Concrete

Concrete is a composite material composed of cement, water, and aggregates (such as sand, gravel, or crushed stone) mixed together in specific proportions.

Introduction

Concrete is a composite material composed of cement, water, and aggregates (such as sand, gravel, or crushed stone) mixed together in specific proportions. It is one of the most widely used construction materials in the world, due to its strength, durability, and versatility. In tunneling and underground structures, concrete is often used for the construction of tunnel linings, floors, walls, and other structural elements.

Uses/benefits:

Provides structural strength and stability to tunneling and underground structures.
Can be cast in any shape or size, allowing for customization and flexibility in design.
Can withstand high pressures, making it suitable for use in tunnel linings where the ground above exerts significant pressure.
Has good fire resistance, making it suitable for use in underground structures.
Provides insulation against heat and cold.
Can be made with a range of textures and finishes to suit various design aesthetics

Specifications:

Strength (IS 456): The strength of concrete is typically measured as the characteristic compressive strength of 150 mm cube at 28 days (IS 456). The strength required for tunneling and underground structures depends on the specific application and the surrounding geology.
Density: The density of concrete is typically measured in kg/m3 (kilograms per cubic meter). The density required for tunneling and underground structures depends on the specific application and the surrounding geology.
Workability: The workability of concrete refers to how easily it can be mixed, transported, and placed. The workability of concrete required for tunneling and underground structures depends on the specific application and the surrounding geology. Use of air entraining agent (up to 4 %) for the improving workability of concrete is suggested as per IS :5878 (part v)-1976. Also, the slump of concrete should not be less than 10 cm. Additionally, slump should be reduced to 5 cm whenever it is placed directly in Inverts.
Set time: The set time of concrete refers to how long it takes for the concrete to harden after it has been mixed. The set time required for tunneling and underground structures depends on the specific application and the surrounding geology.

Types:

There are several types of concrete used in underground structures and tunneling, including:

Ordinary Portland Cement (OPC) concrete: This is the most common type of concrete and is used in a wide range of applications, including tunneling and underground structures.
High-strength concrete: This type of concrete has a higher compressive strength than OPC concrete and is used in applications where higher strength is required.
Self-compacting concrete: This type of concrete has a high flowability and can be poured into tight spaces without the need for compaction.
Fiber-reinforced concrete: This type of concrete contains fibers (such as steel or synthetic fibers) that improve its strength, durability, and toughness.
Lightweight concrete: This type of concrete has a lower density than normal concrete and is used in applications where weight is a concern.

General requirements (Transporting Concrete):

For the tunnels having short length and also, where requirement of concrete is large in volume, a suitable site for batching plant is necessary so that mixed concrete conveyed (through truck mounted mixers, agitator cars, belt conveyors) without any delay in the time of transportation which may affect the quality of concrete by overmixing.
For the longer length of tunnels, batching and mixing of concrete in dry condition and then, convey it in the locations by means of dumpers or tipping wagons should be advantageous for the effectiveness of the quality. The dry mixed aggregates are then remixed at site as per the requirement and to obtain specified slump and W/c ratio.