Admixtures

Definition:

Admixtures are substances added to concrete or grout during mixing to alter its properties and improve its performance in tunneling and underground structures. They are carefully formulated to achieve specific objectives, such as workability, strength enhancement, reduced permeability, or protection against aggressive environments.

Uses/benefits:

Admixtures in tunneling and underground structures offer several uses and benefits, including:

Workability Enhancement: Admixtures improve the workability and flow properties of concrete or grout, making it easier to handle, place, and consolidate in complex tunnel geometries or narrow spaces.
Setting Time Control: Certain admixtures help regulate the setting time of concrete or grout, allowing sufficient time for placement and ensuring proper curing even in challenging underground environments.
Strength Development: Admixtures accelerate or enhance the strength development of concrete or grout, allowing for faster construction progress and early loading of the tunnel or underground structure.
Durability Enhancement: Admixtures improve the resistance of concrete or grout to various detrimental factors, such as chemical attack, freeze-thaw cycles, abrasion, or corrosion, thereby enhancing the overall durability of the structure.
Waterproofing and Sealing: Admixtures can enhance the water impermeability of concrete or grout, reducing the risk of water infiltration and ensuring the long-term integrity of the tunnel or underground structure.
Shrinkage Control: Certain admixtures minimize the drying shrinkage of concrete or grout, reducing the potential for cracking and improving overall structural performance.
Pumpability: Admixtures improve the pumpability of concrete or grout, allowing for efficient placement in deep or complex tunneling operations.
Environmental Friendliness: Some admixtures are formulated to meet environmental standards, promoting sustainable construction practices and minimizing the impact on the surrounding ecosystem.

Specifications:

Specifications for admixtures in tunneling and underground structures can vary depending on the specific project requirements, environmental conditions, and applicable standards. Some common specifications include:

Dosage: The recommended dosage or addition rate of the admixture based on the desired performance and the characteristics of the concrete or grout mixture.
Compatibility: Admixtures should be compatible with other components of the concrete or grout mixture, including cement, aggregates, and other admixtures, without causing adverse reactions or loss of performance.
Setting Time Adjustment: Specifications may include the required range of setting time adjustment achievable with the admixture to suit the specific construction schedule and conditions.
Strength Enhancement: Admixtures may have specifications related to the expected increase in compressive strength or other mechanical properties of the concrete or grout.
Permeability Reduction: Specifications may define the expected reduction in permeability or water absorption achieved by the admixture to enhance the durability and water resistance of the structure.

Types:

There are various types of admixtures used in tunneling and underground structures, including:

Superplasticizers: Superplasticizers, also known as high-range water reducers, improve the workability and flowability of concrete or grout without sacrificing strength.
Accelerators: Accelerating admixtures expedite the setting and early strength development of concrete or grout, allowing for faster construction progress.
Retarders: Retarding admixtures delay the setting time of concrete or grout, providing extended workability and flexibility in placement and finishing operations.
Water Reducers: Water-reducing admixtures reduce the water content required for a given workability, improving the strength and durability of concrete or grout.
Air-Entraining Admixtures: Air-entraining admixtures create tiny air bubbles in concrete or grout, enhancing freeze-thaw resistance and improving workability.
Waterproofing Admixtures: Waterproofing admixtures enhance the water impermeability of concrete or grout, protecting the structure from water penetration.
Corrosion Inhibitors: Corrosion inhibitors reduce the corrosion of embedded steel reinforcement in concrete, extending the service life of tunnel structures.
Shrinkage Reducers: Shrinkage-reducing admixtures mitigate drying shrinkage in concrete or grout, minimizing the risk of cracking and improving long-term durability. The primary function of a shrinkage-reducing compound is to lower the surface tension of the water in concrete pores, which in turn lowers the shrinkage stress that results from the pores losing water.
Fiber Reinforcement Admixtures: Fiber reinforcement admixtures introduce fibers into the concrete or grout mixture, enhancing crack resistance and impact resistance.

Admixtures significantly contribute to the performance and durability of concrete or grout in tunneling and underground construction. They enable the production of high-quality, workable, and durable mixes that can withstand the challenging conditions and requirements of tunnel projects, ensuring long-term structural integrity and reducing maintenance needs. IS 9103: 1999 provides the specification for concretes admixtures.

S. No.	Types of admixtures	Substances used
1	Superplasticizers	Modified lignosulphonate, synthetic polymers
2	Accelerators	Calcium chloride, Alkali carbonates, Sodium chloride, Potassium hydroxide, Sodium hydroxide
3	Retarders	Sugar, Calcium sulphate, Lignosulphonates, Sodium hexametaphosphate
4	Air-Entraining	Vinsol resin, Alkyl sulphonate
5	Waterproofing	Soda, potash soaps, fat, coal tar residue, resin, NT-1, CX-SUN waterproofing agents
6	Corrosion Inhibitors	Cationic rust inhibitor (nitrite, chromate and benzoate), thiohydroxyaniline, sodium carbonate and sodium hydroxid
7	Shrinkage Reducers	Alcohol shrinkage, polyoxyethylene shrinkage, low molecular weight alkylene oxide compounds, high molecular weight comb polymers containing polyoxyalkylene chains
8	Fiber Reinforcement	Steel fibers, Polypropylene fibers, glass fibers, asbestos, carbon and organic fibers