Bridges that stay structurally sound for longer despite normal and ex- traordinary wear-and-tear can help ensure safe passage for vehicles and pedestrians. Additionally, if these structures do not require substantial maintenance or repair over their service life, they can help maximize the investment made into the infrastructure. While proper design can help create long-lasting bridges, engineers and construction profession- als can further enhance a concrete bridge’s lifelong structural integrity with internal curing processes. Internally cured concrete cures from within to reduce the poten- tial for early-age cracking caused by shrinkage stress. To provide internal moisture, construction professionals often turn to pre- wetted lightweight aggregate made from expanded shale, clay or slate (ESCS). Because this material has an internal network of unconnected pores, it can hold water for evenly distributed curing throughout the concrete mix. In an effort to reduce chloride attack, bridge designers have reduced the permeability of the concrete used to construct a structure. Unfor- tunately, the measures used to reduce the permeability have also led to concrete that shrinks and subsequently cracks more. With cracks in the bridge deck, the chlorides now have a pathway to the reinforcing embedded in the concrete. Concrete bridges that are internally cured more readily resist cracking, chloride attack and reinforcing corrosion, which can make them more durable for longer. Further, if repair is needed, utilizing internal curing in the patching materials will reduce shrinkage contributing to a more viable patch. As such, it can mitigate the need for repeated maintenance on damaged bridge sections. When civil and structural engineers plan bridge projects, it is important to maximize the potential of all materi- als involved to solve immediate and long-term design challenges. The following will look at how internal curing can be incorporated into bridge construction to create safer and more durable structures. Easier done than said: incorporating internal curing in bridge construction First recognized in 1948 and researched extensively in the early 1990s, internal curing can help create a stronger, more resilient concrete by reducing permeability and mitigating shrinkage. It curtails hydration- related stresses and the potential for early-age cracking. In fact, the Indiana Department of Transportation explains in a 2013 report that internally cured concrete reduced the early age autogenous shrinkage by more than 80 percent compared to non-internally cured concretes. The Key to Safer and Longer Lasting Bridges is Inside: How Internal Curing Improves Concrete By Corey Coffelt
Engineers can take advantage of these benefits by specifying structural lightweight concrete made with ESCS aggregate or by replacing a portion of sand with a pre-wetted fine ESCS lightweight aggregate in normal weight concrete. Prewetting the lightweight fine aggregate for a minimum of 48 hours allows the material to absorb moisture into its network of internal pores. When the pre-wetted aggregate is added to the concrete mixture, it releases this captured moisture to cure the concrete from the inside, increasing cement hydration and reducing the potential for shrinkage- induced cracks. Due to the extra moisture provided, internally cured concrete can also make construction in hot and dry locations more feasible. This was first noted in a 1965 field report presented to the National Sand and Gravel Association and later confirmed in several different studies—from Robert Philleo’s “Concrete Science and Real- ity” to Wei and Hansen’s 2008 research into hydration and slab warp - ing. Increased cement hydration efficiency and wider optimum curing parameters allow concrete to reach its maximum potential, thereby helping to improve bridge construction. To reap the benefits of internal curing, a concrete mix typically needs seven pounds of internal curing water per 100 pounds of cementitious material, so incorporating internal curing processes into concrete bridge construction can be easily done with the right preparations. While prewetting lightweight aggregate and batching it correctly does require some additional efforts, its ability to significantly improve the performance of concrete outweighs the additional steps needed to in- corporate it into bridge construction. Internal curing defends against chloride attack One of the most common reasons for continued bridge maintenance and repair is to prevent chloride attack, which can corrode the metal reinforcement within concrete. This can weaken a bridge, making it de- teriorate over time. Internal curing sidelines chloride attack developed from early-age micro- and macro-cracking caused by shrinkage. This can prolong a bridge’s service life and increase its durability, especially in locations near saltwater or where salt-based deicers are frequently used, as these increase the speed at which chloride attack can cause significant damage.
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December 2022 csengineermag.com
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