Blocks

A concrete block, also called a breeze block, cinder block, or CMU (concrete masonry unit), is a precast building material made from a mixture of aggregate, cement and water.

The blocks come in numerous sizes and shapes, offering various types for different building applications.

Concrete blocks are made by creating a slurry comprising cement, sand, aggregates and water.

The slurry is poured into moulds, which undergo a steam or autoclave curing process.

The ratio of raw materials used in the slurry and the conditions of the curing stage result in a block with a specific strength and durability. In the UK, concrete blocks are primarily manufactured in the Midlands, South East, and North West to ensure a nationwide supply.

What is the difference between a concrete and a cement block?

It’s worth noting that concrete blocks are sometimes imprecisely referred to as cement blocks. While these terms are used interchangeably, they refer to different building materials.

The main difference is that cement blocks, as the name suggests, are made primarily from cement, whilst concrete blocks are made from a mixture of water, cement and aggregates (gravel, sand, or crushed stones). As cement blocks often lack aggregates, they are less durable than concrete blocks.

Solid vs. hollow concrete blocks

When it comes to concrete blocks, there are two main categories: hollow and solid. Solid blocks are dense, with no holes in them. They have high load-bearing capabilities and sound insulation properties. On the other hand, hollow blocks are lighter and have voids. As they’re lighter, they can be easier to work with. The voids offer increased thermal insulation compared to solid blocks, but they shouldn’t always be used for load-bearing applications.

Hollow concrete blocks are graded from A to C:

A

Suitable for load-bearing walls with a minimum density of 1500 kg/m3

B

Can also be used for load-bearing walls and has a density below 1500 kg/m3

C

Should not be used for load-bearing walls; this grade of block has a density below 1000 kg/m3

Types of solid concrete blocks

Fully solid concrete blocks are ideal for creating walls that resist adverse weather. They are commonly used for retaining walls, foundations, steps, and more. Although heavier than hollow blocks, these blocks are often less expensive. Below are the common types of solid concrete blocks (also known as dense blocks) and their respective uses.

Solid Concrete Block

• Characteristics: Solid and dense with no holes
• Uses: Load-bearing walls, building foundations, partition walls, retaining walls, and basement walls

Cellular Lightweight Block

• Characteristics: Solid but lightweight. It has tiny air pockets distributed throughout the material
• Uses: Non-load-bearing walls, insulation

Expanded Clay Block

• Characteristics: Solid but lightweight with thermal properties. It has a textured surface due to the inclusion of expanded clay pellets
• Uses: Non-load-bearing walls, insulation

Fly Ash Block

• Characteristics: Made from fly ash, cement and water. The fly ash gives the block a smooth and uniform texture. More environmentally friendly
• Uses: Thermal insulation, sound insulation, fire resistance, load-bearing and non-load-bearing walls

Paving Block

• Characteristics: Comes in various shapes, colours, sizes and thicknesses. Highly durable and strong
• Uses: Driveways, pavements, walkways

Aerated Autoclaved Block

• Characteristics: Lightweight, air-filled concrete. These blocks have a slightly porous texture due to the air bubbles trapped within the concrete
• Uses: Partition walls, thermal insulation

What is fly ash?

Fly ash is a powdery by-product of burning coal in electric power plants. The ash has a fine consistency and cementitious properties, which reduce water consumption during manufacturing. This makes fly ash blocks a more environmentally friendly option. There are two types of fly ash separated by their chemical compositions:

Class F

Reacts with lime in the presence of water to create compounds with cementitious properties.

Class C

Contains the same cementitious properties as Class F, but is also capable of self-cementing.

Types of hollow concrete blocks

As mentioned before, hollow blocks benefit from being lighter than their solid counterparts. The holes also facilitate the routing of wiring or piping. Additionally, a rebar can be inserted through the holes for enhanced building stability. Below are the different types of hollow concrete blocks and their common uses.

Hollow Concrete Block

• Characteristics: Features a hollow cavity in the middle
• Uses: Non-load-bearing walls, load-bearing walls, partition walls, insulation, fire-resistant structures, fencing, cavity walls

Column Block

• Characteristics: Available in various shapes and different surface textures
• Uses: Structural columns

Corner Block

• Characteristics: L-shaped block that is often larger than standard blocks for increased stability
• Uses: Building corners

Concrete Blocks with Insulation

• Characteristics: Standard rectangular shape with a core made of insulating material, such as foam
• Uses: Exterior walls, thermal efficiency

Insulated Concrete Form (ICF) Block

• Characteristics: Standard rectangular shape with built-in insulation filling some of the block’s hollow core
• Uses: Energy-efficient buildings

Jamb Block

• Characteristics: Rectangular shape with a notched edge to fit into wall openings
• Uses: Fits around window and door openings

Lintel Block

• Characteristics: Wider than standard concrete blocks to offer adequate structural support
• Uses: Used above door and window openings to reinforce the structure

Pillar Block

• Characteristics: Can be square or rectangular and may have a decorative texture
• Uses: Used for columns and pillars to provide structural support

Partition Block

• Characteristics: Lighter and thinner than standard hollow blocks
• Uses: Interior partition walls to separate interior spaces

Splitface Block

• Characteristics: One side of the block has a textured decorative appearance
• Uses: Facades, decorative walls

Stretcher Block

• Characteristics: Hollow with parallel faces. This block has a longer length than its height and width
• Uses: Structural walls

Bullnose Block

• Characteristics: Has at least one rounded corner for aesthetic appeal
• Uses: Corners, doorways

Should hollow concrete blocks be filled?

