ACI posted the following problem to students in engineering and construction management programs around the country. In all, 38 teams of 146 students from 10 colleges and universities entered the competition. The top three won a trip to the ACI meeting in Charlotte, N.C., in March.
Your company constructed a cast-in-place concrete wall for a building complex in Charlotte, N.C. The wall, 25 feet high, 80 feet long, and 14 inches thick has two curtains of reinforcing bars. Your company cast the wall in two separate 40-foot sections.
The wall drawings and project specifications require a smooth-form finish in accordance with Section 126.96.36.199.b of ACI 301-05, “Specification for Structural Concrete.” The walls will receive a colored architectural coating.
Your crew placed the initial concrete wall section per Table 1 with a concrete pump and filled the form at an average rate of 10 feet per hour in 5-foot lifts consolidated by internal vibration. The form surfaces were a smooth polymer-coated wood composite. The form-release agent was diesel fuel, applied two days before placement.
After form removal, the general contractor informs you that the wall is not acceptable. The coating contractor states there are numerous visible voids on the formed surface, some up to 5/8 inch in diameter. The coating contractor will not guarantee the performance of the coating product unless the wall has no surface voids larger than 1/8 inch in diameter.
Your company says that the wall meets the specification and the coating installer should pay for any remedial work to prepare for the coating. But the coating installer's position is that you, the concrete contractor, are responsible for providing a wall that can be coated.
What will your approach be?
A team from the University of Greenville Technical College, Greenville, S,C., decided to first address the portion of the wall already placed. They recommended a portland-cement-based patching compound after affected areas were pressure-washed. Labor, rentals, and materials would cost $1620. For the remaining work they recommended that concrete be placed in 3-foot lifts instead of the 10-foot lift stated in the problem. Instead of diesel fuel, they would use a chemically reactive release agent. They also recommended internal vibration at 9-inch increments to ensure proper consolidation and release entrapped air.
The students from Missouri State University, Springfield, Mo., recommended a grout-cleaned finish according to ACI 301 paragraph 3.3.4.b and estimated the cost to be $700 to complete the work. For the second placement they recommended using a superplasticizer at the rate of 56 ounces per cubic yard and decreasing the mix water by 1.5 gallons per cubic yard—replacing the midrange specified. They also thought that lifts shouldn't exceed 20 inches and that a 15x2¾-inch rubber-tipped vibrator would do a better job. Finally they would change the diesel oil form release to a barrier type agent, adding $15 to the project cost. For future projects they recommended that laborers should become certified by ACI and the use of superplasticizers should be continued.
Conan Bear, a civil engineering student from the University of Missouri Kansas City, submitted an individual entry. He advocated using a grout patch material. Because the project already required patching tie holes, additional labor to patch the bug holes wouldn't add much cost. For the second wall placement, he recommended a non-petroleum form-release agent, the addition of a super-plasticizer to the concrete, and better training for the workers doing the vibration work.