The Theodore Roosevelt Bridge in Washington, D.C. is being repaired with rapid-set latex modified concrete overlay. Photos: CTS Cement Manufacturing Corp.
Rapid-set cement is being used for the overlay of the 79-year-old Lewis and Clark Bridge over the Columbia River between Washington and Oregon.
Although this cement initially costs somewhat more than the portland cements predominately used for concrete bridges and bridge decks, the lower life-cycle cost due to reduced maintenance and repair over the long run would more than compensate for any increased construction costs.
The cement was invented when a manufacturer combined an expansive component with portland cement. This caused the concrete to expand slightly initially, but then, as the concrete bonded to the reinforcing steel, tensioned the steel. This tension (as per Newton's third law of motion that every action has an equal and opposite reaction) placed the concrete in compression.
In compression, the concrete can resist the tensile stresses from shrinkage. When the concrete later shrinks, the compression is relieved. However, at the same time, the concrete has developed enough tensile strength to resist the tensile stresses put upon it from shrinkage. The combination of this innovative expansive component and portland cement is called a shrinkage-compensating cement, which is recognized in ASTM C 845 as Type K cement. Concrete containing Type K cement is called shrinkage-compensating concrete.
Though shrinkage-compensating concrete requires slightly more water than PCC to hydrate the expansive material, it does not bleed, so there are no water channels to the surface. The combination of no cracks and no bleed water channels results in low chloride permeability and, therefore, longer life.
Over the past four decades, shrinkage-compensating concrete has been used in millions of cubic yards of concrete with excellent results in a broad range of installations. Although it cannot guarantee total freedom from cracking, any cracking will be less frequent and less severe.
For example, since 1988, all bridges under the auspices of the Ohio Turnpike Commission have Type K concrete decks for their new bridges and replacements. Since then, several states have adopted use of Type K cement for bridge decks, including Michigan, Indiana, and Pennsylvania. And, in 1995, a new bridge was built in Barstow, Calif., using Type K.
Shrinkage-compensating concrete is also cost-effective and a smart choice for large flat slabs, such as in warehouses, office buildings, and parking structures. Areas up to 50,000 square feet can be installed without saw-cutting any control joints. The only joints required are construction joints, and the floors stay flat and curl-free. The use of the material in walls enables extended joint spacing and broad, virtually crack-free concrete surfaces.MINIMIZE FUTURE CORROSION
When it comes to repairing crack-damaged bridge decks, there are effective products that can reduce the permeability of PCC bridge decks in order to minimize chloride-induced corrosion of the reinforcing steel. One such product is latex; added to PCC, the concrete is almost impermeable.
While latex-modified PCC (LMC) is quite impermeable, it requires several days to gain sufficient strength to be able to sustain traffic. With Type III cement (LMC-HE) it still requires one to two days before the roadway can be returned to use.
Rapid-hardening cement overcomes this limitation, as it sets and gains strength very rapidly. Developed specifically to capitalize on a different chemistry that allows it to set quickly, when rapid-hardening cement combines with water (hydrates), it forms crystalline compounds much faster than compounds in portland cement form when combined with water. It shrinks less and gains strength faster than portland cement and therefore gets traffic back on the road in less time.