UWE

Blocks & Blockwork

Contents

1 Concrete Blocks - Introduction

2 Dense Concrete Blocks

3 Lightweight Concrete Blocks

4 Aerated Concrete Blocks - Aircrete

5 Aerated Concrete Blocks - Properties and Mortar Mixes

6 Aerated Blocks - Practice

7 Sound Insulation: Party Walls & Partitions

1 Concrete Blocks - Introduction

Concrete blocks have been in common use since the 1930s. Early blocks were often made from local aggregates, most of which are no longer available. The aggregates were often industrial waste products such as breeze and clinker. The density of the aggregate affects the thermal and sound insulation qualities of the blocks. Blocks can be used in either leaf (or both leaves) of a cavity wall. Here, the outer leaf is dense concrete. The wall will be finished with 2 coats sand/cement render.  Insulation is in the form of cavity boards and a lightweight block inner leaf. Blocks (dense and lightweight) are also used for internal loadbearing walls and partitions.
These blocks are made from lightweight, aerated concrete. They form the internal leaf of a cavity wall and will be finished with plasterboard fixed on plaster dabs. These blocks are also suitable for plastering.

Aerated blocks can be used for lightweight partitions and loadbearing internal walls. They have been available for about 40 years or so and have replaced earlier lightweight blocks made from a variety of lightweight aggregates. Aerated concrete blocks can be used in the external leaves of cavity walls (they are usually rendered). It's even possible to built solid (non cavity walls) in aerated blocks as long as an external cladding is fixed to the wall for reasons of appearance and weather protection.

Both these photographs show dense concrete blocks. In the left-hand picture they are being laid in cavity construction to form the external walls below ground level and flat to form the party walls. In the right hand photo 100mm dense concrete blocks form a loadbearing partition (left side of photo); 125mm aerated blocks form the internal leaf of the cavity wall.  

 

2 Dense Concrete Blocks

Concrete blocks have been used since the 1930s although it was not until the 1950s and early 1960s that they completely replaced bricks as the internal leaf in cavity walls. In their early years (30s and 40s) they tended to be used for internal partitions, both loadbearing and non-loadbearing. The picture on the left shows a development of houses from 1998 with 75mm dense blocks being used for the internal loadbearing walls. At the opening a timber door lining is nailed or screwed to the blockwork. The lining is proud of the blockwork to receive the plaster work. The joint between the two will then be covered with an architrave. 
Dense concrete blocks have barely changed in the last 60 years or so. They are usually made from cement, fine aggregate and coarse aggregate. They can be produced in a range of crushing strengths and, nowadays, tend to be used for loadbearing partitions, foundations and, possibly, party walls. They generally have poor thermal insulation and readily absorb water. However, 20 years ago they were common in the internal leaf of cavity walls (when insulation requirements were less onerous) and nowadays they can also be used to form the external leaf providing some form of cladding or render is provided. Dense blocks should be laid in mortars of average strength; 1:1:6 or 1:2:9 (or their equivalent). Stronger mortars may limit movement and may cause cracking in the blocks rather than the joints. Weaker mixes may compress under loading. Stronger mortars are sometimes specified below ground level. Both these picture show dense blocks forming part of the substructure.
A standard block is the equivalent of three bricks high and two bricks long. They are available in a range of widths from about 50mm to 300mm. 

Dense blocks provide a good key for most plasters and renders. However they should be allowed to dry properly before plastering as the blocks absorb rainwater and will shrink on drying. This can cause cracking. 
Party walls, above or below ground level, can be formed from dense blocks. There are no thermal insulation requirements for party walls so block density is not an issue. On site you will find party walls in solid construction and in cavity construction. Look at the pages on Aerated block practice (AB practice) to see how party walls are connect to main loadbearing walls. Click here for clip of party wall and click here for cavity blocks being laid.
During the last 50 years or so there have been several other types of block used in the UK. The ones featured here are no longer available but will be found in older buildings during rehab works etc. Both these blocks had disappeared by the early 1980s.

The left hand block is made from clay. These provided good fire resistance but were difficult to cut on site. The right hand block, formed from concrete, sometimes had the voids filled with polystyrene insulation. They shattered easily when cut or chased (for wiring etc). The polystyrene beads littered many sites in the 1970s and 1980s. 

