CPD 03 2021: Identifying suitability of concrete blocks

This Lignacite sponsored CPD examines the performance of various types of concrete blocks in key areas such as fire, thermal and sound insulation, with suitable compliant constructions

Concrete aggregate blocks

A material familiar to many and tried and trusted in building construction, concrete blocks’ popularity is evident in the new housing market in England, with masonry solutions contributing to more than 80% of the housing market (as evidenced by National House Building Council statistics).

Output has been increased in recent years to meet rising demand, and new capacity is already in place to maintain and increase output to help meet the government’s aspirational housing targets.

Concrete blocks are widely used in walling construction, above and below ground, as well as being used as infill units in beam and block concrete floors. They are inert, robust and extremely durable, making them suitable for applications where resistance to moisture and freeze/thaw conditions are required.

Concrete blocks are generally manufactured from cement and various types of lightweight and dense aggregates; partial cement replacements like granulated ground blast-furnace slag and recycled and secondary aggregates can be used

Sourcing and quality

Generally concrete blocks are manufactured from cement and various types of lightweight and dense aggregates. Where they are available, partial cement replacements – such as granulated ground blast-furnace slag and recycled and secondary aggregates – will be used.

Blocks are produced using a fully automated process. The process from raw material to the cured product normally takes approximately 24 hours. Once the final quality control tests have been completed – checking, for example, compressive strength, dimensions and density – the product is ready for shipment. Concrete blocks are manufactured to comply with BS EN 771‑3:2011 Specification for masonry units: aggregate concrete masonry units (dense and lightweight aggregates).

Additional assurances on quality are widely available from producers, such as manufacturing in accordance with a certified ISO 9001 quality system, and product certification such as that provided by BSI kitemarking scheme.

Fire resistance

Concrete blockwork provides a high level of fire resistance. This is due principally to its constituent materials (cement and various types of 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, or heat transfer, that allows concrete blockwork not only to act as an effective fire shield between adjacent spaces, but also to protect itself from fire damage. With its fire-resisting properties, blockwork provides more than just life-safety protection; it can reduce the damage done to the building as a result of fire (see table 1 below).

Fire resistanceConcrete blockwork performance
Ability to continue to carry the loads on the structureExcellent
Ability to act as insulation against the heat from a fireExcellent
Ability to prevent fire spread through the structureExcellent
Table 1: Blockwork has several features in terms of fire resistance

Other materials, such as timber, rely on linings to provide fire protection. This additional process makes them intolerant of workmanship errors. Additionally, any future changes, such as changing of building fittings, have to be carefully managed so that the integrity of the element is maintained.

Concrete blockwork makes a significant contribution towards satisfying the requirement to limit the risk of internal fire spread

Fire properties of concrete blocks

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

For non-combustibility, concrete blocks have 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.

The fire resistance of concrete blockwork is normally based on tabulated values provided in British standards, as well as Eurocode 6 (Masonry). Tabulated values are derived from fire tests, and Eurocode 6 fire data is derived from a significant number of tests on load-bearing masonry walls, mainly from Belgium, Germany and the UK.

Concrete blockwork can provide fire resistance of up to six hours with relatively thin walls, although such high levels of performance go beyond the requirements of the Building Regulations for all building types. For 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 is provided by manufacturers for their products. Some examples are provided in table 2.

Non-load-bearing wallLoad-bearing wall
100mm solid blocks180 minutes120 minutes
140mm solid blocks240 minutes180 minutes
Table 2: Typical fire resistance of medium-density concrete blocks

Note: Fire periods assessed in accordance with BS EN 1996 Part 1-1 (Eurocode 6)

Internal fire spread

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

Periods of fire resistance depend on many factors, including the building purpose group, the height of the building and whether sprinkler systems are in place. In the case of the last factor, sprinkler systems are mandatory for 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 blocks in load-bearing or non‑load‑bearing walls will achieve at least 120 minutes’ fire resistance.

Stability requirements often mean walls must be greater than 100mm thickness, and typically fire resistances of 180-240 minutes will be achieved, thereby providing a greater margin of resistance.

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 these 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-service penetrations, a fire panel system can be used to fire-stop mechanical, electrical and plumbing services where they pass through fire-rated walls.

Thermal insulation

Over time, the concrete block industry has responded to increasing standards of energy efficiency. Meaningful U-values for elements were first introduced in the 1980s, followed by a procession of changes leading up to the current 2013 Part L thermal standards.

The wall solution is often achieved by a combination of block product enhancements coupled with the improving performance of cavity insulation materials. This strikes a balance between minimal wall spread and cost-effectiveness. For housing, the required U-value of thermal elements is to be derived from the dwelling’s SAP calculation. Typically, this will result in wall U-values of 0.26W/m2K or below.

For a designer, the make-up of the wall will usually comprise a cavity wall, with either full or partial cavity fill. Partial fill solutions will be required in areas of high exposure. Some typical wall constructions are shown below.

