Making Concrete Blocks

This article is about making Concrete Blocks for good looking walls with character and quality. Anyone can make an ugly lump, but with a little more thought a block wall can be pretty, characterful and a pleasure for the eye.

Ugly blocks need rendering & painting just to make them look ok. Save yourself years of repainting and make them look good in the first place.

Uses

External Walls
Partition walls
Outbuildings
Retaining walls
House walls
Plant bed edging

Mix

The standard mix for Concrete Blocks is 1:3:5, 1 part cement to 3 parts sand and 5 parts stone aggregate, by volume.

Note that the ratio is different by weight, since all ingredients do not have the same density.

Many other mixes are also possible, some of which incorporate other materials such as

clay
subsoil
brick
tiles
ash
other pozzolans
expanded polystyrene
straw
glass
chalk
fibres for crack control
clinker

and so on.

Bulky additives such as brick, tile & glass should be broken down in a mixer first.

A stack of blocks is very heavy, so a cement mixer is strongly recommended for block making.

There are also non-concrete mix options, such as adobe, papercrete, etc.

Beauty

Concrete blocks can be basic and functional or pretty & decorative. Its a matter of choice, imagination, and a little extra work. While concrete blocks are too often ugly, there is no need at all to make them so. The only reasons for ugliness are 'I cant be bothered to make them look decent' and 'I didnt know it could be done.'

Beautiful concrete block walls come from these aspects:

Shape
Colour
Finish
Added decoration
Overall design

Shape

As well as decorative finishes, decorative block shapes can also be used to add character to a wall.

If you're building an external wall you may wish to mould a decorative topping for it. Imagination is usually the limit here. A simple easy option is to pour concrete into lengths of 4" pipe. These smooth round sections are mortared onto the top of the wall.

A special small mould can be made of decorative shapes, and cast with cement to produce a few pieces to add character to a wall. Items like gargoyles, roses and so on can be made this way. Even jelly moulds can be used if you find a suitable shape.

If you're more adventurous you could make blocks of an interlocking non-rectangular shape to construct the wall. Ensure the shape leads to a stable block stack, is not too hard to mould or work with, that neat vertical wall ends are straightforward to produce, that the mould is not fiendish to make, and that the pattern tolerates slight inaccuracies in block dimensions.

Not all blocks need to be the same shape, and not all mortar lines need be the same thickness. A minority of differently shaped blocks can be used to make a geometric design.

The apearance of small blocks can be achieved without the extra labour by moulding in grooves which are pointed as the wall is built. This can be done to for example make interlocking rectangular blocks look like square non-interlocking blocks.

Simple novel low garden walls can be made by casting blocks in short plastic pipe sections, and stacking them in staggered or overlapping layers with the flat ends up & down. It is important to get the length of each block the same with this design.

As well as bulk shape there is detail shape. For instance a shaped groove can be moulded into blocks so it runs round the edge of the wall. Or some blocks could be cast with one round edge (4" pipe is handy), so the top of the finished wall is rounded, and any exposed wall end too.

Planting holes may be made for small rock plants or trailing strawberries. Ensure a good freezeproof concrete mix is used for these blocks, as water in the holes will not drain away readily. Ensure hole bottom is sloped a little to minimise pooling. The easiest hole mould items are probably cut off plastic drinks bottles, as they flex easily to come out of the block. Less easy moulding pieces such as a very short piece of plastic pipe, a plastic plant pot or a greased tapered drinking glass can also be used.

For a flamboyant example of how concrete blockwork can be highly decorative, take a look at the famous Concrete Castle. Constructed in 1851 out of 17 different (then experimental) mixes of concrete, it is every bit as elaborate and richly designed as any king's castle.

There really is no reason to make visible concrete walls ugly.

Colour

Block walls do not have to use all blocks of the same colour. A decorative line of a 2nd colour block, or a line of extra-thin blocks (eg 1" or 2" high), can all help construct the wall's character.

A little decoration can also be made using a mixture of block colours. An area of chequer pattern is sometimes seen on old brick walls.

