One business that is certainly season-driven is skiing. When the owner of an East Coast ski resort realized that a fairly new condominium/hotel facility had construction deficiencies and was in need of repair, finding a quick solution became imperative.
Constructed in 2006, the condominium/hotel facility offers high-quality lodging with studio through three-bedroom condominium units. Upon completion of the construction, the owner learned that the building had been under-designed and there were construction deficiencies. In fact, the building was not in code compliance and structural remediation was needed. To ensure the safety of patrons, the owner had to shore the ceiling of the underground parking garage, which eliminated 75% of the parking spaces and ant important amenity for the facility.
A local general contractor, who had previous experience working at the resort, was brought in to perform the structural remediation on the building. Since they were not a specialty repair contractor, the general contractor hit a roadblock in coming up with a procedure for placing the concrete in the overhead beams without creating a cold joint.
The structural engineer suggested contacting Structural Preservation Systems (SPS)—the largest concrete repair company in the United States—since they had extensive experience with repair and strengthening projects. Upon inspecting the structure, SPS developed an innovative repair strategy using self-consolidating concrete (SCC) for the 59 beams that needed to be constructed underneath the plaza level of the building, which was also the ceiling of the parking garage. Upon developing this strategy, SPS subsequently was asked by the owner and structural engineer to take over the remediation project from the previous general contractor. SCC has been used for a variety of new construction and precast manufacturing projects since its inception in the 1980s. Recently, design and construction professionals have found that the features and benefits related to SCC can solve many constructability and placement challenges that most standard concrete mix designs can not. The main feature of this unique type of concrete relates to having very high slump and flowability properties without segregation. Successful SCC applications are not only seen in new construction projects that present difficult placement or finish challenges, but also in concrete repair and strengthening projects where the material must be placed under pressure into confined, highly reinforced forms.
Many of the characteristics that have made SCC popular in early uses for new construction are also advantageous for repair and strengthening projects. These include reduced labor costs, ease of pumping and placement into very congested formwork, creating specialty surface finishes, as well as the high quality of the end product. The SCC mix design uses similar concepts as traditional concrete (strength, accelerated or retarded set times, and aggregate size), except specialty admixtures radically modify the slump and hence flowability.
Why use SCC for repair/strengthening project?
Also known as self-compacting concrete, SCC is a highly fluid, nonsegregating concrete mixture that consolidates under its own weight. According to the National Ready Mixed Concrete Association (NRMCA), "the development of high-performance polycarboxylate polymers and viscosity modifiers have made it possible to create 'flowing' concrete without compromising durability, cohesiveness, or compressive strength." Translation: A concrete so flowable that when testing for slump, instead of measuring the height of the slump cone, you will need to classify the slump flow as opposed to the slump by measuring the diameter of the circular "puddle" (20 to 28 inches) that pours out of the slump test cone. This is not an easy concept after years of working with standard concrete mixes.
Many repair/strengthening projects have tight formwork space constraints that make concrete pours challenging. These projects usually have tighter spacing between the reinforcement and the existing concrete surface, reinforcement, and formwork. SCC's flowability lends itself well to concrete beam, slab, and column "form and pump" repairs that commonly are found on structural restoration and strengthening projects. Frequently, these repairs involve enclosed formwork and require the material to be placed under pressure to ensure a good bond with the prepared, "open" substrate. This placement process and material alleviate problems associated with using traditional concrete where voids and honeycombs can occur and often are not revealed until the forms are stripped, if ever.
Examples could include filling a repair void created by chipping out deteriorated concrete or enlarging the sides and bottom of a beam to carry additional loads. In addition, repair projects often require crews to work in extremely tight conditions with many mechanical/electrical/plumbing (MEP) components, oftentimes a maze of shoring to negotiate and due to access and building operations, the pumping distance can be hundreds of feet away.
