By Jennifer Cordes, Colby Stodden and Alex Stone
Colorado State University has always used a traditional cast-in-place foundation system for its campus buildings. Since university officials were mostly unfamiliar with shotcrete foundations, the responsibility rested on our design-build team to demonstrate how and why this construction method was the best approach for the new $54-million CSU biology building.
In spring 2016, the design-build team completed the shotcrete foundations for the 152,000-gross-sq-ft, steel-framed classroom and biology research facility, which is spread across four stories.
The decision to use shotcrete on the project stemmed largely from site constraints. Given the building’s proximity to street access and adjacent buildings, our team began discussions early on about how to construct the full-height basement as if it were a zero-lot line project.
After considering multiple options, we agreed that a beam-and-lag shoring system would effectively shore the earth during excavation of the basement, while consuming less space and proving less disruptive to campus than traditional lay-back excavation. However, this also dictated the need for an alternative method for placing the foundation walls.
After researching other projects in the region with basement-wall details similar to those proposed at the CSU biology building, we determined that shotcrete would provide the best opportunity for success and ultimately improve the project timeline, reduce costs and enhance the quality of the final product.
The decision to use shotcrete did not alter the primary design of the concrete walls; however, additional considerations were given to items such as rebar size and placement. The shoring, consisting of cantilevered I-beams and wood lagging, was tight to the exterior of the basement and acted as the backside form. This factor affected several key details, including foundation insulation, waterproofing, and perimeter drainage.
- Foundation Insulation. An unfaced extruded-polystyrene drainage panel—which retains its insultating value, even when wet—was used to ensure the basement is properly insulated and meets current energy codes.
- Waterproofing. Because the waterproofing was installed before the shotcrete application, a “blind-side” waterproofing system was specified.
- Perimeter Drain. Our geotech report recommended a perimeter drain to alleviate hydrostatic pressure against the foundation wall. However, because the shoring acts as the outside form for the structrual concrete wall, a traditional perimeter drain detail could not be installed.
Our team designed a perimeter drainage system that allows water to flow from the foundation drainage panels (installed between the shoring and the shotcrete wall) into a fabric-wrapped gravel bed at the base of the foundation wall. It extends from the drainage panel under the wall (and between the piers) to a perforated perimeter drainpipe installed inside the structural pier caps. The perforated pipe is in a gravel burrito and sloped to guide the water to a sump on the east side of the building.
The shotcrete installation was essentially a six-part process (independent of the perimeter drainage system installation). After installing the shoring system, excavating, and drilling the foundation caissons, the work sequence was as follows:
- Install the drainage board directly to the shoring wall
- Install the insulation over the drainage board
- Install spray-waterproofing application
- Drill anchors through all membranes and into lagging
- Install rebar
- Apply shotcrete
The shotcrete technique involves wet concrete propelled through a hose at a high velocity onto walls reinforced by rebar. Due to the force with which it leaves the nozzle, the shotcrete is placed and compacted simultaneously. The walls are finished via screeding and troweling after the concrete is applied.
Ensuring structural integrity in a shotcrete wall is more challenging than in a formed concrete wall. The installer has to depend on the “vectoring” skill—basically, hose maneuvering and concrete placement—of the nozzle person shooting the concrete, rather than vibration, to fill all the voids in the wall and prevent future failure.
The process of applying shotcrete is a physically exhausting task, so the team anticipated that multiple nozzle persons would be needed throughout the application. Though all were ACI (American Concrete Institute) certified for shotcrete application, to ensure the quality and physical integrity of the walls, we tested the skills of each nozzle person in three complex mockup situations with varied wall sizes and different interior foundation elements.
Each four-ft-tall mockup panel simulated expected reinforcement congestion for each condition and contained a steel-embed plate for a steel-beam attachment.
Each nozzle person applied shotcrete to the mockup panels to establish their ability to encapsulate the reinforcement with proper consolidation of the shotcrete. The mockups were then cored in five selected locations and evaluated by a qualified third-party special inspector.
This third-party inspector also provided continuous inspection of the shotcrete operations, including creating test panels every day to test for strength and additional properties of the concrete mix. To validate proper consolidation behind steel-embed plates, we used a hammer sound test to determine the presence of any voids behind the plates. If a hollow spot was found, a hole was drilled into the plate and grout was pressure-injected into the cavity.
While shotcrete was certainly the best option for this project, our team came away from the effort with several key lessons learned about ensuring structural integrity, the benefits of a design-build team, safety considerations related to weatherization, and aesthetics.
Many challenges were presented at the steel superstructure to shotcrete interfaces. Braced-frame columns were anchored at the top of the shotcrete walls with a horizontal embed plate that transfers shear into the wall, with high-strength anchor rods embedded into the wall to resist uplift forces.
At these locations, there was considerable reinforcement congestion around the anchor rods. It was critical that not only the reinforcement and anchors be properly encapsulated with shotcrete but also that the anchor rods be precisely placed to set the columns.
Through effective communication and coordination among the design and construction teams, all 12 locations were successfully placed. Additionally, throughout the building, steel columns bear directly on integral concrete columns and pilasters.
During design, shotcrete versus cast-in-place construction of the bumped-out pilasters was thoroughly discussed. Due to potential shadowing of the shotcrete material caused by the congested rebar, the team decided the highest-quality assurance would be shotcrete applied through the backside of the pilaster—essentially a continuation of the wall—with the bump-out pilaster supporting the steel column to be cast-in-place.
Because our team had worked together from the onset of the project, we were able to determine the best waterproofing process, product and subcontractor. Had the design and construction been done independently, the possibility of rework in the field would have been substantial.
Another lesson learned involved the necessity to weatherize the basement because we started in the winter. The shotcrete area had to be fully tented in order to be properly heated for the concrete, which also meant increasing the air exchange to provide adequate ventilation for the workers, specifically regarding potential exposure to silica dust. To ensure safety standards were met, respirator controls were put in place and we brought in third-party testing agencies to ensure proper air quality.
Lastly, the aesthetic quality of the finished product was a pleasant surprise. With cast-in-place concrete, the finish must be finessed once the forms are removed. With shotcrete, this process is performed in tandem with the application and results in a superior finish.
The final verdict from our team and the owner is that the shotcrete process was a tremendous success and provided for a solid foundation for the new biology building.
Jennifer Cordes is a principal at Hord Coplan Macht, Colby Stodden is a senior project manager with Haselden Construction and Alex Stone is a structural engineer at KL&A.