Durable concrete for tunnelling application
In this first part of the two-part series,
EugenKleen of Mc-Bauchemie Mueller GmbH and Co.KG
spells out the properties required in concrete and the types of concrete used in tunnelling application.
Over the 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 tunnelling 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 tunnelling 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
• Surface finish
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
• 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 Sillica 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 adapted for tunnelling segments, considering special properties as well as low cost maintenance strategies.
What type of concrete do we use?
Concrete used in tunnelling 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 tunnelling applications can be classified into:
• Self compacting concrete / high workability concrete
• Concretes resistant against aggressive media
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 aggregated should be fully soaked in the concrete matrix. In addition, the concrete should be self-levelling type and self-defoaming without any external compaction.
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 SCC. Self compacting concrete has the following special advantages.
• Saving of cost 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 adhesive between cement binder and aggregates
• Reduction in de-moulding 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.
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.
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 the 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.
(Source: Paper presented by the author at the Construction Chemicals International Conference 2012 held in Mumbai)