T 20.015 concrete blocks
Rev 03 [Jan 09]

Designing to make management of hazards associated with concrete blocks easier

INTRODUCTION

1. Designers can play a major part in making it easier to manage the hazards associated with using concrete blocks.

2. Decisions that affect how a concrete block wall is constructed are made very early in the design process. The criteria for the selection of the blocks to be used are many and will include, for example compressive strength, fire resistance and thermal and sound insulation. Whatever the criteria, designers should not forget that people have to work with these blocks: getting them to site, around the site and to the work face, usually on scaffolds or in trenches. Often little thought is given to what these blocks weigh.

3. Many block layers are asked repetitively to handle blocks far in excess of the recommended maximum of 20 kg, resulting in a wide range of injuries where the damage although gradual is progressive over a period of time.

4. The single-handed handling of blocks in excess of 20 kg is not prohibited as long as the process is not repetitive. In particular quoin and reveal blocks may fall into this category and it would not be expected that different measures be used to lay these types of block.

5.  However, where large numbers of blocks greater than 20 kg are specified they should be handled mechanically or lifted by two people, in accordance with industry guidance that has prevailed for many years through manufacturers’ literature.

6. And, while it is desirable that all block handling is confined to units weighing less than 20 kg, there will always be circumstances where it is necessary to specify blocks in excess of 20 kg. Concrete blocks that weigh more than 20 kg have traditionally been used for very specific purposes including:

• a) Structural stability;
• b) Fire resistance;
• c) Acoustic performance; and
• d) Radiation shielding.

7. This guidance note is to help designers to understand their duties and to be aware of how they can help to make the handling of blocks safer.

HAZARDS ASSOCIATED WITH USING HEAVY BLOCKS

8. The main hazard associated with building with concrete blocks is lifting heavy blocks. This puts the body under strain, which is exacerbated when blocks have to be placed away from the body and/or above waist level. The effects of having to work repetitively in this manner manifest themselves as musculo-skeletal injuries: back injuries, upper limb injuries, etc, which are very debilitating. The effects are cumulative.

9. It should be noted that there are no hard and fast rules about what is a safe load to handle, because this varies depending on where the load is held. Further information is given in General Information Series I 001 Manual Handling.

10. Other hazards associated with use of concrete blocks include:

• a) Noise, when ties are connected by shot-firing; 
• b) Contact hazards from protruding ties;
• c) Structural stability problems; and
• d) Laying with an awkward posture.

WHAT DESIGNERS SHOULD DO

11. Designers should give careful consideration to the hazards listed above, in 8 and 10, either to remove them or lessen their impact.

Manual handling: lifting heavy blocks

12. As the repetitive use of concrete blocks in excess of 20 kg in single-handed laying presents a significant risk to the health and safety of the person carrying out the task, designers should wherever possible:

• a) Eliminate this hazard, by specifying lighter weight blocks, where heavy blocks are not necessary; or
• b) Reduce the risk by using alternative detailing such that lighter weight blocks can be used that achieve the same performance criteria that heavier blocks would give.

13. If block substitution or alternative detailing is not possible, designer’s should identify this hazard, to allow a contractor to:

• a) Provide and use mechanical handling devices; or
• b) Make provision for two person teams to be employed in the lifting of the blocks.

This information should be included on drawings, in specifications and bills of quantity. And it must also be included in the pre-tender health and safety plan.

14. When a design requires properties that blocks greater than 20 kg would supply, there are available a number of options utilising lighter blocks, which a designer should consider. The appropriate choice depends on the specific application and the characteristic performance requirements of the wall but essentially the choice is as follows:

• a) Choose a cellular or hollow block instead of a solid block that has identical properties;
• b) Re-assess the design and use alternative construction details such as:
  • i) laying blocks flat to achieve the 190 or 215 mm width wall suitable for plastering;
  • ii) collar-jointing* to form a 190 or 215 mm wall. This type of detail is unsuitable for use as a party wall in dwellings;
    • * collar-jointing is the laying of blocks back to back in normal construction incorporating a 10-15 mm mortar joint between the adjoining block faces. The two leaves are then tied together using appropriate brick ties or bed joint reinforcement.
  • iii) internal sound insulation, which satisfies the Building Regulations, can be achieved using lightweight blocks and cavities;
  • iv) in basements, using reinforced hollow blockwall construction** to achieve strength;
    • ** Re-bar passes through the block and the voids filled with concrete producing composite action between the block and the reinforcement.

15. In addition, blocks with built in lifting aids are available and should be specified when their properties meet the requirements of the design.

Noise

16. Where it is necessary to tie walls together, built in ties should be used in preference to ties shot fired to the blocks on site – see H 20.002.

17. Also, to reduce the need for cutting blocks on site, designers should draw a contractor’s attention to the fact that cutting blocks is a hazard.

Contact

18. The prevention of contact hazards is a site management problem, which designers can do little about. However, if ties that can be bent up and still supply the requirements are available, they should be specified.

Structural stability

19. “Green” blockwork is susceptible to collapse, especially when the erection requirements are different from normal erection procedures. This is usually only a problem when:

• a) Blockwork cannot be built in conventional 2m lifts. But this hazard can be managed as long as designers inform contractors about such limitations;
• b) The blockwork is loaded before it has achieved sufficient strength, which usually occurs when:
  • i) joist hangers are used to support floors, which allows floors to be installed very soon after the wall has been built, providing a surface on which to stack blocks, which frequently occurs; and
  • ii) pc floor or roof units are placed on external cavity walls where the inner [supporting leaf] is built up too far in advance of the outer leaf.

To prevent these types of instability, designers should make a clear statement about:

•  
  • when the walls can be loaded,
  • what loads the walls can carry at these early stages,
  • detailing pc units to be supported on the full width of the inner leaf;
  • checking floors for the loads floors that may be applied by any propping scheme, to allow a contractor to back-prop the floors safely.
    
• c) Infill panels are not properly tied in to the framing members, which may be avoided by informing the contractor of the necessity to tie in;
• d) The mortar used is different from what the designers specifies, eg, highly plasticised. While this is out with a designer's control, if the type of mortar is critical to the performance of the blockwork, designers should emphasise this;
• e) Structural rules-of-thumb, eg, that you can build a block wall up to 10 times its thickness without support, do not apply, eg, lightweight walls or walls subjected to local wind conditions.

Laying in awkward positions

20. If ducts and other services are installed before internal walls are built, blocklayers are, sometimes, forced to adopt awkward postures to lay blocks around these obstacles.  In addition, it could be difficult to erect scaffolds to legal standards, because these obstructions get in the way. To overcome this, designers should leave penetrations in walls, which can be filled in later with appropriate sound and fire-retardant material. Such penetrations should, preferably, have whole-block dimensions.

Note: In order to achieve this, designers will have to give consideration to grouping service runs and to hanging them closer to ceilings.

BACKGROUND INFORMATION ON BLOCKS

Market availability

21. The market for concrete blocks is divided into two distinct sectors:

• a) Aggregate concrete blocks that take around 70% of the market of which 95% weight 20 kg or less. These blocks are made up of small-scale solid blocks, lightweight aggregate blocks and cellular or hollow blocks; and
• b) Aerated concrete blocks that account for 30% of the market share.

USEFUL REFERENCES

Guidance Notes on the Use of Dense Concrete Masonry - Concrete Block Association

Manual Handling Operations 1992 - Guidance on Regulations - Health and Safety Executive

Handling heavy blocks - Health and Safety Executive Information sheet No37