3 Lightweight Concrete Blocks

Since the 1930s a wide range of lightweight blocks have been produced. In many cases the early blocks were meant to be light, not because of their insulation properties, but because they were light and easy to handle. However, as fuel prices rose in the late 1960s more emphasis was placed on thermal insulation. Nowadays, the big advantage of lightweight blocks is their thermal insulation characteristics.
Before the 2nd World War a few houses were built with blocks on the inner leaf; a very few had solid walls built in lightweight block and rendered. (below)
Early aggregates included breeze, blast furnace slag, clinker and expanded shale. They were often mixed in about 1:6 - 1 part cement 6 parts aggregate. 
During the last 80 years or so a variety of aggregates have been used to make lightweight blocks. These would often depend on the availability of local industrial wastes. Breeze blocks (no longer available) for example, were made from coke. Clinker blocks were made from furnace clinker (8 parts clinker to 1 part cement was typical). Blast furnace slag (from the iron and steel industry) was also a common aggregate. These slags contained hydraulic limes and so only small amounts of cement were required to make the blocks. Some blocks have been made from naturally occurring lightweight aggregates. Pumice, for example, is a lightweight volcanic material. These are fairly rare in modern construction. 
In the 1970s and early 1980s there were a number of blocks available which, although made from dense concrete, incorporated thermal insulation. Two are shown here; the one on the left had two or more cells filled with polystyrene beads. On site these were a bit of a nuisance. They were difficult to cut, eg, to make a three quarter block, and the pellets often escaped and drifted over the site. They were also difficult to chase for wiring and sockets. The one on the right had a foamed insulation bonded to the inner face, ie the cavity side. Both these blocks have been generally superseded by aerated concrete blocks.  These were introduced in the early 1950s when they were known as cellular concrete or gas concrete blocks. They are now often referred to as aircrete blocks.

4 Aerated Concrete Blocks - Aircrete

Aerated blocks are made from cement, lime, sand, pulverised fuel ash (from power stations) and water. First the PFA sand and water are mixed to form a slurry. This is then heated and mixed with cement and lime and finally a small amount of aluminium powder is evenly dispersed through the mixture before it is poured into moulds. The aluminium powder reacts with the mix to form millions of tiny pockets of hydrogen. These subsequently diffuse from the material to be replaced by air (see photo right). When the mixture has partially set the long strips of aerated concrete are wire cut into blocks of the right size and the cut blocks are transferred to an autoclave for high pressure steam-curing. During this process calcium silicates are formed which bind all the ingredients together. 
 

Different manufacturers produce blocks with slightly different characteristics. Celcon, for example, produce a range of blocks, some of which are described below.

 
Standard blocks are available in a range of thicknesses from 75m to 230mm. Thermal conductivity is 0.15W/mK and compressive strength is 3.5N/mm2. When used in modern cavity walls with a brick external leaf they will usually require additional insulation to achieve the requirements of the Building Regulations. These blocks can be used in internal and external leaves of cavity walls, solid walls, party walls, partitions, beam and block floors, and foundations.

A Solar block is also available in thicknesses of 100 to 250mm. Its strengths is 2.8N/mm2 and its thermal conductivity is 0.11 W/mK.  Both these blocks are available in face sizes of 440x215, 610x215, 610x140, and 610x270mm.  These blocks are suitable for cavity walls, solid walls, partitions and foundations

Hi-seven blocks are 75 to 230mm thick, thermal conductivity is 0.19W/mK and strength is 7.0N/mm2. There is a black stripe on one end to facilitate recognition on site. They are suitable in the same situations as Standard blocks.

Hi-ten blocks are 100 to 200mm thick, thermal conductivity is 0.19W/mK and strength is 8.4N/mm2. There is a red stripe on one end to facilitate recognition on site. Both these blocks are used where greater strength is required, for example in houses of 3 storeys or more.
Most aircrete manufacturers make blocks suitable for solid foundations. These blocks are quick to lay and don't require the rather slow and expensive cavity fill normally needed to stop two separate leaves being squashed together during backfill. Celcon's blocks are available in strengths of 3.5N/mm2,  7 N/mm2 and 8.4N/mm2. They are resistant to frost attack and will also resist most sulfates. 

Thin joint systems are starting to become a more common sight on UK building projects. The blocks are typically 610mm long by 215mm or 270mm high. They can be laid with normal sand/cement mortar joints. When laid with thin joints (using special quick setting mortar) considerable heights can be achieved in one day. The system is designed for cavity walls, solid walls, party walls, partitions, and foundations.

5 Aerated Concrete Blocks - Properties and Mortar Mixes

 

Aerated blocks are popular for a number of reasons other than their light weight and their good thermal insulation. These reasons include:

 

  • Their closed cell structure means that they have good resistance to water penetration. 
  • They provide good fire protection. 
  • They are easy to chase for cable runs and socket boxes etc. 
  • They can be cut with a saw. 
  • They will directly take fixings such as screws and nails. 
In addition they can be tiled and painted directly. The blocks are also suitable for drylining, either applied directly (plaster dabs), or via timber battens or lightweight steel channels. All the pictures on this page, and the video clips, have been supplied by Celcon.
Strong mortars are not suitable for aerated blocks
Mortar mixes should not be too strong. Mixes of cement sand 1:3 are unsuitable because they will not accommodate movement in the blockwork. Celcon, for example, recommend the following mortars above ground level:
  • Cement:lime:sand 1:1:6 
  • Cement sand with plasticiser 1:6 
  • Masonry cement:sand 1:5 
  • Cement:lime:sand 1:2:9 

Below ground level a slightly stronger mortar can be used, say 1:0.5:4, cement:lime:sand. 

Note : Strong renders should also be avoided. The shrinkage of a dense mix can place too much stress on the blocks resulting in a detached or cracked render.

 

Click link below to see Celcon blocks: 

Celcon and other aerated blocks are easy to cut so working around floor joists is easy. Joists can be built in or supported on hangers.

