Durable precast concrete: the long-term solution for above-ground and below-ground application.
By Gary K. Munkelt, P.E.
Durability is an important concept to consider when it comes to specifying requirements for precast concrete products. Products used above or below ground may be exposed to harsh environmental conditions. Without durability, the concrete product may deteriorate at a rate much faster than the anticipated life span of 50 to 100 years.
Technology in concrete mix design has advanced rapidly for several decades. The need for higher strengths in high-rise buildings has been one motivation. Another driving force is the demand of the precast concrete industry for a mix providing durable concrete that will cure faster and provide high-early compressive strengths. Chemicals added to the “old-fashioned cement, sand, stone and water concrete” provide the industry with a low water/cement (w/c) ratio concrete having acceptable flow characteristics. Low w/c ratios in concrete lead to low permeability.
What makes a durable product?
Permeability plays an important role in making concrete long-lasting. A concrete with high permeability will allow fluids to pass into and affect the internal structure. Water reaching the reinforcing steel can promote corrosion and ultimately damage the concrete. A concrete with low permeability will resist the penetration of fluids and be more durable.
Compressive strength of the concrete, in addition to low w/c ratios, will provide low permeability and affect the resistance of the concrete to penetrating fluids. Higher compressive strengths with low w/c ratios provide a dense concrete with low permeability. The simplest method for obtaining increased compressive strength is to add cement. The additional cement fills more of the cross section with fine particles, reducing the size and number of microscopic voids for penetrating fluids to enter. A concrete with a compressive strength of 5,000 psi will be denser than a concrete with compressive strength of 3,500 psi. Assuming w/c ratios are the same, the denser concrete has the potential to provide longer life.
Additives have helped precast concrete manufacturers remain competitive by allowing them to make products containing low w/c ratios and higher compressive strengths than many cast-in-place structures. In many cases, the forms used to make the precast product provide narrow spaces for wet concrete to flow through. It is necessary to maintain relatively high slumps to enhance flowability characteristics and minimize voids. High slumps insinuate high water levels and resulting high w/c ratios. But water-reducing chemicals enable the water to flow more efficiently so that higher slumps are possible without adding extra water. For a relatively workable mix without chemicals, the w/c ratio would be greater than the 0.45 desired to provide low permeability. With chemicals, precast producers can create mixes with w/c ratios below 0.4, and at the precast plant, the accurate amount of chemicals is easily added to a mixer producing identical batches of concrete. One benefit of precast over cast-in-place is the ability to produce quality products on a consistent basis using new advances in additive technology.
Precast manufacturers commonly use concrete with compressive strengths in the range of 4,000 psi to 5,000 psi. When the industry was in its infancy, 3,000 psi was the common 28-day compressive strength specified. Precasters learned that they needed to wait until concrete compressive strength was approximately 2,500 psi before they could strip the forms and handle the product. The concrete with 3,000 psi 28-day compressive strength couldn’t be stripped from the forms for several days under ideal curing conditions. However, precast concrete products needed to be stripped within one day to be competitive with cast-in-place concrete. The solution was to increase the 28-day compressive strength of the concrete mix to between 4,000 and 5,000 psi.
Today, some precasters are getting 28-day compressive strengths in excess of 6,000 psi, with the potential to reach strengths even well beyond that. By increasing mix design strength, the 2,500 psi necessary for stripping could be reached within one day and forms reused more efficiently. Some precasters utilize chemical accelerators and apply steam to reduce curing time, enabling them to use forms twice daily or more. The higher compressive strengths in addition to chemical additives and controlled curing practices allow precasters to make a product more durable by reducing permeability.
Plant certification promotes quality
All of the technology and experience does not guarantee durable concrete products if proper manufacturing techniques are not followed on a day-to-day basis.
To promote consistent quality in precast products, the industry has embraced the concept of plant certification programs. These programs, which employ independent engineering firms, provide two advantages for both the precaster and the end user. First, the associations who manage the certification programs provide precast concrete facilities with a Quality Control Manual to be used as a guideline to prepare their plant-specific manual. A plant’s manual must identify record keeping procedures to ensure use of proper materials and define the procedures for properly manufacturing, pouring and curing concrete. A written document provides continuity of production practices even though personnel may change.
The second advantage of certification programs is that physical in-plant inspections are made by independent engineering firms to confirm compliance with the procedures identified in the Quality Control Manual. This process provides assurance to end users that unbiased inspections are performed at the manufacturer’s plant. It also ensures that manufacturers don’t establish a Quality Control program and then “forget” to enforce it.
Plant certification programs provide specifiers with assurances that certified precast concrete facilities conform to a minimum set of industry-accepted standards.
Quality, value and permanence are the keys to long-term success of building components on the job site, and precast concrete is well-positioned to fulfill this requirement. With great technical advances of concrete chemical additives over the past decade and overseeing of quality by associations offering plant certification programs, precast concrete is an excellent long term solution for aboveground and below-ground applications.