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Concrete blo
Apr
30
Concrete blocks 
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Amongst the concrete blocks there is an enormous variety of shapes, sizes, colours, and textures now available, with all the major concrete paving manufacturers producing their own versions of the standard rectangular block, and also special shapes, "olde-worlde" looking tumbled or antiqued units, and an ever increasing array of textured blocks.
Amongst contractors, specifiers and manufacturers, the blocks are often referred to as "CBP" - Concrete Block Paving
Typical concrete block Most rectangular blocks are moulded to a specific size, 100mm x 200mm x 50-100mm, with exactly 50 blocks per square metre, and, because of this standard size, blocks from different manufacturers may be interchangeable, depending on tolerances and position of any spacer lugs. The typical rectangular blocks tend to have one presentable face, although some blocks are reversible, which makes easy work of replacing stained paving.
The colour of concrete blocks is achieved by the use of concrete dyes, usually some form of metallic oxide. Even the best dyes are prone to some fading in natural daylight and the quality of the dyes is a significant factor in block price; cheaper blocks are usually coloured by cheaper, inferior dyes. The manufacturers rarely entertain any complaints of fading, so choose carefully. Make your selection by looking at blocks that have been in place for at least a couple of years, rather than from glossy publicity photos or newly-laid paving.
Face Mix allows the money to be spent only on that part of the block that will actually be seen once the paving is laid.
Example of a face-mix block
The face mix is clearly visible above the backing mix concrete that makes up the bulk of the block.
Face Mix technology enables the manufacturer to use cheaper, possibly recycled aggregates and alternative cements such as PFA and GGBFS for the backing mix, and to use amuch smaller quality of very high quality fine aggregates, cements and colours in the face mix.
While there may be cost savings, the production cycle-time for face mix manufacturing is greater than that for the through-colour process, and so less block per hour can be manufactured. There are also other costs involved in creating separate bins for face mix aggregates and separate hoppers for the face mix concrete once mixed.
Through-colour and face-mix
There are two manufacturing processes used to produce colour (and, to a lesser extent, texture) in a concrete block.
The first process, known as 'through colour', relies on placing all of the concrete (even though it may be three or more different colours of concrete) into the mould in one operation, and then pressing and compacting to create a unit that has the colour running right through the block, even though it's most likely that only the top surface will ever be seen once the paving is laid.
Through colour concrete block
The colour is present throughout the whole block While this use of expensively-coloured concrete throughout the block may be considered wasteful, there are manufacturing cost savings, that have to be considered.
The alternative is to use a technique known as "Face Mix", in which a no-frill, un-coloured 'backing mix' or 'base mix' is first placed into the mould, partially pressed and then topped with a high-quality, coloured 'face mix' concrete. The whole is then pressed and compacted before being de-moulded and passed into the curing chamber.
There is a delay of only a very few seconds between placing the base and the face, so there is no day joint or construction joint, no plane of weakness, and absolutely no risk of delamination between the two. The block cures as one, single, monolithic mass of concrete.
It's not possible to say that one production technique is better than the other. Both have their advantages and disadvantages, and some special products, such as tumbled blocks, need to be through-coloured otherwise they would have just the one serviceable face. However, most production in Ireland, in continental Europe, and in North America use face-mix technology far more wideley than has been the case in Britain to date
Amongst contractors, specifiers and manufacturers, the blocks are often referred to as "CBP" - Concrete Block Paving
Typical concrete block Most rectangular blocks are moulded to a specific size, 100mm x 200mm x 50-100mm, with exactly 50 blocks per square metre, and, because of this standard size, blocks from different manufacturers may be interchangeable, depending on tolerances and position of any spacer lugs. The typical rectangular blocks tend to have one presentable face, although some blocks are reversible, which makes easy work of replacing stained paving.
The colour of concrete blocks is achieved by the use of concrete dyes, usually some form of metallic oxide. Even the best dyes are prone to some fading in natural daylight and the quality of the dyes is a significant factor in block price; cheaper blocks are usually coloured by cheaper, inferior dyes. The manufacturers rarely entertain any complaints of fading, so choose carefully. Make your selection by looking at blocks that have been in place for at least a couple of years, rather than from glossy publicity photos or newly-laid paving.
Face Mix allows the money to be spent only on that part of the block that will actually be seen once the paving is laid.
Example of a face-mix block
The face mix is clearly visible above the backing mix concrete that makes up the bulk of the block.
Face Mix technology enables the manufacturer to use cheaper, possibly recycled aggregates and alternative cements such as PFA and GGBFS for the backing mix, and to use amuch smaller quality of very high quality fine aggregates, cements and colours in the face mix.
While there may be cost savings, the production cycle-time for face mix manufacturing is greater than that for the through-colour process, and so less block per hour can be manufactured. There are also other costs involved in creating separate bins for face mix aggregates and separate hoppers for the face mix concrete once mixed.
Through-colour and face-mix
There are two manufacturing processes used to produce colour (and, to a lesser extent, texture) in a concrete block.
The first process, known as 'through colour', relies on placing all of the concrete (even though it may be three or more different colours of concrete) into the mould in one operation, and then pressing and compacting to create a unit that has the colour running right through the block, even though it's most likely that only the top surface will ever be seen once the paving is laid.
Through colour concrete block
The colour is present throughout the whole block While this use of expensively-coloured concrete throughout the block may be considered wasteful, there are manufacturing cost savings, that have to be considered.
The alternative is to use a technique known as "Face Mix", in which a no-frill, un-coloured 'backing mix' or 'base mix' is first placed into the mould, partially pressed and then topped with a high-quality, coloured 'face mix' concrete. The whole is then pressed and compacted before being de-moulded and passed into the curing chamber.
There is a delay of only a very few seconds between placing the base and the face, so there is no day joint or construction joint, no plane of weakness, and absolutely no risk of delamination between the two. The block cures as one, single, monolithic mass of concrete.
It's not possible to say that one production technique is better than the other. Both have their advantages and disadvantages, and some special products, such as tumbled blocks, need to be through-coloured otherwise they would have just the one serviceable face. However, most production in Ireland, in continental Europe, and in North America use face-mix technology far more wideley than has been the case in Britain to date
