Prolonged internal curing promises to help concrete resist shrinkage and cracking.
BYU researcher Dr. Spencer Guthrie is comparing and evaluating the performance of concrete on two concrete bridge decks – one is made of regular concrete and the other is made with “pre-saturated lightweight aggregate fines,” explains Guthrie. “My particular task is to quantify the differences between this type of concrete and conventional concrete in bridge deck applications.”
Adding the wet, fine aggregate causes prolonged internal curing which has been shown to reduce shrinkage and cracking in concrete.* Internal curing also makes the concrete less porous and therefore delays the intrusion of water and dissolved chemicals and minerals that eventually cause the steel reinforcement to corrode.
A detailed explanation of internal curing can be found in a YouTube video of a presentation given by Dr. Jason Weiss of Purdue University School of Civil Engineering. According to Weiss, “internal curing increases hydration and ‘densifies’ the system.”
Concrete (basically cement and aggregate) cures through a chemical reaction, called hydration, which occurs when water saturates the cement. Internal curing is especially useful when it comes to high performance concrete that has a very dense aggregate matrix. Concrete mixes that are designed to be dense and structurally robust restrict water movement during the curing process.
Pre-wetted aggregate temporarily holds water in the concrete mixture without increasing the water-cement ratio. As the hydration unfolds, tiny pockets of water in the aggregate continue to react with the cement. “When we get more internal curing water, we keep the system moist, we keep the system reacting, we keep hydrating the cement which means we’re going to have lower overall porosity in the system,” says Weiss.
Guthrie and engineers at UDOT will eventually have objective data that will show the differences between conventional concrete and concrete designed for internal curing. Sensors embedded in the concrete measure time, water content, temperature, and electrical conductivity, which is a good representation of permeability. Guthrie and others will also conduct strength and durability tests in the lab.
Weiss claims that internal curing can effectively double the life of bridge decks, making it possible to use transportation resources more wisely.
*According to Guthrie: “Shrinkage always happens before cracking (and is often the cause of the cracking).”