Title: High-Performance Cement Grout for Post-Tensioning Applications
Author(s): K. H. Khayat, A. Yahia, and P. Duffy
Publication: Materials Journal
Appears on pages(s): 471-477
Keywords: bleeding; cohesion; freeze-thaw durability; post-tensioning; rheological properties; stability
Grout mixes used to fill post-tensioning ducts to protect wire strands against corrosion are made with portland cement, water, and some chemical admixtures to enhance rheological and hardened properties. Such grouts should be fluid enough to facilitate pumping and spread into place, hence insuring proper coating of the prestressing steel with minimum zones of the ducts that are imperfectly grouted. Unstable grouts can exhibit sedimentation of cement particles and bleeding of some of the free water that can propagate upwards and fill upper zones of post-tensioning ducts. Bleeding can also occur whenever there are variations in elevation between different grouted areas, such as in vertical ducts where bleeding water can also result from fluid loss between the wire strands that can rise upwards by capillary action. Free water accumulated in larger voids that is not reabsorbed can freeze and lead to some delirious expansion. Cement grout can be exposed to freeze-thaw cycles during their service lives, as in the case of grouted ducts in the surf and tidal zones of marine structures. Consequently, it is important, in some cases, to insure adequate air-void system in the hardened grout to resist freeze-thaw actions. A laboratory investigation was undertaken to develop highly fluid, yet stable cement grouts with adequate and stable air-void system and good mechanical properties. This paper reports the test results leading to the recommendation of high-performance structural grout containing 8 percent silica fume replacement and balanced combinations of high-range water reducer and rheology-enhancing admixture. Such grout can be properly air entrained to develop adequate and stable air-void system for frost durability.