Building your walls with concrete usually means using the popular block build, but other options exist.

In this article we cover:

• What concrete is and how it’s improving its eco credentials
• Cavity wall construction (blockwork)
• How to insulate blockwork walls
• Types of blocks, from aerated to clay
• Single skin construction
• Cast concrete, including flooring
• Insulating Concrete Formwork (ICF) construction

In the UK, concrete remains the most common building method. But what is it, how is it used, and what are its pros and cons?

What is concrete?

Concrete is made from cement, water and aggregates, often crushed rock or eco-friendly materials. It differs from mortar, which combines cement, water, lime and sand. Concrete can be mixed on-site, delivered in a truck for pouring or precast into specific shapes.

Concrete plays a major role in self-build projects. It is often used for foundations, floors, screeds, window sills, lintelsand sometimes even roof coverings.

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Most UK builders are familiar with masonry. Concrete materials are locally available, and lenders, insurers and warranty providers readily cover concrete builds.

Concrete is strong, fire-resistant and durable. It does not rot, decay, absorb moisture, corrode or suffer from UV damage or infestations. Once crushed, it can be reused.

However, concrete is heavy and requires strong foundations. It also struggles with movement, making it unsuitable for poor ground conditions.

Concrete and masonry absorb heat, storing and releasing it later. This thermal mass is beneficial for low-energy homes. Masonry also allows design flexibility due to its small unit construction.

Concrete has high embodied energy. Cement production requires significant energy, but manufacturers are improving. The European cement industry aims for carbon neutrality by 2050. Cement mixes now incorporate waste by-products to reduce impact.

Quarrying affects water use, but the industry is investing in sustainable extraction methods.

Cavity wall construction (blockwork)

Blockwork, or cavity wall construction, is the most common masonry method in the UK. It consists of two layers: an inner structural leaf, often made of concrete blocks and an outer leaf. Wall ties join the two layers.

Originally, cavities prevented moisture from soaking through. Today, they also hold insulation, either partially or fully.

Proper installation is crucial. Insulation must be tightly fitted to prevent gaps, thermal bridging and air circulation issues.

To manage water penetration, weep holes are installed at the base of the outer layer. Cavity trays with stop ends and weep holes are used above openings.

The most traditional masonry build technique in the UK continues to be what’s called a block build, or cavity wall construction.

Insulation methods

The most common insulation methods are:

• Pumped cavity insulation: EPS beads are injected into the cavity.
• Rigid board insulation: PIR or phenolic boards are installed as the wall is built.
• Spray foam insulation: More expensive but effective for full cavity fill.

Insulating externally is also more expensive and therefore not common on new builds, unless it is an intrinsic part of the building method (see Single skin construction below). However, for existing buildings, it has become a go-to for solid masonry and some older cavity wall types of construction.

Whether or not you also insulate the internal wall, the one facing inside the house, usually with insulated plasterboard, will depend on your insulation strategy. Structural concrete blocks are therefore usually surfaced with plaster, sand and cement mortar, or covered with (often insulated) plasterboard
to produce the internal wall finish required and add a layer of airtightness. Synthetic fibres incorporated into the sand and cement finish can increase the strength of a block wall considerably.

Your energy assessor will evaluate your options; this is specialist work that takes into account your insulation strategy as a whole – for the entire dwelling, so that there are no weak
links.

Types of concrete blocks

Concrete blocks come in three main types:

• High-density blocks – Cement and aggregate-based.
• Lower-density blocks – Made with cement and fly ash.
• Lightweight blocks – Made from aerated concrete, easy to handle and highly insulating.

Standard blocks measure 440mm x 215mm x 100mm, with larger sizes available. Hollow blocks can be reinforced with steel and concrete for added strength.

Clay blocks, gaining popularity, provide better insulation and can sometimes replace cavity walls.

Blocks must be stored properly to prevent damage. Weather can also impact construction, requiring protection from frost and rain.

Single skin construction and cast concrete

Single skin construction uses prefabricated concrete wall units, once common only in commercial buildings. Precast units are quick to install, thermally efficient and require less on-site storage. However, they are expensive and rare in self-builds.

A UK system uses 200mm-wide blocks with thin bed mortar. These blocks are externally insulated and finished with multiple layers to create a watertight, airtight wall.

Concrete can also be poured on-site, allowing for custom architectural details. Foundations, floors and screeds are commonly cast in place, requiring a cement truck for large pours.

Insulating Concrete Formwork

ICF consists of lightweight polystyrene blocks stacked to form a mould. Concrete is then poured inside, creating structural walls. Some systems can also form roofs.

ICF blocks lock together and use spacers to maintain position. They can be cut on-site but are designed to minimise waste. Special lintels and flooring systems allow simultaneous casting of walls and floors.

ICF builds are airtight and nearly free of thermal bridging. U-values can be as low as 0.11W/m²K with added insulation.

Advantages include:

• Quick assembly
• DIY-friendly with professional oversight
• Weather resistance
• Flexible design, including curved walls

In terms of finishes, the outer skin of the polystyrene blocks has to be clad (using built-in fixings or wall ties) with something – it can’t be left open to the elements. This outer skin can, though, be easily chased to make room for rainwater downpipes and even other utilities.

Once built and roofed, interior work can begin faster than with other methods. Manufacturers claim ICF can cut three months off a build schedule.

What about flooring?

The most common option nowadays for upper storeys is to use concrete hollowcore or wide slabs, craned into place on site. A concrete upper floor is considered more suited to underfloor heating, as a screed covers the piping, which may mean that you will need to use radiators if your upper floor
is solely made of timber although nowadays timber solutions are also robust. Remember that a concrete floor will be heavy and will require adequate support from the foundations, i.e. more concrete.

Another option, especially in the case where accessibility is an issue, is the concrete beam and block method whereby concrete blocks are supported on reinforced concrete beams. The joints are grouted and a screed is then applied on top. The strength of the beam and block method allows wider spans of flooring to be constructed without needing extra support.

It is important to research this area prior to construction and select the type of flooring that is most suited to your needs and to select a supplier to consult them on your design. They will be able to advise which method is most cost effective for your requirements, e.g. to support upper floors, requirement for open spans, etc.

The supplier will normally turn around an order very quickly once as-built measurements can be taken on site. Builders may also wish to incorporate balconies and stairs when ordering precast units which will also be manufactured to exact dimensions.

Waste materials as cement substitutes

In the UK, the most common cement substitutes in concrete are fly ash (Pulverised Fuel Ash – PFA) and ground granulated blast-furnace slag (GGBS).  A replacement rate of up to 70 per cent (so 30 per cent Portland cement in the cement mix that goes on to make concrete) is possible under I.S EN 206-1.

An Australian company, meanwhile, is substituting sawdust and other recycled timber, e.g. from discarded pallets, to its mix of cement to make eco-friendly concrete bricks and panels, among other products.

Mostly still at the development stage is Ferrock, which uses steel dust and fly ash to make a cement alternative, capturing CO2 during the curing process. The availability of the waste materials is a barrier to further development. The patent is held by the University of Arizona.

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