Placing shotcrete in hot weather can be a risky proposition if you don’t follow basic guidelines. Success under these conditions, calls for controlling concrete temperature and mixture design while also managing details of the installation and curing processes.
The summer is always busy, but that may be a huge understatement this year. The country is coming back to life with consumers very aware of their personal space and wanting to do more with it – which, often times means adding watershapes.
As a result, the heightened demand for our pool industry’s products will likely lead to all sorts of logistical and supply issues. The availability of concrete may well be one of those critical issues. Ready-mix suppliers have informed me that during this forced downtime, many homeowners, do-it-yourselfers, are taking on their own concrete work. Suppliers caution that when the contractors fully come back online during the summer, there could be delays in product supply and even shortages of ready-mix delivery trucks. To varying degrees, it’s often true that hot weather and elevated temperatures drive up demand.
Especially with the current extraordinary circumstances, the first step in hot-weather concrete work, whether you’re doing wet-mix or dry-mix shotcrete, is you need to communicate with your concrete material suppliers and place orders well in advance. That’s the only way you’ll know what to expect for availability and delivery, so you can schedule accordingly and in turn let the client know what to expect.
Beyond that basic caveat, there are specific measures and conditions required whenever you’re placing concrete in hot weather. Shotcrete placement in hot weather requires temperature control, environmental control, and jobsite control.
ENVIRONMENTAL CONDITIONS
I currently serve as Chair of American Concrete Institute’s (ACI) 506-H Pool Shotcrete Committee. We are writing guidelines for the shotcrete placement process, as it pertains to both wet- or dry-mix for pools. In the hot weather shotcrete section, we refer to the existing hot weather concreting documents from ACI 305, which define acceptable concrete temperatures and procedures.
According to the ACI 506 pool shotcrete document, you should always keep concrete as cool as possible. You should not apply shotcrete when ambient temperatures are over 95°F (35°C), unless special precautions are taken. The precautions should be reviewed by the pool designer or other experienced engineer. You can cool the concrete by using cold water to mix it, misting systems, wetting blankets, and shade. Each of these options slow down evaporation and help reduce the potential for early age plastic shrinkage cracking on the surface of the concrete.
Considering what’s happening in the fresh concrete matrix helps explain why these measures are necessary. When you mix water with portland cement hydration occurs. The cement hydration produces changes in the cement particles that lead to strengthening as the particles interlock. The hydration process leads to a volumetric change in the concrete, as well as additional heat from the chemical reaction.
When you’re already working in warm conditions, you can easily wind up generating too much heat if you’re not taking measures to cool down the environmental temperatures and the concrete itself. If temperature of reinforcement, embedments, or forms is greater than 120°F (49°C), use a fine mist of water to moisten and cool hot surfaces. Remove standing water before shotcrete placement.
Yes, it is possible to successfully place shotcrete in temperatures over 105°F (41°C) or even higher – consider Arizona and Nevada. But you must take recommended precautions. Do everything you can to avoid the heat of the day by working in the morning, if possible. You may also consider erecting a shade structure to keep the concrete, forms and reinforcing out of direct sunlight.
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