Hollow blocks offer thermal and acoustic insulation. When the voids are filled with concrete, the insulating properties of the blocks decrease. But, if higher strength and fire resistance are required, filling hollow blocks may be advantageous.

Advantages of filling hollow blocks:

• Can increase structural integrity and load-bearing capacity
• Improved resistance to moisture ingress
• Enhanced fire resistance

Disadvantages of filling hollow blocks:

• Increased weight makes the blocks harder to handle
• Decreased thermal insulation

Why do people use concrete blocks?

People choose concrete blocks as they’re a versatile building material that offers strength, insulation and fire resistance. Blocks are also straightforward to work with, and because of their large size, projects can be completed quickly.

Below, we’ve compiled some advantages of using concrete blocks:

• Highly durable
• Non-combustible
• Good thermal insulation (depending on the type)
• Provides effective soundproofing (depending on the type)
• Various sizes and types of blocks are available to suit different purposes
• Relatively inexpensive
• Quick to build with
• Can be made of recycled materials
• Can be rendered, painted or finished
• High load-bearing capacity makes blocks ideal for structural walls and building foundations
• May improve the energy efficiency of buildings

Common uses of concrete blocks

Here is an overview of building projects using concrete blocks.

Commercial buildings

Partition walls, firebreaks

Infrastructures

Retaining walls, sound barriers

Landscaping

Garden walls, paving

Residential buildings

Building foundations, load-bearing walls

What are dense concrete blocks used for?

Dense concrete blocks are used to build foundations, infrastructure, reinforced concrete blockwork, and walls (basement, load-bearing, retaining, firewalls, and external).

What are hollow concrete blocks used for?

Hollow concrete blocks are used for non-load-bearing partition walls, thermal insulation, sound insulation, retaining walls, infills, formwork and for landscaping.

Can concrete blocks be used underground?

Yes, concrete blocks can be used underground. But waterproofing, drainage, structural integrity, chemical resistance and building regulation requirements should be considered.

UK Building Regulations for blocks

UK Building Regulations include requirements for masonry work. These include specifications for thermal performance, structural integrity and fire resistance. Failure to comply with UK Building Regulations can result in fines, unsafe buildings and property insurance being voided. Some regulations refer to the blocks themselves, which manufacturers adhere to, whilst others are met by constructing structurally sound buildings.

Regulation A1

Buildings constructed from concrete blocks must be structurally stable

BS EN 771-3

Ensures concrete blocks are suitable for different applications. Size, compressive strength, density, dimensional stability, freeze/thaw resistance, sulphate resistance, water absorption, insulation, and more physical properties are tested to ensure blocks are manufactured to the correct standards

BS 5628-1

Relates to the use of masonry in buildings for load-bearing and non-load-bearing structures

Approved Document L

States the thermal transmittance limits for walls to ensure energy efficiency

Regulation B3

Specifies how walls should be constructed to prevent fires from spreading

Approved Document C

Gives guidance on moisture protection and the use of damp proof courses and membranes with concrete blocks

Categories of compressive strength

Concrete blocks are split into Category 1 and Category 2 units.

Category 1

Units have a mean compressive strength and a coefficient of variation not exceeding 15%

Category 2

Units do not meet Category 1 requirements but can be used for non-critical (non-load-bearing) applications

Construction blocks and durability

Not all concrete blocks are suitable for use in adverse weather conditions.

• Solid concrete blocks may require waterproofing if you use them in damp environments
• Aerated blocks are less durable in freezing conditions
• Hollow blocks shouldn’t be used in areas with heavy rainfall to avoid water ingress

Concrete blocks and fire resistance

Concrete blocks are non-combustible and therefore inherently fire-resistant. Solid concrete blocks are ideal for constructing fire-resistant walls and firebreaks.

Are concrete blocks sustainable?

Blocks can be reused and last a long time, making them a more sustainable building material. Aerated and lightweight blocks are even more eco-friendly due to their insulation properties. Improved insulation reduces the energy consumption expended on interior temperature control.

Are concrete blocks porous?

Concrete blocks are porous and need to be sealed to avoid water absorption. Lightweight concrete blocks and aerated autoclaved blocks are more porous than solid blocks.

Concrete blocks were developed in the 19th and 20th centuries, but their origins date as far back as the Roman Empire (when they were made using earlier forms of concrete). Modern concrete blocks became popular after World War 2, due to the high demand for cost-effective and efficient building materials to rebuild homes.