The California Department of Transportation (Caltrans) uses rapid strength concrete (also known as high early strength concrete) to repair or rehabilitate failed concrete pavements. The rapid strength of concrete is vital to mitigate short traffic closures in California and achieve proper performance within a few hours of slab placement. The short closure time severely limits the opportunity to use conventional external curing. Further, the large thickness of concrete slabs curbs the reach of surface water into the body of the concrete. These limitations contribute to the premature failure of early-age slabs and often cause microcracking that leads to long-term exposure to corrosive environments.
The intention of slab replacement or patching in isolated locations of failed pavement is to minimize further deterioration and extend the life of the existing pavement. The failed concrete slabs are removed and then replaced as a measure used to improve the structural integrity and performance of an existing concrete pavement. However, Caltrans has considered a potential short service life of slabs constructed with RSC due to insufficient concrete curing within the severely short traffic closure. Hence, internal curing (IC) with prewetted fine lightweight aggregate was employed to compensate for the inadequate surface curing and the lack of external curing of the RSC beyond the traffic closure time.
The American Concrete Institute defines internal curing as “supplying water throughout a freshly placed cementitious mixture using reservoirs, via pre-wetted lightweight aggregates, that readily release water as needed for hydration or to replace moisture lost through evaporation or self-desiccation.” Simply put, internal curing is referred to as “curing concrete from the inside out” [1]. Internal curing improves the durability and performance of concrete mixtures by reducing early-age cracking, chloride ingress, and curling and warping, all of which extend the concrete’s service life.
A section of pavement on Route 680 in the Bay Area was identified as the location for test sections. Rapid strength concrete slabs with internal curing were placed next to slabs using the conventional rapid strength concrete without internal curing. Caltrans plans to monitor, test, and report the expected improvement in the performance of internally cured rapid strength concrete in the future.
A trial batch was performed in March 2019. At the time of slab fabrication, the ambient temperature was around 15° C. The results of concrete flexural strength (modulus of rupture) testing for both RSC and RSC-IC mixes met the specification requirements.
The field trial using conventional RSC and RSC-IC showed very little statistically significant difference between the flexural strength of the two mixes in the first six hours but a significant increase in RSC-IC strength at the age of 24 hours. In terms of constructability, there was no noticeable difference between RSC and RSC-IC when mixing, transporting, and placing the concrete.
An independent testing lab conducted testing for conformance to the specified design strength of 600 psi modulus of rupture (MOR) in 3 days with an average of 764 psi MOR for the RSC-IC mix.
A visual inspection was performed in the field in late August 2019. Considering that the slabs were placed under traffic for a relatively short amount of time, there was no indication of any difference in their performance with and without internal curing. However, the internally cured slabs are expected to outperform the conventional slabs in the long run. In October 2019, cores were taken from multiple slabs (both RSC and RSC-IC mixes), and a close visual inspection did not show any apparent difference.
Caltrans concluded that field experience showed that RSC-IC has similar workability, strength, and mechanical property development to conventional RSC. The short-term performance of the two types of mixes has been almost identical. Caltrans is anticipating that using RSC-IC will reduce shrinkage and residual stress development, less cracking, and improved durability compared with RSC in the long term [2].
