Q: We are pouring a 60,000 square-foot indoor facility in Northern Ohio that is being heated with a 2 million btu salamander. We have good ventilation but are concerned about carbonation. How can we check carbon dioxide levels? At what level does it become detrimental to the concrete?
A: Many companies offer handheld gas detectors, but you may need to use more than one. These detectors report levels of either a single poisonous gas, such as carbon monoxide (CO), or multiple gases. You have to make a selection depending on what gases you expect because the sensors are different for each gas. Bacharach, New Kensington, Pa., offers a simple CO monitor that measures from 0 to 1999 parts per million (ppm). To put that in context, the EPA exposure guideline is 9 ppm averaged over eight hours and 35 ppm for one hour.
Checking for carbon dioxide (CO2) is a bit different. From a personal safety perspective, CO2 is not a poisonous gas, but because it displaces oxygen it can lead to asphyxiation. One federal standard allows a maximum of 5000 ppm, but the ASHRAE guideline for indoors is 1000 ppm. Outdoor air often has 300 to 400 ppm. Dual-beam infrared absorption technology is one way to levels, and is available monitor CO2 in a monitor offered by CEA Instruments, Westwood, N.J.
What level of CO2 adversely affects concrete? Remember that concrete forms as the result of chemical reactions occurring as cement hydrates over time. Carbon dioxide and water in the atmosphere enter into that reaction; in enclosed spaces combustion heaters can raise the levels of both. As a result, construction specifications for some of the Big Box stores now prohibit the use of unvented natural gas or propane heaters in the building during placement, require monitoring the levels (with maximums CO and CO2 of 15 and 4500 ppm, respectively), and require extra ventilation to pull clean air across the slab. They also say to minimize the use of unnecessary combustion equipment during placement and to place curing blankets or curing compound quickly.