Sprayed concrete mix process consists of dry mix and wet mix. Methods, equipments and advantages of these mix processes of sprayed concrete is discussed.
- Process of Sprayed Concrete Mix
- The Dry Process of Sprayed Concrete Mix
- Wet Process of Sprayed Concrete Mix
Process of Sprayed Concrete MixThere are mainly two different processes that are involved in sprayed concrete mix construction. They are:
- Dry process sprayed concrete
- Wet Process sprayed concrete
- Strength gain at early stages
- Dense - flow delivery during pumping
- Workability i.e. user-friendly
- Concrete with correct characteristics in setting
- Good pliability i.e. spraying ability
- For Wet sprayed concrete: the dense-flow process
- For the dry sprayed concrete: the thin flow process
- For the wet sprayed concrete: the thin flow process
The Dry Process of Sprayed Concrete MixA predetermined amount of cement and aggregate is mixed in correct ratios without water in the case of dry process. Into a purpose-designed machine, the mixture is fed that is pressurized and taken at a high-velocity airstream that makes the mix to be conveyed through the hose to the spraying nozzle. At the nozzle area, a spray of clean water is provided by atomized spray arrangement to hydrate the dry mix conveyed. The spray will provide the right water content to have the right consistency. This quality of consistency will help in the spraying of the material without any kind of interruption. The dry process makes use of the principle that the aggregate /cement ratio of the mix at a suitable amount with lower water-cement ratio can facilitate good transportation capability and workability, without the higher amount of water content or admixtures. The slump characteristic is not taken as criteria. Hence for limited thickness and overhead construction, the dry process works best. If necessary the admixtures are added in dry form to the dry cement and aggregate content, in the liquid form it can be added to the water that is sprayed or a separate injection for admixture can be provided in the equipment. Incorporation of steel or another type of fibers can be introduced in the pre-mix. Based on the throughputs, a wide variety of machines are available. They provide an accurately controlled rate of application i.e. low rates of application that can be applied to thin layers or in the case of intrinsic structures. High rates are also facilitated for the construction in tunnel slope stabilization and to cover larger areas.
Sprayed Concrete Dry Mix Process Equipment TechniqueThe nozzle application is controlled by the hand. The nozzle man will direct the stream of materials and controls the water addition in accurate measures. The variation of water amount can be applied within a limited range. So, it must be kept in mind that less water will not provide adequate compaction and more water will cause excessive slumping of the mix. In areas where it is hazardous for the nozzle man to apply the sprayed concrete, automated spraying machines are available which work based on robotic systems. This technique is very flexible and capable in a wide range of application. All type of cement, conventional aggregates, the light-weight aggregate can be employed. The aggregate to cement ratio of the mix lies within the limited range of 3.5 /1 to 4.0 /1 by weight. Because the rebound is aggregate, the cement content in the placed mix will be higher compared to the batched pre-mix. The characteristics of dry performance on its performance are:
- Good Density range
- High strength gain from 40 to 50N/mm2
- Higher bonding with the substrate
Sprayed Concrete Dry Mix Process EquipmentThe traditional method of having sprayed concrete mix is using twin chamber machines, for example, the Boulder Gun and their variations. This machine performed well with the use of refractory materials or mixes of dry sand and cement. This machine found to have a disadvantage when it was necessary to have a mix of water content greater than 3%. More water content results in blocking of the material in the traditional machines described before. Other Boulder gun types were developed that follows the sealed chamber principle that allows a constant feed by means of a rotary valve that feed the material into the chamber.
Fig.1: Dry Process Equipment and Application for Sprayed Concrete MixThe dry mix machines available present are the Rotor Type. This machine has an open hopper, into which most of the dry mix is fed. The material is dropped by gravity into a revolving barrel that is blown by means of compressed air into the spraying hose. Here they pass the water injection nozzle, where they are treated with water. The water amount is based on the judgment of the nozzle man. The machine to the nozzle distance can be far apart, say almost up to 600 meters. The presently developed machines for the dry process have the capacity to handle moisture content to the maximum of 10% giving output volume of 10m3/hr. To avoid the blockage problems due to high moisture content, liners are provided in the rotor as well as in the air chamber. To perform application only by using pre-bagged engineering materials, special machines are developed. To have greater tension than other machines mentioned before, lubricated sealing plates and motors that are oversized are employed. This arrangement help in avoiding the escape of dust from the sealing plates to a higher degree. The dry spray machines are the only ones that facilitate the feeding the mix (sand: cement: aggregate) at a steady rate into the air stream. The nozzle man finds it difficult to maintain the water cement ratio if the feed rate is not at a constant rate. The applications of dry sprayed concrete mix are Fire damage structural repairs, tunneling, mining, swimming pools, zoo enclosure construction, bridge strengthening etc.
