CONTRACTING: ABSORPTION AND SLURRY/MICROSURFACING EMULSIONS IS IT A PROBLEM?   In Hotmix applications, absorption by aggregate is a serious problem in some sources of stone. It can lead to an effective reduction in film thickness and so a reduction in durability, cohesion and adhesion. If the aggregate has a tendency to be difficult to coat, the effect can be made worse. It must be allowed for in mix design!!! DOES THIS HAPPEN IN MICROSURFACING OR SLURRY? To answer this question requires a knowledge of how absorption occurs. Aggregates which are abstractive have the following in common: high surface texture open and dry surfaces Interconnected capillaries- often caused by secondary mineralisation. When a liquid encounters such a surface it can flow into the surface nooks and crannies and into the capillaries. The limit of this action is the "SURFACE TENSION" of the liquid, its viscosity and the energy difference between the aggregate and the liquid. The viscosity is obvious, if it's too high, then the surface will not be wetted out and the the liquid will not go far. If its low enough, and the surface tension is low enough, and the energy difference is low then it can go right in. LIQUID ASPHALT is almost exclusively controlled by the viscosity and the compatibility of the asphalt with the aggregate. CUTBACKS are controlled by viscosity. EMULSIONS are controlled by surface tension and the charge on the emulsion. (see page on emulsion stability) That is the emulsion CANNOT penetrate the surface voids more than a short distance. The higher the charge density the higher the surface tension, the more emulsifier the thicker the double layer and the more difficult it is to go in. As the liquid asphalt is mobile at even temperatures of 30-40C ( depending on grade and whether its polymer modified). It may penetrate even after the mix is formed. That is in service. In an emulsion asphalt after its broken asphalt may penetrate at elevated service temperatures but NOT in making the mix. This secondary absorption is generally lower than that which occurs in mixes during manufacture, storage and transportation. SO FOR EMULSION APPLIED SYSTEMS LIKE SLURRY AND MICROSURFACING only the most highly absorbing aggregates are of concern and as these usually will not pass abrasion or soundness requirements it is NOT a practical problem. For Contracting Products and Services contact pavements@slurry.com Copyright © 2000 Valley Slurry Seal Co All Rights Reserved.