Lightweight concrete density: strength vs insulation

Why does density change three things?

In a lightweight concrete block, the pores are filled with air. As the share of air rises, the solid skeleton of the material shrinks. That has three direct consequences: compressive strength falls, thermal insulation improves and unit weight goes down.

These three cannot be tuned separately. If you want better insulation, you give up strength. If you want more strength, you give up lightness and insulation. So when you write a specification, first decide which one is critical: the load the wall must carry, the insulation, or the mass added to the building.

Redblok density classes and design targets

Lightweight concrete is a broad family; the values below apply to the Redblok lightweight concrete block classes. The number in the class code is the target dry density in kg/m³. Strength is 28-day compressive strength, conductivity is dry thermal conductivity, and water absorption is by volume. All of them are design targets.

KB300: 300 ± 50 kg/m³; strength 0.5–0.9 MPa; conductivity 0.08–0.10 W/mK; water absorption 8–12%. The lightest block class, for insulation first.

KB400: 400 ± 50 kg/m³; strength 0.9–1.6 MPa; conductivity 0.10–0.12 W/mK; water absorption 8–12%. A light partition block.

KB500: 500 ± 50 kg/m³; strength 1.5–2.5 MPa; conductivity 0.12–0.14 W/mK; water absorption 10–14%. A partition wall block.

KB600: 600 ± 50 kg/m³; strength 2.2–3.5 MPa; conductivity 0.14–0.17 W/mK; water absorption 10–14%. A partition and infill wall block.

KB700: 700 ± 50 kg/m³; strength 3.0–4.5 MPa; conductivity 0.17–0.20 W/mK; water absorption 12–16%. An infill wall block.

KB800: 800 ± 50 kg/m³; strength 4.0–6.0 MPa; conductivity 0.20–0.24 W/mK; water absorption 12–16%. An infill wall block where higher strength is needed.

KB1000: 1,000 ± 50 kg/m³; strength 6.0–9.0 MPa; conductivity 0.26–0.32 W/mK; water absorption 14–18%. A non-structural, high-density block.

We use a separate mix design for each density class; the admixtures and their proportions are specific to our company. These values are not a commercial specification. Every class is verified by trial mixes and laboratory testing, and project-specific values are given in a test report.

How do you pick a density class?

The design bands break down roughly as follows. The 300–500 kg/m³ band is for light partitions where thermal and acoustic insulation come first. The 500–800 kg/m³ band is the typical block density for non-loadbearing partition and infill walls. The 700–1,000 kg/m³ band is for non-loadbearing walls that need higher strength.

Start from the job the wall has to do. Wall height, hanging loads such as cabinets and worktops, and, for external walls, the heat calculation to TS 825 (the Turkish thermal insulation standard for buildings) are assessed together. The lowest class that meets these needs is usually the right choice. A higher class than needed makes the wall heavier and insulates less. A lower class than needed may not give the wall the strength it requires.

The conductivity values in the list are for dry material. When water gets into the pores, conductivity rises, because water conducts heat far better than air. That is why blocks are stored covered, walls are not built with wet blocks, and external faces are protected by render or cladding.

How does density affect seismic mass?

Choosing a density is not only an insulation decision; it is also a mass decision. Normal concrete weighs about 2,400 kg/m³. A lightweight concrete block at 500 kg/m³ weighs roughly a fifth of that for the same volume.

Seismic force is directly proportional to the mass of the building. In an analysis to TBDY 2018, the Turkish Building Earthquake Code, partition and infill walls count towards the building's mass. Building these walls with lighter blocks reduces the seismic mass entering the structure and the demand on the structural frame. But that difference alone does not decide how the building behaves in an earthquake. The responsible structural engineer decides how the lighter block is reflected in the design.

How are the values checked in production?

The relationship between density and strength shifts with the mix design and the curing conditions. The exact compressive strength and thermal conductivity for a project are therefore set by project-specific testing. Cellular lightweight concrete falls outside the scope of TS EN 206, the Turkish adoption of the European concrete standard, so its performance is defined by the project specification and laboratory testing.

The main production risk is a pore structure that drifts from one mix to the next. A small change in the amount of air shifts the fresh density and gives an unexpected result in the hardened block. That is why we measure fresh density on every mix. We also record every batch: fresh and dry density, compressive strength, water absorption, shrinkage and a dimensional check. On request, we add these records to your project file.

The known weak point of non-autoclaved concrete is lower strength and higher drying shrinkage at the same density. We developed the Redblok mix design around that point and measure it in every batch.