At the forefront of seismic testing on structural concrete is a specially constructed strong wall designed to function as an immovable object against which forces can be reacted during research at the Mid-America Earthquake Center, University of Illinois, Urbana-Champaign. Serving as headquarters for the Mid-America Earthquake Center besides offering one of the nation's leading civil engineering programs, U of I has been a driving force behind the George E. Brown Network for Earthquake Engineering Simulation (NEES), a national earthquake engineering resource that enables world-class laboratories around the country to collaborate remotely on experiments, computational modeling and education.
As part of the NEES program, Illinois' Department of Civil and Environmental Engineering received funding to establish a Multi-Axial Full-Scale Sub-structure Testing and Simulation (MUST-SIM) facility, including a strong wall within its Newmark Civil Engineering Laboratory in Champaign. An imposing edifice, the 80-ft.-long _ 28-ft.-high _ 5-ft.-thick strong wall is an L-shaped structure comprising 50-ft. and 30-ft.-long wings. Constructed of high-strength, densely reinforced concrete, it is engineered to withstand testing that simulates the impact of earthquakes of varying magnitudes.
The strong wall was designed to support the forces from three very large loading units that can be attached to any point on the wall, explains Assistant Professor Daniel Kuchma. Each of these loading units has a self-weight of 34 tons and can be used to exert forces of up to a million pounds to test specimens such as bridge piers and other structures.
To prevent its lifting off the Newmark floor when exposed to these extreme forces, the strong wall was secured to the slab using 81 2.5-in.-diameter, high-strength post-tensioning rods in sleeves. A heavy grid of reinforcing bars was installed on front and back wall faces to resist pressures exerted by the loading units. In more heavily reinforced regions, the grid consists of #9 and #11 bars on 4.5-in. centers.
In addition, more than 800 large tubes on 18-in. centers were placed throughout the thickness of the structure to support the connection of loading units to the strong wall. As tubes were required to be within 18-in. of specified locations in the horizontal, vertical and out-of-plane positions, a precise arrangement was achieved through the use of vertical strands to affix the horizontal reinforcement to which the tubes were anchored.
Ready mixed producer Champaign Builders Supply satisfied specifications calling for a minimum 8,000-psi, high performance concrete as well as a highly flowable mix to accommodate congested steel reinforcement within the structure. Project success also hinged on mix placement without disturbing precise tube positioning. This requirement, coupled with the sheer height of the wall as well as concerns regarding indoor noise levels, ruled out the use of vibration equipment during placement. Notes Champaign Builders Supply General Manager Bernie Hinkle, The need to achieve a very high break, use a highly flowable mix that didn't affect the concrete's strength, and yet place it without vibration posed a significant challenge.
A further complication was the necessity of placing concrete in a single day. Speed of placement was important to avoid cold joints, says David Lange, P.E., associate head of Illinois' Department of Civil and Environmental Engineering. Because finishing the job in one day was critical, a continuous pour was mandatory. To do this, we required a consistent supply of concrete that met all the design parameters.
The Champaign Builders team worked with Grace Construction Products to design a self-consolidating concrete, using the company's proprietary SCC technology to produce a highly flowable mix that could be placed without vibration equipment and aggregate segregation. A continuous line of mixer trucks enabled pour completion in approximately seven hours. The concrete was pumped to a tremie hose lowered into the form from a single location on top of the wall. As the concrete level rose, the hose was pulled up and, after removal of a section, once again lowered into place. In all, the job took more than 50 truckloads of concrete totaling 320 yards.
According to Lange, the SCC mix met all expectations. Despite initial reservations about the possibility of placing concrete in a 28-ft.-tall structure without vibration, people were impressed with the truck-to-truck consistency of the delivered mix, the fluidity of SCC and the way the concrete moved through the forms, flowing smoothly around the rebar and tubes, he affirms. An additional bonus was the relatively quiet pour. Using SCC, indoor noise was restricted primarily to the pump and trucks.
In the past year, the Network for Earthquake Engineering Simulation has transitioned from a developmental to operational state. Illinois' strong wall is being put to the test in research that stands to save lives by making structures more earthquake resistant. Û www.graceconstruction.com