Facing brick or similar

100mm full cavity fill
100mm medium-density block
12.5mm plasterboard on dabs

U-value = 0.17-0.26, depending on the λ of the
cavity fill
Facing brick or similar

100mm cavity with
50mm partial cavity fill
100mm medium-density block
12.5mm plasterboard on dabs

U-value = 0.24-0.28, depending on the λ of the
cavity fill

Thermal bridging

A thermal bridge, also known as a cold bridge, is an area of the building where there is a significantly higher heat transfer than the surrounding materials. This is usually where there is either a break in the insulation, reduced insulation, or the insulation is penetrated by an element with a higher thermal conductivity.

Unless controlled, heat loss by thermal bridging can account for over 15% of the heat loss through the building fabric. The heat loss associated with these thermal bridges is expressed as a linear thermal transmittance (ψ-value) – pronounced (and alternatively written) as “psi-value”.

Energy calculations, such as SAP assessments, must account for thermal bridges. Tougher energy targets have resulted in better U-values of walls and floors, as well as improved airtightness. It follows that enhanced bridging details must be adopted to contribute towards a lower heat loss.

The masonry industry has responded to this issue, and design information is available through masonry trade associations, including the Concrete Block Association. Thermal bridging details are provided for all common junctions with the corresponding psi-value, which is refined to take account of common block densities available.

Sound insulation

Current acoustic standards have been in force for a number of years and have been successful in alleviating noise complaints, resulting in a better quality of life for home occupants. Specific sound performance targets, particularly those limiting sound transfer between adjoining dwellings, are incorporated into Approved Document E1, under Part E of the Building Regulations.

The performance requirements cover a range of sound frequencies from 100Hz (low frequency) to 3,150Hz (high frequency). These requirements apply to separating walls in attached houses or flats and separating floors in flats. The standards also apply to rooms used for residential purposes.

Demonstrating compliance

There are two methods for demonstrating compliance:

  • Robust details Before works have commenced the builder is required to register a “robust detail” (RD) (see www.robustdetails.com) and the construction is then executed in accordance with the RD specification. The use of an RD will avoid the need for pre-completion testing. There are numerous RD specifications available for masonry walls; these are predominantly of cavity masonry construction, with cavity widths of between 75mm and 100mm.
    RDs are the most popular method of compliance for housebuilders and help to avoid any potential pitfalls when using non-RD specifications. Evidence obtained through the RD evaluation process also identifies constructions that are capable of achieving a significant uplift in performance, with specifications that can perform 3dB, 5dB, and 8dB better than the minimum performance standard.
  • Pre-completion testing (PCT) This is where sound tests are undertaken at the end of the build and a test report is produced that details the performance levels achieved.
    Approved Document E provides a number of compliant solid and cavity wall constructions to select from, but other constructions can be adopted as long as the designer has evidence to prove the construction has a realistic prospect of achieving the performance standards when tested.

Choice of masonry wall specification

Party walls in dwellings can be constructed using dense or lightweight blocks. This generally encompasses concrete blocks in the density range of 1,350kg/m3 to 2,000kg/m3. Compliance can be achieved with a cavity width of 75mm- 100mm. There is little to distinguish between the performance of dense and lightweight blocks using these cavity widths. Walls can usually be finished with plaster, or a parge-coat – basically a thin coat of mortar – and plasterboard. However, in the case of RD specifications it is necessary to refer to the specific details provided.

Most party walls will incorporate full fill cavity insulation. This is done primarily for thermal compliance, avoiding heat bypass upwards via the
cavity, and allowing a zero-heat loss to be applied to the party wall element in heat loss calculations.

Particular attention must be given to wall ties to minimise sound transfer. Approved Document E: Resistance to the passage of sound specifies the requirements for type A wall ties for use in separating walls of new-build dwellings. It specifies ties must have a measured dynamic stiffness of less than 4.8MN/m³ to minimise the transfer of sound across a cavity. This requirement also refers to RD specifications. Wall ties with this performance are readily available and can cater for cavity widths of 50mm, 75mm, 100mm, 125mm and 150mm.

Some types of construction allow blockwork to be used fair-face on one side, for example an internal residential stairway, while providing a high level of sound insulation in conjunction with an acoustic panel on the unseen face

Achieving higher levels of sound insulation

Often there is a requirement for walls in buildings other than dwellings to achieve a higher level of sound insulation. The performance of such walls can be conveyed as a weighted sound reduction index, Rw. This term describes the airborne sound insulating power of a building element. It is a laboratory-measured value as defined in ISO 717: Part 1. It can apply to walls, ceiling/floors, ceiling/ roofs, doors, or windows. The higher the number, the greater the sound insulating power of the building element. It is measured over the frequency range 100Hz‑3,150Hz.

Depending on its composition, density and any applied finishes, concrete blockwork can provide an Rw of up to around 57dB. An example at the higher performance end can be achieved by a 215mm-thick dense block wall with a plaster finish. Where performance in excess of this is required, consideration should be given to using concrete blockwork with an acoustic panel to one side.

Such a panel typically comprises a 50mm C-stud frame, set back about 15mm from the face of the blockwork, with acoustic quilt between the studs and finished with high-density plasterboard. The advantage of this construction is that it allows blockwork to be used fair-face on one side, for example facing a sports hall, as well as meeting fire and stability requirements.

Together, the blockwork and acoustic panel act to provide elevated levels of sound insulation. Tests in a UKAS-approved laboratory have shown it is possible to achieve a weighted sound reduction index, Rw, of up to 65dB.

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