It’s not often that you see a 43-foot tall wall placed with self-consolidating concrete (SCC) in a single lift. It’s even rarer when you consider that the walls were placed at a rate of 24 feet per hour. But that is what the construction team accomplished last year at the Central Baptist Hospital Parking Garage project in Lexington, Ky.
Original design
The three-story below-grade parking structure has a footprint of 127x355 feet in a chorded-radius layout — meaning each wall section is straight, but angled or kinked between sections to form a radius wall. Cast-in-place interior and perimeter foundation walls have precisely positioned corbels to support prestressed double-tee deck sections. Perimeter walls are 12 inches thick and range from 28 to 43 feet high.
The original wall design submitted for bid required using continuous lengths of vertical reinforcing steel cast within a single lift of normal concrete. After reviewing the layout, congestion, and depth of perimeter walls in the bid documents, Baker Concrete Construction raised concern over placing the walls with normal concrete in a single lift. To minimize placement and consolidation issues associated with the tall, congested walls, Baker bid the job recommending lapping vertical bars and using horizontal construction joints to limit pour heights to 14 feet.
Creative approach
Post-bid, Baker approached the project construction manager with the possibility of going back to placing the walls in a single lift but using SCC instead of normal slump concrete. SCC was not considered as part of the original design, but Baker had previous experience placing SCC and was up to the challenge of taking an aggressively creative approach to the wall pours. Switching to SCC eliminated vertical bar splices and horizontal construction joints. The project schedule was another important consideration since full-depth pours significantly reduced the total number of planned placements.
Using SCC relieved Baker’s consolidation concerns, but the challenge of 43-foot single-lift placements was still uncharted territory for the project team. Although Baker had previously placed 40-foot-tall SCC walls, they were well aware of form pressure and mixture stability concerns associated with the fluid nature of SCC compared to conventional concrete. In preparation for the pours, Baker researched formwork options and worked with the concrete supplier and North S.Tarr Concrete Consulting to develop a stable, but highly flowable SCC mixture.
Formwork
Formwork design for SCC mixtures must account for the liquid head of the highly fluid concrete. ACI 237, Self Consolidating Concrete, informs contractors that formwork designs accounting for full liquid head can allow them to take advantage of the rapid casting rates possible with SCC. While formwork designed for full liquid head cost more than conventional formwork, on projects such as this, costs are offset through scheduling improvements, cutting the number of placements in half, and reduced labor. From the design and placement standpoint, specialized forms would eliminate horizontal joints, allow the use of continuous vertical reinforcement, and minimize concerns associated with consolidation and cold joint issues that must be taken into account with slower placements.
After reviewing the project details, Baker contacted EFCO and their recommendation was to use their HP2400 form system rated for form pressures up to 2,400 psf. EFCO assured Baker that the HP2400 form system would maintain a full liquid head and allow wall placement rates up to 24 feet per hour.
For the chorded radius walls the inside form panels are 17 feet wide and the outside are 23 feet wide. It was necessary to use plywood fillers between the steel panels at the angles on both the inside and outside and where the forms met the previously placed wall section. The panels were set as 8-foot-wide sections 43-feet tall due to the crane capacity and then bolted together to form the 17- or 23-foot-wide sections.
Each formwork section had to be customized for its location since there was an angling down portion of the top of the wall and also a stepped up portion of the footing on the bottom. And each form had to be very close to the final height of the wall since it needed to be finished at the top because the SCC tended to be a little foamy and the top needed to be at exactly the correct grade. To set the forms at the bottom, shim packs were used and backer rod, some as much as 2 inches diameter, was placed on the footing and compressed by the weight of the forms to create a seal at the bottom. There wasn’t a single pour where workers didn’t need to adjust the panels to get them to fit the specific location. And also the corbel blockouts needed to be very carefully positioned, since the corbel needed to be within 1/8 inch of the specified elevation.
To form the shallow blockouts for the 300 corbels, a 2x2-foot piece of ¾-inch plywood was attached to the steel EFCO forms and six #6 DBRs (form savers) were attached to the plywood. After the wall was stripped, the DBR bars were screwed in and additional reinforcement was placed into the corbel box form. Crews cast the 2x2-foot, 10-inch-deep corbels with the same SCC mix used in the walls.
