By Mike Rispin
Our European tunneling colleagues prefer to call wet shotcrete “sprayed concrete” and the reasoning for this is sound: shotcrete is concrete and all the basics apply. Yes, the primary difference of shotcrete vs. concrete is the method of placement, and there are substantive differences in mix design, aggregate gradation and, in most cases, the addition of an accelerator at the nozzle to optimize placement, as well as vastly different equipment requirements. The combination of these involves specialized knowledge, but the basics of concrete still apply.
When considering economies of scale, applied shotcrete volumes, and in-situ quality, it is clear why wet shotcrete is now the dominant choice over the dry method. Industry average rate of application is in the range of 12-15 m3 per hour, with peak volumes exceeding 25 m3 per hour, and with a good mix design, equipment selection and experienced operators, the expected rebound rate is 5-7%. Where necessary, applied thicknesses can be built up, even overhead, to 300 mm in a “single” layer. Of greatest importance is that the water/cementitious (w/c) ratio is prescribed and managed at the batch plant and this allows for the engineering of more rapid initial and final set, and of optimized ultimate strength development. Workability of plastic shotcrete can be economically achieved at low w/c ratios (0.38 – 0.42) and the hydration process can be managed to provide working times of up to 72 hours.
A “holistic approach” for wet shotcrete is recommended because of the myriad factors from batch plant to workplace to placement to performance that must be considered in order to achieve a durable concrete structure:
- Expected load conditions
- Buildable design
- Relevant specification
- Raw materials
- Batch plant production
- Robotic equipment
- Accelerator selection
- Sand/aggregate grading
- Cement type and amount
- Hydration control admixtures
- Type of plasticizer/superplasticizer
- Accelerator type
- W/C ratio
- Accelerator dosage
- Nozzle systems and set up
- Nozzle distance
- Nozzle angle
Before the Batch Plant
It’s a given that your raw material selection and mix design have to be right. ACI 506 has readily available a Guide for Specification of Underground Shotcrete and the author recommends the document as a very substantive work. However, it does not answer all questions and it is recommended to consult your cement and admixture suppliers in mix design, as the pace of technology can change rapidly, and local knowledge can be important in optimizing your cost efficiency.
Regardless of the makeup of your mix design for achieving 28-day strength, you need to tailor your plastic concrete to a w/c < 0.45 (preferably 0.42 or lower) and a workability measured in slump (>18 cm/6.5 – 8 in.) or flow (> 50 cm/20 in.). Stiffer shotcrete mixes, while possibly placeable, will not fill a shotcrete pump’s cylinders properly and this will result in a host of inefficiencies including reduced output, increased accelerator consumption, and layering, a phenomemon caused by poor matching of accelerator dosing to shotcrete flow.
At the Batch Plant
Water-reducing admixtures and superplasticizers, or today’s hyperplasticizers, are the key to workability, pumpability and mix consistency at low water/cement ratios. Polycarboxylic ether technology sets the standard today, and dosages are typically in the 0.5 to 1.5% bwc (by weight of cementitious material).
An important decision for underground shotcrete is to apply a hydration control admixture (HCA) at the batch plant. Hydration control admixtures are differentiated from conventional retarders in that they act on all four phases of cement hydration (as opposed to retarders which act on 1-2), effectively putting the concrete to sleep until the dosage wears off or they are counteracted with the application of an accelerator during spraying. Typical dosage to allow an additional 4-6 hours of open time, beyond an untreated mix which gives a maximum of 2.5 hours, is 0.5-1.0 % bwc. Given the myriad challenges in getting plastic shotcrete to the underground workplace, this is very cost-effective “insurance” and also enhances the performance of the shotcrete, effectively keeping it “fresh” until the point of application.
Microsilica, or silica fume, is an important constituent for improving pumpability of the plastic shotcrete and to achieving durability of the in-situ structure.
Finally, you may wish to consider a pumping aid, available from your admixture supplier, which helps overcome weaknesses in mix design due to less-than-superior raw materials. These pumping aids also improve consistency.
Over the last 15 years, the author has seen steel fibers become a preferential alternative to wire mesh, as a reinforcing element and to resist spalling in the event of a fire. Now, Macro synthetic fibers are the evolutionary alternative of choice from a cost/performance benefit perspective over steel fiber.
A surprising amount of wet shotcrete application continues to be achieved in North American tunneling today via the hand application method. In particular, when doing detailed work around reinforcement such as rebar cages and, in some cases, lattice girder installation, this can be beneficial and at the same time relies heavily on the skill and experience of the nozzleman. This is not a job for the faint of heart (or arms). It is best to seek the services of an ACI Certified Nozzleman.
In most cases, the better application approach is through a robot, complete with arm, shotcrete pump and accelerator dosing system, and also possibly with an on-board compressor to ensure proper air pressure and volume.
The advantages of robotic application are clear:
- Increased safety and production
- Good compaction
- Outputs up to 30 m3/hr
- No need for lifting platforms
- Attractive overall in-situ cost
Shotcrete Pump: Not all sprayers are created equally; indeed, nor are all pumps. Should you choose a robotic sprayer, you can be assured of getting a properly specified shotcrete pump. However, should you opt for a stand-alone pump and a crawler mounted robotic arm, for example, be sure to select a built-for-purpose shotcrete pump. A concrete pump that is on-site that is otherwise fine and readily available would not be the right choice for shotcreting.
