One solution is iCrete, a proprietary mix design technology that uses the Internet to link the company's mix-optimization algorithm to a ready-mix plant. The company licenses that technology to the plant to identify the materials, measure moisture contents, and factor in environmental conditions, optimizing the proportions of all raw materials — aggregates, cement, slag, fly ash, and other industrial byproducts — to produce a mix that meets cost, strength, permeability, workability, and shrinkage/cracking performance requirements. Through batch-to-batch monitoring and an improved moisture control system, iCrete produces a consistent mix.SPECIFYING PERMEABILITY AND SHRINKAGE PERFORMANCE
Self-consolidating concrete is ideal for when there is congested reinforcing steel and to create very smooth architectural surfaces. These are very sophisticated mixes that use higher amounts of fines to prevent the aggregate from segregating during placement and a high-range water reducer, usually a polycarboxylate, to create high flowability. The biggest challenge with this 20-year-old innovation has been robustness: variations between batches that result in problems on the jobsite.
“Every mix design engineer has a unique characterization technique for the raw materials used to meet a certain flowability and strength requirement,” says iCrete Director of Concrete Development Jack Holley. “The ideal isn't necessarily to go for maximum particle packing, but for optimum particle packing. The result is a matrix that's very stable and allows the producer to batch a mix with self-consolidating properties a lot easier, and to do it in a repeatable way.”
Low permeability and low shrinkage (leading to less cracking) are also better addressed by performance rather than prescriptive-based specifications. Permeability may be reduced by using supplementary cementitious materials, especially fly ash and silica fume, but if the concrete is cracked, water moves easily through unless expensive waterproofing membranes are applied. Thus, impermeable, crack-free concrete is important for any water-retaining structure. It's also important for preventing chlorides from migrating in and corroding reinforcing steel.
“You can have a very good impermeable concrete but it's only impermeable between the cracks,” says Holley, a member of the National Ready Mix Concrete Association's P2P (Prescriptive to Performance) Task Group. “So even if you come up with a system to reduce permeability with high cement contents that results in higher shrinkage, you haven't accomplished your objectives.” He suggests using lower cement contents to reduce shrinkage and cracking.
Another way to waterproof is to plug the concrete's pore structure. All concrete, even fairly dense mixes, has pores through which water migrates. Two companies provide materials and systems using two different technologies. Kryton, which has been around for more than 35 years, is an integral system; while Hycrete is a relatively new admixture that reacts with water to form a polymer globule within the pore structure.
Kryton's material may be used as an admixture or applied to either the positive or negative sides of the concrete, where it penetrates and leads to crystal growth. Water is part of the reaction, so the crystals grow more rapidly where there are leaks to create a self-healing process that bridges over small cracks. For larger cracks, Kryton's patching compound pushes the crystals into the surrounding concrete and seals the wall even under high hydrostatic pressures. Kryton's KIM admixture has performed well on water treatment tanks, waste-water tanks, and water reservoirs.
Hycrete provides concrete with “hydrophobic” properties, meaning it reduces concrete's ability to absorb water. This prevents water from penetrating and can reduce corrosion of reinforcing steel. Hycrete has not, however, been shown to reduce water migration under hydrostatic pressure, although the company claims that it's effective in these applications.
Performance specifications result in better concrete at a lower price. But this approach only works when the owner, designer, specifier, contractor, and concrete producer work together to get the best possible end result.
It's like buying a new truck. You don't specify what kind of iron to use in the engine block or the design of the fuel injectors; instead, you ask for a certain level of performance backed up by a warranty. That allows the manufacturer to provide the best product at the best price. Concrete is a manufactured product that benefits from the same approach.Other concrete materials innovations
- Elemix (www.elemix.com) — This concrete additive of tiny polymeric spheres reduces the unit weight of concrete, increases R-value, enhances durability, and increases pumpability.
- Ekocrete (www.ekocrete.com) — This company has developed a system for making concrete with recycled crushed concrete as the only aggregate and high doses of fly ash and nano-fibers.
- BASF Glenium Sky (www.basf.com) — This high-performance superplasticizer allows concrete producers to make concrete with high percentages of carbon neutral supplementary cementitious materials.
— William D Palmer Jr is with Complete Construction Consultants in Lyons, Colo., where he develops technical and educational resources for the construction industry. He is a licensed professional engineer and former editor of PUBLIC WORKS.