Concrete pavements and bridge decks face different demands than concrete floor slabs and walls. These parts of the nation’s infrastructure are subjected to standing moisture in the form of rain, snow and ice. For the latter two, departments of transportation (DOTs) often use salt-based deicers to keep the roads safe. The combination of moisture and salt creates a higher risk of chloride attack, which can be responsible for almost 40% of concrete failures. The likelihood of chloride attack increases when bridge decks and concrete pavements are located near bodies of salt water.
As engineers and concrete producers work together to develop long-lasting, resilient infrastructure, the question becomes — how can concrete structures more readily resist chloride attack?
One answer is to use concrete mixes that facilitate internal curing. This process helps concrete reach its maximum potential for chloride resistance by mitigating early-age shrinkage cracking and reducings permeability. Engineers and concrete producers can readily facilitate internal curing through the use of expanded shale, clay and slate (ESCS) lightweight aggregates.
What is chloride attack and how does it impact concrete pavements and bridge decks?
When chlorine gains an electron, it becomes a chloride ion. High concentrations of chloride ions break down the passive layer of reinforcing steel, leading to corrosion throughout these reinforcements. When steel reinforcements are eaten away, concrete pavements and bridge decks will crack, spall and eventually fail.
While chloride attacks can impact all concrete structures, horizontal slabs that remain exposed to the elements are most at risk. Concrete pavements and bridge decks that experience chloride attack will have shortened lifespans and increased costs of upkeep. They can also put motorists at risk unnecessarily. Not only does this put a strain on the sustainability of concrete infrastructure, but it can also represent a sustained cost for DOTs.
Using concretes with ESCS lightweight aggregates can fight against chloride attacks to improve the longevity and resilience of concrete pavements and bridge decks—making bridge and pavement maintenance as cost-effective as possible.
How does internal curing improve resistance to chloride attack?
Saturated ESCS lightweight aggregates supply water throughout the concrete mix. After the concrete has set, it slowly releases the water to cure the concrete from the inside out. The process helps reduce chemical shrinkage to a point where the concrete can gain enough strength to minimize cracking, which is a common cause of chloride penetration. Additionally, internally cured concretes are less permeable as they create a more dense paste structure due to more complete hydration of the cementitious materials. This minimizes the chance of corrosion due to chloride attacks, helping to prolong the lifespan and durability of the concrete in general.
How to start internally cured concrete production?
For concrete producers looking to improve their concrete’s resistance to chloride attack, ESCS lightweight aggregate can be an economical and easy way to facilitate internal curing. However, internal curing does require a couple more steps than conventional curing. But these steps are easily incorporated into normal preparation and batching.
The first step is to plan to replace a small portion of normal weight concrete sand with an equal volume of fine ESCS lightweight aggregate. For almost all applications, scientific evidence indicates that seven pounds of internal curing water per 100 pounds of cementitious material is appropriate, but it is encouraged to confirm with ESCS suppliers if this ratio is correct for a particular application.
Another essential step is utilizing a well-saturated lightweight aggregate for internal curing. Consult your ESCS supplier for the appropriate method of soaking. Some ESCS suppliers can provide a prewetted material for internal curing.
These simple actions can help producers improve the quality of their concrete mixes to support concrete pavements and bridge decks that can withstand chloride attacks for longer than conventionally cured concrete products.