Precast Inc. - May/June 2008 - (Page 18) temperature will increase the concrete temperature by about 1 F (0.5 C). Aggregates. If you consider that most mix designs include 60 percent to 80 percent aggregates, then the temperature and moisture content of aggregates should have the most significant impact on the concrete. A 1 F decrease in concrete temperature can be obtained by lowering the aggregate temperature 2 F, for example. Consequently, extra efforts should be taken to keep aggregates cool during hot weather. Aggregate factors such as shape, size and grading all affect the amount of water needed in a mix to produce the required slump. Crushed coarse aggregates provide better resistance to cracking than round aggregates, but they also require additional water. Blending two or more sized aggregates can reduce the mixing water demand and increase workability. Cementitious Materials. Adding supplementary cementitious materials (fly ash, slag, etc.) should be considered when it is necessary to delay the setting time or lessen the temperature rise from hydration. Formwork/Reinforcement. Misting the forms and reinforcement immediately before placement can help cool them and prevent unwanted temperature increases. However, ensure that form release agents are not adversely affected, and always avoid pooling water anywhere within the forms. Post-Pour. After placing and finishing concrete, you can prevent moisture loss by immediately covering fresh concrete with any moisture-retaining material such as burlap or a curing compound described in ACI 306. Retention of moisture will optimize the cement hydration process and allow the concrete to develop its full strength potential. Failure to keep exposed surfaces from drying excessively fast may result in cracking and shrinking, and jeopardizes the integrity of product. Air content and temperature As concrete temperature increases, entrained air decreases. Reduction in entrained air content is typically a result of slump loss. An increase in concrete temperature will require additional air entraining admixtures in order to maintain the air content of the mix. In general, it is impractical to establish a maximum ambient temperature as your upper limit for production practices because of the other factors affecting the mix – concrete temperature, solar radiation, relative humidity and wind. Plants in climate controlled facilities, obviously, are not seriously affected by these factors. If your plant is not climate controlled, ACI 305 advises creating a set of measures that would include all the factors and testing your limits. BOOTH 903 BOOTH 1049 18 MAY/JUNE 2008 | PRECAST INC. http://www.pennsylvaniainsert.com http://www.grifcote.com http://www.pennsylvaniainsert.com http://www.grifcote.com
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.