Concrete Block

Property

Constituent

Concrete blocks are made of the following ingredients:

• Water
• Portland Cement
• Blended cement
• Aggregates

Aggregate

A great variety of aggregate type is used in the production of concrete blocks, includes:

• Natural aggregates
• Air-cooled blast furnace slag
• Furnace clinker
• Foamed/ expanded/ granulated blast-furnace slag
• Bottom ash from boilers and milled softwood chips

Types

1. Solid blocks are largely void less but may have grooves or holes to reduce weigh or facilitate handling. These must not exceed 25% of the gross volume of the block.
2. Hollow blocks have voids passing right through. They can be ‘shell-bedded’, that is the mortar is laid in 2 strips adjacent to each face, so that there is no continuous capillary path for moisture through the bed. Load bearing capacity is reduced when blocks are laid in this manner. The strength of hollow block work can be increased by filling the cavity with concrete, especially if reinforcement is included. Sound insulation is improved in this way.
3. Cellular blocks are a special type of hollow block in which the cavities are closed at one end. The solid edge would normally be laid upwards and, in the case of thin blocks, this makes it easier to produce an effective bed joint.
4. Facing block specially made or selected to have consistent shape/ texture.
5. Architectural masonry block; often used to describe a higher than normal specification facing block. May also have special finish such as rock finish or exposed aggregate.
6. Insulating block; usually manufactured from lightweight aggregate or autoclaved aerated concrete to give better than normal inherent insulating properties. May also have insulating material incorporated within the block and be referred to as composite insulated blocks.

ASTM C90:

1. Type 1: Moisture controlled
2. Type 2: Non-moisture controlled

Compressive Strength

Concrete blocks range in compressive strength from 2.8 N/mm² to 35 N/mm²

The strength categories are:

BS 6073 gives minimum strength for block types:

Density

420 to 2200 kg/m³

Thermal Conductivity

0.10 to 1.5 W/m K at 3 % moisture content

Drying Shrinkage

Shall comply with BS 6073 Part 1: 1981 and are typically in the range 0.03-0.05%.

Maximum permitted shrinkage of concrete masonry units:

Fixability

Aerated and lightweight concrete blocks offer a good background for fixings. For light loads, cut nails to a depth of 50mm are sufficient. For heavier loads, wall plugs and proprietary fixings are necessary.

Fire Resistance

Concrete block construction offers good fire resistance. Solid un-plastered 90 mm blocks can give up to 60 minutes’ fire protection when used as load-bearing walls; certain 150mm and most 215mm solid blocks can achieve 6 hours’ protection.

Sound Absorption

The majority of standard concrete blocks with hard surfaces are highly reflective to sound, thus creating long reverberation times within building enclosure.

Water Absorption

Generally, water absorption value of concrete block ranges from 7-20%. A good indication of the density of the block is obtained from the absorption value. Absorption varies widely, from 160- 400 kg/m3 depending on type of aggregate and amount of compaction of the block. Permeability, thermal and acoustical properties are also related to absorption. Although high absorption is not desirable, it may accompany where lightweight and acoustical properties are desirable. Block for exterior walls should have low absorption.

Volume Changes

Concrete blocks undergo changes in dimensions due to changes in temperatures and moisture content. One change is the original drying shrinkage of green concrete. If blocks are placed into a wall before this initial shrinkage has occurred, cracks may occur in the wall due to tensile stresses induced by the shrinkage. Proper curing and drying of the block in the yard will reduce the moisture content to an equilibrium condition with the surrounding exposure and minimize cracking.

Texture and Colour

Texture is influenced by the distribution of particles and voids in the surface. A satisfactory texture can be obtained by adjusting the aggregate gradation, amount of mixing water and degree of compaction in the mold. Special finishes can be obtained by use of face molds, rubber or plastic liners, grinding of the face with an abrasive, exposing aggregate by chemical means. The desire for colour and textures has led to the use of bonded faces composed of thermosetting resin binders with silica sand and glazed surfaces.

Colour depends on the chosen cement, aggregate, use of pigment and whether the product is autoclaved. Cement from different sources will vary in shades of gray. Available block cement is usually lighter in colour. The choices become critical for split face block which depend on the colour of the mortar.

Size

Coefficient of Linear Thermal Expansion/ºC

7-14 X 10^-6/ºC