混凝土内养护(NIST)美国国家标准实验室.ppt

Dale P. Bentz dale.bentz@ ESCSI Special Workshop May 7, 2008 ? 5 cm ? 4.6 mm on a side Question: What is internal curing (IC)? Answer: As being considered by ACI, “internal curing refers to the process by which the hydration of cement occurs because of the availability of additional internal water that is not part of the mixing water.” For many years, we have cured concrete from the outside in; internal curing is for curing concrete from the inside out. Internal water is generally supplied via internal reservoirs, such as saturated lightweight aggregates (LWA), superabsorbent polymers (SAPs- think baby diapers), saturated wood fibers, or saturated crushed (returned) concrete aggregates (CCA). Question: Why do we need IC? Answer: Particularly in high performance concrete (HPC), it is not easily possible to provide curing water from the top surface (for example) at the rate that is required to satisfy the ongoing chemical shrinkage, due to the extremely low permeabilities that are often achieved in such concretes as the capillary pores depercolate. Capillary pore percolation/depercolation first noted by Powers, Copeland and Mann (PCA-1959). Question: How does IC work? Answer: IC distributes the extra curing water (uniformly) throughout the entire 3-D concrete microstructure so that it is more readily available to maintain saturation of the cement paste during hydration, avoiding self-desiccation (in the paste) and reducing autogenous shrinkage. Because the generated capillary stresses are inversely proportional to the diameter of the pores being emptied, for IC to do its job, the individual pores in the internal reservoirs should be much larger than the typical sizes of the capillary pores (micrometers) in hydrating cement paste and should also be well connected (percolated). Internal curing is not a substitute for external curing. As a minimum, evaporative moisture loss (after set) must be prevented using conventional external measures (misting, fogging, curin