Shotcrete

Shotcrete is mortar or concrete that is projected pneumatically at high velocities onto a surface. There are two different application processes- wet-mix and dry mix. Both procedures produce the same outcome, but the applications for both differ slightly. Wet-mix uses cement, sand, aggregate, admixtures, and water. It combines all of the ingredients before it is introduced to air from a pneumatic sprayer. Dry-mix, however, uses cement, damp sand, and aggregate which are delivered through a hose. When the mixture gets to the nozzle, pressurized water and air are introduced.

History
Wet- and dry-mix shotcrete are historically connected, both coming from the application of concrete mixture using compressed air while experimenting with pneumatic machinery for grout and plaster application. In 1908, a taxidermist by the name of Carl E Akeley applied for a patent for a machine that applied wet plaster or mortar using compressed air. Three years later he obtained another patent for modified handling equipment that could apply plastic adhesive materials. In Allentown, Pennsylvania, the Cement Gun Company acquired Akeley’s machine and developed the modern day dry-mix shotcrete industry. Between the 1910s and the 1920s, experimentation with wet-and dry-mix shotcrete began. Dry-mix became more standard in the 1920s. The wet-mix lacked in popularity for several reasons, the most pertinent being that it tended to clog machinery. A True-Gun, a device for gunning a wet mortar mix, brought the wet-mix back into use around the 1950s. Gunite was developed by the Cement Gun Company to describe the sandy cement material produced by their cement gun. The term “shotcrete” was developed in the 1930s by the American Railway Engineering Association, and it was usually applied to the dry-mixture process. This term was all-encompassing for any gunned application of mortar or concrete materials and is still acceptable today.

Manufacturing Process
Shotcrete is made of the same material as regular concrete, some include aggregate, sand, and Portland cement and are usually mixed on sight immediately before application. It is important that aggregates must be sized appropriately and the amount of water should be carefully regulated. For most part, if the material is plastic enough to be pumped, it can be projected into place as shotcrete. Prepackaged mixes are usually more expensive than purchasing materials separately and may include admixtures like synthetic fibers, air-entraining agents and water reducers. The use of admixtures appeared after World War II, although there is some evidence of mild use before this time, such as iron fillings.

Uses and Installation
Shotcrete is applicable in a variety of industries. Industrial applications include mine shaft tunnel coatings and lining for refractory, furnaces, and boilers. Some architectural shotcrete can be used over masonry, concrete, or steel or for replacing stucco or brick. Also, thin, reinforced sections of shotcrete can be used to construct domes and thin roofs. In 1911, using shotcrete for coating buildings and for use on existing structures was suggested. The Field Museum of Natural History in Chicago ( now known as the Museum of Science and Industry) was one of the earliest known applications of shotcrete. In 1910, Akeley’s cement gun was used to repair lime stucco coating. Shotcrete was also used for fireproofing in early industry years. By the 1920s, standard procedures were born from experimentation Gunstone, which is a mixture of gunite and concrete, and was formed by dry-mix, large aggregate shotcrete. Shotcrete was used in the Hayden Planetarium dome of the American Museum of Natural History in New York City. The dome utilizes a shell construction, a subdome hung from the main reinforced concrete dome, and shotcrete that has been blown into cracks and small segments of the inner dome. It is commonly used for engineering applications but has been known to be found in the architectural industry as well. The roof truss for St. John’s Abbey Church was created using reinforced, folded plates and wet-mix shotcrete. To apply shotcrete properly, it is important that the accepting surface be prepared properly. Shotcrete can be applied by itself over a prepared surface or over reinforcing bars. It is recommended that reinforcement be used when the surface is even and unfinished, rather than an uneven surface. Reinforcement is important because the substrate cannot provide a proper bonding surface or for crack repair. Shotcrete is usually thin, being between 1/8 to 4 inches in depth. When reinforcing, a greater thickness may be required. Shotcrete is usually applied to surfaces without formwork, but one- or two-sided forms can be used to help form sharp corners or decorative elements. The skill of the applicator can greatly affect the quality of the finish of shotcrete. It is generally applied by two people, a nozzleman, and a partner to handle the hose. In both mix styles, the shotcrete is shot, with high velocity, onto the surface. Rebound, or the amount of mixtures that bounces back during the application. Larger aggregate and some sand bounce off the receiving surface leaving a cement paste. Because of the risk of rebound, it is important to look for trapped aggregate that may leave unfilled pockets when shotcrete is being blown onto a reinforced surface. Shotcrete, when finished, should appear compact, creamy, and rich, never dry or sandy. Brooming the finished surface or an acid wash can achieve different textures in the finished project. Shotcrete must have adequate protection to cure properly, similar to poured-in-place concrete.

Conservation
Shotcrete can fail in several ways, the primary two being cracking and delamination. Durability can be affected by poor surface preparation, insufficient detailing, water infiltration, and substrate flaws. It is necessary to properly prepare the surface to ensure that the shotcrete will bond with the substrate. It is also essential that the substrate be stable. A qualified applicator with proper equipment will eliminate some of the negligible issues.

