As autumn settles in, many of us have started dressing in warmer layers and donning thicker coats. We’re also more conscious of the thermostat – and that drafty door – both at home and in the precast plant. Throughout the year, and especially during times of seasonal transition, it’s imperative to remember how Mother Nature affects concrete and the materials used to produce it.
Now is an ideal time to review the procedures in your plant and potentially increase the frequency of certain tests to ensure consistency in your concrete, materials and products.
NPCA’s technical services personnel recommend reviewing the following requirements and standards to help prepare for the colder weather:
- Per Section 220.127.116.11 of the NPCA Quality Control Manual for Precast Concrete Plants, when producing conventional and/or dry-cast concrete without the use of moisture probes in aggregate bins, aggregate surface moisture content must be determined at least once per day in accordance with ASTM C70, “Standard Test Method for Surface Moisture in Fine Aggregate,” or ASTM C566, “Standard Test Method for Total Evaporable Moisture Content of Aggregate by Drying.”
- When aggregate bins are fitted with moisture probes or meters that are used in conjunction with automatic mixing water adjustment systems, the aggregate moisture content shall be determined in accordance with one of the two above ASTM standards at least once per week.
- Per Section 18.104.22.168 of the NPCA QC Manual, when producing self-consolidating concrete, and without the use of moisture probes in aggregate bins, aggregate surface moisture content shall be determined at least once per day prior to making the first batch of SCC, plus at least once every four hours of elapsed time after the first batch while SCC is being produced. These moisture tests shall be performed in accordance with ASTM C70 or ASTM C566.
- If aggregate bins are fitted with moisture probes or meters that are used in conjunction with automatic mixing water adjustment systems, the aggregate moisture content shall be determined in accordance with ASTM C70 or ASTM C566.
- In addition to stringently monitoring aggregate moisture content, reviewing ACI 306, “Guide to Cold Weather Concreting,” can also provide assistance in preparing for seasonal changes. Section 4.4.7 of the NPCA QC Manual also outlines precautions for cold weather concreting. The temperature of everything from the ambient environment to mix water, aggregates, chemicals, steel and formwork all play a role in concrete curing and fresh concrete shall not be less than 45 degrees Fahrenheit at the time of placement.
- During this time of year, accelerated curing procedures become more common. Section 4.5.3 of the NPCA QC Manual outlines requirements and guidelines for curing with heat and moisture. ACI 308, “Guide to External Curing of Concrete,” provides additional information.
Don’t be caught off guard by a late arrival of autumn weather or an early assault of harsh winter temperatures. Review your plant-specific manual and the cold weather precautions and update your procedures as needed. If you have any questions on cold weather concreting or are in need of additional resources, please contact NPCA’s technical services team.
GIANNI f DIFRANCO says
Cold weather concrete is often something required with construction in the colder states. Unfortunately, I’ve found most general contractors want to continue construction in the cold weather months but don’t want to pay the concrete contractors the extra cost associated with cold weather placement. This leaves us concrete contractors in a situation we don’t want to be in.