Why Shotcrete? Features and Benefits Although the hardened properties of shotcrete are similar to conventional cast-in-place concrete, the nature of the shotcrete placement process provides additional benefits, such as excellent bond with most substrates and instant or rapid capabilities, particularly on complex forms or shapes. The properties of both wet- and dry-process shotcrete can be further enhanced through the addition of many different additives or admixtures such as: Silica Fume — Provides reduced permeability, increased compressive and flexural strength, increased resistance to alkali and chemical attack, improved resistance to water washout, reduced rebound levels, and allows for thicker single pass applications; Air-Entraining Admixtures — Improve pumpability and adhesion in wet-process shotcrete and freeze-thaw durability in both wet and dry processes; Fibers — Control cracking, increase toughness values, improve impact resistance and energy absorption; and Accelerators — Improve placement characteristics in adverse conditions, allow for thicker single pass applications, increase production capabilities, and reduce the occurrence of fallouts on structures subjected to vibration. Shotcrete Applications.
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These innovations enhanced the utility, flexibility, The following definitions cover terms used in shotcreting: Nozzle-Attachment at end of delivery hose from which Air ring-Perforated manifold in nozzle of wet-mix shot- shotcrete is projected at high velocity.
A for scouring surfaces in preparation for next layer of nozzle tip is attached to the exit end of the nozzle body. Nozzle liner-Replaceable insert in nozzle tip, usually Alignment wire-See Ground wire.
Blowpipe-Air jet operated by nozzlemans helper in Nozzleman-Worker on shotcrete crew who manipulates shotcrete gunning to assist in keeping rebound or other the nozzle, controls consistency with the dry process, and loose material out of the work. Board butt joint-Shotcrete construction joint formed by Nozzle velocity-Velocity of shotcrete material particles sloping gunned surface to a l-in.
Overspray-Shotcrete material deposited away from Buildup-Thickness of shotcrete. Bulking-Increase in volume of sand in a moist condition Pass-Distribution of stream of materials over the receiv- over the same quantity dry. A layer of shotcrete is built Conveying hose-See Delivery hose.
Cutting screed-Sharp edged tool used to trim shotcrete Pneumatically applied concrete-See Shotcrete. See Rod. Pneumatically applied mortar-See Shotcrete. Delivery equipment-Equipment which introduces shot- Pneumatic feed-Shotcrete delivery equipment in which crete material into the delivery hose. Delivery hose-Hose through which shotcrete materials Positive displacement- Wet-mix shotcrete delivery pass on their way to nozzle; also known as material hose or equipment in which the material is pumped through the conveying hose.
Air is introduced into the material flow at ing water is added at the nozzle. See Pneumatic feed. Feed Wheel-Material distributor or regulator in certain Predampening-In the dry-mix process the addition of types of shotcrete delivery equipment.
See Flash coat. Prewetting-In the dry-mix process the addition of a por- Flash coat-Thin shotcrete coat applied from a distance tion of mixing water to shotcrete materials in the delivery greater than normal for use as a final coat or for finishing; hose at some distance prior to the nozzle. Puddling-Placement of shotcrete wherein air pressure is Ground wire-Small-gage, high-strength steel wire used decreased and water content is increased, usually an unde- to establish line and grade for shotcrete work; also called sirable method of shotcreting.
Pump-Wet-mix delivery equipment. Gun-Shotcrete delivery equipment. Pump operator-Workman on wet-mix shotcreting crew Gun casting-Placing concrete or mortar using a special who operates delivery equipment. Rod-Sharp-edged cutting screed used to trim shotcrete Gunite-Term sometimes used for dry-mix shotcrete. Gunman-Worker on shotcreting crew who operates de- Rolling-Result of applying shotcrete at angles less than livery equipment; also known as Gun operator.
Gun operator-See Gunman. Sagging-See Sloughing. Hamm tip-Flared shotcrete nozzle having a larger diam- Sand pocket-A zone in the shotcrete containing fine eter at midpoint than either inlet or outlet; also called Pre- aggregate with little or no cement. Screed wire-See Ground wire. Hydro nozzle-A special prewetting and mixing nozzle Shooting-Act of applying shotcrete; see Gunning.
Lance-An extended nozzle of various configurations Sloughing-Subsidence of shotcrete, due generally to ex- consisting of a length of metal pipe with nozzle and body cessive water in mixture; also called Sagging.
Wet-mix shotcrete-Shotcrete in which all of the ingre- dients including water, are mixed before introduction into TABLE 1. If accelerator is used it is normally added at the nozzle. Dry-mix process Wet-mix process Wetting-In the dry-mix process, the addition of mixing 1. Instantaneous control over 1. Mixing water is controlled at water to shotcrete materials just prior to exit from the mixing water and consistency the delivery equipment and nozzle.
Better suited for placing mixes 2. Better assurance that the 1. Less dusting and cement loss coarse or fine. See Table 2. Normally has lower rebound lowing steps: characteristics are better with resulting in less material waste 1 A cementitious binder and aggregate are thoroughly minimal waste and greater placement flexibility mixed.
Capable of higher strengths 5. Capable of greater production 2 The cement-aggregate mixture is fed into a special mechanical feeder or gun called the delivery equipment. Some equipment uses air pressure alone orifice Although information on shotcrete construction and ap- feed to deliver the material into the hoses. The nozzle body is fitted inside available shows a wide range of values. This is attributable, with a water ring, through which water is introduced under in part, to a lack of standard testing procedures, variations pressure and intimately mixed with the other ingredients.
