Engineers are frequently required to specify some type of support system in order to maintain the stability of an underground opening. In most cases, this support system is a composite structure made up of rock bolts, welded wire mesh, steel sets, lattice girders, and/or shotcrete. Shotcrete can serve a variety of functions when used as part of a support system. It can first be used as a simple protective layer to keep the exposed rock safe from the elements. It can also be used as a structural layer in conjunction with rock bolting, steel sets, and/or lattice girders.
Shotcrete is structural in this case because it provides support for the rock bolts and steel sets or lattice girders. This type of support is used to keep the excavation stable and prevent rock blocks from falling out.
Finally, shotcrete can be used in some openings to provide complete structural support to the surrounding ground, known as a shell action. Shotcrete shells that are generally thicker and work primarily in compression are usually reinforced and may include fibres.
Batching, Mixing, and Pumping Wet-Mix Shotcrete
This is probably the part of the shotcrete process that has benefited the most from recent advances in concrete admixture technology.
Indeed, thanks to the use of high-range water reducers and superplasticizers that allow mixtures to be designed with water/cementitious material ratios (w/cm) as low as 0.35, concretes with compressive strengths of 60 MPa (8700 psi) can now be mixed and pumped.
Even with significant amounts of steel or synthetic fibres in the mix, it is now possible to create robust and flexible mixtures that will not cause unwanted surprises when it comes time to pump.
In order to increase the robustness against potential pumping problems, two important approaches have been increasingly used in recent years: the optimization of the aggregate phase and the use of the concept of temporary high initial air content.
An Introduction to Mix Design for Shotcrete
When compared to conventional form-and-pour concrete mixes, a properly designed concrete mix design for shotcrete has higher compressive strength, lower permeability, and improved durability. Here’s a quick primer on modern shotcrete mix design, whether you are an engineer specifying shotcrete or a contractor inspecting work in the field.
To develop a concrete material that meets or exceeds project goals, a qualified and approved shotcrete contractor should first communicate with the engineer and ready mix QC. Mix design expectations and goals are increasingly being set using performance-based specifications (PBS).
Prescriptive specifications, in the past, simply told a contractor what to provide, ignoring operational characteristics like pumpability and workability. Allow the manner in which your shotcrete contractor begins this process to reveal their capabilities and expertise.
A Few Shotcrete Mix Design Considerations
Water, cement, and aggregate are the three basic ingredients in any concrete mix. The use of concrete admixtures is the fourth important component of a shotcrete mix design. The water-to-cementitious-material (W/CM) ratio in shotcrete concrete is much lower than in traditional form-and-pour mixes.
Shotcrete material’s water W/CM is typically in the 0.35–0.45 range. A water reducing (WR) agent is frequently used in shotcrete mix designs. There are two advantages to this inclusion. As previously stated, the reduction of water in the mixture allows for a lower W/CM.
A WR agent also improves the ability of a shotcrete contractor to pump material through hoses and pipes. Due to freeze/thaw cycles, entrained air extends the life of concrete by increasing hardened, in-place durability. Entrained air also aids the contractor’s ability to pump material through the system more easily.
Hydration control is a third admixture that is frequently used during shotcrete operations. During the warmer months or for long transit times, a set retarder can be used, and during the colder months, a concrete accelerator can be used.
Due to freeze/thaw cycles, entrained air extends the life of concrete by increasing hardened, in-place durability. Entrained air also aids the contractor’s ability to pump material through the system more easily. Hydration control is a third admixture that is frequently used during shotcrete operations. During the warmer months or for long transit times, a set retarder can be used, and during the colder months, a concrete accelerator can be used.
Shotcrete Mix Raw Materials
Portland cement or ordinary Portland cement should be preferred.
The coarse aggregate should be gravel or pebbles that are hard and durable. The particle size should not exceed 16mm, the aggregate grade should be continuous, the fine aggregate should be hard and durable medium or coarse sand, the fineness modulus should be greater than 2. 5, and the water content should be between 5% and 7%.
The appropriate accelerator should be chosen based on the cement type, water-cement ratio, and other factors, with an initial setting time of no more than 5 minutes and a final setting time of no more than 10 minutes.
Concrete water should meet the relevant engineering water standards, and it should not contain harmful substances that interfere with normal cement condensation and hardening. Use sewage and acidic water with a pH less than 4 and a high sulphate content. This proportion is based on plain water.
