CLSM for backfilling in confined spaces

Nagesh Veeturi

Executive Director - Civil, KEC International

Sumanta Sahu

Chief Manager - Quality, KEC International

Backfilling is often a neglected activity in construction industry. Improper backfilling leads to settlement of structure and seepages due to water stagnation in backfilled materials. Normally, murum soil is used as backfilled materials. The process involves layer-wise spreading of backfilled materials, mixing of required amount of water followed by compaction by rollers. Conventional method of backfilling is tedious and unsafe process in confined areas. Deployment of machinery such as grader or roller is not feasible. The process leads to improper compaction of backfilled materials. To nullify the compaction issue and proper backfilling, controlled low strength materials (CLSM) can be used. CLSM is low cementing materials consist of cement, water, fine aggregates, mineral admixture such as fly ash, and pond ash. Locally available materials are used for producing CLSM.

Ingredients in CLSM

Fly ash: In thermal power plant, electricity is generated by burning coal. Fly ash is produced from burnt coal and is hazardous to environment. Disposal of fly ash is a major concern in present scenario, and require huge area for storage.

Due to the presence of aluminous and silicious components in fly ash, it can be used as a cementitious material for production of concrete.

Benefits of using fly ash

Enhanced workability: Due to the spherical particle of fly ash, internal friction between particles is decreased. This leads to improvement in workability of concrete/CLSM mix.

Minimised heat of hydration: Partial replacement of cement with fly ash leads to reduction in heat of hydration of concrete. Less heat of hydration subsides temperature and thermal crack issue in mass concrete.

Improving concrete durability

• Silicious materials present in fly ash react with calcium hydroxide produced from cement hydration. This increases the strength, impermeability of concrete at later stage.
• Improved resistance to alkali silica reaction. Fly ash reacts with available alkali in concrete and minimise the reactive silica minerals present in aggregate.
• Improved resistance to sulphate attack: Fly ash consumes the free lime and making it unavailable to react with sulphate. Reduced permeability prevents sulphate penetration into concrete. Replacement of cement reduces the amount of reactive aluminates available in cement.
• Improved resistance to corrosion: The reduction in permeability increases the resistance to corrosion.

Crusher dust: It is produced from crusher along with aggregates. Crusher dust is byproduct of manufactured sand and is hazardous for environment.

Cement: OPC 53 grade cement

Water: Used for construction activity

Admixture: Superplasticizer is used 0.1-0.5% of cementitious content for improving the flowability property of CLSM.

CLSM is highly flowable materials with self-compaction properties. It is produced in batching plant by mixing cement, fly ash, sand and water as per mix design. Fly ash is major ingredient in CLSM which is a waste material produced from thermal plant. Fly ash makes the mix cohesive, reduce permeability and improves the strength at later stage.

Applications of CLSM

Use of CLSM for backfilling in confined space: CLSM is used for backfilling in confined space, trench. CLSM is placed easily by pumping, chute. The process is quite safe and mechanised, reduces cycle time drastically.

CLSM for thermal insulation, bedding materials: CLSM is used as an alternative of brickbat on terrace slab water proofing and act as a thermal insulative materials. It is also used for backfilling conduits, sewer pipelines, cables. As a bedding material, CLSM provides uniform support.

CLSM for embankment protection: CLSM is placed on slop surface to prevent erosion of soil, used to fill the voids caused due to erosion of soil, abandoned structure, underground structure, mines.

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