Advantages of Dry Process of Sprayed Concrete Mix
- Early strength is high, facilitating sealing and stabilization works at preliminary stages
- The holding time is unlimited
- There is no concrete waste generation
Wet Process of Sprayed Concrete MixThe wet process makes use of the mixture containing cement, aggregates, and weighed volume of water for the whole mix. The whole mixture is made before the pumping process is carried out. To bring the whole mix into position, high-velocity air is employed. This high-pressure air that is introduced into the nozzle plus the resultant velocity will spray the concrete to the area where it must be applied. The mix sprayed will be placed in a compacted state due to the momentum it gains through spraying. The supply of wet mixed sprayed concrete can be either ready mix or carried out through site batching. These can be also supplied in dry condition (pre-blended materials) in the bags. For low volumes, these bags work best. Also in areas, large wet concrete volume delivery is restricted, this method is a good choice. The water/cement ratio can be easily controlled with the help of wet process. A water/cement ratio of 0.45 can be easily attained using reducing plasticizers. The wet process gains higher strength characteristics due to the higher cement content. Mainly the cement content used range from 350 Kg to 450 Kg per cubic meters. The cube strength that is finally obtained is 30N/mm2 and 60N/mm2 at 28 days. The working hours of the mix can be extended up to 72 hours, with the help of hydration control admixtures. This provides greater flexibility. The cleaning of the whole system after each application can be prevented by this method. The mix should attain good quality to facilitate better strength and durability. A proper balance between the fine and the coarse aggregate must be maintained to have optimum performance. This check will also help in the reduction of the rebound effect. All the above-mentioned requirements must be attained by the specialist contractor in charge.
Wet Process Technique of Sprayed Concrete MixBased on the nature of the work, the spraying technique is varied. Basically, the thickness of the concrete layer is constructed up to 150mm. Once the final layer has set properly, more thickness can be made over it. The spraying of the new concrete layer over an existing one should be performed with care. The existing layer must be free from any deleterious substances. If any, they must be cleaned by means of jet washing with water from the top of the structure to the bottom. The wet sprayed concrete mix can be either structural component or a decorative finish. This will define which system is employed. The wet process does not facilitate the making of sharp details. But wherever they are required special specifications must be defined by the engineer in charge. These critical sections require proper attention while placing.
Sprayed Concrete Wet Mix Process Equipment
Fig.2: Wet Process Equipment and Process for Sprayed Concrete MixThe pumps used in the wet process include a machine that has piston and worm pumps. This is the area where the concrete is delivered in a dense stream form. The air and the accelerator arrangement at the nozzle will compact the material to the substrate. The worm pump machines can take concrete with aggregates of maximum size 4mm. This mix is mainly used for repairs and finishing works. The output of 4m3/hr is mainly obtained. A dense stream of concrete is also provided by the double piston pump. These seem to be pulse free while pumping. Higher outputs from 4m3/hr to 25m3/hr are obtained from the same system. Wherever large volume of concrete for structural works are required like tunnel construction, these methods are the best. Synchronized dosing is facilitated by providing accelerator dosing units. Aggregate size up to 20mm can be employed in the mix performed by the wet process. The rotor pump that is used in the dry process can also be employed in the wet process. These give an output of 4 and 15 m3/hr having a mix with an aggregate size of 20mm. Constant rate spraying with no pulsation effects is one of the best performance of the spraying machine. The pulsation effect causes segregation problems. Before the pumping process is carried out, all the delivery lines must be sealed with lubrication. The transport lines should be kept straight with only gentle curves. The pipes that are used for transporting should not have lips, kinks, and dents. The diameter of the transportation pipe should be appropriate for the mix to travel. The concrete build-up with regular use in the hopper and delivery system have to controlled by regular cleaning and maintenance.
Surface Finishes in Dry and Wet Process of Sprayed Concrete MixThe concrete is placed in a protruded slightly. This is later screened by means of a timber or steel derby. For thin coatings, trowelling is undesirable that it will disturb the impaction bond. Finished surface should also be kept as sprayed. Any other finishing other than by light trowelling methods will cause plastic cracking. To aid finishing, no additional amount of water should be used. Steel float or a wooden trowel is best for finishing. The finer the aggregate used in the mix, easier is the way to float.
Advantages of Wet Sprayed Concrete Mix
- The spraying output is more, up to 25m3/hr
- The level is rebound is reduced to a factor of two or four
- There is less dust generation. Hence the working environment is improved
- Spraying with less air demand
- High quality sprayed concrete