Shotcrete Mix Design
The parameters include cement dosage, gray bone ratio, sand ratio, water-cement ratio, and slump.
The amount of cement used to control the amount of cement should be kept between 400 and 450 kilogrammes per square metre. If the amount of cement used is insufficient, the amount of rebound is high, and the initial strength is slow to increase. The strength of shotcrete increases as the amount of cement used increases, while the rebound decreases. Excessive cement use, on the other hand, is uneconomical. Second, the amount of dust generated during construction increases, the construction environment degrades, and concrete shrinkage increases as it hardens due to condensation.
Gray to Bone Ratio
Depending on the construction process, the cement to aggregate ratio should be different. The weight ratio of cement to sand and stone should be 1:4 -1:4.5 when dry spraying is used. The weight ratio of cement to sand and stone should be 1:3.51:4.0 when wet spraying is used.
The sand rate should be kept within the range of 50% to 60%. The sand rate has been shown to affect both the shotcrete’s construction performance and its mechanical properties in practise. The rebound rate is high when the sand rate is less than 50%, the pipeline should be blocked, and the shotcrete strength is highly discrete. When the sand ratio is too high, greater than 60%, the shotcrete’s strength is reduced due to a lack of coarse aggregate. The amount of cement used to achieve the desired slump and fluidity should be increased, but neither the economy nor the concrete shrinkage will be affected.
Water to Cement Ratio
The water-cement ratio of wet-spraying constructions ranges between 0.42 and 0.50. The rebound rate and strength of shotcrete are affected by the size of the water-cement ratio. The rebound rate can be reduced if the water-cement ratio is too large, but the strength is reduced; if the water-cement ratio is too small, the strength is high, but the rebound rate is also increased.
If the surface of the sprayed concrete is flowing, slipping, or cracking, the water-cement ratio is too large, and the amount of water added needs to be reduced; if there are dry spots on the surface, there is a lot of dust in the operation, and there is a lot of rebound, the water-cement ratio is too small.
It is necessary to gradually increase the amount of water used; when the surface is flat, shiny, shiny, and free of dust and rebound, the water-cement ratio is appropriate.
When evaluating concrete fluidity, cohesiveness, and water retention, the slump is an important indicator. The slump is 8 to 12 cm when wet spraying is used. The slump has the lowest shotcrete rebound rate in this range, according to practise.
Selection of Concrete Mix Ratio
The shotcrete mix ratio is generally determined by a combination of empirical formulas and construction techniques.
Spray Concrete Mix Design Method
The shotcrete mix design should include two parts: a traditional mix design and a shotcrete field test spray. The first part is based on shotcrete requirements, and the reference ratio is suggested based on conventional concrete mix design. The latter part, which is based on the reference ratio, adjusts, verifies, and determines the mix ratio on site, is based on the reference ratio. Both steps are necessary and complement each other.
|Grades of Concrete||Ratios of Concrete mix design|
(Cement: Sand: Aggregate)
Before making a mix design for shotcrete or sprayed concrete, it is needed to get sufficient information from the clients. Adequate support information will result in a technically and economically effective for the work. A proper mix design assures a successful trial and thus, successful application on-site.
Hereafter is a list of 9 questions to ask the clients prior to making a shotcrete mix design:
1. The location of shotcrete to be applied? Slope stabilization, overhead and wall in the tunnel.
2. Wet or Dry process will be applied? Normally, dry process for slope stabilization, the wet process for tunnel lining.
3. The requirement of early strength (8 hours, 72 hours, 7 days, 28 days) and the final strength at each location.
4. Slump requirement (for wet process only)
5. What accelerator admixture to be used? Please note that it would be better to use liquid admixture in the rainy season.
6. What is the concrete spraying machine to be used? Rotor Machine (for both wet and dry process) or Pump Machine (wet process only)
7. What kind and type of cement to be used (cement should be super fine. Blain Surface fineness is greater than 4000 square centimetres per gram) PC cement is most suitable for early strength requirement.
8. A dispenser system needs to be installed in case the wet process is used. Is such a liquid dosing unit available?
9. Any other information is helpful in designing the shotcrete mix?
Read More – https://shotcrete.org/wp-content/uploads/2020/01/2003Sum_jolinbeaupre.pdf
Once these questions get answered, the mix designing process can be started. For shotcrete or sprayed concrete, the trial is a must be prior to the issue of mix design for the actual application.