So much depends on concrete’s durability yet early-age cracking is a common and damaging issue structures face. These cracks expose internal steel reinforcement to chemicals and elements, accelerating corrosion and jeopardizing a concrete component’s structural integrity and service life. Factors like surface drying and environmental stressors like temperature can increase the risk of early-age cracking.
Carefully considering the mix design can address these issues and support efforts for more resilient concrete. Internally cured concrete (ICC) made with lightweight aggregates (LWA) such as expanded shale, clay or slate (ESCS) can help mitigate early-age cracking, chloride ingress, curling and warping—resulting in more durable and longer-lasting concrete products.
Reducing shrinkage and permeability through internal curing
Internal curing refers to curing concrete from the inside out by supplying water that is not part of the mixing water. ESCS’ network of unconnected pores slowly releases absorbed water to hydrate the cement. This results in a more uniform curing process that minimizes cracks and curling. It also creates a less permeable and more robust concrete.
Generally, permeable concrete more readily allows the ingress of water, chlorides and other harmful chemicals, which can corrode steel reinforcements and weaken the overall structural integrity of concrete. Internal curing strengthens the bond between the aggregate and the cementitious mix by reducing the potential for shrinkage and early-age cracking. Further, ESCS has an irregular surface, which provides more area for mechanical adhesion. The surface of ESCS is also considered pozzolanic, which increases strength, impermeability and sulfate resistance while also reducing expansion from the alkali-silica reaction that might otherwise take place.
With fewer cracks and a less permeable makeup, ICC components are more resilient to early-age cracking and the ingress of damaging materials. This helps producers deliver a more durable product and more easily satisfy clients and secure more business.
Minimizing cracking due to thermal fluctuations
Besides curing, temperature fluctuation may also make concrete structures or components crack. Typically, frequent freeze-thaw cycles can cause internal stressors in the matrix bond. Compounding the problem, when water or snowmelt fills these gaps, it solidifies and increases in volume by 9 percent. This causes pressure forces inside the concrete to weaken joints and form internal cracks.
Because ESCS in ICC products mitigates microcracking, it reduces spaces for water to fill. Furthermore, ESCS’s pores allow water to expand more easily and distribute temperature-related stresses more uniformly. This translates to concrete components that can cushion the impact of volume expansion without cracking.
Sidelining common concerns with the right aggregate
Concrete producers can also adjust the aggregate ratio in the mix to achieve structural lightweight concrete (SLC), which can be up to 35 percent lighter than traditional concrete. SLC’s lower density reduces structural dead load while enabling the concrete to carry a higher load, which helps with cost efficiency.
Despite its many benefits, SLC has a lower tensile strength than standard concrete, which might raise concerns about cracking under live loads and limit its application. However, cracks are often caused due to the difference in stiffness between the aggregate and cementitious paste. With ESCS, SLC has improved strain compatibility, which means all parts of the component deform proportionally under external loads. This harmony nearly eliminates excessive stress concentrations, cracks or failures.
According to a 2003 paper by Frank Dehn, building materials and concrete construction professor at Karlsruhe Institute of Technology and director of the Material Testing and Research Laboratory, SLC made with ESCS LWA displays fracture behaviors somewhere between other SLCs and standard concretes. This translates to a ductility that is closer to that of normal weight concrete, which can be further improved by incorporating fibers or steel reinforcements. Though these methods for improving ductility apply to all types of SLC, a mixture made with ESCS requires minimal adjustments, reducing any significant changes to the material’s performance capabilities.
Bolstering resilience for long-term performance
Producing durable and resilient concrete is not just about reputation; it’s about the safety and longevity of buildings and infrastructure. ICC made with ESCS offers a path to achieve both. By mitigating early-age cracking and improving overall performance, ICC made with ESCS lightweight aggregate empowers producers to deliver concrete with superior durability and resilience.
Clint Chapman is Arcosa Lightweight Western Region Marketing & Technical Manager. An NRMCA Certified Concrete Technologist – Level 3 with 20-plus years in concrete—the majority in lightweight aggregates, focusing on structural lightweight concrete—he has been involved in high-profile projects throughout the West, including the Wilshire Grand and Gerald Desmond Bridge in Los Angeles, along with numerous internally cured water tanks throughout the Denver area.