Definitions – Source of Cracking
June 17, 2010
Plastic Shrinkage Crack Reinforcement would be appropriate for slabs-on-grade and on elevated deck and thin precast concrete elements where the weather conditions (temperature, wind speed, and humidity) could produce plastic shrinkage cracks. The cause of plastic shrinkage cracking is a rapid loss of moisture from the surface of the concrete element.
Plastic Settlement Crack Reinforcement reduces the cracking directly over rebars by reducing the plastic settlement of the concrete. The 3-dimensional fiber reinforcement network restricts the settlement of the coarse and fine aggregate and cement reducing the displacement of the free water in the concrete.
Drying Shrinkage Cracking Reinforcement provides for the cracking that occurs with volume change in the hardened concrete as the moisture leaves the concrete as it ages. The three-dimensional reinforcement provided by the fibers modifies the stress distribution resulting from this volume change thus producing micro cracking versus macro cracking, which is common when WWF is used. There is also a companion reduction in the volume change.
Impact Resistance, Surface Abrasion Resistance & Volume Change Reduction are all durability benefits derived from the 3-dimensional fiber reinforcement that cannot be provided by the single plain of WWF. These durability enhancements translate into longer concrete life with much lower maintenance costs. Higher dosage levels will yield higher test values and should be considered from some applications.
Ultra-Thin Whitetopping is a technology where concrete is placed on distressed hot mixed asphalt (HMA) pavements that have been milled down to a depth where there is good HMA in place. This technology is used at high trafficked intersections and other areas which show accelerated traffic wear in the form of pushing, rutting and shoving in the HMA. The FHWA specification calls for 3.0 lbs/cy of Microsynthetic Fiber per cubic yard of concrete. The purpose is multifold and includes increased fatigue strength and reduced curling and warping.
Post-First Crack Reinforcement looks at the use of the new generation Macrosynthetic Fibers and their ability to hold the concrete together after the formation of cracks generated by an external load. This is much different than holding cracks together created by internal stresses. The physical properties of Macrosynthetic Fibers are much different than Microsynthetic Fibers plus the dosage level is much higher thus producing an elevated performance level.
Applications include industrial and warehouse floor slabs and septic tanks plus precast panels. There are three ASTM consensus test methods that can be used in quantifying the precise performance level desired. Thus we are developing a performance level specification predicated on test derived data.
-R.C. Zellers, PE/PLS, Director, Engineering Services