Every year, plants spend millions of dollars repairing or replacing failed concrete. The corrosive environment in many plants subjects concrete to attack from a wide range of acids and alkalis. In addition, fuel oils and chlorinated hydrocarbon based products, while not always corrosive, must be contained and prevented from passing through the concrete and entering into soil or ground water. Repair, replacement and high-remediation costs for a failed system can be astronomical.
Standard Portland cement concrete has very little resistance to acids and salts. Acids readily attack the calcium-based binder in portland cement. This leaves behind nothing more than weakly-bound aggregates.
Because standard concrete is porous, non-corrosive chemicals pass through it easily contaminating the concrete itself and the ground below. Once this happens, replacement costs soar, since the concrete must be removed.
The cost to dispose of contaminated concrete and the contaminated soil around it continues to rise. Even when contaminated concrete can be repaired, the required, extensive surface preparation also is costly and owners incur disposal costs for a certain amount of debris.
A wide variety of coating and linings systems are available to protect concrete. The alternatives include thin-film coatings, filled and reinforced monolithics, membranes, acid brick, polymer concretes and sheet or molded liners made of rubber or plastics.
Generally, most of the coating and lining systems are epoxy, vinyl ester, polyester urethane, furan or silicate-based. The liners, for the most part, are natural or synthetic rubbers, polyvinylchloride, plastic or FRPs based on a variety of resins.
The cost for these systems varies tremendously depending upon surface preparation, plant conditions and available downtime. Even on new work, the cost ranges from only a few dollars per square foot to $30 or more per square foot. Repair work may be even higher.
Now, a breakthrough in concrete technology for precast structures eliminates many of these problems. Scientists at Drexel University developed a high performance, acid and corrosion resistant concrete that is completely free of portland cement.
[pullquote]The chemistry of this material is unique since it contains no portland cement. The process combines a silicate solution and a chemical activator with Class F fly ash, sand and lime-free aggregate. The result is a high-performance concrete having a dense, impermeable microstructure with excellent resistance to most corrosive solutions.
The chemical reaction that takes place during the formation of this material is a polymerization reaction that creates a stone — like matrix with the strength and durability of naturally occurring elements.
The interaction of the sand and stone particles with the binder paste — fly ash, silicate and the alkaline activator — cause the dense structure. The result is a material structure that is mechanically and chemically resistant to aggressive environments. Portland cement concrete has a greater degree of porosity and a binder system that does not resist chemical attack.
New path to old technology
The basic silicate chemistry of this system is not new. Silicate-based cements, mortars, refractories and concretes have a long history of use in aggressive environments.
Power plant chimneys, scrubbers, acid containment vessels and high-temperature refractory applications in refineries and power plants are only a few of the applications that use silicate-based products.
The chemical and temperature resistance of silicates is far superior, not only to portland cement, but to epoxies, vinyl esters, polyesters, urethanes and furans. It is also superior to other calcium-based products such as calcium aluminate cements. The main drawback to their use has always been the cost, which ranges up to $1,000 per ton.
This new technology provides the advantages of silicate chemistry without the high cost. The patented process allows existing manufacturing plants — precasters — to produce high-performance, cementitious structures comparable in price to Portland cement structures with reasonably good lining systems.
However, with this new technology, instead of a thin, corrosion resistant barrier protecting concrete that is easily susceptible to damage, the entire mass of the structure is corrosion resistant. Concerns about seams that can break open or disbondment are no longer issues. Further, mechanical damage that destroys the integrity of a lining system is also of very little concern. A chip or gouge in this concrete only exposes more concrete with the same physical and chemical properties as the surface.
Structures produced with this advanced concrete material are manufactured under controlled conditions. Experienced production management teams assist precast plants, using conventional mixing, placing and forming equipment. Production management works with the precaster for optimum use of existing plant assets and modification where necessary. This ensures rigid quality control — something that is not always possible with structures that require additional measures for corrosion protection.
When installing protective lining systems, many variables determine the quality and effectiveness of any given system. Some of these critical factors are surface preparation, temperature — both ambient and substrate — moisture, seam or joint irregularities, pinholes, thickness variations and curing of both the concrete and the lining system. If difficulties occur in any of these areas, the lining system can be rendered useless and the concrete comes under attack.
One can eliminate this problem with the use of acid resistant concrete. Once a structure is precast with the silicate-based material and cured, it requires no additional measures for protection and is ready for immediate use. There are no delays waiting for concrete to cure or waiting for linings to be applied and cured.
For example, consider the following application. Suppose an owner specified a chemical waste tank of precast, portland cement concrete with an epoxy liner. The nominal tank dimensions are nine foot by nine foot by seven feet and five inches deep and would weigh approximately 15.7 tons. This design is for holding a variety of acids and other chemicals, including concentrated sulfuric acid, caustic cavities, solvents and phosphoric acid.
Typically, a general contractor would have this tank precast at a local precaster's plant, have it shipped to the jobsite, set the tank in place and connect it. After the concrete cured properly, a specialty contractor would tent in the tank, sand blast the tank, apply 1/8 inch of specified epoxy lining system and allow it to cure.
The time required for this is considerable. A precaster can cast a tank in 1 day, it takes 28 days to cure the concrete before installing an epoxy lining. Shipping, site installation and hook up of the tank requires three to four days. Then, typically the lining contractor needs three to four days to apply the epoxy followed by seven days at 70 °F to cure the epoxy fully before opening the tank for service. In the meantime, since this new tank replaces an original waste sump and goes in the same location, a temporary sump has to be put into service.
An alternate approach
If engineers and owners review their options, they will find that silicate-based concrete provides more chemical resistance than the epoxy.
Further, an alternate proposal for precasting the same tank of silicate-based, acid resistant concrete eliminates both the portland cement concrete and the epoxy lining and much of the time.
With the alternate tank system, the time line for completion of the project can be reduced significantly. If a precaster cast the tank on a Friday, it would be fully cured in 24 hours with a compressive strength of 7,900 psi. To lend credence to the this example, we'll impress a three-day rain delay on the contractor and make him wait until Thursday to install the tank. Tank hook-up can be completed on the following Monday.
Despite rain delays and weekends this tank installation requires only 11 days to complete. If need be, a tank such as this can be manufactured and installed in four to five days. The use of this technology in acid resistant concrete allows a general contractor to shorten the schedule required for this tank replacement by at least 30 days.
This means projects can move forward with minimal downtime, much to the relief of owners and engineers. Also, waste disposal now can operate normally and temporary sumps can be eliminated a month sooner than anticipated when using conventional precast units. The unique properties of this patented concrete enables plants to save money and shorten their construction schedule.
The superior chemical resistance of this silicate-based product make it an easy choice for chemical resistance. The rapid strength gain and cure allow tanks to be installed and opened to service immediately. Lining systems become unnecessary, yet the life expectancy of these tanks is greatly increased and this concrete does not require maintenance. In many ways plants receive far more for their money than they expect. Instead of 1/8 inch of corrosion protection, they now have eight inches of total acid resistance. The costs are lower and projects can be completed in a fraction of the time.
This material is ideal for waste disposal structures in both industrial and municipal waste. Piping, trenches, manholes, sumps, storage tanks, containment tanks, lift stations — in short, any precast concrete structure that requires corrosion protection or high strength is an ideal candidate for this material.