What Is Quality Concrete?
To fully appreciate the benefits of fly ash in concrete, the basics of producing exceptional concrete must be understood. Concrete is a composite material, which essentially consists of two components: aggregates and cementitious paste. To produce exceptional concrete, it is extremely important to have a smooth gradation of material from rock down to the finest particles (in other words, a good mix of particle sizes, so that the largest practicable rock fills the majority of the volume, while the progressively smaller rock and sand fill the voids left between the larger particles).
Ideally, it is best to have as much volume as possible filled with strong, durable aggregate particles, with enough paste (comprised of as much CSH and as little lime as possible) to coat every particle. Also, voids should not be present in the paste unless they are specifically provided as microscopic entrained air bubbles to provide durability in freeze-thaw environments.
In real life, though, economics and local aggregate sources dictate the quality of materials used. The result is that excess voids often exist between the aggregate particles that must now be filled by paste and air. The challenge becomes producing an appropriate amount of the best possible quality paste, so that the resulting hardened paste will fill the excess voids with durability and strength approaching that of the aggregates.
How Fly Ash Contributes to Concrete Durability and Strength
Most people don’t realize that durability and strength are not synonymous when talking about concrete. Durability is the ability to maintain integrity and strength over time. Strength is only a measure of the ability to sustain loads at a given point in time. Two concrete mixes with equal cylinder breaks of 4,000 psi at 28 days can vary widely in their permeability, resistance to chemical attack, resistance to cracking and general deterioration over time — all of which are important to durability.
Cement normally gains the great majority of its strength within 28 days, thus the reasoning behind specifications normally requiring determination of 28-day strengths as a standard. As lime from cement hydration becomes available (cements tend to vary widely in their reactivity), it reacts with fly ash. Typically, concrete made with fly ash will be slightly lower in strength than straight cement concrete up to 28 days, equal strength at 28 days, and substantially higher strength within a year’s time. Conversely, in straight cement concrete, this lime would remain intact and over time it would be susceptible to the effects of weathering and loss of strength and durability.
As previously described, the paste is the key to durable and strong concrete, assuming average quality aggregates are used. At full hydration, concrete made with typical cements produces approximately 1/4 pound of non-durable lime per pound of cement in the mix. Most people have seen concrete or masonry walls or slabs with the white, chalky surface coating or streaks called efflorescence. Efflorescence is caused by the face of the concrete being wetted and dried repeatedly, or by the movement of water vapor from the damp side of the concrete to the dry side through the capillaries (voids), drawing out the water soluble lime from the concrete, block or mortar.
A typical 5 sack concrete mix having 470 pounds of cement per cubic yard has the potential of producing 118 pounds of lime. Fly ash chemically reacts with this lime to create more CSH, the same “glue” produced by the hydration of cement and water, thereby closing off the capillaries that allow the movement of moisture through the concrete. The result is concrete that is less permeable, as witnessed by the reduction
Other evidence of the contribution fly ash makes to strength and durability includes:
– Cement has an upper limit of roughly 7.5 sacks (7.5 x 94# sack = 705#) when using
1 " maximum size aggregate, above which the psi per pound of cement strength contribution in a concrete mix diminishes rapidly. The tallest concrete structures in the world are made with concrete where fly ash
is a necessary component. Its ability to contribute to additional CSH, lower water demand, reduced heat of hydration and its fine particle size are crucial to making high-strength concrete (8, 000 psi to over 20,000 psi).
– Cement was invented in 1824, over 170 years ago. There are examples on the west coast of Italy, in a town named Cosa, where a mixture of natural pozzolans (volcanic) were combined with lime to produce concrete that has withstood waves and attack from seawater for over 2, 000 years and is still intact.
– The Pantheon in Rome is a pozzolan and lime concrete structure built around 300 B. C. and still stands today. It features a cast concrete dome 124 feet in diameter and was the world’s largest domed structure until modern times.
How Fly Ash Contributes to Concrete Workability
First, fly ash produces more cementitious paste. It has a lower unit weight, which means that on a pound for pound basis, fly ash contributes roughly 30% more volume of cementitious material per pound versus cement. The greater the percentage of fly ash “ball bearings” in the paste, the better lubricated the aggregates are and the better concrete flows.
Second, fly ash reduces the amount of water needed to produce a given slump. The spherical shape of fly ash particles and its dispersive ability provide water-reducing characteristics similar to a water-reducing admixture. Typically, water demand of a concrete mix with fly ash is reduced by 2% to 10%, depending on a number of factors including the amount used and class of fly ash.
Third, fly ash reduces the amount of sand needed in the mix to produce workability. Because fly ash creates more paste, and by its shape and dispersive action makes the paste more “slippery”, the
amount of sand proportioned into the mix can be reduced. Since sand has a much greater surface area than larger aggregates and therefore requires more paste, reducing the sand means the paste available can more efficiently coat the surface area of the aggregates that remain.
Evidence of the contribution fly ash makes to workability includes: – Lightweight concrete including fly ash is much easier to pump.
– Finishers notice the “creamier” texture when working. They also see reduced “bug holes” and segregation when stripping forms. Slip form pavers eliminate rock pockets and voids in an otherwise harsh, no-slump paving mix.
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