Designing the SCC mixture
Project-specific formwork characteristics, reinforcing layout and congestion, and placement techniques influenced the target values for the SCC properties. After confirming allowable form pressures and filling rates with EFCO, Baker’s focus shifted to designing an SCC mixture that would be used to fill the heavily reinforced concrete walls. Baker worked with North S.Tarr Concrete Consulting, and ready-mix concrete supplier Harrod Concrete and Stone to develop a mixture with plastic properties to fill the tall, wide, congested walls. The SCC goals identified at the group’s initial meeting included:
- Ability to fill forms from one placing location without using mechanical consolidation.
- Maintain good dynamic stability so that SCC could flow laterally around obstacles up to 24 feet away without segregation of mix components.
- Achieve good passing ability to efficiently flow around and encapsulate reinforcing steel.
- Maintain high static stability so that mix components would not segregate vertically once the tall walls were filled.
Baker and North S.Tarr conducted numerous trial mixtures at Harrod’s batch plant to identify the maximum slump flow that could be achieved with available material combinations. ACI 237 recommends a target slump flow of 26 to 30 inches for wall placements because a higher fluidity is desirable to transport concrete far from the point of placement and around obstacles.
SCC used in tall, wide walls must also be stable so that the mixture components do not segregate during or after the concrete has been placed. Unfortunately, higher flow values tend to decrease stability. The team developed a highly stable mixture with slump flows up to 24 inches, but mixtures became unstable when higher slump flows were attempted. Because the desired target range for the wall placement was 26 to 28 inches, a viscosity-modifying admixture was used to stabilize mixtures with flows in excess of 24 inches.
In addition to slump flow, trial batch properties were also evaluated using the J-Ring slump flow and column segregation tests. The J-Ring is a variation of the slump flow test where the mixture is forced to pass through a series of closely spaced bars. When slump flow and J-Ring results are within 1 inch or less of each other, it indicates that the SCC will flow well around obstacles. Column segregation is a means to evaluate segregation potential of the SCC mixture once it has been placed. Test results for the SCC mixture indicated that all properties were within the target range.
Placing concrete
After thorough planning and design, Baker and the construction team began placing walls with SCC during summer 2014.
The inside face of the form panels with the corbel blockouts attached was set first and braced with deadmen. The rebar installer then flew in the jobsite-built pre-tied inside rebar curtain followed by the outer curtain of rebar. The blockouts and DBRs for the corbels were installed and then the outside face form was set in place and buttoned up.
To maximize productivity, steel placement was happening while the previous section was being poured, which meant there needed to be enough formwork for one-and-a-half placements: one complete set of front and back wall gangs plus a second complete front gang for the next pour.
Construction proceeded with each new section placed adjacent to the most recent previously placed section. Two 23.5-foot wide chords were cast at a time. To limit freefall, a 20-foot-long pipe was connected to the end of the pump boom and positioned at the center of the wall placement where the concrete then flowed in each direction to fill the forms. The single laborer in charge of the pour then directed the pipe to be pulled up as the wall filled.
All walls were pumped and the only direct access was with a 48-foot straight-boom manlift (for safety, no ladders were used). There was no internal or external vibration used on the project and the walls all came out smooth and dense.
There were 18 two-sided pours and three one-sided pours in all. The one-sided pours were 28 feet tall and 35 feet wide and were placed against an adjacent radiation vault with a 3-foot thick wall. Crews placed insulation against the vault wall then epoxied anchors into the vault wall on a 4x4-foot grid and installed she-bolts to tie the forms back.
All walls and corbels were built using the SCC mix. Pours were typically about 65 cubic yards of concrete placed at 16 yards per hour at a rate up to 24 feet per hour — what the forms were designed to handle. On average there was a pour every four days.
This project proves that tall concrete walls can be successfully placed in a single lift when the right team works together with the proper mix, forms, and methods.
Dave Schaible started with Baker Concrete Construction in 1978 and is currently an operations manager in Baker’s Midwest region. Darrin Farr started with Baker Concrete in 1987 and is currently a project manager in Baker’s Midwest region. Ron Kozikowski is vice president of North S.Tarr Concrete Consulting and has more than 15 years experience as a materials and construction engineer.
