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 & 5 parts stone aggregate, by volume. 1 part cement to 8 parts mixed aggregate is much the same thing.

Note that the ratio is different by weight, since the ingredients don't have the same density.

Many other mixes are also possible. As well as standard concrete, mixes that incorporate other materials are also possible, including:

• clay
• subsoil
• brick
• tiles
• ash
• 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 cement mixer is strongly recommended for block making, mixing half a ton of concrete by hand isn't much fun.

There are also non-concrete mix options that are very occasionlly used, 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 can't be bothered to make them look decent' and 'I didn't 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.

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Simple fancy block pattern with 2 block types

For a flamboyant example of how shaped 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 need to make visible concrete walls ugly.

Non-rectangular blocks

If you're more adventurous you could make blocks of an interlocking non-rectangular shape to construct the wall. The shape should:

• lead to a stable interlocking block stack
• be not too hard to mould or work with
• neat vertical wall ends should be straightforward to produce, or else partial blocks can be cast for this
• the mould should not be fiendish to make
• the pattern should tolerates slight inaccuracies in block dimensions and laying position.

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 grooves into the blocks, which are then 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. Split pipe is used for the mould.

Shaped detail

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.

Topping

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 split 4" pipe. These smooth round sections are mortared onto the top of the wall.

Gargoyles

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.

Plant holes

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 could also be used.

Colour

A wide range of concrete colours is possible using the colourants listed below.

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 0.5", 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 basic patterns, with imagination you can do far 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 - all but red 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 produces a wide array of colours
• chalk

Where colourants cost significantly 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. To do this its shovelled in holding the shovel close to the mould to minimise drop, and a layer of bulk put in before completely filling, to minimise sideways movement of the mixture.

Large pieces in the concrete mix 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 in a cement mixer with a hard brick.

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 don't look good if exposed the wrong way up. The concrete should then be tamped or vibrated to eliminate air pockets. Banging the mould with a shovel is the usual way.

When using surface sprinkled blocks, don't forget you'll need some blocks with 2 fair faces for most jobs. These take significantly longer to mould.

Colour combinations

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

• white cement with black stone chips (or maybe red)
• black cement (ash & dye) with clear glass sparkling in it.
• red cement with blue 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
• assorted pale pastel coloured blocks
• white wall peppered with grey cement dots.
• Red rectangular blocks with a black dot pattern in each half like dominoes.

1. The dots are premoulded separately and placed in the mould
2. Bulk is scooped over each dot first to minimise chance of movement.
3. 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.

Mould accuracy helps keep the mortar lines straight and consistent. If the blocks are just a mm or 2 off, this can't be done.

Textured Finishes

Textured finishes get dirtier faster than smooth wall surfaces, so either need more maintenance or look worse. Textured finishes are also obvious imitations, and inaccurate ones, and aren't always appreciated.

For a wood grain finish, wire-brushed wood can be used to create the mould base. A rotary wire brush creates the slightly 3d surface on the wood by removing the softer parts of the wood. A release agent must be used when casting.

For a riven stone finish, riven slate or riven concrete slabs can be used as the mould base. Release agent or thin polythene must be used to prevent bonding. Thin polythene is more likely to bend to shape than thick.

Various textured surfaces can be used to create different textures if required.

Portholes may be moulded in with rigid pipe, but expect to leave it in place permanently. Holes can also be made by moulding blocks with a corner missing.

Moulding glass bottles into blocks 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 thinner wood if needed.

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. Thick polythene is ideal.

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

One moulding tip is to 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 block wall, you can save work by fitting small 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 and accept cavities - ok for indoor use
• 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)
• keeping the concrete damp for its 1 month curing time

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 slightly tapered to make withdrawal easy.

If a non-tapered plug is wanted, the plug is made in 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 commercially by adding aluminium powder to the mix. There must be very few diyers with experience of doing this though.

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 with no fine aggregate (sand) are slightly lighter and cheaper, significantly weaker, and not freeze cycle tolerant. They're good for interior use, and have rarely been used for greenhouse floor drainage where they won't freeze.

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 a way to reduce costs by using unwanted hard materials such as broken concrete, brick, removed plaster and so on.

Low strength blocks for dry use may have a high percentage of garbage throughout the mix, but all garbage must be thermally and chemically compatible. Suitable materials include

• shredded paper
• sawdust
• broken brick
• crushed concrete
• hardcore
• stone waste

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
• thick 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. Distribute the weight evenly over the block's surface, and keep your toes out the way when it collapses. 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, loading must always be kept far below the failure figure. A large loading margin is required as failure must never be reached even in the worst of conditions, eg a heavily 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

Problem additives

Glass reacts with cement in time. ASR products (alkaline silica reaction) expand and break blocks. Precautions are needed to avoid ASR.

Porous types of stone make a freeze-thaw vulnerable block.

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.
• use a good mould release agent, such as polythene

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 can be poured to make low 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 (steel can cause rust staining). 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
• Though visible holes could be filled en masse, interior voids still remain, and such blocks should not be exposed to freezing water.

Alternatives to Concrete Blocks

• Poured concrete
• Adobe
• Papercrete
• various non-block construction methods.

Paving slabs

The same approach can be taken to paving slabs if you want something special. There are a few additional issues to consider.

Longer set time in the mould is needed, as the slabs are thin. Or you can simply lift the mould outer frame off as soon as the mould is filled and compacted. You'll need plenty of very flat space when doing this.

Cavities in the slab are ever an issue. The standard way to avoid them is to use a vibrator of some sort. Its also possible to simply let cavities happen and fill the top surface ones with a dark mix afterwards.

Get the mould squareness accurate, if its 2mm out it looks rather poor when laid. This is a common mistake.

A slight taper on the mould sides aids release. Another option is to make the frame come apart in 2 pieces. Ready made paving slab moulds are also available online.

If you want a smooth surface, thick polythene is the material of choice to line the mould. Textures can be made with wood, routed wood, or oiled stone.

Sufficient curing time is more important for slabs as they're thin. Until sufficiently cured they're very weak.

See Also

Cement mixing

Concrete & Mortar

Disposal Options

Papercrete

Concrete Castle: 2nd & 3rd pics

Lots of Decorative blocks in Puerto rico