An integrated accelerator dosing system is essential.
Nozzle Set-Up and Air Supply: This is a key factor for ensuring that proper mixing of accelerator and projection of the shotcrete at the nozzle occur.
Compressed Air: Air for shotcrete must be at 6-7 bar pressure and, often overlooked, the supplied volume must be at 10-12 m3/minute.
Application Parameters: Keeping the nozzle at a 90 degree angle to the substrate is critical in that this optimizes compaction and minimizes rebound, getting more of your shotcrete in place and doing what you have designed it to do.
Nozzle distance to the substrate, when robotically spraying, should be from 1-2 m, with the average of 1.5 m being a good target.
Start low in application, filling in the first 0.5-1 m on both sides of the excavation. Avoid trapping rebound and forming pockets of poorly compacted material. Work up to the back of the excavation.
Where steel arch or lattice girder emplacement is required, “weld” the support structure to the substrate with oscillation of the nozzle turned off. Use the support structure to support further shotcrete emplacement. Make use of the oscillator once “welding” is finished.
Shotcrete Sprayer: Key operational features in the sprayer selection process are:
- Tramming capability
- Ease of operation
- Boom stability and reach
- Pump hopper synched in height to the shotcrete delivery system
- After-market parts and service availability
- Training from the supplier
There is a wide selection of commercially available sprayer configurations available, allowing robotic application of shotcrete from large headings to cross passages and smaller, more detailed work.
Accelerator Selection: Today, there is no reason not to choose an alkali-free accelerator (AFA) for your project. While silicates are still available on the market and, in some jurisdictions globally highly unsafe, antiquated aluminates, AFAs provide improved working safety, less strength differential to a base shotcrete mix, less dust and rebound, and improved sulfate resistance when using ordinary Portland cement. The pH of AFAs is in the range of 3-4, comparable to the acidity of orange juice.
Note that due to the pH being acidic, all AFA accelerator tanks must be made of non-corrosive material such as stainless steel or plastic.
EFNARC Robotic Nozzleman Training: While the ACI Nozzleman Certification program, referenced above, is alive and well in North America and substantially covers the field for hand nozzling, no current programs exist domestically specifically for robotic application.
In Europe, the EFNARC body has designed a training program for robotic nozzlemen and is currently systematically offering certification for trainers. A number of North Americans have been certified as instructors, and their qualifications include substantial experience and actual hours on sprayers.
Testing: A variety of tests exist for both green and hardened shotcrete, many of them ASTM specified.
Early age strength development is typically tracked at its earliest stages with a penetration needle test. This is followed after some hours of hardening with either a stud driving test or end beam compressive test.
Mature shotcrete is tested for compressive strength with cylinders retained at the time of placement or of coring of the in-situ structure. Toughness is tested through beam or panel (such as ASTM C-1550 Round Panel) testing.
Curing: Referring back to the introduction, whereby “shotcrete is concrete,” this is no more true when considering curing. Shotcrete structures need to be cured or the following may occur:
- Rapid moisture loss and drying resulting from tunnel ventilation systems
- Relatively high cement contents in the shotcrete mix and early reaction temperatures due to acceleration may cause rapid drying and shrinkage cracking
- Without proper curing, poor bonding, low strength and reduced durability may result
The secret is to prevent the moisture in the shotcrete from evaporating too quickly. This can be achieved with the application of a curing compound or with moisture blanketing or water sprays.
Single Shell Lining
High-performance shotcrete is enabling the global drive to consider single shell linings (shotcrete/spray-on waterproofing/shotcrete) as an economic alternative over double shell linings. This represents a step-wise change in the philosophy of tunnel lining design and, in some cases, the use of the inner shotcrete lining as the final lining. Time and cost savings are the drivers for this but final lining shotcrete can also provide a versatile and aesthetically pleasing finish to underground infrastructure.
If sulfate resistance is required for your project, it is preferable to manage it through mix design and the judicious use of microsilica, as opposed to defaulting to sulfate resistant cement, as this will have a mitigating effect on strength development.
Knowledge of the moisture content of your aggregates is critical. Excessive resident water will alter your w/c ratio.
The storage location of your chemical admixtures, including accelerator, and the resultant impact on their temperature cannot be neglected. Cold constituents will hamper your applicability and strength development.
Never forget the importance of lighting in the underground workplace. This can be such a critical determinant of effectiveness of your shotcrete program; simple, yet often overlooked.
Length of reinforcing fibers in your shotcrete mix should not exceed 60 percent of pump line diameter. This is particularly true in the case of steel fibers as macrosynthetic fibers may be a little more forgiving.
Shotcrete, or sprayed concrete, is a relatively complex discipline when considering all aspects of a state-of-the-art operation, but it is also relatively well understood and applied regularly and successfully in today’s tunneling industry. In North America, it has become a wholly accepted ground support system of choice, and further development in ideology will allow it to go even further.
Mike Rispin is Vice President – Americas for Normet Group.