Internal Curing: Minimizing Cracking

In 2010, the early use of internal curing for concrete began on bridge decks in Indiana and New York. Unlike conventional curing, which supplies water from the concrete’s surface, internal curing provides curing water from the aggregates within the concrete. While external water can only penetrate to a limited depth, using expanded lightweight aggregate for internal curing disperses water throughout the depth of the concrete.

“Internal curing has proven to reduce shrinkage cracks and fluid transport and increase hydration,” said Jason Weiss, Ph.D., Professor of Civil and Construction Engineering at Oregon State University. “In Indiana, the use of internal curing and high-performance mixture has tripled the life of the bridge.”

Following Indiana and New York, states including Ohio, Iowa, Utah, and many others began adopting internal curing methods for bridge decks. Pavements have been constructed in Texas and by the Illinois Tollway, he said. “There are now numerous states allowing internal curing of concrete pavement, and more state Departments of Transportation are open to the technology.

Porous, lightweight aggregate is plentiful in most regions of the country, so supply is not a challenge,” says Weiss. “The greatest challenge is controlling the moisture of the aggregate. While the Illinois Tollway was a mainline pavement, some of the most common uses for internal curing include patching, bridge decks and continuously reinforced concrete pavement (CRCP).”

According to a Federal Highway Administration Tech Brief, “The water that is absorbed in the lightweight aggregate does not contribute to the classic definition of the water-to-cement ratio. The water-to-cement ratio is a descriptor of the structure of the matrix and pores that develop in the fluid concrete system. Once the concrete sets, the structure and pore network have been established, and water can only aid in hydration. The water in the lightweight aggregate will desorb (leave) the pores of the lightweight aggregate as the negative pressure in the pore fluid develops with setting and increases thereafter.”

“Studies by Rao and Darter also suggested several benefits of using internal curing in pavements,” said Weiss. One of the benefits includes a reduction in curl in CRCP.

Field tests of internal curing on two test sections containing lightweight fine aggregate (LWFA) performed by the National Concrete Pavement Technology Center found:

  • Internal curing improved the degree of hydration over time.
  • The inclusion of LWFA did not affect maturity.
  • The internal curing reduced temperature and moisture differentials in the system.
  • Hence, warping and curling were reduced significantly. This is a benefit, as ride is improved and the risk of corner breaks is reduced. Based on this observation, it is likely that slab sizes can be extended for thinner sections, thus keeping saw-cuts out of the wheel path.
  • The permeability of the mixture containing LWFA was found to be improved—potentially increasing the pavement’s longevity.
  • Structural design modeling did not reflect any changes for traffic loadings on these pavements.
  • A life-cycle cost analysis indicated a long-term financial benefit to the technique based on reduced frequency of rehabilitation work and extended predicted life.
  • Reports from construction sites indicated that storing and preconditioning the LWFA would be a challenge in larger applications; otherwise, no significant changes were observed.

This story originally appeared in the 2022 Fall edition of Concrete Pavement Progress.