Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blogs or magazine articles on precast.org.
If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.
Terry writes:
Would using a water mister directly on a product after producing polymer-concrete channels aid in the reduction of crazing or cracking?
NPCA Technical Services engineers answered:
In order to provide a thorough response, we would need more information on the formulation used in your polymer concrete. In general, polymer concrete is a composite material in which the aggregate is bound together in a matrix with a polymer binder. Most times, these composites don’t contain hydraulic cement; however, portland cement can sometimes be used as an aggregate or filler. Polymer concrete composites possess a unique combination of properties dependent on the formulation, so it’s impossible to assess performance or give proper advice without knowing additional details. ACI 548.1R-09, “Guide for the Use of Polymers in Concrete,” may provide some information on specific manufacturing procedures.
For hydraulic, cementitious-based precast concrete, fine map cracking, commonly referred to as crazing, is usually caused by surface shrinkage. Anything that can cause rapid surface drying, such as low humidity, high air temperatures or wind, can cause crazing. To help prevent crazing in this type of concrete, curing procedures should include processes that maintain a high level of humidity. Misting portland cement concrete provides excellent curing conditions by maintaining humidity levels that are ideal for proper hydration. It also slows evaporation from the concrete surface and reduces the potential for plastic shrinkage cracking. It should only be used when the air temperature is above freezing and once the concrete has achieved sufficient setting to avoid damaging finishes and potentially raising the water-cementitious ratio. Intermittent misting should be avoided if the concrete surface is allowed to dry between periods of wetting.
Mike writes:
What temperature and moisture levels are required for the curing process?
NPCA Technical Services engineers answered:
Assuming you’re talking about conventional portland cement concrete, the ideal temperature and moisture levels will vary based on many factors. Generally, concrete temperature should fall between 50 and 70 degrees Fahrenheit and relative humidity should exceed 80% to 85%. This would apply to conventional curing and not accelerated curing processes. The reaction between cementitious materials and water, otherwise known as hydration, is a series of chemical reactions. The rates of those reactions are temperature dependent. The rate of reaction can double for each 18 F rise in concrete temperature. Low concrete temperatures can result in stunting the hydration process. Higher concrete temperatures can result in faster strength gain at early ages, but lower ultimate strength at later ages. A study by Paul Klieger in Portland Cement Association Research Bulletin 103 concluded that Type I portland cement concrete cured at about 55 F for the first 28 days ultimately reached the highest strength. Again, these ideal conditions will depend on many factors including what admixtures and cementitious materials are used and whether this is a mass structure or thin wall.
When pouring concrete in cold or hot temperature extremes, added precautions need to be employed to ensure quality. Those special guidelines can be found within ACI 305R-10, “Guide to Hot Weather Concreting,” and 306R-16, “Guide to Cold Weather Concreting.”
David writes:
What is the requirement for concrete slabs that cannot be strapped in the middle, is it two on the end and front or every 10 feet?
NPCA Technical Services engineers answered:
According to the Federal Motor Carrier Safety Administration, “When an article of cargo is not blocked or positioned to prevent movement in the forward direction, and the item is longer than 10 ft in length, then it must be secured by two tiedowns for the first 10 ft of length, and one additional tiedown for every 10 ft of length, or fraction thereof, beyond the first 10 ft… If an article is blocked, braced or immobilized to prevent movement in the forward direction by a headerboard, bulkhead, other articles that are adequately secured, or other appropriate means, it must be secured by at least one tiedown for every 10 ft of article length, or fraction thereof.”
If the slabs will crack under those requirements, then this should be brought up with the company’s quality control manager. According to Section 4.8.4 in the NPCA Quality Control Manual for Precast Concrete Plants, “Trucks and other conveyances used to transport precast concrete products from the plant to the location designated by the customer shall be equipped and maintained to deliver those products without damaging them to the extent that they must be repaired or rejected.”
In short, if the slabs are longer than 10 feet in length, they must be secured by at least one tiedown every 10 feet. If the slab is not braced to prevent forward movement, then it must have two tiedowns in the first 10 feet as well as a tiedown every 10 feet afterward.
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