Water is essential not only for life but also for producing high-quality, durable concrete. One of the most significant advances in modern concrete technology is internal curing, which uses pre-moistened lightweight aggregate to supply moisture from within the concrete matrix throughout the hydration process. This technique, supported by the ASTM Standard Specification for Lightweight Aggregate for Internal Curing of Concrete, addresses longstanding challenges including shrinkage, cracking, and durability limitations. For construction professionals seeking to understand advanced curing methods, Curing of High Performance Concrete Methods and Duration provides context on how curing practices have evolved. Internal curing represents a shift from traditional external curing, which relies on surface-applied water, to a system where curing water is distributed uniformly throughout the concrete itself.
The Science Behind Internal Curing with Lightweight Aggregate
Internal curing works by replacing a portion of the conventional aggregate with pre-moistened lightweight, porous aggregate. These particles act as internal water reservoirs, releasing moisture gradually as cement hydration consumes available water within the concrete matrix. This continuous supply extends the hydration of cementitious materials over a longer period, producing a denser microstructure.
How Internal Moisture Distribution Works
Conventional external curing relies on water applied to the concrete surface through ponding, fogging, or wet coverings, drawing moisture inward by capillary action. This leaves deeper sections with less available moisture. Internal curing eliminates this gradient by embedding water sources uniformly throughout the mixture.
- The lightweight aggregate is pre-moistened before batching, absorbing water into its porous structure
- As hydration reduces internal relative humidity, stored water is drawn out of aggregate pores by capillary suction
- Moisture release occurs gradually over days and weeks, matching the hydration demand of cementitious materials
- Because this water is absorbed within the aggregate, it does not count as mixing water and does not affect the w/cm ratio
Key Benefits of Internal Curing
Jeff Speck, vice president of sales and marketing for Big River Industries and chair of the ASTM Subcommittee C09.21 on Lightweight Aggregates, notes that internal curing results in only slightly higher initial costs, but these are far outmatched by the value internally cured concrete provides over an extended service life.
- Reduced shrinkage and cracking: Even distribution of additional moisture leads to greater uniformity throughout the concrete thickness, reducing internal stresses from differential drying
- Increased hydration: Higher percentages of cementitious materials undergo complete hydration, resulting in greater strength development
- Lower permeability: The denser microstructure makes concrete more resistant to chloride penetration
- Improved durability: Reduced cracking and lower permeability translate to longer service life and lower maintenance
The w/cm Ratio and Internal Curing
Modern concrete designs have progressively reduced the water-to-cementitious materials ratio to achieve higher strengths. However, lower w/cm ratios leave insufficient water to fully hydrate all cementitious materials, causing self-desiccation that creates internal voids. Internal curing provides supplemental water that is not part of the mixing water, allowing the benefits of low w/cm ratios while maintaining complete hydration.
ASTM Standardization and Industry Adoption
The formalization of internal curing through ASTM standardization marked a turning point for industry adoption. The ASTM Standard Specification for Lightweight Aggregate for Internal Curing of Concrete, developed by Subcommittee C09.21, gave engineers and specifiers a codified framework for incorporating internally cured concrete into project specifications. Since the standard’s approval, numerous projects have specified internally cured concrete across the United States.
Industry Adoption Statistics
Since 2003, more than 2 million cubic yards of internally cured normal weight concrete have been placed in the United States, including 1.3 million cubic yards in low-slump pavements. High-performance concrete projects have increasingly specified internal curing as a standard practice rather than an experimental technique.
| Property | Conventional Concrete | Internally Cured Concrete |
|---|---|---|
| Hydration completeness | Partial at low w/cm ratios | Near-complete at same w/cm ratios |
| Autogenous shrinkage | Significant | Minimal to none |
| Permeability | Moderate to high | Low to very low |
| Crack resistance | Susceptible to early-age cracking | Highly resistant |
| Form stripping time | 2 to 3 days | 1 day or less |
| Long-term durability | Standard | Enhanced |
Materials for Internal Curing
Expanded clay lightweight aggregate is the most commonly used material for internal curing in North America. Products such as Riverlite, produced by Big River Industries, provide the porous structure necessary to absorb and retain water during mixing and release it during hydration. The lightweight aggregate replaces a calculated portion of conventional coarse or fine aggregate, determined by the water demand of the cementitious materials and the absorption capacity of the lightweight aggregate.
For projects considering various approaches to concrete curing, understanding the relationship between internal and external methods is important. Does Cold Curing Water Cause Concrete Surfaces to examines how external curing practices can introduce risks such as thermal shock from cold water. Internal curing eliminates this concern since the curing water is already within the concrete at placement temperature. The Curing Method chosen should account for performance requirements, environmental conditions, and structural demands.
