Seeking long-term life for concrete structures.
By Gary K. Munkelt, P.E.
Durability is a term that has grown in importance for several decades in the concrete industry. More than ever, owners and designers are looking for a long-term life of 100 years when using concrete structures.
A durable concrete will not only withstand harsh mechanical damage but will also resist deterioration from chemicals. Chemicals include deicing salts used on highways and sulfates that exist in soils. Sanitary sewer structures can be exposed to industrial chemicals or the sulfuric acid created in a hydrogen sulfide atmosphere. A concrete product that can withstand adverse conditions such as these without showing signs of deterioration is considered durable.
Prior to the 1960s, brick, concrete block and cast-in-place concrete were materials of choice when building projects sub-terra. It was a slow process that required building round manholes block by block or installing forms and waiting until the concrete gained enough strength to withstand the load from the soil. Then precast concrete became more popular for these underground applications because of its lower cost and speedier installation. Although cost and convenience were prime attractions for precast, it was its durability that accelerated public acceptance.
Durability of cast-in-place products vs. precast
The “cast-in-place” structure was the main method of choice until precast structures became popular in the 1960s. The generally accepted codes for reinforced concrete have been published by the American Concrete Institute (ACI) with input from volunteer committees. They are a summary of experiences in the construction field and research performed in the laboratories. From the beginning of the 20th century, failures and problems in the construction of cast-in-place concrete resulted in additions or changes to these codes.
Durability of precast concrete products
The success of precast as the best underground option was not based on cost alone, but also on the durability of the product. A durable product is created not because all producers adhere to available codes but because of the way the product is manufactured. First, steel forms are required to produce the same product over and over. The forms are used to manufacture two, and sometimes three products per day using customized concrete mixes that allowed the forms to be stripped within eight hours. Manufacturing the same product over and over naturally provides a durable product. The industry is now known for providing cost-effective products that are more durable than the cast-in-place concrete.
Manufacturers of precast products know that a poor-quality product will end up in the trash pile. Handling and transportation places stresses on the concrete that are greater than those placed on it after installation at the site. To compensate, manufacturers have adjusted the quality of the concrete mix and devised methods of handling to minimize rejects. Concrete mixes can produce a 4,000 psi to 5,000 psi strength in 28 days. These strengths, combined with good curing practices, produce a concrete strength of 2,500 psi in one day. Steam curing is also used when 2,500 psi stripping strength must be reached in six to eight hours. Each refinement improves durability of the product shipped to the site.
Specifications for reinforced concrete mixes such as those contained in “Building Code Requirements for Structural Concrete,” ACI 318, and “Environmental Engineering Concrete Structures,” ACI 350, promote a low water/cement ratio where site conditions are corrosive and a durable concrete is required. Low water/cement ratios make moving and placing fresh concrete difficult, but chemicals have been developed to promote the flowability of concrete and still keep water/cement ratios low. Applying these chemicals to the concrete mix requires safeguards to avoid misuse. The precast concrete plant environment is better suited for regulating chemicals in the mix than on site.
Specifications for precast concrete products were created during the last half of the 20th century. These prescriptions created by the ASTM International include ASTM C478 for sewer manholes, ASTM C913 for water and wastewater containers, and many others. ASTM rules for the volunteer committees that generate specifications are quite rigorous. Only 50 percent of the voting members can be producers. The remaining 50 percent must be members with a general interest or members representing users of the product covered by the committee. These specifications are updated as needed but no less than once in five years. Rules such as these have produced specifications that reflect experience and research by both producers and users to create uniform and durable products throughout the industry.
In recent years, the industry has aggressively promoted quality assurance programs to gain acceptance from a demanding public. The National Precast Concrete Association administers a rigorous plant certification process. This inspection-based process is performed by an independent engineering firm, with annual formal inspections and regularly-performed random inspections. The inspection process includes review of quality control procedures to ensure that proper methods are employed on a consistent basis and that a durable concrete product is shipped to the public.
A new material gaining acceptance in the industry is self-consolidating concrete (SCC). Chemicals added to the mix create a concrete with improved flow characteristics that eliminates the need for vibration. The end product is free from bug holes and honeycomb problems associated with the normal concrete mix. Internally, the absence of air pockets provides a denser concrete that protects the reinforcing steel. Self-consolidating concrete benefits the precast industry by making it possible to create products that are more cost efficient and more durable than in the past.
Since the 1960s, precast concrete has continued to prove its superiority in underground construction applications. It has proven to be both cost effective and long lasting in spite of hard wear and frequent use. And that is the definition of durability.
ACI Building Codes
ACI 318, “Building Code Requirements for Structural Concrete,” was primarily written for cast-in-place concrete in building construction. As precast concrete products became more widely used, some areas of the code were amended to take advantage of improved quality controls available in an industrialized environment. ACI 318-02 has an entire chapter relating to precast concrete products, along with a chapter on durability.
ACI 350, “Environmental Engineering Concrete Structures,” was created to address problems stemming from cast-in-place concrete in sanitary sewer treatment plants. These structures are exposed to a variety of chemicals that cause premature deterioration. Durability of the final concrete product is the main emphasis of this code.
These codes have addressed durability issues with the cast-in-place concrete. They are written based on experiences with cast-in-place projects and research in specific areas. The durability problems inherent in poured in place concrete have to do with the lack of repetition by those fabricating the structures and the lack of and sometimes absence of inspection. In most cases, the contractor and its crew will not build the same product repetitively. The contractor does not have the opportunity to gain from the experience of past projects. The codes make allowance for this lack of perfection in such areas as protective cover of concrete over reinforcing steel and water/cement ratios. The more stringent requirements of ACI 350 include special formulas for crack control. Thus, the code requirements have been created to promote durability of the cast-in-place concrete.