Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blog posts or magazine articles posted on precast.org.
If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.
David writes: How can I heat forms in the winter?
NPCA Technical Services engineers answer:
Whatever option you choose to heat your forms, it is important neither to overheat nor unevenly heat forms. Hot spots on forms may cause flash setting in parts of the concrete. The goal is to provide an ideal curing temperature for the concrete rather than heating the forms.
Gas heaters may be used to keep the ambient air around forms warm, which may help keep the concrete inside the forms at ideal curing temperatures. Do not point heaters directly onto forms, however. The goal is to keep the ambient air around the form at an elevated but controlled temperature.
Tarps and covers may help keep frost, ice and snow off outdoor forms prior to using the forms, since melted snow and ice – if not removed prior to pouring the form – could impact the water-to-cement ratio of the fresh concrete that is poured into the form.
The ambient temperature around forms kept indoors can better be controlled with typical indoor heat sources, but the same principles apply: Do not point heat sources directly onto formwork.
Form suppliers may aid with any systems designed to heat their specific forms, and you should always consult a supplier for recommendations, whether your forms are indoors or outside.
Other methods for maintaining an adequate concrete curing temperature in cold weather include using heated mix water and storing aggregate supplies indoors or underground so they’re protected from outdoor temperatures and the elements. Check out NPCA’s “Troubleshooting Cold Weather Concrete” article and ACI 306, “Guide to Cold Weather Concreting,” for more information
Eric writes: What are the measured values for sound transmission loss for precast concrete panels that are 6-, 8- and 12-inches thick?
NPCA Technical Services engineers answer:
Sound attenuation can be measured in a few ways. Sound transmission loss (STL) may be measured using ASTM E90, “Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building
Partitions and Elements.” The other option is to measure the sound transmission class (STC), which is determined by ASTM E413, “Classification for Rating Sound Insulation.” This is more widely used and provides a rough decibel reduction level of a partition. The higher the STC, the greater the sound attenuation. STC only measures to a low frequency limit of 125 Hz. There’s also outside inside transmission class (OITC), which takes into consideration lower frequency noises such as the ones you would expect from trucks, heavy equipment and pumps.
Mass is a primary means to attenuate sound. Usually, the higher the material density, the higher the STC rating for the same partition thickness.
In a Portland Cement Association study, researchers found that an 8-inch concrete wall with a density of 145 pounds per cubic foot has an STC of approximately 58.
The Precast/Prestressed Concrete Institute also has a resource on precast acoustics that shows a 4-inch wall has an STC of 49 while 6-inch and 8-inch walls have an STC of 55 and 58, respectively. The latter matches the rating found in the PCA research.
There is no specific data available for a 12-inch-thick wall. The STC rating for the same material is not linearly related to the material thickness, so the STC rating for the 12-inch-thick wall would not be much higher than the 58 you might get with the 8-inch panel. PI
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