Real-World Application: Denver Water Lone Tree Tank Project
Denver Water’s second water tank installation in Lone Tree, Colorado provides compelling evidence of internal curing’s practical benefits. Denver Water, serving 1.3 million people, needed to install a second 10 million-gallon tank at a site originally planned for two tanks but built with only one in the 1980s. The project faced unique challenges that made internal curing an attractive solution.
Project Constraints and Challenges
The construction site was in the middle of a new residential neighborhood near an elementary school. The city restricted delivery trucks during hours before and after school. The tank’s floor and roof slabs had to be placed monolithically, without construction joints that could become leak paths.
- Colorado’s cold winters and hot, dry summers make concrete placement more challenging
- Post-tensioned tank design required exceptional crack control to maintain watertightness
- Delivery restrictions meant slab placements had to occur during weekend hours
- Adjacent residential area required noise and traffic management
Testing and Mixture Development
Erik Holck, Denver Water’s construction engineering manager, proposed internal curing to Bates Engineering. The team contacted three local concrete suppliers for trial mixture work. Aggregate Industries and Trinity Lightweight partnered with Denver Water to run trials and a comprehensive testing program. Results showed that a 0.40 w/cm mixture exceeded the 4,500 psi strength requirement at 28 days. The contractor, Garney Construction, requested using internally cured concrete for the two large vaults beneath the tank before proceeding with the main structure.
Construction and Placement
Based on vault performance, the design team relaxed the w/cm ratio to 0.42, providing a wider finishing window for large slab placements. On October 10, 2012, Garney placed the floor slab, pouring 1,500 cubic yards over 10 hours with three boom-style pumpers, at a rate of 150 to 200 cubic yards per hour.
- Floor slab: 1,500 cubic yards, 5 inches thick
- Walls and columns completed in 16 segments over subsequent months
- Roof slab: 1,800 cubic yards, 8.5 inches thick, placed March 4, 2013
- Cold weather delayed roof placement by six to eight weeks
Performance Results and Practical Implications
The performance data from the Lone Tree project demonstrates practical advantages of internal curing across multiple metrics. The contractor benefited from faster form cycling and improved workability, while the owner received a higher quality structure with reduced cracking and enhanced durability.
Strength Development and Form Cycling
The 28-day compressive strength of the floor slab averaged 7,130 psi, far exceeding the 4,500 psi requirement. The vault placements achieved 6,300 psi (0 percent fly ash) and 6,900 psi (10 percent fly ash) at 28 days. The speed of strength gain had a direct impact on form cycling. With conventional concrete, wall forms needed two to three days before stripping. With internally cured concrete, forms could be stripped the next day, allowing the contractor to maintain a continuous placement schedule with only two sets of forms.
Temperature Management and Crack Reduction
The rapid strength gain raised initial concerns about elevated internal temperatures from hydration heat. Holck monitored temperatures using embedded sensors. The floor slab peaked at 77 F. The massive ring section, 1.5 feet thick by 5 feet wide, reached 106 F. Wall temperatures averaged 86.7 F, and the roof slab reached 92.1 F.
| Structural Element | Thickness | Peak Temperature | 28-Day Strength |
|---|---|---|---|
| Floor slab | 5 inches | 77.0 F | 7,130 psi |
| Ring section | 18 in x 5 ft | 106.0 F | N/A |
| Walls | 24 ft high | 86.7 F (avg) | Exceeded requirement |
| Roof slab | 8.5 inches | 92.1 F | Exceeded requirement |
The moderate temperature rise, combined with reduced shrinkage, resulted in significantly less cracking. Inspection estimated approximately 40 percent fewer cracks, with those that formed being substantially smaller. When the tank was filled and monitored for leaks before backfilling, no significant cracks or leaks were found.
Economic Considerations and Future Adoption
The initial material cost of internally cured concrete is slightly higher, but extended service life, reduced maintenance, fewer crack repairs, and faster construction schedules contribute to a favorable total cost of ownership. The Lone Tree tank was completed and put into service in June 2012, remaining watertight without leaks.
Denver Water specified internal curing for additional projects starting in 2014, including two 10 million-gallon tanks at the Ashland facility and one at the Highland facility. Bates Engineering began specifying internally cured concrete for its clients based on the Lone Tree experience. This pattern of adoption followed by broader specification reflects growing industry confidence. For professionals working with moisture-sensitive joining processes, Selective Soldering Strategy How to Solder Pipe Valves demonstrates how precision moisture and heat management principles apply across diverse construction disciplines. The fundamental lesson is the same: controlled, evenly distributed moisture management yields superior results.