Overhead Shotcrete

When you say overhead, people invariably think of fixed costs, salaries, and the general costs of running a business.

Curing Silica Fume Shotcrete with Wet-Sprayed Cellulose

Chloride-induced corrosion is the primary cause of deterioration in many concrete bridges in Canada and the northern United States. For corrosion to take place, the chloride ions permeate through the interconnected pores of the concrete toward the embedded steel reinforcement and attack the passive layer that forms around the steel during the hydration process of concrete. Once this passive layer is destroyed, corrosion proceeds, resulting in a reduction in the structural integrity of the structure in addition to the onset of spalling of the concrete cover. To restore the serviceability of the structure and extend its service life, the deteriorated concrete is usually removed, the reinforcement is replaced if necessary, and the concrete is replaced with a durable repair material that has high resistance to chloride ion transport. In the province of Ontario, Canada, partially deteri-orated concrete bridge soffits are typically repaired with shotcrete containing silica fume.

Quality Management of Shotcrete in North America

As for all construction materials, a proper program of quality management should be implemented in shotcrete construction to protect the owner™s investment. This article briefly examines typical shotcrete Quality Management (QM) practice in North America. QM can be considered to be comprised of Quality Assurance (QA) and Quality Control (QC) functions.

Specified Dilemmas

On shotcrete sites, it is not unusual to hear comments starting with œIn a perfect world, … Of course, in a perfect world,there wouldn™t be any shotcrete because there wouldn™t be anything to repair or strengthen. Fortunately, there are plenty of imperfections in the jurisdiction where most of my work comes from. We find them on cracked and fissured rock slopes next to highways, in ground to be tunneled for infrastructure improvements, in old masonry walls, washed-out bridge foundations, and in marine piers where the chloride concentration at the rebar level in the concrete exceeds the reasonable limit set for the onset of corrosion.

Shotcrete – The Repair Solution for the Memorial Tunnel

In 1953, the Memorial Tunnel in Standard, WV, was constructed as a two-lane, 2800 ft (853 m) tunnel with semi-transverse ventilation. The owner operated the tunnel until the mid-1980s when a four-lane bypass was constructed to upgrade the turnpike to current Interstate standards. The tunnel was abandoned until 1989 when the Federal Highway Administration (FHWA), in conjunction with the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE), embarked on the Tunnel Fire Ventilation Test Program using funding from the Central Artery Project.
The test program consisted of performing controlled test fires up to 100 megawatts. These intense fires then provided valuable information for the design of ceiling wall partitions and the protection of facilities for power, ventilation, and lighting. In addition, it provided the opportunity to develop and evaluate methods of proper ventilation control of a tunnel under various fire scenarios.
Parsons Brinckerhoff was retained to perform the test program. As part of that test program, an evaluation of the structural condition of the tunnel was performed and structural repairs were designed. A critical part of the design was to insulate structural portions of the tunnel for temperatures in excess of 2000 °F (1143 °C).
The rehabilitation program for the reuse of the tunnel required the sealing of all cracks in the tunnel liner because the bedrock around the tunnel contained low-flashpoint cannel coal. In addition to sealing the cracks, extensive structural rehabil-itation of the liner was performed to repair damage caused by the excavation for the bypass on the adjacent highway. Numerous products were evaluated to determine which would provide suitable fire protection for the structural elements of the ceiling and for mechanical equipment anchorages. The test program included the use of traditional venti-lation with a tunnel ceiling and tests with the ceiling removed for the use of jet fans. The construction contract for the rehabilitation of the tunnel and the removal of the ceiling had a projected cost of $10 million.
During the test program of 98 fires, routine inspection of the tunnel was performed to evaluate the performance of the fireproofing. Based on the performance of certain structural elements, changes were made in the use of structural fire-proofing and code requirements for the protection of equipment. After the test program in 1991, the

History of Shotcrete in Seismic Retrofit in California

The widespread use of structural shotcrete actually began long before the first appli-cation was made. Its rise was politically motivated and its continued development dictated by the occurrence of earthquakes. Responding to a school fire in the 1920s, the Los Angeles School Board directed that all future school buildings be constructed of masonry. However, masonry of the day was not reinforced, and several hundreds of these buildings were destroyed or damaged in the great Long Beach earthquake of 1933. Fortunately, the quake occurred in the early morning hours when the schools were unoccupied; had it been during the day, hundreds of deaths and thousands of injuries would have likely resulted.

Surface Preparation for Shotcrete Repairs

Surface preparation is an important element of the repair process, both with shotcrete and cast-in-place concrete. It covers a large scope, including concrete removal, saturation of the substrate, the use of bonding agents (rare with shotcrete), and cleaning of the surface. These operations are influenced by the local conditions (surface position: vertical or overhead, the presence of reinforcement) and are very important for both the short- and long-term bond strength and, thus, the repair integrity. When all steps involved in surface preparation are considered, it is obvious that these operations represent a large part of the repair cost and may reach up to 50% of the total repair cost. For this reason, it is important not to neglect surface preparation.

Washington State’s Capitol Seismic Repair

Washington State’s Capitol Seismic Upgrade will surely rank as one of the top restoration projects of this decade and shotcrete proved to be essential to its success. As with most complex rehabilitations, many of the hurdles faced arose after the project had begun. The ability of the contractors, engineers, and architects working together to overcome these issues proved once again to be the crucial factor in the success of the project.