Concrete units have been made since the 1920s and were widely used, in the form of the ‘breeze block’, to build partitions in houses in the building boom of the 1930s. In the past 40 years, however, the range of products has expanded enormously to cover facing bricks and blocks, high-strength units, simulated stone units, thermally insulating blocks and pavers.

In this article we will discuss units formed from concrete containing aggregates, which are produced by pressing specially designed mixes. Other processes are used to produce so-called concrete that does not contain aggregates, such as autoclaved aerated concrete (AAC or aircrete), which we will discuss in the next section. 

Production Processes for Concrete Units

Casting Concrete

Concrete blocks can be manufactured by pouring or vibrating a concrete mix into a mould and demoulding after a curing process. While this method is used, particularly for some types of manufactured stone and stone-faced blocks, it is not favoured because of its slowness and labour demands.

Pressing of Concrete

This is a widely used method for producing solid and frogged bricks and solid, cellular and hollow blocks either in dense concrete or as a porous open structure by using gap-graded aggregates of varying density and not compacting fully. The machine is basically a static mould (or die) that is filled automatically from a mixer and hopper system, and a dynamic press-head that compacts the concrete into the die.

After each production cycle the green block is ejected on to a conveyor system and taken away to cure. The press-head may have multiple dies. A variation of the method is the ‘egg layer’. This performs the same basic function as a static press but ejects the product straight on to the surface on which it is standing and then moves itself to a new position for the next production cycle.

Curing

All aggregate concrete products may be cured at ambient temperature or at elevated temperature. Elevated temperatures are usually achieved by the use of steam chambers, and allow the manufacturer to decrease the curing period or increase the strength or both. Products cured externally should not be made when the temperature is near or below 0°C since they will be damaged by freezing while in the green state.

Concrete Products

Dense Aggregate Concrete Blocks and Concrete Bricks

These are generally made from well-graded natural aggregates, sands, pigments, and CEM I or white Portland cement by static pressing to a well-compacted state. Figure 1a illustrates the principle of such materials, in which the voids between large particles are filled with smaller particles.

Fig. 1 Types of aggregate: (a) well graded; (b) gap graded (schematic).

They are strong, dense products and are often made with a good surface finish suitable for external facing masonry. They are also suitable for engineering applications. Bricks are produced mainly at the standard size (215 × 102 × 65 mm) in the UK but in a wide range of sizes in continental Europe. Blocks are produced as solid, cellular or hollow by varying the quantity of mix and the shape of the press platen.

In order to facilitate demoulding the hollows will always have a slight taper. The hollows or cells in UK products are all designed to run vertically in the finished masonry as this gives the optimum strength to weight ratio. The face size of UK units is generally 440 mm long by 215 mm high but the thickness may vary from 50 mm to 300 mm. Some of the important properties are summarised in Table 1.

Table 1 Properties of some typical dense aggregate (DA) and lightweight aggregate (LWA) concrete
blocks.

Manufacture should comply with BS EN 771-3 (2003), and testing to the relevant BS EN 772-XX series of standards. BRE Digest 460 Parts 1 and 2 (2001) give some useful guidance and background.

Manufactured Stone Masonry Units

These have an essentially similar specification to dense aggregate concrete blocks, except that the main aggregate will be a crushed natural rock such as limestone or basalt and the other materials will be chosen such that the finished unit mimics the colour and texture of the natural stone. Also the production will often be by casting. The relevant standard is BS EN 771-5 (2003).

Lightweight Aggregate Concrete Blocks

These are generally produced as load-bearing building blocks for housing, small industrial buildings, in-fill for frames and partition walling. High strength and attractive appearance are rarely the prime consideration but handling, weight, thermal properties and economy are important. Inherently low-density aggregates are used and are often deliberately gapgraded, as illustrated by Fig. 1b, and only partly compacted to keep the density down.

They will frequently be made hollow as well to reduce the weight still further. The aggregates used are sintered pfa nodules, expanded clay, furnace clinker, furnace bottom ash, pumice or foamed slag together with sand and binder. Breeze is a traditional term for a lightweight block made from furnace clinker.

Often low-density fillers or aggregates such as sawdust, ground bark or polystyrene beads are incorporated to further reduce the density. They are produced either by static pressing or in egg-layer plants. Some of the important properties are summarised in Table 1.

