Building professionals will recognise that concrete blockwork provides good levels of fire resistance and contribute much to the fire strategy of buildings. This is important as buildings must be designed and built to minimise risk to both people and property as effectively and efficiently as possible.

Due to the inherent material properties of a concrete blockwork it can be used to minimise fire risk for the lowest initial cost whilst requiring little in terms of on-going maintenance. It is unlikely that it will require any additional fire-protection due to its built-in resistance to fire. It is a non-combustible material (i.e. it does not burn), and has a slow rate of heat transfer. Specifying concrete blockwork ensures that structural integrity remains, fire compartmentation is not compromised and shielding from heat can be relied upon.

Concrete blockwork’s excellent performance is due in principle to its constituent materials (cement and various types aggregates) which, when chemically combined, form a material that is essentially inert and, crucially for fire safety design, has relatively low thermal conductivity. It is this slow rate of conductivity (heat transfer) which allows concrete blockwork to act as an effective fire shield not only between adjacent spaces, but also to protect itself from fire damage. With its fire-resisting properties, blockwork can provide more than just life-safety protection, it also provides a reduction to the damage done to the building as a result of fire.

Other building materials rely on fire protection, fire safety engineering or rate of loss of combustion. This dependence makes blockwork intolerant of workmanship errors, future changes as simple as changing building fittings, compliance with management procedures and human behaviour.

Sadly, fire disasters often result in a rapid change in legislation and construction methods. This was the case for the 1666 Fire of London. In its aftermath, the London Building Act of 1667 banned the use of timber framed houses in the City of London and stated that masonry be used as the structural material. This was extended to Westminster in 1707, and to the rest of London by 1774. There has been no great fire in London since, due in significant part to the restrictions laid down in the London Building Act of 1667 and subsequent Acts. Masonry was recognised as an important material in resisting fire way before test methods and national building regulations were introduced.

Fire properties of concrete blocks

The key benefits of using blockwork in fire resisting elements arise from its non-combustibility and the high periods of fire resistance it can provide.

For non-combustibility, concrete blockwork has the highest fire resistance classification (Class AI) under BS EN 13501-1. This Standard specifies the method of fire classification for construction products and building elements. Materials that are classified as A1 are non-combustible and satisfy the requirements of all other classifications. Concrete blockwork is classified as combustibility Class A1.

Concrete blockwork has excellent fire-resisting capacity and provides fire resistance for up to six hours with relatively thin walls, although such high levels of performance go beyond the requirements of Building regulations for all building types. As an example, a standard 100mm-thick aggregate block wall will be more than sufficient to provide the one-hour fire separation between apartments. The exact performance varies between block types and load-bearing conditions, and detailed information can be provided by Lignacite for their concrete products.

Using blockwork to meet Part B Fire Standards

The importance of fire design is reflected in the 2019 changes to Part B of the Building Regulations with additional amendments published in May 2020. Guidance is published in Approved Document B – Volume 2: Buildings other than dwellings. The scope of the Guidance encompasses:

• Requirement B1: Means of warning and escape
• Requirement B2: Internal fire spread (linings)
• Requirement B3: Internal fire spread (structure)
• Requirement B4: External fire spread Requirement
• B5: Access and facilities for the fire service

The non-combustibility and fire resisting properties can be used to good advantage in helping to satisfy Requirements B2, B3 and B4.

Internal fire spread (linings)

Limiting fire spread is a key specification decision for the materials to line walls and ceilings. It is important for the building fabric generally, however more so for circulation areas where linings offer the potential for rapid fire spread which can jeopardise the safe evacuation of occupants. Satisfying Part B2 is achieved by specifying linings with a minimum classification which are dependent on its location in the building.

Reaction to fire relates to the extent to which a product will contribute, by its own decomposition, to a fire under specified conditions. Products, other than floorings, are classified as A1, A2, B, C, D, E or F (with class A1 being the highest performance and F being the lowest) in accordance with BS EN 13501-1. Class F is assigned when a product fails to attain Class E. Untested products cannot be classified in accordance with BS EN 13501-1. Concrete blockwork will achieve Class A1 without the addition of any surface finishes and therefore surpasses the minimum requirements for linings. This makes the use of the Lignacite fair-face blockworkan attractive proposition for circulation areas as well as escape routes with the added advantage that the block material is very robust and its limited contribution to fire spread will not lessen over time.

Internal fire spread (structure)

Concrete blockwork makes a significant contribution towards satisfying the requirement to limit the risk of internal fire spread within the structure. It can be used to construct external and compartment walls, fire enclosures, protected shafts etc.

The periods of fire resistance depends on many factors such as the building purpose group, the height of the building and whether sprinkler systems are in place. In the case of the latter, sprinkler systems are mandatory to a number of building types over 30m in height. The most onerous fire resistance period specified is 120 minutes, however for the majority of buildings up to 30m in height, minimum fire resistance periods of 60-90 minutes apply. These levels of fire protection can be met by all types of concrete blocks. For example, walls built with 100mm solid Lignacite blocks in loadbearing or non-loadbearing walls will achieve at least 120 minutes fire resistance. Stability requirements often require walls to be greater than 100mm thickness, and typically fire resistances of 180-240 minutes will be achieved thereby building in a greater margin of resistance.

External fire spread

External walls and roofs of buildings are required to adequately resist the spread of fire over these elements and from one building to another, having regard to the height, use and position of the building. Specifying materials that restrict flame spread is key to compliance.

Combustible materials and cavities in external walls and attachments to them can present such a risk, particularly in tall buildings. The minimum reaction to fire for materials is specified as a means of satisfying this requirement and is dependent on the building type, height and proximity to the relevant boundary.

For the most onerous condition – less than 1000mm from a boundary - the external surface of walls should have a reaction to fire of Class A2-s1, d0(1) or better to Class B-s3, d2(2) or better, depending on the building type. Use of concrete blockwork will satisfy a Class A1 rating, making this material an excellent choice for limiting external fire spread.

Maintaining the integrity of fire walls

As with all fire detailing, the detail of the joints and junctions of a wall require special attention, but can be simply and effectively constructed.

For example, the fire integrity when forming movement joints in blockwork can be assured by specifying a fire resistant joint filler, such as an intumescent movement joint seal.

Penetrations through the wall can be fire protected using a fire collar, or for multi services a fire panel system can be used to firestop mechanical, electrical and plumbing services where they pass through fire rated walls.

Lignacite’s block manufacturing plants located in Norfolk and Essex, are responsible for producing over 100,000 blocks per day, enough blocks to build approximately 100 houses. Working closely with several of the leading Architects, Specifiers and Builders’ Merchants throughout East Anglia, London and the South East, we supply many of the major house builders and Commercial contractors.

Lignacite offers a comprehensive range of concrete blocks from ultra-lightweight to dense suitable for a wide range of building applications. The sustainability of our blocks is core to the Lignacite philosophy and they all have a high level of recycled aggregate content, some blocks exceeding 55%.

Lignacite also offers extensive technical support for its products, the website has an extensive library with product data sheets, CAD Library, Design Guidance booklets and Site Work guide and much more. All may be accessed for free.