A line of alternating colour blocks may be run around the perimeter of the wall.

These are common basic patterns, with imagination you can do much more.

Colourants

Many materials can colour blocks:

White cement - white
Grey cement - grey
Stone chips - many colours available
Clinker - dark
Glass - crushed glass available in many colours. Sparkling finish.
Tiles - any colour you like
Brick dust - red brick is high in iron oxide and pozzolanic
Iron oxide - dull red, makes pink, red, & brown
Cement dyes - some are prone to fade and streak over time, better avoided
Coal ash - makes black mortar. Some black mortar can fail prematurely, so use it sparingly.
Clay & subsoil - brown
coloured emulsion
chalk

Where colourants cost more than bulk concrete, which is true in most cases, the coloured mix is laid in a thin layer in the mould then bulk concrete added to fill the mould. The first layer of bulk concrete must be laid in the mould gently to avoid it pushing the thin coloured layer aside. A scoop is used to do this.

Large pieces need breaking up before use. Bricks, tiles, clinker, glass bottles should all be broken up. An easy way to do this is put the material plus a hard brick in a cement mixer.

Where it is required to concentrate solid colourants at the face of the block, the pieces are sprinkled or laid out in the mould and concrete poured on. Gently scoop the first layer of concrete mix in so it does not disturb the coloured pieces. This method is especially suited to tile pieces, which dont look good if exposed the wrong way up. The concrete should then be tamped or vibrated to eliminate air pockets.

When using surface sprinkled blocks, don't overlook the fact that you will need some blocks with 2 fair faces. These take significantly longer to mould.

If your imagination still hasn't fired up, try thinking about:

a white cement finish with black stone chips
black cement (ash & dye) with clear glass sparkling in it.
red cement with purple sparkly glass pieces
dull red cement (brick colour) 2" blocks with white cement pointing.
brown cement with mixed colour tile mosaic pieces
an almost all white wall with pieces of 1" high red block scatterd here and there
white cement with marbled colour streaks in pale pink, pale grey etc
white wall peppered with grey cement dots.
Red rectangular blocks with a black dot pattern in each half like dominoes.

The dots are premoulded separately and placed in the mould
Bulk is scooped over each dot to minimise chance of movement.
the moulds are then bulk filled.

Finishes

Quality of finish is maximised by compacting the mix well in the mould, and by using accurately made moulds.

Sparkling finishes are obtained by adding crushed glass.

Textured Finishes

Textured finishes get dirtier faster than smooth wall surfaces, so will need more maintenance. Textured finishes are also obvious imitations, and inaccurate ones, so are not always liked.

For a wood grain finish, use wire-brushed wood as a mould base. A powered rotary wire brush creates the slightly 3d surface on the wood by removing the softer parts of the wood. A release agent must be applied to the mould base.

For a riven stone finish, riven slate can be used as the mould base. Release agent must be applied to prevent bonding.

For another textured finish, try polythene over gravel. In this case to prevent leakage the poly sheet is stapled to the base of the wood frame and draught excluder foam rubber strip is applied to the underside of the poly around each mould cavity. Tiny pinprick holes in the polythene enable release.

Portholes may be moulded in with rigid pipe. Multiple holes may be made with cut sections of rigid pipe.

It is possible to mould glass bottles into blocks, but this is best avoided as the bottles are breakable and not replaceable.

Moulds

Moulds may be made from 2x4 and lined with polythene. Other wood sizes and mould release agents can also be used if preferred. Dividers between mould cavities may be made with 1" wood.

To get a good flat smooth face on one side, use a mould base of sheet wood with polythene on it, and sit the polythene lined frame onto this. Take care the polythene is fully flat, and does not ruck or fold.

For an easier, faster and perfect finish, it is possible to use a sheet of glass resting on wood sheet as the base. The glass must be well supported. For cost reasons this is generally only practical if you already have a large sheet of glass to hand, or will be making a lot of blocks.

Cut and assemble wood moulds accurately to obtain blocks of accurate shape, which will always look better than inaccurately shaped blocks.

