Freeze-thaw deterioration is dependent on the concrete being saturated in multiple freezing/thawing cycles. In an overhead application, where water can’t stand on the surface, the concrete can’t be saturated unless water permeates through from the upper surface. And with good quality concrete in the tunnel, water shouldn’t permeate through, so it should be functionally watertight. As a result, freeze-thaw likely isn’t a critical durability issue.
A steel trowel finish does require extra working of the surface and would require the contractor to be very attentive to the proper time to obtain the finish yet not overly disturb the fresh concrete. Gravity is working against the overhead concrete staying in place.
Having a smooth steel trowel finish would make minor shrinkage cracks more noticeable. However, in the tunnel without exposure to sunlight or much wind exposure, and with proper attention to curing, perhaps surface cracking will be minimal.
Shotcrete, both dry-mix (gunite) and wet-mix are a placement method for concrete. Wet-mix uses premixed concrete while dry-mix simply adds water to the concrete materials at the nozzle. Both dry-mix and wet-mix with proper materials, equipment, and placement with produce quality concrete sections. The embedded reinforcement in the pool shell is designed to carry tensile loads. This may be bending stresses from structural loadings (settlement or water/backfill), or volume changes from drying shrinkage and temperature changes. Cutting a reinforcing bar would certainly negate its ability to carry loads in the vicinity of the cut and reduce the load carrying capacity until the development length allows the reinforcing bar to start carrying it full load.
The layout of your cracked section isn’t clear from your description. An 8 to 10 mil (2 to 2.5 mm) crack is sizable in a water-containing structure. Fixing the existing crack with a reinforcing bar epoxied in place across the crack may be effective. However, that solution would only carry any additional load on the section (structural or volume change), as the existing loads have already created cracks. Thus, you should also address filling the crack as part of the solution. This may be with epoxy injection or swellable polyurethane grouts. You should consult with the pool design engineer for their recommendation on the best method for repair.
Shotcrete is simply a placement method for concrete. Thus, fire resistance of any shotcreted concrete section can be evaluated by consulting ACI CODE-216.1- 14(19) Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies. There are many factors that affect the fire resistance so you will need to review the code to establish what’s appropriate for your structure.
Since shotcrete is pneumatically placed against a one-sided form there is no liquid concrete pressure against the form. When bench shooting vertical walls the majority of pressure from the high velocity impact is carried by the previously placed concrete as the concrete is stacked. Plus, this pressure is very localized, only affecting a small zone immediately adjacent to the impact area of the material stream. Here is an article on a research project that quantified the force https://shotcrete.org/wp-content/uploads/2020/05/2007Sum_TechnicalTip.pdf
From the paper conclusion “In normal spraying conditions, wet- and dry-mix shotcretes produced a force on the panel of about 45 and 20 lb (200 and 90 N), respectively. The maximum load recorded is 87 lb (389 N) and it was observed in simulating a water plug.”
If your shotcrete form is exposed to wind forces before placing shotcrete, you may want to consider the wind pressure that may be expected across the entire form during construction to keep the form intact. This should be much less than the 50 lb/ft2 pressure you mention. ACI formwork design is intended for cast concrete where liquid concrete is contained within a two-sided form and is NOT applicable to shotcrete placement. Some sources show a 60 mi/hr (100 km/ hr) wind exerting about 10 lb/ft2 (50 kg/m2) on a vertical wall.
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.
BRIDGE HISTORY Listed on the National Register of Historic Places in 1989, the 10th Avenue Bridge in Minneapolis features seven reinforced
concrete arches to span the Mississippi River. The historic 1,141-ft (348 m) open spandrel column arch structure carries over 10,000 vehicles daily, as well as hundreds of pedestrians and cyclists between downtown Minneapolis and an area dominated by the University of Minnesota on the east bank.
We got a call from one of the local drilling contractors, Braestone Rockworks to check out a shotcrete shoring project down at Deep Cove
in North Vancouver, British Columbia.
In the summer of 2018 Morningstar Homes retained GeoPacific Consultants for the geotechnical design of a proposed development of 109 single family residential lots.
UMA Geotechnical Construction Inc. (UMA) typically finishes off the soil nail walls it constructs with a
standard shotcrete finish. But sometimes the owner wants a more artistic approach.
That was the case for North Carolina Department of Transportation (NCDOT) on the I-40/I-77 interchange
improvements project in Iredell County. UMA was contracted by Lane Construction to install roughly 45,000
ft2 (4200 m2) of various sized soil nail walls with aesthetically appealing sculpted shotcrete finishes. UMA’s earth retention
work was first required to hold up cuts for widened roadway sections and to facilitate new bridge construction.
UMA is working as a geotechnical subcontractor to a Fluor-led joint venture (JV) with United Infrastructure Group, UMA to Break NCDOT Record for Largest Square Footage of Soil Nail Wall By Brian DeSpain, President, UMA Geotechnical Construction Inc. The project, formally known as NCDOT I-26 Exit 40 to I-40 Interstate Expansion Project, will help to alleviate traffic congestion and improve the safety and operational efficiency of this vital stretch of interstate in the Asheville area.