6 Aerated Blocks - Practice

Coursing blocks

 

Most aerated block manufacturers produce coursing blocks to reduce the amount of cutting necessary on site.

 

Ties

Conventional butterfly type or double triangular wall ties are best suited to aerated blocks. Some rigid, twist ties may not be suitable - they do not allow for differential movement between leaves.   

Movement Joints

Movement in building materials can be caused (structural movement apart) by changes in moisture content and changes in temperature. The design of internal walls must, therefore, accommodate movement.    
Concrete block walls (internal) should have movement joints approximately every 6 metres, the first joint should only be 3 metres away from an internal or external angle. In addition the use of strong mortars should be avoided - they restrict the wall's ability to accommodate movement. Movement joints are not normally required below DPC level because the moisture content and temperature will be relatively constant. 
In most houses movement joints are not required because few walls exceed 6 metres.  

Straight unbonded vertical joints are the most common way of providing movement (left). Blocks can abut a 10mm rigid filler which can be removed when the wall is complete. Where a wall requires lateral stability across the joint special slip ties should be used. One end of the tie is bedded in the blockwork, the other end is free to move along the wall's length but not side-to-side. These ties might, for example, be required in long internal corridor walls a single-block wide.  
Openings are potential weak areas in walls and cracking, caused by movement, may be concentrated around these areas. One option is to reinforce the bed joints (left) with masonry-grade steel reinforcement to distribute stresses into the blockwork either side of the opening. This method can also be used to reduce the risk of cracking caused by movement (deflection) in a suspended concrete floor.   aerated blocks on floor

Finishes

Aerated blocks can successfully be plastered, rendered or painted as long as manufacturer's guidance is followed. Celcon provide clear detailed guidance on all aspects of finishing. Before rendering Celcon blocks, for example, there are some key points to remember:
  • Avoid strong cement-based mixes
  • Blockwork to be rendered should have raked out joints
  • Rendering should not be carried out in direct sunlight
  • Dry blocks should be lightly wetted prior before application to reduce their natural suction. In dry weather the render should be kept damp for a few days.
  • Let each coat dry thoroughly and slowly before applying subsequent coats (to prevent the risk of cracking)
  • Each coat should be weaker than one which precedes it. 
Celcon blocks:

Fire protection

Aerated blocks have good resistance to fire. They are classified as non combustible and have a Class 'O' surface spread of flame (the best rating). A 100mm Celcon wall for example, in Solar blocks, provides 1 hour fire protection, a 200mm wall provides 4 hours.    
Connections The image on the far right shows how party walls can be connected to the internal leaf of cavity walls. The walls can either be bonded in or connected with ties. In modern construction the internal leaf of a cavity wall is normally always formed in aerated blocks. The party walls can either be in aerated blocks (certain types) or dens blocks. More details of party wall construction can be found on the next page. 

7 Sound Insulation: Party Walls & Partitions

Sound Insulation

The Building Regulations require good insulation against airborne sound for party walls (separating walls). There are no requirements for impact sound.  The former includes noise from radios, talking etc, the latter from hammering nails into the wall or drilling. Until 2004 the Regulations specified minimum densities which would provide suitable insulation. Although dense blocks could easily these densities many aerated blocks did not. However, lab and field tests proved their suitability for use in party walls and Celcon Blocks, for example, have an Agrement certificate showing that aircrete blocks can match the performance of heavier block or brick walls.
More specific details of sound insulation can be found in the B Regs section.
 
In 2004 the Building Regulations changed with regard to sound insulation. The Regs now specify performance standards for party walls (separating walls) and some walls within dwellings (walls adjacent to bathrooms, WCs and bedrooms). Many aerated blocks (and of course most solid ones) can meet these requirements.

The Regs divide separating walls are grouped into four main types:

  • Wall type 1 Solid masonry (see right)
  • Wall type 2 Cavity masonry (see right)
  • Wall type 3 Masonry between independent panels
  • Wall type 4 Framed walls with absorbent material.
  To prove that the walls are satisfactory there are three approaches
  1. For new houses and flats only, from July 1st 2004, use Robust Details in accordance with the rules and procedures of Robust Details Ltd. (www.robustdetails.com).
  2. Build using construction details contained in the guidance sections of the Approved Document, then sound test to achieve the specified performance requirements.
  3. Build using alternative designs, materials or products supported by advice from a manufacturer or other appropriate source, then sound test to achieve the specified performance requirements.

The image on the right shows a typical detail for an internal wall, ie within 1 single dwelling.

The far-right image shows a party wall after applying a render coat of plaster. The wall will be finished with drylining.
In addition there are some basic areas of good practice to help improve the sound insulation of walls:
  • Rooms either side of party wall should have similar use
  • Services should not pass through, or be chased into, separating walls
  • Mechanical equipment such as cooker hoods should not be fixed to separating walls
  • Joists should not, if possible, be built into the separating wall
  • All mortar joints should be well filled
  • Electrical sockets on either side of the wall should be staggered.

©2006 University of the West of England, Bristol
except where acknowledged