One moulding tip comes readily to mind: make the mould such that a small percentage of half blocks get produced. This saves work during construction.

If you plan to run wiring in a 4" block wall, you can save work by fitting 1"x1" strips of wood to some of the moulds so as to make ready cast wiring channels.

Compacting

Several options exist for compacting the mix in the mould:

do nothing
tamp & level with a strip of wood - most common option
vibrating poker - common on commercial sites
low pressure press - may be done with weights
high pressure press - probably not practical for diy use

Block Types

High vs low strength blocks

Final block compressive strength is determined by

the mixture (1:3:5 is strongest)
the amount of water used in the mix (the less the stronger)
how well the mortar is compacted in the mould (stiff mixes are difficult to compact)

The highest possible strength blocks are made with a semi-dry mix and compacted by a hydraulic press.

Solid vs hollow

For light duty use such as most internal dividing walls, hollow blocks are strong enough, as well as being cheaper to make and lighter to carry. The central plug in the mould is usually slightly tapered to make withdrawal easy.

When a non-tapered plug is wanted, it may be made in 2 or more pieces such that withdrawing one tapered piece loosens the other pieces in contact with the concrete.

When a reinforced wall is wanted, hollow blocks may be used, reinforcing bar fitted, concrete poured into the wall, and the poured mix poker vibrated to release air pockets. Each pour should be limited to 1m depth, and fall of the concrete during pour must also be similarly limited to avoid separation of the components.

Aerated

Foamed or aerated concrete is made by adding aluminium powder to the mix. There are probably few diyers who have experience of doing this.

Another way to make light blocks is to use LECA, perlite or expanded polystyrene as the aggregate or part of the aggregate.

Lightweight blocks have much lower weight, compressive strength and freeze cycle resistance, and better thermal insulation.

No fines

Blocks made without fine aggregate are slightly lighter and cheaper, weaker, and not freeze cycle tolerant.

Garbage fill

Hollow blocks can be filled with a mix of non-expanding garbage and cement mortar, or solid blocks can have garbage dumped in the centre during pour. This gives a greater total load carrying ability than a hollow block while reducing cost.

This is an effective way to reduce cost by using unwanted hard materials such as broken concrete, brick, removed plaster and so on.

Testing Strength

Where a high strength block is needed, it may be necessary to make some test blocks ahead of time and test their strength.

To determine the strength of your test block, let it cure fully first (which can take a month). Then pile up the following in this order on a strong flat level surface:

sheet wood
your test block
sheet wood
weights

The sheets of wood are to even the load over the whole surface of the block.

Just keep adding more weight onto the block until it crumbles. (Don't stand close enough for toe injury.) Knowing the weight you added and the top surface area of the block, you can calculate the pressure at which it failed.

In real life applications you must keep loading far below this figure. A substantial loading margin is needed as this failure load figure must never be reached even in the worst of conditions, eg a fully loaded structure in a hurricane.

Maximising Strength

Optimum 1:3:5 mix
low water content, a semi-dry mix
good compression of mix in mould
fibres to reduce cracking

Troubleshooting

Edge breakup

leave in mould longer before removing
protect curing blocks from frost
Ensure mould sections separate easily to avoid scraping the newly moulded block during mould removal.

Insufficient strength

see [#Maximising Strength]
allow a full month for blocks to cure.
keep blocks damp while curing
protect from frost during cure. A tarp is adequate for very mild frost.
Do not use concrete that was wetted more than 30-40 minutes before pour. Out of time concrete will make lower strength blocks.

Cement smeared over face decorative features

brush or wipe the surface before the block is cured
Gentle use of a copper scourer on the surface will remove cement from features. Take care not to damage the infill.
After cure, remove a very thin surface layer by brushing on a little brick acid. Don't overdo it.

Air pockets

tamp, vibrate or compress more thoroughly in mould to prevent this
to repair faulty blocks, fill pocket holes with more cement mix
Badly holed blocks can be trowelled with a layer of plain cement mix and used as plain blocks.

Alternatives to Concrete Blocks

Poured concrete
Adobe
Papercrete
various non-block construction methods.