A PRESENT PRESENTA ATION ON
SHOTCRETE CONCRETE TECHNOLOGY
UNDER THE GUIDANCE OF ASST PROF PROF.. R.L.SIRALE Submitted by SINGH NITESH J. ROLL NO. 49
INTRODUCTION
Shortcreting has proved to be the best method for construction of curved surfaces. surfaces. Domes are now much easier to construct with the advent of shotcrete technology. technology. Tunnel Tunnel linings li nings are also becoming easy with this technology techn ology.. Not only are these but there a wide range of applications where this technology has been a leading one. This technical technica l paper includes includ es the concept of shotcrete and how it differs differ s from conventional conventional concrete. It also enumerates the different types of process involved in shotcreting
DEFINITION OF SHOTCRETE
Shotcrete is a mortar or high performance concrete conveyed through a hose and pneumatically projected at high velocity onto a backing surface. It is the force of this spraying action that leads to compaction of the concrete or mortar which then forms layers of concrete to the required thickness. Shotcreting has been an acceptable way of placing cementitious material in a variety of applications.
Usually patented polypropylene fibers are included in the shotcrete which increases the cohesive nature of the shotcrete through mechanically binding the cementitious materials together. This mechanism reduces the rebound waste that occurs through the shotcreting process and these fibers also resist plastic shrinkage and cracking through their ability to enhance the early stage tensile strength of concrete.
SHOTCRETE VERSUS CONVENTIONAL CONCRETE
Unlike conventional concrete, which is first placed and then compacted in the second operation, shotcrete undergoes placement and compaction at the same time due to the force with which it is projected from the nozzle. Shotcrete is more dense, homogeneous, strong, and waterproof than is possible to obtain by any other process. Shotcrete is not placed or contained by forms. It can be impacted onto any type or shape of surface, including vertical or overhead areas. It forms an excellent bond with the substrate and can be given a variety of surface finishes.
DRY PROCESSES
Simplified sketch of a typical dry mix shotcrete system
DRY PROCESS - AN OVERVIEW Working of dry process proceeds as per the following steps:
Step1: Pre blended, dry or semi-dampened materials are placed into shotcrete equipment and metered into a hose. Step2: Compressed air conveys materials at high velocity to the nozzle where the water is added. Step3: Then the material is consolidated on receiving surface by high impact velocity .
The dry process can be used for any shotcreting applications from the smallest patching and sealing works to largest projects. The maximum production achievable with dry process equipment ranges from 10-12 yards per hour of dry mix depending on the conditions. To achieve ultimate capacity it is necessary to use 2 -2.5 hose and larger compressors. Most applications have production rates of 2-6 cubic yards per hour of mix. ”
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WET PROCESSES
One typical type of wet-mix shotcrete system
WET PROCESS - AN OVERVIEW Working of wet process proceeds as per the following steps:
Step1: All ingredients, including water, are thoroughly mixed and introduced into the shotcrete equipment. Step 2: Wet material is pumped to the nozzle where compressed air is introduced to provide high velocity for placement and consolidation of the material onto the receiving surface. Step 3: Mostly wet-process shotcreting is done with premixed mortar or small aggregate concrete. The mix design and consistency of supply are very important in order to provide a mix with the workability or plasticity to be pumped through a small-diameter hose.
Sand gradation is the same as for the dry process and for coarse aggregate mixes; 20-30 percent of 20mm aggregate is added. Cement content will vary according to the aggregate gradation and surface area. However, most mixes contain approximately 700 pounds or more of cement.
NOVEL SHOTCRETE MATERIALS
POLYMER MODIFICATION POZZOLANIC ADMIXTURES FIBER REINFORCEMENT LOW VELOCITY SHOTCRETE STEEL FIBRE REINFORCED MICRO SILICA SHOTCRETE
EQUIPMENTS USED IN SHOTCRETE
Batching and mixing equipment Admixture dispensers Air compressor Nozzles
APPLICATION OF SHOTCRETE
Support of underground openings in tunnel, mines, drainage adits and exploratory audits. Rock slope stabilization and support for excavated foundations, often in conjuction with rock and soil anchor systems. Channel linings, protection of bridge abutments and stabilization of debris-f low prone creeks. Rehabilitation of reinforced concrete structures such as bridges ,chemical processing and handling plants. Rehabilitation of deteriorated marine structures such as bulkheads, piers, sea wall and dry docks.
Shotcrete finds a major application in tunneling. Tunnel lining is made much easy by this process
Rehabilitation of subway tunnels
Shotcrete technology proved to be more efficient and easy for the construction of domed roofs
Domes
ADVANTAGES
Little or no framework is required. Cost effective method for placing concrete. Ideal for irregular surfaces application. Allows for easier material handling in areas with difficult access. Easy start up, shutdown and clean up. An increase in load bearing capacity due to redistribution of stresses. Excellent corrosion resistance.
DISADVANTAGES
Rain may wash out the cement leaving a sandy surface, or it may saturate the shotcrete and cause sloughing or sagging Strong wind will separate the material between the nozzle and the point of deposit, reducing the strength It requires skilled and experienced labours. A greater degree of geotechnical knowledge is required Improperly applied shotcrete may create conditions much worse than the untreated condition
CONCLUSION
The use of Shotcrete to build new concrete structures as well as to restore and repair existing structures is well known and documented and it is perhaps the most diverse method available for concrete construction. The use of this innovative technology is increasing day by day and procedures for its proper performance are well developed and high quality work is regularly obtained. Shotcrete is a viable option for doing superstructure repair (unformed) and substructure repair (unformed) if done strictly according to specifications and best practices. It is a good substitute for the manual lay-up method for vertical and overhead applications using rapid set mortar.
REFRENCES
Shotcrete in concrete restoration by Ivan Razal (Ph.D., P.Eng, Technical Director Gemite Products Inc.) Bieniawski Z.T. 1989. Engineering Rock Mass Classifications. New York:Wiley. 251pages. Mahar, J.W., Parker, H.W. and Wuellner, W.W. 1975. Shotcrete practice in under ground construction. US Dept. Transportation Report FRA-OR&D 75-90.Springfield, VA: Nat. Tech. Info. Service. Morgan, D.R. 1993. Advances in shotcrete technology for support of underground openings in Canada. In Shotcrete for underground support V , proc. engineering foundation con f ., Uppsala, (eds J.C. Sharp and T. Franzen), 35 Standard practice for shotcrete concrete (U.S.ARMY MANUAL)
