How is the Modulus of Elasticity (Ec) of lightweight concrete calculated?

It's calculated using the formula Ec = w^1.5 * 0.043 * sqrt(fck), where 'w' is the oven-dry density in kg/m³ and 'fck' is the characteristic strength (Clause 5.2.1).

Do I use the same shear strength values for lightweight concrete as for normal concrete in IS 456?

No. The design shear strength (τc) from IS 456 must be multiplied by a modification factor 'λ' (e.g., 0.80 for all-lightweight concrete) as per Clause 8.2.1.

What is the primary advantage of using structural lightweight concrete?

The main advantage is the reduction in the dead load of the structure, which can lead to smaller foundations, longer spans, and improved seismic performance.

What is meant by 'pre-wetting' lightweight aggregates?

It is the process of adding water to the aggregates before mixing them into the concrete to satisfy their high water absorption, preventing them from drawing water from the mix. (Clause 9.1.2)

IS 9142 Part 2

: 1979

Methods of Test for Lightweight Aggregates for Structural Concrete - Part 2: Properties of Aggregates

CurrentSpecializedCode of PracticeBIMMaterials Science · Aggregates and Sand

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This code of practice provides guidelines for the use of lightweight aggregates in structural concrete. It outlines the classification, properties, and testing of these aggregates, and details the mix design, properties, and special design considerations for lightweight concrete, such as modifications to the modulus of elasticity and shear strength calculations.

Describes methods for determining various properties of lightweight aggregates, including loose bulk density, specific gravity, and water absorption.

Quick Reference Values

Oven-Dry Density Range of Structural Lightweight Concrete1440 to 1840 kg/m³

Shear Strength Modification Factor (λ) for all-lightweight concrete0.80

Shear Strength Modification Factor (λ) for sand-lightweight concrete0.85

Typical Water Absorption of Lightweight Aggregates5 to 25 percent by weight

Minimum Cement Content for Moderate Exposure300 kg/m³

Maximum Free Water-Cement Ratio for M20 Concrete0.55

Key Formulas

Ec = w^1.5 * 0.043 * sqrt(fck) — Modulus of Elasticity for lightweight concrete

τc' = λ * τc — Modified Design Shear Strength of concrete

Practical Notes

Lightweight aggregates have high water absorption; pre-wetting them before mixing is essential to control workability and the effective water-cement ratio.

Due to the lower density of aggregates, lightweight concrete is more prone to segregation. Careful handling, placing, and controlled vibration are crucial.

Structural design must account for the lower modulus of elasticity, tensile strength, and shear strength compared to normal weight concrete of the same compressive strength, using the modification factors provided in this code.