Shotcrete is a method of placing concrete and properly placed shotcrete should have a service life similar to cast concrete. Generally, concrete structures in normal environmental exposures are expected to have service lives from 50 to 100 years. With particular attention to materials and construction methods, some concrete structures, such as the new San Francisco Bay Bridge, have been designed for a service life up to 150 years. The first step in achieving a long-lasting, high-quality installation is to engage a highly qualified and experienced shotcrete contractor. There are many other factors that influence service life, including using the right mixture design for the anticipated exposure conditions.
We are in the process of designing retaining walls that will be supported by either concrete piers or steel piles. We would like to see some typical details on how the reinforcing is secured to either the piers or piles.
For concrete piers, the reinforcing steel is generally secured to the piles with reinforcing bar grouted dowels. For steel piles, the reinforcing bar is generally secured with Nelson studs.
I am a Civil Engineer working on a hydropower project. Is it possible to place shotcrete at a thickness of 24 in. (600 mm) inside a tunnel that will be used as a water tunnel to generate power?
Yes, it is possible to shoot 24 in. (600 mm) thick tunnel linings. There are various ways of doing this, depending on the reinforcing steel configuration. One method we have successfully used for shooting tunnel linings this thick with a double mat of reinforcing bar (1 in. [20 mm] diameter bars at 6 in. [150 mm] on center, vertically and horizontally) is to bench gun shoot the walls up to the spring line with a wet-mix silica-fume-modified shotcrete (without accelerator) and then ribbon-shoot (2 ft [0.5 m] wide strips) overhead using the same mixture but with the option of using an alkali-free accelerator added at the nozzle.
If the shotcrete requires a smooth finish (equivalent to a cast-in-place concrete finish), then the initial shotcrete is shot to within about 1 in. (30 mm) of the final shotcrete thickness and allowed to set and harden. Following that, a final non-accelerated finish coat can be applied that can be trimmed to shooting wires with a cutting rod, closed up with a darby, and then trowelled with either a magnesium or steel trowel, depending on the required finished surface texture.
Such work can be done with a remote-control manipulator arm (robot) or, for more precision, with hand nozzling out of a basket on a manlift (provided the tunnel floor is sufficiently smooth for operation of a manlift). The bottom line: hire a contractor who has experience in conducting such work.
Is it critical for the early and intermediate compressive strength at 3 and 7 days, respectively, to be met for shotcrete applications for a rock fall face if the 28-day compressive strength is met?
Compressive strength at 1, 3, and 7 days can be important to all for subsequent operations. In general, 7-day strengths are a good indicator of the ultimate 28-day strength. The need for early strength is an engineering and construction sequence issue, not a normal or typical shotcrete requirement.
We are working on a geotechnical project in the northwest to repair an existing rockery retaining wall. The wall is around 750 ft (229 m) in length and up to 12 ft (4 m) in height. The issue is that some of the basalt boulders within the wall are weathered soft and falling apart. The total weathered rocks that are falling apart comprise approximately 7% of the wall. Can we use shotcrete on the weathered rocks to give them more stability as a repair process? If not, is there a process we can use with shotcrete to repair the wall without having to rebuild the entire wall?
Shotcrete has been used in the Northwest to strengthen and overlay existing rockery walls. The need to remove the weathered material is dependent upon the need for the overlay to bond with the existing wall, which is an engineering issue and not a shotcrete issue. Shotcrete can and is shot successfully against soil and other weathered surfaces.
In regards to the strength of shotcrete and weather is there a Mine Safety and Health Administration (MSHA) regulation requiring our paste cylinder sample results to be above a certain psi (MPa)?
Shotcrete is normally expected to meet or exceed 4000 psi (27.6 MPa). We are not aware of any specific MSHA requirements. We would suggest you consult a tunnel or mining engineer who is well-versed with shotcrete. Cores taken from field-shot test panels are generally used for the evaluation of compressive strength of shotcrete (ASTM C1140/C1140M-11).
Can shotcrete be effectively used to repair holes in an old 8 ft (2.4 m) diameter storm sewer tunnel constructed of stone/brick/mortar? One of the holes is completely worn through to the earthen backfill material. The other two holes are missing the innermost layer of stone masonry, but the outer layer of masonry is still in place.
