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Durable Concrete for Tunneling Applications

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Over the past decade, the use of concrete admixtures, especially plasticizers and superplasticizers, is showing upward trend in India.  The advent of concrete pumps and transit mixers has also contributed to this, as the use of superplasticizers enables trouble-free pumping operations and minimizes pipe blockages. With the advent of Major Metro Projects across India, durability of concrete used especially for tunneling segments is of prime importance. The earlier attitude of taking recourse to the use of admixtures only after facing problems is changing fast, and now, in most tuneling projects, high performing admixtures are already included in the specifications and the mix is designed to achieve the necessary properties.

The concrete for tunnel segments necessitates the concrete to have the following properties:

–    Compressive strength
–    Workability
–    Surface Finish
–    Durability

As part of the durability requirements, concrete is or should be generally tested for the following properties:

–    Chloride migration
–    Sulfate resistance
–    Water absorption
–    Acid resistance
–    Porosity
–    Freeze Thaw Resistance

This can be achieved using the latest technologies available for concrete. Concrete is now no longer a material consisting of cement, aggregates, water and admixtures but it is an engineered material with several new constituents like PFA, GGBSF, Microsilica, Metakaolin, Colloidal Silica and several other Binders, Fillers and Pozzolanic materials. The concrete today can take care of any specific requirements under most exposure conditions.

The mix designs are getting relatively complex on account of interaction of several materials and mix design calls for expertise in concrete technology and materials. High Performance Concretes will have to be adopted for tunneling segments, considering special properties as well as low cost maintenance strategies.

3What type of Concrete do we use?

Concrete used in tunneling applications need the following outstanding properties viz. Compressive Strength, High Workability, Enhanced Resistances to Chemical or Mechanical Stresses, Lower Permeability, Durability etc.This will necessitate the use of High Performance Concrete. Some HPC types which will hold the key for tunneling applications, can be classified into:

–    Self Compacting Concrete / High workability concrete
–    Concretes resistant against aggressive media

1. Self-Compacting Concrete (SCC)

Self-Compacting or Consolidating Concrete (SCC) as the name signifies should be able to compact itself by its self-weight under gravity without any additional vibrations or compaction. Self Compacting Concrete should be able to assume any complicated formwork shapes without cavities and entrapment of air. The reinforcement should be effectively covered and the aggregates should be fully soaked in the concrete matrix. In addition, the concrete should be self-leveling type and self-defoaming without any external compaction. Figure 1shows SCC.

The formulation of Self Compacting Concrete has the latest concrete technology and it requires in-depth knowledge of materials and meticulous testing procedures before the concrete is designated as Self Compacting Concrete (SCC).Self Compacting Concrete has the following special advantages.

–    Saving of costs on machinery, energy and personnel for vibrating the concrete
–    Considerable improvements to exposed surfaces (Fair Faced Concrete), less efflorescence.
–    Marked improvements in durability on account of better compaction
–    Extremely suitable for slim and complicated moulds
–    Covers reinforcement effectively.
–    Better adhesion between cement binder and aggregates.
–    Reduction in demoulding time
–    Advantage with respect to sound pollution

Therefore while calculating the costing and economics of Self Compacting Concrete all the above mentioned advantages should be converted to cost parameters. This kind of concrete can give advantage of good Compressive Strength, workability and finish to the tunnel segments and may prove suitable.

2. Durable Concrete resistant against aggressive media

One major application of HPC is to increase the durability of concrete where aggressive underground conditions are anticipated. This can be achieved physically by resorting to very dense aggregate packing. The packing curve is shown in Figures 2a and 2b. This is practically possible by selecting a very smooth sieve line from largest aggregate to the smallest grain of Mineral Additives like Microsilica or New Generation Aluminosilicate slurries. Chemically, cement by itself is not acid resistant. The acid resistant binder is formed by combination of cement, microsilica / aluminosilicate and flyash.To control permeability very low water cement ratio has to be adopted. So as to provide the essential concrete properties a high-performance PCE (polycarboxylate ether) needs to be incorporated in the mix. By adjusting the particle size distribution on a micro scale the permeability of the concrete is reduced which minimizes the penetration of aggressive substances. Depending on the degree of dispersion these material particles more or less completely fill the spaces between the cement particles. During hydration the pozzolanic silica reacts with the free calcium hydroxide to form calcium silicate hydrates. This gives a denser concrete structure.