Deterioration
Shrinkage and movement are generally the largest problems because they can cause severe cracking. Hairline cracks are often a cause of shrinkage during the initial setting but do not usually require much attention. These cracks can be beneficial to the shotcrete’s structural integrity. Vertical cracks that are continuous can indicate damage from expansion or contraction of either the substrate or the shotcrete. Large areas of shotcrete-covered surfaces are more prone to this type of cracking when there is no room left for material movement. These should be anticipated and should be provided in the details. Also, these types of cracks can be due to flaws in the substrate. Delamination and cracking are usually related. The open cracks allow water to creep in, allowing carbonations and resultant corrosion of reinforcing bars. Expansion due to corrosion can cause shotcrete coatings to delaminate. Open cracks allow water to travel deep into the coating of shotcrete, causing the cement rich layer to leach out the cracks. Deterioration can eventually lead to delamination of the surface and spalling. Shotcrete deterioration can be visually investigated up close or with the use of binoculars. A “crack map” is used to record the cracking pattern. Also, visible cracks can we traced with chalk and photographed. A steel hammer is used to sound the concrete, this helps to check for delamination. Hollow sounding areas may be delaminated and outlining with chalk for photographic evidence should be done. Static cracks are usually formed as a result of shrinkage during the initial set. Some cracks are active due to material expansion. Active cracks are tracked over time with calibrated crack monitors. With an accuracy of nearly 1 millimeter, these gauges record both horizontal and vertical movement of cracks over a period of time. Construction Testing Laboratories tested the durability of shotcrete in 1965, 1966, and 1989. The company found that if dry-and wet-mix shotcrete was installed properly (whether air- or non-air entrained), both would be extremely durable. ASTM has published five shotcrete standards and performance testing remains ongoing. Core samples should be analyzed by a qualified testing laboratory in order to find if it has any weaknesses. When extracting samples at bonded areas, it is important not to break the bond between the shotcrete and its substrate. These samples can be visually inspected by comparing two samples, microscopically, or with a chemical analysis to determine its constituents. ASTM standards for concrete may be performed by testing laboratories.

Conservation Techniques
Shotcrete and finished concrete share similar maintenance procedures. Shotcrete can experience surface soiling and, depending on the type and exposure or aggregate, the amount of lime in the mix, and the presence of nonoriginal coatings will determine the best cleaning process. Early shotcrete applications may have received a waterproof coating and testing should be done before cleaning the entire surface is cleaned. Little is known about the coatings that may have been applied to earlier applications of shotcrete. These coatings may connect to the surface over time, and removal could leave a sticky residue that will collect dirt. Previous treatments may too difficult to remove chemically and mechanical means may be implemented. Sandblasting will alter the appearance of the exposed shotcrete, and this should be taken into consideration before further steps are taken. The bond of exposed aggregate to the cement mixture may also be affected. The integrity of historic shotcrete finishes and acidic masonry cleaners should be avoided. Water filtration should be identified, especially is the water is collecting behind the deboned shotcrete surfaces. Freezing and thawing increases cracking and debonding can occur. Water should be prevented from entering the shotcrete through large cracks because it can wash away the cement in the mix and weaken the bond between the concrete and the substrate. Breathable water repellents may help prevent water intrusion. If cracks are greater than a hairline crack (more than 1/80th of an inch), the use of a repellent will be ineffective. Repairing and conserving shotcrete techniques are rare to find. Modern shotcrete installations are on industrial structures and are removed or replaced as they become damaged. Concrete consolidants may be applied to severely deteriorated shotcrete surfaces but may be more costly than selectively removing and replacing the coating. Deteriorate shotcrete surfaces are usually removed, the base material prepared, and shotcrete is applied again. Small repairs cannot usually be made using the pneumatic applied concrete. Shotcrete is designed to be applied in a large, sweeping motion in large areas. Repairs to shotcrete should be made with a mortar mixture that matches the surrounding area but in patch form. Visual compatibility can be difficult. When spalls occur, removing large panels of the material and reapplying a shotcrete coating may be necessary. An alternative to removal or replacing, is reaffixing delaminated material to a substrate using a structural adhesive. Epoxies can be injected through ports placed strategically along the crack. The adhesive then spreads behind the delaminated material and reaffixes to the substrate. Cracks are dammed to prevent epoxy from trickling out of the rear and should be monitored carefully to prevent materials from being forced off the substrate.

Replacement
Replacing the shotcrete with a like material is most frequent replacement method used, especially for engineering applications. Test panels of at least 3 feet are erected to ensure quality of materials, workmanship, and the new materials ability to match existing surfaces. Testing is a necessity and the price of it should be added in the bidding process. Nozzlemen may also be prequalified during this trial period. Hairline shrinkage cracks are prevented through proper mix and application. Shotcrete panels should allow for expansion and contraction to prevent cracking due to material movement.