Concrete Construction Competition Continues

The second Concrete Construction Competition was held before the ACI Spring 2006 Convention in Charlotte, NC. As with the first competition that took place in conjunction with the ACI Fall 2005 Convention in Kansas City, MO, teams of undergraduate students with an interest in construction were asked

Again, schools with construction management, construction technology, civil engineering, and architectural engineering programs were sought out. The response was quite positive, as a total of 39 teams (totaling about 180 students) from 13 schools submitted responses to ACI contact Luke M. Snell. Judges Joe Nasvik, Scott Anderson, Kari Yuers, and Charles Nmai selected the winners.

The first-place team was from Greenville Technical College, Greenville, SC (see "Greenville Technical College"). Team members included Ariel Hartman, Aaron Howell (project manager), Blake Hughes, and Gina Koenig; George Ishak, Faculty Advisor. A summary of their entry is found in the section, "The Winning Response." The second-place team was from Missouri State University, Springfield, MO. Team members included Brant Malan, Stephen Monsanto, Brian Ross, and Jeff Sly; Steven McCrary, Faculty Advisor. The third-place team was from the University of Missouri-Kansas City and comprised Conan Bear; Ganesh Thiagarajan, Faculty Advisor.

The three teams presented their solutions to the Construction Liaison Committee, attended the Student Luncheon, and attended technical sessions during the convention.

THE PROBLEM STATEMENT

Your company is the concrete subcontractor responsible for construction of a cast-in-place concrete wall for one section of a building complex in Charlotte, NC. The wall is is 25 ft high by 80 ft long. Your company has elected to cast the wall in two separate sections, each section being 40 ft long.

The drawings and project specifications for the wall require a smooth-form finish in accordance with Section 5.3.3.3.b of ACI 301-05, "Specification for Structural Concrete." The project specifications also indicate (in a different section) that the finished walls will receive a colored architectural coating.

Your crew placed the initial wall section using concrete per Table 1. The ambient and concrete temperatures were 75 and 72 ?F, respectively; the concrete was placed using a concrete pump; and the form was filled at an average rate of 10 ft per hour in 5 ft lifts that were consolidated using internal vibration. The form surfaces were a polymercoated wood composite with a smooth finish. The form release agent was diesel fuel, applied 2 days before the concrete placement.

Two days after the placement of the first wall section, the forms were stripped. Shortly thereafter, you received notice from the general contractor that the wall was not acceptable. The coating contractor had inspected the wall and noted that there were numerous visible voids on the formed surface, some up to 5/8 in. in diameter. The coating contractor had then informed the architect that, because pinholes would form in the coating that bridged over these voids, there would be a loss of bond and the coating could be breached. Therefore, the coating contractor would not guarantee the performance of the coating product unless he was provided with a wall with no surface voids larger than 1/8 in. in diameter.

Although your company's position is that the wall meets the project specification and the coating installer should pay for any remedial work needed to prepare the wall for coating, the coating installer's position is that you, the concrete contractor, are responsible for providing a wall that can be coated.

The president of your company wants your recommendations for resolving this, and has asked you to respond to the following specific questions. State your recommendations, with cost justifications (use RSMeans Building Construction Cost Data), in the form of a memo to Mr. John Dow, President, Quality Assured Concrete Co.:

* Assuming that your company may be held responsible for the costs of either remediation or of coating failure, what should you do with the wall section that has been completed?

* Under the same assumption, what actions do you recommend for construction of the second wall section?

* What actions do you recommend to avoid this problem in future projects?

THE WINNING RESPONSE

We recommend patching the voids using a portland cement-based patching compound called Panel Patch, manufactured by Nox-crete.(TM) We selected this product because it requires no special surface preparation and is easy to apply. To remove form oil residue, the surface only needs to be sprayed with a degreaser, followed by pressure washing to remove any dust or other foreign material. The actual patching can be accomplished in 1 day by three of our cement finishers. A test area 4 by 4 ft should be completed, and the test area should be approved by all parties before moving forward with the rest of the wall patching.

Total cost for repair, based on data from RSMeans, is as follows:

* Pressure wash (two laborers), 1000 ft^sup 2^ total, material/ft^sup 2^ = 0, labor/ft^sup 2^ = 0.46, daily output = 2400, total hours = 3.3, total cost = $460;

* Patching (three finishers), 1000 ft^sup 2^ total, material/ft^sup 2^ = 0.24, labor/ft^sup 2^ = 0.92, daily output = 900, total hours = 8.9; total cost = $1160;

* Total hours = 12.2; and

* Total cost = $1620.

In order to prevent or minimize these voids in future work as well as the second section of this job, we should limit our lift heights to no more than 20 in. (per ACI 309R). We also need to avoid the use of diesel fuel as a release agent. In this application, the use of the diesel fuel was unfortunate, as it sometimes is the root cause of voids in formed concrete. We recommend the use of a chemically active release agent in all new work. Chemically active agents not only assure a clean release, but they also reduce drag, allowing voids trapped between the form face and the concrete to easily slide up the face of the form and out the top. This will produce a better finish and save time in vibration and form removal cleanup.

Finally, the crew foreman will conduct training for crew members on vibrating and placing concrete. Based on a 1-3/8 in. head, the vibrator should be placed in 9 in. increments (ACI 309R). This will ensure proper consolidation of concrete and the release of entrapped air from the wall.

GREENVILLE TECHNICAL COLLEGE

By providing associate degree programs in engineering technology, Greenville Tech's Architectural and Construction Engineering Technology program provides an important resource for the regional and national construction industry. The program is accredited by the Technology Accreditation Commission of the Accreditation Board for Engineering and Technology.

Students who complete the architectural program obtain employment with architectural and engineering firms, construction companies, and building materials suppliers. The program trains students to convert designs into working drawings and specifications as well as plan, supervise, and estimate costs of construction projects. Graduates may continue their training for 2 or more years at 4-year institutions offering Bachelor of Engineering Technology programs.

Students who complete the construction engineering technology program are prepared for site supervision, construction, estimating, scheduling, and assistant project management. Graduates are prepared to take the South Carolina General Contractors and the South Carolina Residential Contractors exams, and may continue their education toward a Bachelor of Science degree at Clemson University, Clemson, SC.