mm (32 in) thickness. There are national consulting firms that provide these investigation systems. Each requires a highly trained, experienced operator so be sure to verify the firm can document successful experience with the method.

Can shotcrete be considered as structural concrete with wire mesh and rockbolts in tunnel linings?

Shotcrete is a placement method for concrete. It is routinely used for a wide variety of structural applications. It has been used for both initial and final linings in tunnels where it is commonly reinforced with wire mesh, fibers, or reinforcing steel. You may want to review our past Shotcrete magazine articles on tunnel shotcrete at https://shotcrete.org/archive-search/
using keywords such as “tunnel,” “underground,” and “linings.” Also, we have two position papers from our underground committee: “Spraying Shotcrete Overhead in Underground Applications,” and “Spraying Shotcrete on Synthetic Sheet Waterproofing Membranes,” that you may find informative. Also, ACI 506.5R-09, “Guide for Specifying Underground Shotcrete,” can provide insight into topics important for using and specifying underground shotcrete.

Do you have any reference documentation for the application of using concrete stamps/textures or hand carving on vertical shotcrete applications for architectural finishes?

Unfortunately, I’m not aware of any specific references to guide you in stamping or carving of fresh shotcrete for vertical surfaces. In many ways carving and stamping is a technique that requires more of an artistic flare that a finisher develops by hands-on experience. One key aspect is to minimize the working of the surface as much as possible to prevent tearing or delaminating the shotcrete surface layers. Carving or stamping when the concrete has reached an appropriate level of set (not too hard or soft) must also be judged by the experienced shotcrete finisher.