Page 48

2013SprShotcreteEMag

Goin’ Underground Rapid-Setting Cement in Shotcrete By Mike Ballou esulfoaluminate (CSA) cement in lieu of a Time SavingsI won’t get into the “time is money” clichés,Where Do You Use It and Why?volving technology in the shotcrete industryusing a rapid-setting mixture with calcium more common portland cement mixture is helping but we all know that the more effective we are to meet the challenges for shotcrete repairs and with our work force, the better. Time spent underground projects throughout the world. watching over concrete/shotcrete while it cures is Repairs performed with ordinary portland a thing we’ve grown accustomed to but have never cement (OPC) concrete mixtures can take up to accepted—we’ve just dealt with it. 28 days before they reach design strength. This So, if we have a cement that cures in about an can slow down projects and cycle time for mining eighth of the time that OPC cures—and if we don’t and tunneling. Shotcrete using rapid-setting CSA need to stand over it for hours, days, or weeks keeping cement can reach full design strength in only a it wet so that it won’t develop shrinkage cracks—well, few hours. In about 15 minutes—without any perhaps that’s something we ought to consider for accelerator—dry-mix shotcrete commonly some applications. Oh, sure, you still need to cure it, reaches strengths of 2200 psi (15 MPa); in an hour, and water curing is good for that, but it doesn’t take the strengths can be close to 3600 psi (25 MPa). nearly as long. Rapid-setting shotcrete, if care is used in placing, needs only a few hours of water curing. What Makes It Different andhow You can do it longer than that, but the hydration Does It Work? water helps to cool the hydrating CSA cement butprocess or the chemical reaction is completed. The First of all, CSA cement is not a new product—it is not a critical factor like it is for OPC shotcrete.1 has been around for years and is manufactured in a way similar to OPC. Most of the same raw materials Strength and Durability or ingredients are used to make rapid-setting cement, There’s been a lot of very intensive testing done such as limestone, clay, gypsum, silica, and alumina. to show how well rapid-setting shotcrete gains The materials are mixed, the clinker is made, and the strength and holds it in compressive tests. The results clinker is ground to fine powder—much like OPC. are incredible—3600 psi (25 MPa) in an hour, The hydration or chemical reaction is similar, but the easily—without any accelerator. Shotcrete accelera- reaction time with the CSA cement is much quicker. tors can promote cracking due to early-age plastic shrinkage and should be avoided if possible unless used with care.1 Rapid-setting cement is not only strong in compression but also very durable. It has been used with steel fibers and several types of Fig. 1: Shotcrete repair on bridge abutment Fig. 2: Screeding new rapid-setting, low- using low-permeability, rapid-setting cement permeability shotcrete 46 Shotcrete • Spring 2013


2013SprShotcreteEMag
To see the actual publication please follow the link above