The properties commonly measured include compressive strength with the test face ground flat or capped. Flexural strength has also been used to evaluate partition blocks that bear lateral loads, but only self-weight compressive loads. It is a simple, three-point bend test for clay units with an aspect ratio greater than 4. Other properties measured include dimensions, water absorption by the method of vacuum absorption, density and drying shrinkage. These are manufactured to the same standard as dense aggregate blocks and are also covered by BRE Digest 460 (2001). 

Aircrete (Autoclaved Aerated Concrete – AAC)

Aircrete is the current term for AAC and it is made by a process, developed originally in Scandinavia, that produces solid microcellular units that are light and have good insulating properties. Fine sand or pulverised fuel ash or mixtures thereof is used as the main ingredient.

The binder is a mixture of Portland cement (A CEM I), to give the initial set to allow cutting, and lime, which reacts with the silica during the autoclaving to produce calcium silicate hydrates and gives the block sufficient strength for normal building purposes.

Manufacturing Process

The method involves mixing a slurry containing a fine siliceous base material, a binder, some lime and the raising agent, aluminium powder, which reacts with the alkalis (mainly calcium hydroxide) to produce fine bubbles of hydrogen gas:

Ca(OH)2 + 2Al + 2H2O → CaAl2O4 + 3H2

This mixture is poured into a mould maintained in warm surroundings and the hydrogen gas makes the slurry rise like baker’s dough and set to a weak ‘cake’. The cake is then cured for several hours at elevated temperature, de-moulded, trimmed to a set height and cut with two orthogonal sets of oscillating parallel wires to the unit size required using automatic machinery.

The cut units are then usually set, as cut, on to cars which are run on rails into large autoclaves. The calcium silicate binder forms by reaction under the influence of high-pressure steam. Additional curing after autoclaving is not necessary all the units can be incorporated in work as soon as they have cooled down.

Properties 

The binder reaction is conventional, as given for sandlime (equation 33.2). The structure, of small closed spherical cells with walls composed of a fine siliceous aggregate bound together by calcium silicate hydrates, gives the product a good resistance to permeation by water, good thermal properties and a high strength:density ratio.

The nature of the principal siliceous material is identifiable from the colour: ground sand produces a white material and pulverised fuel ash a grey material. Because of its light weight (low density) the product can be made into large blocks while remaining handleable. Some units are available with (double) face size, 447 mm by 447 mm, designed for building thin-joint masonry (see BRE Digest 432 (1998)).

Table 2 Properties of Aircrete.

The most common size for normal work is 440 × 215 × 100 mm (or thicker). Some of the key properties are summarised in Table 2. Manufacture is to BS EN 771-4 (2003), testing to the BS EN 772 series or RILEM (1975). BRE Digest 468 Parts 1 and 2 (2002) give some useful guidance and background.

Natural Stone Units

Stone units are either naturally occurring flints from chalk deposits (used to make rubble masonry), pieces formed by weathering from the original deposit or partly trimmed pieces used widely for domestic coursed or semi-coursed masonry or precisely cut blocks used to make ashlar stonework, usually for prestige or heritage buildings.

To ensure optimum performance the layered rocks are usually cut to maintain the bedding plane perpendicular to the compressive stress field in the building, e.g. horizontal in normal load-bearing masonry. This also gives the optimum durability.

Specification and testing should comply with BS EN 771-6 (2005). Sourcing the stone and a useful set of references are covered in BRE Digest 420 (1997), and the Geological Society SP16 (1999) has comprehensive coverage. Specification and use of stone for conservation work is covered in BRE Digest 508 Part 1 (2008).

Table 3 Properties of some types of stone unit

Some performance characteristics of well-known types are given in Table 3.

Ancillary Devices 

In order to ensure their stability, masonry elements need to be connected either to other masonry elements to form stable box structures or to other structural elements such as frames, floors, roofs, beams and partitions. There is a huge range of ties and other connecting devices. Most of these devices are made from metal, predominantly galvanised mild steel, austenitic stainless steel and bronzes. A few light-duty tie products are made from plastic. Masonry walls require openings for doors, windows and services. The masonry over the openings has to be supported by constructing a masonry arch, by use of a lintel (beam), or by reinforcing the masonry in-situ with bed-joint reinforcement to form a beam.

Lintels are prefabricated beams made from steel, timber, concrete and clay-ware designed and sized to co-ordinate with and support masonry in walls. They are covered by BS EN845-2 (2003). Bed-joint reinforcement is typically prefabricated metal meshwork elements sized to be embedded in the mortar joints to increase the overall strength of the masonry. Joint reinforcement is also used to resist out-of-plane loading and to tie masonry together to resist accidental damage.

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