Table 1. These favorable properties 4 Accelerator is usually added at the nozzle. However, differences in capital and maintenance cost normally falls within a range of 0.
Most published values aggregate, and placement characteristics may make one or the for day strength are in the range of to psi 20 other more attractive for a particular application.
Differences to 48 MPa , although dry-mix shotcrete in some tests has in operational features and other properties which may merit developed strengths in excess of 10, psi 69 MPa.
It is consideration are given in Table 1. Shotcrete offers advantages over conventional concrete in a variety of new construction and repair work Fig. Reinforcing details may complicate the use of shotcrete cores, Bond strength in shear, Sample No. The excellent bond of shotcrete Table Dry-mix shotcrete on old wet-mix shotcrete 8 Wet-mix shotcrete on old wet-mix shotcrete 11 It is presented for illustrative purposes only.
All tests on 6 in. Shotcrete placed by dry method. Base shotcrete in Section B was by wet mix. Shear test conducted by guillotine method where load is applied parallel to the bonded surface.
This is slightly higher than most low slump conventional concrete which can be placed in heavier sections using larger aggregate and leaner mixes. Most shotcrete has a high cement factor and, therefore, has a greater potential for drying shrinkage cracking which may require a closer control joint spacing or increased use of reinforcing mesh or steel. The coefficient of thermal expansion of shotcrete approx- imates that of reinforcing steel, thereby minimizing internal stress development.
The modulus of elasticity generally falls between 2. The freeze-thaw durability of shotcrete in laboratory tests and under field exposure has generally been good.
However, problems can develop with even the best shotcrete when it is applied to a nondurable or improperly prepared substrate. It should be stressed that the properties and performance of shotcrete are largely dependent on the conditions under which it is placed.
They may also be dependent on the characteristics of the particular equipment selected, and ul- timately on the competence and experience of the applica- tion crew. Deteriorated concrete in bridges, prietary combinations of binder and aggregate, or conven- culverts, sewers, dams, reservoir linings, grain elevators, tional shotcrete with special admixtures. Concrete 1. Roofs, thin shells, walls, pre- beams, columns and slabs, concrete and masonry walls, stressed tanks, buildings, reservoirs, canals, swimming steel stacks, tanks, and pipe.
Because of the speed of installation and general effective- ness of the process it has found great favor in the refractory industry. Today it has become a major method of installation for all types of linings from several inches to several feet thick. It is used in new construction and for repair and maintenance in steel, nonferrous metal, chemical, mineral and ceramic processing plants, steam power generation, and incinerators.
Portland cement and aggregate with admixtures are used to produce special properties. Included among the for- mer are the sodium and potassium silicates, magnesium phosphates, and polymers.
Among the latter, are portland cement shotcretes using accelerating, retarding, and pozzolanic admixtures. It also includes a new group of mate- rials classified as polymer modified shotcretes. These special Fig. Over brick, masonry, earth, and rock; underground support, tunnels, slope pro- Fig.
The fibers can provide improved flexural and shear strength, tough- ness, and impact resistance. For refractory shotcrete, fibers increase resistance to thermal shock, temperature cycling damage, and crack development.
Some specific future uses where fibrous shotcrete can be cost effective are slope pro- tection Fig. Although most uses of steel fiber reinforcement has been Fig. Special care, and sometimes spe- cial equipment, may be required in adding fibers to the shotcrete mix to prevent clumping or kinking of the fibers, and to assure that they are properly proportioned. The use of fibers in shotcrete is a relatively new development, and much has still to be learned about the optimum size and shape of the fibers, methods of addition to both wet- and dry-mix shotcretes, and other factors.
Refer to ACI The addition of certain latex formulations to a conventional portland cement shotcrete mix improves flex- ural and tensile strengths, and may improve bond as well as reduce absorption and penetration of chlorides. A potential major application of this material is the repair of concrete Fig. Final lines are uneven plants that are under chemical attack.
In place of the standard nozzle, a casting head is chimneys and stacks, process vessels, chemical spillage used in conjunction with standard dry-mix equipment. This areas, sumps, trenches, pollution control systems, and con- slows the velocity of the material on exit, almost eliminating crete repair in other highly aggressive environments.
It has found extensive use in certain sectors of the refractory industry, 1. It increases potential the utility of the dry-mix gun, permits placement of con- crete in close restricted quarters and at distant or inaccessi- 1.
Form requirements are minimal while labor the speed of development of new materials, equipment, and and material costs are usually reduced Fig. A prime example of recent expansion in the use 1. Improvements in accelerating admixtures, the use of Composites of shotcrete and polystyrene or polyurethane steel fibers, and specially designed equipment including foam, special fiberglass and other plastics forming a sand- robot and remote shotcrete devices have spurred the devel- wich construction is another possibility.
These innovations enhanced the utility, flexibility, The following definitions cover terms used in shotcreting: Nozzle-Attachment at end of delivery hose from which Air ring-Perforated manifold in nozzle of wet-mix shot- shotcrete is projected at high velocity. A for scouring surfaces in preparation for next layer of nozzle tip is attached to the exit end of the nozzle body. Nozzle liner-Replaceable insert in nozzle tip, usually Alignment wire-See Ground wire. Blowpipe-Air jet operated by nozzlemans helper in Nozzleman-Worker on shotcrete crew who manipulates shotcrete gunning to assist in keeping rebound or other the nozzle, controls consistency with the dry process, and loose material out of the work.
ACI 506R-16: Guide to Shotcrete
ACI guide to Shotcrete