The friction coefficient n of well-finished shotcrete for use in Kutter’s equation (and, more streamlined, Manning’s equation) is generally used as 0.012. Shotcrete is used not only to improve flow characteristics of brick, corrugated metal, or any other pipe construction but can also be conventionally reinforced as a structural liner to eliminate the need for liner plates or other pipe-lining alternatives.
We are a local agency considering the repair of a number of older culverts with shotcrete. Like most agencies, we are trying to be creative about maximizing our funds. We work with a federal agency when it is determined that a “new” culvert is needed. In other words, the agency will not pay for maintenance repairs but will pay for “new” culverts. We are wondering if the shotcrete method has ever been viewed as a means of creating a “new” culvert. Could the existing culvert be considered as merely a form for the new culvert? Could you also speak to the life expectancy of shotcrete (life cycle) versus a new concrete culvert?
Shotcrete has been used extensively for the purpose of relining existing culverts. You are correct to visualize the existing culvert as a form for building a new structure. Because it is a stay-in-place form, it may actually act as a composite structure. Shotcrete is a method of placing concrete and will have similar, if not better, durability and life span if installed professionally with good mixtures. You can locate numerous past articles on durability of shotcrete that have appeared in Shotcrete magazine in the magazine’s archive on the ASA website, shotcrete.org. Similar work has been done in California, Colorado, and other states. This approach is currently being used as permanent tunnel lining in many places, including many of the current New York Transit projects.
A knowledgeable contractor will develop a mixture and procedures to ensure that the timing of the subsequent blast is compatible with the set time of the shotcrete. Preconstruction testing should be required to establish the set time (both early and final set) to assist in developing the sequence of operations. The set time will also be impacted by the site conditions, such as temperature.
We are considering a shotcrete lining of a new corrugated metal pipe to improve the smoothness and hydraulic capacity. What is the minimum thickness over the corrugations and should we be looking at any reinforcement or studs to support the shotcrete?
This has been done in the past to improve hydraulic capacity and provide better wear resistance in the invert. Typically, the minimum cover over the corrugation is 2 to 3 in. (51 to 76 mm) with a welded-wire fabric either welded or otherwise attached to the corrugated pipe. The cover could likely be reduced with the use of structural fibers of either steel or synthetic material. Steel fibers and wire mesh should not be used together. Care must be taken to specify the required finish. This application would likely benefit from a smooth trowel or light broom finish. A light broom finish is preferable from a safety standpoint, as a trowel finish creates a very slippery surface both during construction and for the maintenance at a later date. In addition to the hydraulic and wear characteristics, once shotcreted, the entire pipe will become a composite section with improved structural characteristics. It should be noted that the pipe must be large enough for workers to work in safely and productively. This would mean an absolute minimum of 48 in. (1219 mm) and preferably larger.
A knowledgeable contractor will develop a mixture and procedures to ensure that the timing of the subsequent blast is compatible with the set time of the shotcrete. Preconstruction testing should be required to establish the set time (both early and final set) to assist in developing the sequence of operations. The set time will also be impacted by the site conditions, such as temperature.
We will be shotcreting the inside of a tunnel entrance. The plan is to apply 3 ft (0.9 m) of shotcrete on the walls and ceiling to support a large section of limestone rock 22 ft (6.7 m) high, 30 ft (9.1 m) wide, and 20 ft (6.1 m) deep above the tunnel at the entrance that has moved and is wedged and supported with an existing steel structure. We would like to test the shotcrete and are wondering what type and quantity of tests you recommend and which testing labs are in our area that would be able to conduct the testing?
The article “Shotcrete Testing—Who, Why, When, and How” in ASA’s Summer 2011 issue of Shotcrete magazine should help answer most of your questions on testing of shotcrete. ACI 506R, “Guide to Shotcrete,” and ACI 506.2, “Specification for Shotcrete,” also have helpful information on shotcrete testing. Most competent testing labs should be able to test the compressive strength of cores extracted from shotcrete panels or sections, as they are very similar to concrete cylinder tests. If conducting more advanced testing, you may want to consider selecting a lab experienced with shotcrete.