Structural lightweight concrete normally designates concrete, with an air dry density of 1440 kg to 1840 kg/m³ as determined by ASTM 567 - 05 Standard Test Method for Determining Density of Structural Lightweight Concrete, that yields a 28 day compressive strength of at least 17 MPa when measured in accordance with ASTM C 330 / C 330-09M Standard Specification for Lightweight Aggregates for Structural Concrete, and that is made from lightweight aggregates conforming to ASTM C 330–09M Standard Specification covering expanding, pelletizing or sintering products such as blast-furnace slag, clay, diatomite, fly ash, shale or slate. They can also be made with aggregates prepared by processing natural materials, such as pumice, scoria or tuff. The lightweight aggregates generally used to produce structural lightweight concrete fall within three broad categories: aggregates from volcanic sources, byproducts from coal combustion, and manufactured products. They generally have densities ranging from 560 to 1100 kg/m³ as opposed to normal-weight aggregates with densities in the 1200 to 1750 kg/m³ range. Structural lightweight concrete is typically 25% to 35% lighter than normal-weight concrete, which density hovers in the 2240 to 2400 kg/m³.
Although the aggregate properties (such as type, unit size, shape, unit weight etc...) will affect the ultimate concrete strength, the nature of the cement paste will have a much greater influence. The mixture should have enough fine particles to ensure the cohesiveness of the mixture. Over the last century, lightweight concrete mix designs have been developed to meet strength requirements of 35 MPa, normally specified for standard concrete, while over the last two decades much higher 28 day strengths have been achieved. The concrete strength is correlated for a given slump to the quantity of cement present in a mix and air content rather than to the water cement ratio. Albeit the compressive strength will slightly vary depending on the type of aggregates used, ACI 211.2 provides guidance on the relationship between compressive strength and cement content.
Specifications for lightweight concrete normally state a maximum concrete density, a minimum compressive strength, a maximum slump and an acceptable air-content range. Although material proportions vary significantly for different materials and strength development, as a starting mix design, for structural lightweight concrete, replace one third of the normal weight aggregates by lightweight aggregates. To achieve a given workability, the slump requirements may be lower than for normal-weight concrete. Higher slumps or too much vibration may lead to larger aggregates emerging on the surface rendering finishing more difficult. Testing for air content should be performed by the volumetric method as per ASTM C 173.
Albeit external curing remains necessary as for normal-weight concrete, the porosity of the lightweight concrete matrix entrains water, contributing to internal curing and thus to the development of strength and durability over time while mitigating micro-cracking by increasing the cement aggregate interface area, and improving corrosion and weathering resistance.
Lightweight concrete requires special placement and pumping precautions. Lightweight aggregates have typically an absorption rate of 5 to 20% by weight of the dry aggregate and generally require pre-wetting prior to use to avoid slump loss while pumping. While the design engineer will be concerned by the density and performance characteristics of the mix, the contractor will have a particular interest in monitoring the slump, aggregate content and air-content and will need to ensure pump lines are as large as possible (at least 125 mm) with as few clogging areas as feasible and the pumping pressure applied as low as possible to avoid segregation.
Lightweight concrete improves fire resistance, partly due to the fact that the lightweight aggregates have been subjected to heating processes during manufacture. With lightweight concrete, the coefficient of thermal expansion can be significantly reduced by around one third and thermal conductivity by over 25%, with generally speaking, the lighter the weight of the concrete, the greater the insulating properties.
Structural lightweight concrete have primarily been used to reduce the dead load of concrete structures, having a more efficient strength to weight ratio than normal-weight concrete. They provide civil engineers the flexibility to enhance functionality and architectural designs with structures that are taller, have longer spans, lighter foundations, expressive roof designs, greater elasticity and thus more resistance to seismic movements. They have primarily been used in offshore oil platforms, bridge decks to allow for a wider deck for additional traffic lanes, piers and beams, slabs and wall elements in both steel and concrete frame buildings, parking structures, tilt up walls, topping slabs and composite slabs on metal decks. They have also been successfully used in the pre-cast industry to manufacture bigger size elements without increasing overall weight.
With lightweight concrete, cost reductions are often derived from engineering structures requiring less reinforcing steel, smaller size structural elements, lighter foundations and thinner columns and concrete volume. Reduced dead load results in lower haulage and handling costs. These cost reductions and the benefits otherwise derived from lightweight concrete normally offset the materials’ higher cost.
Holderchem can assist architects and engineers in the optimization of structural lightweight concrete mix designs. Based on specific project requirements, it can assist in sourcing suitable lightweight aggregates and offer to supply the required construction chemicals.