Sixteen bridges with more than 300 columns, 250 bridge caps, and 900,000 square feet of bridge deck have only two years for construction, from start to completion. That's what the Texas Department of Transportation (TxDOT) and the North Texas Tollway Authority (NTTA) proposed for the construction of the Sam Rayburn Tollway.
This $221 million project is segment four of a $700 million, five-segment project that will reconstruct the 26-mile stretch of State Highway 121 on the east side of the Dallas/Fort Worth International Airport. Due to the limited construction time frame, the fabrication and supply of steel girders was impossible, so more than 130,000 linear feet of precast concrete beams are used instead.
To further complicate matters, approximately 35,000 cubic yards of substructure are classified as architectural concrete and thus subject to NTTA special specification Item 850 “Natural Grey Concrete,” which stipulates the concrete's color and finish. Conformity to this specification and adherence to the tight construction schedule had all parties involved honing their communication skills. The general contractor, W.W. Webber LLC, Houston, worked vigorously with concrete supplier Texas Industries Inc., Dallas, and the formwork provider, Efco Corp., Des Moines, Iowa, to provide a concrete finish that meets or exceeds NTTA expectations.
The 850 specification
The 850 specification ensures a uniform and consistent off-the-form (as-cast) finish with little to no touch up aside from light sandblasting. It eliminates the need to paint the concrete surface, reducing future maintenance costs. Uniformity in color occurs by having the final product fall within the range of gray between Federal Standard 595B Color No. 36559 (lightest) and Color No. 36622 (darkest).
Consistency is ensured by casting mock-up panels for comparison. NTTA requires a minimum of five panels: two 5x10-foot, 18-inch-thick samples and three 5x5-foot, 18-inch- thick samples. All samples must simulate the actual construction features as closely as possible, and include demonstrations of proposed patching techniques. Samples also need to be produced on different days to prove color uniformity between samples.
The consistency of the final product is evaluated by direct comparison to the mock-up panels. The quality of the finish and color is reviewed in accordance with the specifications by the engineer and visual quality manager for compliance with the NTTA specifications. Reviewing the final product takes place 30 feet away in direct daylight.
Getting consistent results
Maintaining the color requirements has proven challenging. The initial mockup panels were poured using a straight cement-based mix to help control the consistency of the color. Unfortunately, the high temperatures induced by the heat of hydration made this mix unacceptable. Introducing fly ash at 40% of the cementitious material kept the temperature in check, but contributed to more color variability. To maintain consistent color throughout the project, single sources were used for all concrete materials, including aggregate, sand, cement, and fly ash. This helps to ensure that the color of the first batch of concrete matches as closely as possible the color of the last batch.
This specification also affected formwork design. One of the NTTA requirements was to eliminate or hide as many form joints as possible. This was accomplished by strategically placing form joints at reveal locations in every column and the corresponding flare at the top of the direct connector columns. In many situations, this required the fabrication of customized forms.
In anticipation of the admixtures and cooling requirements, coupled with the stringent product finish requirements, the standard design of the form-work was re-evaluated. A change was necessary due to the potential adverse effects of the multiple concrete design mixes on pour rates and corresponding concrete form pressure. Standard formwork steel facesheets have a thickness of 3/16 inch, but it was determined that a 1/4 -inch thickness would produce the desired results. The extra thickness helps reduce the pillowing effect or any facesheet deformations between the ribs of the formwork that are viewable when pour pressures exceed the rated capacity. A judicious approach must be exercised when evaluating and monitoring the concrete pour pressures, but the extra thickness in the facesheet tends to be more forgiving when there are so many unknowns.
The downturn in the economy played an unforeseen role in the decision to supply 1/4-inch facesheet material. The aggressive construction and completion schedule dictated the need for quickly delivering the formwork. But the consistent and reliable supply of quality 1/4-inch material, especially in what is considered an oversized width (6 feet and wider), quickly dried up. Steel mills only run this size when ample demand makes it beneficial to fire up the kilns to make a run. In most cases, the lead time required by steel mills exceeded the time frame required to supply the material. In the end, material shortages were averted by supplying the 1/4-inch facesheets for structures only where they were critically needed, such as in column formwork. In a portion of the cap formwork where concrete pour pressures are never high enough to make an impact, 3/16-inch facesheet material was used.
Careful evaluation was made to provide the amount of formwork that could effectively be used to meet the accelerated schedule of the project. When possible, form panels were reused in different bolting patterns to obtain various configurations. For example, by changing the bolting location, an 8x6-foot column used the same forms as an 8x6 1/2-foot column. This tactic helped minimize the up-front cost of the formwork system.
This project incorporates many different structural configurations. Rectangular columns range from 5x12x3 feet to 8x71x6 1/2 feet. Bent caps range from standard square 59x3x3 feet to post-tensioned caps up to 143x10 5/6x13 feet. A number of diversely sized inverted “T” caps also are incorporated into the interchange. The varying sizes and shapes of these columns and caps all require different concrete mix designs. Concrete slumps range from 4 to 10 inches depending on concrete class and mix. Concrete placement temperatures vary depending on the mix design, requiring admixtures as well as nitrogen-induced cooling to maintain concrete workability, placement, and curing. All of these variables work against maintaining a consistent and uniform color in each structure.
Some structures are considered standard placements but more than 85% of the project is considered mass placement. In order to prevent thermal cracking in the mass placement pours, the inner core temperature cannot exceed 160º F and the differential temperature between the outside face and the core must remain below a maximum 35º F. To ensure these criteria are met, temperature monitoring occurs for the first four days from at least three strategically placed sensors embedded in each concrete pour. Insulated forms remain in place for these four days to help meet these temperature requirements.
The 850 specification dictates careful attention when making even the most straightforward concrete pours. Prior to a pour, all forms are cleaned and oiled to provide a uniform face. All joints, corners, and seams are caulked with a silicone material to prevent water loss. Painstaking attention is placed on uniform and consistent vibration as the concrete is placed, and the curing process is followed diligently after placement.
Maintaining consistency throughout a project of this size can be a daunting task. Internal quality-control measures at every level from all involved parties have been heightened. From material suppliers to formwork suppliers to construction crews to visual inspectors, everyone involved is working diligently to provide the highest quality material or service. The result thus far has been a reliable final product. The results have been favorable and are expected to continue through the projected completion in January 2011.
Brian Miller, PE, is the Dallas district operations manager for Efco Corp.