New Materials: Potential for microfine cement in tunneling

Potential for microfine cement in tunneling

The different types of materials used in tunnel construction include explosives, concrete, steel, shotcrete material, lattice girders, geomembranes, rock bolts/anchors, admixtures, fibres and rock reinforcement. The demand for a particular type of material depends on its availability, site conditions, transportation costs, project location, manufacturing capabilities and local taxes. Other key factors that play a vital role in the selection of construction materials are tunnel life expectancy and capital costs. External factors too play an important role in the selection of construction materials. For instance, chemicals such as accelerators and grouting agents used during construction contaminate groundwater and cause noise and vibration in transportation tunnels.

At present, explosives are the most important raw material used in tunnel construction. Broadly, the four variants of explosives used are low explosives, high explosives, packaged explosives and bulk explosives. The second most important raw material used in tunnel construction is steel. This is primarily used for steel rib supports, lattice girders, mesh reinforcement, rock bolting and anchoring, and self-drilling anchors.

Another key raw material used in tunnel construction is concrete. At present, various additives are used to improve the durability and strength of concrete. The different types of cement available in the country are ordinary Portland cement (OPC), fly ash-based Portland pozzolana cement, calcine-based Portland pozzolana cement, sulphur-resistant cement, etc. Concrete is used in rail tracks, vertical shafts and underground stations, among others. The material should be leak-proof, crack-proof, corrosion-resistant and durable, with a structural life of over 100 years. Ultrafine additives should therefore be added to increase the durability of concrete.

New types of materials are also being used to improve the durability and strength of tunnels. Steel fibre-reinforced concrete is being used in tunnel construction. It provides ductility in tension and compression as well as high resistance against the spalling of concrete. Micro silica or microfine cement is being used in pre-grouting to effectively control water seepage. Since the development of the New Austrian Tunneling Method, shotcrete in tunnels has been widely applied. Its most important features are durability, speed of application and cost effectiveness. Further, the use of synthetic fibres with micro-silica in shotcreting improves strength and reduces rebound. A case in point is the Allain Duhangan hydropower project where the material was used for grouting. Artificial ground freezing is being used for soft ground tunnelling to create a watertight barrier as a support system. Earlier, rib and lagging were used as principal tunnel liners to strengthen the material on the external part of the tunnels. Today, primary or safety shotcreting is done, which requires no lagging. This is followed by rock bolting.

Recently, tunnel contractors have started using a metakolin mineral admixture for high strength concrete and grouting. Self-compacting concrete has also emerged as a cost-effective option for casting heavily reinforced elements and complex geometrical shapes. Besides, retarder chemical is being used to increase the setting time of cement slurries. Geotextile membranes are also being used to make tunnels waterproof.

Another material which is gaining traction internationally is heat prompt reaction (HPR) fibre. What sets HPR fibre apart from the conventional polypropylene anti-spalling fibre is its outstanding flow capability in the molten state. In the event of a fire, this ease of flow permits quick and efficient formation of a permeable capillary network which vents off vapour pressure from the concrete, therefore reducing spalling.

Focus on microfine materials

As far as tunnelling is concerned, microfine materials hold a lot of significance, though their potential is yet to be explored. The particle size distribution (PSD) is the key factor to be considered while selecting the microfine material. For tunnel construction, microfine grout injection and microfine additives are used. There are two types of grout injections – pre-injection grouting and post-injection grouting. Pre-injection helps in consolidating the strata ahead of excavation. It helps in avoiding ground collapse, intercepting water paths, and hence ensures a speedy and economical process of excavation. Post-injection involves consolidation of the strata after the excavation activity. It is used for stopping water leaks and making the strata firm. It is used in most of the tunnel projects.

A crack/joint is the weakest portion of a concrete structure. A crack needs to be repaired by filling compatible materials using the pressurised grouting technique. OPC grout, due to its larger particle size, cannot fill cracks completely. However, in India, OPC is mostly used for grouting due to cost considerations. Fine sandy soil strata need consolidation to improve soil bearing capacity. It can be done by injecting specially formulated microfine cementitious grouts under pressure. Consolidated soil strata allow safe and effective excavation of the substrate without the fall of adjacent soil.

There are two fundamental types of grouts – particulate and non-particulate grouts. The former comprises cement, microfine cement, bentonite, etc., whereas the latter includes silicates, polyornithine, resins, etc. According to the European Standard for grouting, microfine cements are characterised by a specific surface area>800 m2/kg and d95<20 mm. Microfine cements are highly reactive with a short shelf life of around three months. Globally, the material is being widely used for underground construction; however, deployment in India is still at a nascent stage. The Bureau of Indian Standards has been working on finalising the standards for microfine cement for the past three years and is expected to come out with them soon.

At tunnelling sites (which often are at remote places), the following tests can be conducted to evaluate the suitability of microfine cements:

  • Viscosity: The less viscous the grout, the easier it is to inject into the strata with minimal pressure. The Marsh Funnel Test is a good indicator of the true viscosity of the grout slurry. A marsh cone flow time of 30 to 40 seconds is desirable for a fluid grout.

Penetrability of the grout can be analysed by using the Sand Column test. Fluid grout is passed through a glass tube filled with fine sand. The speed of travel of grout and the quantity of grout collected after passing it through the tube indicates the suitability of a material for the injection process. PSD is an important contributor for superior groutability/penetrability of any microfine cement grout. Before finalising the materials, PSD should be evaluated carefully. Grouts with PSD on the coarser side will not be able to pass through a sand column.

  • Filtration stability of the grout: Often, despite selecting the finest of grout materials, the grout may not get injected in the porous strata or fine fissures. Filtration of the grout is a result of a plug building at fracture constrictions which reduces the penetrability of the grout. Highly fluid grouts may bleed under pressure. Under the filtration stability test, a fluid grout is sucked through a metal tube fitted with steel wire mesh at one end. The grout is then emptied into a measuring cylinder and the volume is recorded. The procedure is repeated for the next fine wire mesh. The efficiency of grout mixing machines influence the result of this test.
  • Bleeding test: Bleeding of grout refers to the separation of solid particles from the fluid. Highly fluid particulate suspension grouts tend to bleed. Due to the particle shape, size and granulometry, water gets trapped in the inter-particle voids. This water gets squeezed out due to pressure or settlement of particles and is termed as bleed water. Microfine and ultrafine cement grouts are less prone to bleeding due to their compact and finer particle size.

Conclusion

Microfine cement grouts are environment friendly, economical, easy to handle and compatible with soil, rock, etc. Certain precautions need to be taken while selecting the microfine cement grout: the finer the grout, the better it is, therefore, d95<20 mm microfine cement grout must be used; the material should be as fresh as possible so that it does not lose its reactivity; date of manufacturing should be less than a month prior; mixed design of grout should be conducted at site; and penetrability of the grout is the most important parameter to be considered. At present, there is a lack of awareness about microfine cement grout in the country which is limiting its application. Further, there is no provision for the specification of quantity of microfine cement injection grouts during tendering.

To improve the durability of reinforced cement concrete structures, steps such as the reduction of water-cement ratio by compatible admixtures, reduction of calcium hydroxide generation in the concrete by using supplementary cementitious material, and increase in the packing density of the paste component by using compatible microfine materials must be taken.

Based on a presentation by Yatin Joshi, Business Head, Alccofine Micro Materials, Ambuja, at a recent India Infrastructure conference