When pumping concrete for repairs on existing structures, the installation of formwork can create challenges. For example, forming and pumping new concrete on an overhead beam or slab with SCC requires stronger formwork design as the head pressure created by the truly liquid material is greater than those seen in standard concrete. Also, the pressurization of the formwork adds even more stress. Traditional ready-mix concrete—because of its high viscosity and limited flowability—also inherently is difficult to pump (especially over long distances) into enclosed/pressurized formwork with tightly spaced reinforcement. This is particularly true in applications that have several structural members intersecting at different angles requiring the material to flow around corners and in several directions at once. Concrete contractors routinely have had challenges with clogged pump lines during a pour and understand the loss of time and money this can create when a pour is shutdown to repair the line. The low viscosity of SCC makes it easier to pump and allows it to flow easily throughout a form, typically from one single port location while preventing voids and honeycombs.
In addition to improved flowability, SCC may offer several cost savings that can easily offset the reasonable premium per yard cost as compared to traditional ready-mix concrete. For example, there is a labor savings because crews can pour larger repair areas at one time all from one port location—sometimes allowing a single pump location and line setup. This minimizes the amount of hard and soft pump lines. The improved finish means crews have to spend less time on rub-out repairs after the forms have been stripped. The flowability also allows for dramatic architectural finishes that mimic whatever finish or shapes are inside the formwork. Forms do not have to be vibrated to consolidate the concrete.
Using SCC For the condominium/hotel at the ski resort, SCC was the only way to place the concrete in the overhead beams. The forms for the beams were completely enclosed, so pouring conventional concrete would not have been possible. The repair strategy suggested by the original general contractor involved pouring the bottom two-thirds of the beam into an open top form and then forming and pumping the last few inches. This approach would have created a cold joint along the length of all beams, which the structural engineer did not want. The flowability of SCC allowed the concrete to flow properly around the tight reinforcing steel. In addition, pumping SCC under pressure achieved the bond between the existing and new concrete.
Although SCC was clearly the best material for completing this strengthening project, SPS had to overcome several challenges to make it work. The most significant challenge faced was that there were no ready-mix plants in close proximity to the project site. The nearest sizeable town was about 55 miles from the resort and there are only two ready-mix suppliers that can deliver materials to the mountain. Of those two suppliers, one was eliminated because the structural engineer was concerned with their aggregate. The remaining supplier had no experience with SCC, so SPS had to work closely with them and Euclid Chemical, the admixture supplier, to develop a new mix for this project to ensure proper strength properties and workability.
The new mix was put through a myriad of tests before it could be approved for use by the structural engineer. Typically, SCC should be placed within 90 minutes of mixing. Because of the distance from the plant and the mountainous terrain, it was a two-hour drive from the ready-mix plant to the jobsite, thus the concrete would begin to set up by the time it reached the jobsite. To combat this challenge, SPS worked with Euclid and the concrete supplier to develop a mix that would delay the hydration reaction of the mixture—essentially putting the concrete to sleep until it arrived at the jobsite.
The concrete had to be pumped from 300 feet away, and several different kinds of pumps were used to get the concrete into the forms. The forms were pressurized to allow for proper concrete placement with no voids and to achieve the required bond with the existing substrate, so special safety precautions were taken. The team carefully monitored the pumps and forms both visually and audibly to ensure there was not a catastrophic failure. The innovative formwork helped ensure that the beams were monolithic structural members.
In addition to these challenges, the facility remained open during the repairs. Crews had to confine materials to a limited laydown area and no work could be done on many weekends. The project began at the end of January, which is the peak of ski season, so crews had to be cognizant of the patrons at all times. There were restricted work hours, but the project had a tight schedule—work had to be completed by November for the upcoming ski season. Even with the challenges, SPS was able to complete the project on time and budget. Both the owner and the structural engineer were extremely pleased with the outcome.
Several elements were critical to the success of this project. First, the whole team ensured there was adequate time in the planning phase and everyone participated actively in this phase. Second, SPS worked closely with the concrete and admixture supplier to develop the right SCC mix for the project. By working together, they were able to tackle the challenging situation and achieve success that others did not think was possible. Finally, upfront testing allowed the team to assess how the materials would perform in real-life conditions, which set the project up for success from the onset. The key ingredient for success was a partnering attitude that created a collaborative environment with all parties involved in the project.
Mike Miller is the chief estimating engineer and Matt Frye is a project manager at the Baltimore Branch of Structural Preservation Systems LLC, a division of Structural Group.
