V2 T-Biscuits Offer Simple, Strong Design to Repair Failed Connections
May 3, 2017
Oxymoron is not a term that is often associated with structural engineering, but then again aerospace technology and exquisitely simple are rarely used in the same sentence either. Those two phrases are the best way to describe an innovative new product for repairing pre-cast parking garages.
It is common knowledge that the double tee is the backbone of any pre-cast parking structure. The 8 to 15-foot widths combined with spans in excess of 60 feet provide a fast and cost-efficient solution for the beam and deck system of a parking structure. While transported and erected individually, the double tee’s are designed to act in concert with each other. More specifically, after final erection, the deck is designed to respond to loads as if it was one homogeneous structure. This is for a number of reasons including structural considerations like load sharing and diaphragm action as well as functional issues like smoothness of ride and waterproofing.
To accomplish this, the individual double tees are attached to each other by a series of discrete connections along both sides of their flanges. These connections allow adjacent tees to act integrally with one another. There are typically seven to 13 of these connections per joint formed by adjacent tee flanges.
While there has been a number of evolutions and improvements over the years, the basic concept of these connections has remained the same. During the fabrication of the tee, a steel section is placed in the formwork at the flange locations called out by the designer. These can be as simple as a section of plain carbon steel angle to proprietary highly engineered stainless steel fabrications. Upon placement of the tees on site, the connections align with the ones on the adjacent tees. Based on the design, the tees have a ¼-inch to 1-inch gap between them. Due to this gap, the connections cannot be directly attached to one another regardless of the connection style. In the end, they all rely on a small piece of steel rod or plate being welded across them in the field. The steel components, as well as the concrete holding them, must be able to securely resist a number of loads in the vertical and horizontal directions. No matter what type of connector is utilized, the final complete connection is required to be very robust.
If properly installed and maintained, all the various tee-to-tee connections will function flawlessly for the service life of the garage. However, due to any number of reasons, these connections fail on a regular basis and have become a constant source of headache for parking garage owners and engineers.
To date, the methodology of repairing these failed connections has been as simple as welding them back together to as complex as full depth in-situ replacements. By far, the most common way has been some sort of galvanized or stainless steel fabrication attached to the underside of the tees by means of through bolting or masonry anchors. The resulting repairs are not only expensive and time consuming, but at times, are aesthetically unpleasing. The parking garage industry has been searching for an economical, efficient and unobtrusive repair since the failure of the first tee connection.
Over the last 20 years, Carbon Fiber Reinforced Polymer (CFRP) has proven to be an extremely lightweight, strong and durable material that has evolved from its aerospace roots to day-to-day use in the transportation, energy and infrastructure markets. With strengths in excess of six times that of steel with only a quarter of the weight and completely corrosion resistant, CFRP is the perfect concrete repair material. CFRP has already been embraced by the engineering and construction community to strengthen and restore the beams and columns in concrete parking structures, but has never been used for the joint connections.
Utilizing materials and fabrication techniques originally designed for the space program, a new and efficient CFRP tee-to-tee repair solution is available. V2 Structural Systems, a division of V2 Composites, Inc. in Auburn, Alabama, has developed the V2 T-Biscuit. Employing custom carbon and glass fabrics co-molded into a high-strength epoxy matrix, the technicians at V2 Composites have produced a simple, yet complex CFRP laminate that is used to repair failed tee-to-tee connections. Shaped like half of a flattened football, V2 T-Biscuits are designed to be installed vertically across the joint adjacent to the failed connection. Backed by extensive coupon and full scale testing, V2 T-Biscuits have been engineered to handle all the vertical and horizontal loads associated with standard pre-cast design. To aid the designer, V2 T-Biscuits are meant to be a direct one-for-one replacement for all the existing flange connections.
The installation of V2 T-Biscuits is as simple as its design. Utilizing a standard 14-inch diameter concrete saw, a ¼-inch wide cut is made 18 inches long transverse to the joint between the adjacent tees. The saw cut is then cleaned out and filled with V2 Biscuit Bond paste, a rapid setting, high-strength epoxy. Once the slot is full of epoxy, the V2 T-Biscuit is simply pushed in and the excess epoxy is cleaned off. From start to finish, a V2 T-Biscuit can be installed in as little as 15 minutes. Depending on the ambient temperature, the deck can be placed back into full service in as little as eight hours. For emergency and cold weather conditions, an acrylic-based adhesive is available that can be used to bring service back in as little as an hour. Once the joint has been re-caulked and the project is cleaned up, the only evidence that a repair has been done is a small line of epoxy in the deck.
Although V2 T-Biscuits have been available since 2009, their use has grown exponentially in the last two years. With over 30,000 biscuits successfully installed in 24 states, parking garage owners, structural engineers and repair contractors have been embracing their inherent strength and simplicity. Sometimes rocket science is really that simple.