2The main materials, which can help change normal concrete to durable aggressive media resistant concrete, are:

–    New Generation PCE Based Admixtures
–    Condensed Silica Fume or Microsilica Slurry or
–    Latest Generation Aluminosilicates

a. PCE Based Admixtures: Most of the new generation superplasticizers are from the Acrylic Polymer (AP) family. Polycarboxylate is a common term for the substances that are specifically used as Polyacrylate or Polycarboxylate ethers (PCE). The PCE based Super Plasticizers are by far superior to the conventional ones with respect to initial slump as well as slump retention with time. The efficient working of these plasticizers is due to the new type of molecule designs. PCE based superplasticizers produce excellent properties when used with cementitious materials. The disadvantages associated with longer setting times of conventional superplasticizers is offset by PCE based super plasticizer and therefore its use in concrete can also attain high early strengths.Figure 3 shows the structure of PCE molecule and its working mechanism – steric hindrance. The development of highly effective super plasticizers with long and consistent duration of action is therefore an important precondition for the production durable concrete, due to low water contents and high early strength requirements.

1Concrete additives based on PCE offer advantages like:

–    Significant reduction of the water demand of the mix
–    Little loss of consistency
–    Short setting times
–    High early strengths
–    Low tendency to segregation

The advantages of these New Generation polymers are very clear, not only in terms of performance but also in terms of the dosages used for similar conditions and this factor balances the disadvantages in economy, as New Generation Superplasticizers are relatively expensive per unit price.Figure 4shows workability comparisons of MSF/SNF against PCE. Figure 5 shows comparative development of compressive strengths and the dosages required are very low.

b. Condensed Silica Fume / Microsilica:The term “Microsilica” is adopted to characterize the silica fume, which is used for the production of concrete. Microsilica or Condensed Silica Fume (CSF) is a by-product resulting from reduction of high purity quartz with coal in the Electric Arc Furnaces used in manufacture of Silicon, Ferrosilicon and other alloys of silicon.

There are three main reasons for the incorporation of Silica Fume as an additive for HPC. Microsilica has a filler effect i.e very fine particle distribute itself in the space between the materials in the concrete in a homogenous way to give rise to more dense concrete. Silica Fume improves the strength of the transition zone between cement paste and aggregates.  CSF is highly pozzolanic in combination with Portland cement. Figure 6 shows structure and effect of Microsilica.

During cement hydration there is surplus of Calcium Hydroxide. The Added Condensed Silica Fume’s SiO2 reacts with surplus of Calcium Hydroxide.  This results in greater amounts of Calcium Silicate Hydrate, which are denser and stronger than Calcium Hydroxide. The pozzolanic reaction and the filler-effect lead to a compaction of the cement paste and the conversion of CH crystals into CSH gel leads to a homogeneous paste. This phenomenon of dense packing in the interface zone of aggregates also contributes to increase in strength of the concrete on account of aggregates fully contributing their strength to the set concrete. Therefore the high strength of concrete with silica fume is greater than those of the matrix, indicating the contribution of the aggregate to the total strength.Experience shows that slurry forms of Microsilica (50:50 with water) have all the benefits in transportation, dispensing methods, mixing times and dispersions to get the desired effect in durable concrete for tunneling segments.

3. New Generation Aluminosilicates:New generation aluminosilicates based on special nano-crystallizers have been developed. These new materials improve the properties that are crucial for the durability of high-performance concrete. In addition to reducing chloride migration, an exceptional chemical and resistance to aggressive media of the concrete can be achieved with Aluminosilicates. The concrete structure is simultaneously reinforced right down to nanoscale, density is improved and compressive and flexural strength as well as abrasion resistance of the high-performance concrete is increased. There is also a significant reduction of micro-crack formation,which makes it particularly suitable for the production of tunneling concrete. Aluminosilicates reduce the proportion of portlandite by way of a pozzolanic reaction that changes it into the aluminosilicate crystals into calcium silicate hydrate. In addition to the unique resistance against acids a crystalline micro-reinforcement within the concrete structure is achieved. This reduces the risk of micro-crack formation, rendering concrete impermeable.

Due to high homogeneity and reduced tackiness compared with microsilica-basedconcrete, workability is improved significantly. In many instances this enables the production of high-performance concrete that can be pumped. In addition, a distinct improvement of the building structure’s aesthetics is gained due to the fair appearance of the concrete surface.Aluminosilicates performs over the some of the disadvantages of Microsilica:

–    Graded for dispersion in concrete
–    Graded particle size
–    Optimizes mixing time within concrete
–    Good dispersion reduces unreacted material in the mix and increases passivation by C-S-H gel on aggregate surface
–    Material if agglomerated improve strength of the mix
–    Reduces risk of Alkali Silica Reaction by Agglomeration of aluminosilicate particles

Table 2 shows some of the key differences between Microsilica and Aluminosilicate slurries. Figure 7 shows the comparison of strength development between Microsilica and Aluminosilicates.

All in all the use of PCE Admixtures and Microsilica or Aluminosilicate Slurries in addition to the standard ingredients in concrete, plus excellent mix-design practices can facilitate the production of high performance concretes resistant to aggressive media, suitable for use in tunneling applications.

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