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Q. We are creating a new mix design for a large mass pour. The engineer is concerned with strength gain and has requested a tight water/cement ratio. The mix may also include a fair percentage of supplementary cementitious materials. The engineer wants a mix design that mitigates the potential of microcracks generated by internal heat gain from hydration.

To accomplish this, we've been trying to develop a new performance mix. We've heard of a new philosophy that encourages substituting lightweight aggregate fines for a portion of our standard river sand.

We've been told that lightweight aggregate will help us achieve a steady strength gain to our target strength with less chance of microcracking through a phenomenon called internal concrete curing.

What is internal concrete curing? How does it promote strength gain? Should we cut back on mixing water? What benefits come from internal concrete curing?

A. Internal curing refers to the time-dependent improvement of concrete strength due to the gradual release of water from aggregate, in which it was absorbed before mixing, to the cement particle to allow continued hydration. To be more specific, there's new terminology currently being discussed by ACI Committee 308, Curing Concrete. The working draft says “internal curing refers to the process by which the hydration of cement occurs because of the availability of additional internal water that is not part of the mixing water.”

Internal curing is not a new discovery. According to John Reis, executive director of the Expanded Shale, Clay & Slate Institute, documenting improved long-term strength gains from saturated normal-weight aggregates was first reported by Paul Klieger in 1957. This strength gaining effect was confirmed eight years later in research on lightweight aggregate (LWA) conducted at the National Ready Mixed Concrete Association's lab.

But in recent years, there's been a renewed interest in developing methods to incorporate internal curing as a tool to reduce cracking, especially in pavements. Dale Bentz, a researcher at the National Institute of Standards and Technology (NIST), says it's often not possible to provide enough curing water from a pavement's surface at a sufficient rate to satisfy the ongoing chemical shrinkage. “Internal curing distributes extra water throughout the entire microstructure, thus maintaining saturation of the cement paste during hydration,” he says. “This avoids paste self-desiccation and reducing autogenous shrinkage.”

Furthermore, there's the need to encourage internal curing, especially in high-performance mixes. Some design applications call for the concrete to be less permeable by including quantities of divided silica, such as fly ash or silica fume. This tends to reduce internal water movement.

The change in definition for internal curing comes from the research effort to position water effectively throughout these highly impermeable mixes, according to Bentz. This can be accomplished with the certain material dispersed throughout the mix. These suppliers act as internal reservoirs, replacing the water throughout the hydration process.

“These special materials include saturated lightweight fine aggregates, superabsorbent polymers, and saturated wood fibers,” according to Bentz.