Concrete – After the Concrete is Laid

Concrete Myths

Some people think that concrete is strong enough the day after it is laid……..WRONG

Some people think that concrete ‘dries’……..WRONG

Hardening of concrete is a complex chemical reaction which requires time and moisture.

Concrete takes at least a month to reach its final design strength.

This time is known as the ‘Curing Period’.

During this time the concrete strength will increase but the slab will also shrink.

Strength

The typical slab laid and exposed to air after finishing will increase in strength as follows:

After 1 Day – The concrete will have about 20% of its design strength. Being still relatively weak it can easily be damaged. If you are going to have saw cut joints now is the time to cut them. You can also remove any formwork.

After 3 days – The concrete will now be at around 60% of design strength. You should be able to walk on it and use wheel barrows without damaging it. This is the earliest time I would like to see frame erection start on a house slab.

After 7 days – Around 80% has now been achieved. At this stage you should be able to use a driveway for cars

After 28 days – Design Strength Reached.

Shrinkage

During the 28 day curing period the concrete will shrink by 0.8 – 1.3mm per metre. That’s 2.4 -3.9mm for a 3m slab. This means:

  • Paving – Unless you have adequate joints this shrinkage will stress the slab causing cracks.
  • House Slabs – The slab is normally cast without joints so its usual to find that there are fine cracks. As long as there is adequate reinforcement in the slab these cracks should be less than about 1mm and are not an issue.

Curing Best Practice

Proper treatment of the concrete will improve the final result.

If treated properly during the curing period concrete can be up to 50% stronger than the Design Strength, and will also be stronger at each stage of the curing period.

There is also less likely to be fine cracking of the surface.

The most important thing during curing is to stop the concrete drying out too quickly.

Ways to achieve this include:

    • Covering with material such as sacking, sand, or Old Carpets, which are kept damp.
    • Covering with Plastic sheet.
    • Apply a Curing Membrane (You may need to go to a specialist supplier for this)
    • Regularly spraying the slab.once the initial set is achieved (wait at least 12 hours.

Frost

Frost can damage curing concrete reducing the final strength.

If you are in one of the colder states during winter protecting the concrete by covering it at night is a good idea.

 

For more posts on on getting your paths and driveways correct see Concreting

 

Concrete – What is It?

Concrete is an artificial stone that you can make yourself.

It has four ingredients that all contribute to the overall strength.

  • Large stones (Aggregate) – The aggregate provides shear strength to the concrete. In other words the concrete has to crack around rather than through the stones. A good aggregate will have a mix of stone sizes rather than just one size of stone, this help the aggregate lock together
  • Sand – acts to fill in the spaces between the aggregate and further helps lock it solidly.
  • Cement powder – Is the basis for the ‘cement paste’ (glue) which coats the surface of the first two components and holds them together.
  • Water – Reacts with the cement powder to form the cement paste

Just like baking a cake its important to get the proportions of the various components right for the best results.

Too Much Aggregate compared with the sand and the cement paste will be be just cement coated stones stuck together where they touch. (known as ‘Boney’)

Too Little Aggregate, or Too Much Sand and the mixis called ‘Fatty’ There will be too little aggregate to provide shear strength and much of the aggregate will sink to the bottom.

Too Much Cement will cost you money without adding to the strength of the concrete.

Too Much Water will dilute the cement paste meaning the ‘glue’ has less strength and will also cause increased shrinkage as the cement cures. (It is very unusual to use too little water as the concrete will be very dry and as a result be hard to place and finish)

If you are going to lay large areas of concrete its probably be best to get premixed concrete which should have the ingredients in the correct quantities.

For small quantities you can buy bags of dry concrete mix where you just add water. Make sure you follow the instructions.

If you have got a job that is of a reasonable size but too small for premixed concrete here is a suggested mix:

  • Cement = 1 part.
  • Sand = 2.5 parts.
  • Stone or gravel = 3 parts.

This should be fine for paths, garden wall foundations, fence posts, washing poles, and driveways.

If you are going to order Ready Mix N25 (Normal 25) should be fine for a driveway, pathway or shed foundation.

For  similar posts see Concreting

 

Concrete – Preparing The Base

It seems to me that many people think that a concrete driveway or path will cover any number of shortcuts and other sins.

Well if you want a good finish, that will last, you need to make sure that preparation isn’t skimped.

Without a good base for the slab to sit on cracks will soon spoil the finished look of your drive or path.

This is what I would regard as the minimum preparation for laying a concrete driveway or path for a new home.

Well prepared base for concrete path.
  1. The area of excavation, and base, should be a minimum of the area of concrete plus 150 mm all round, except where the edge is against a wall.
  2. During dry weather excavate the area to a depth that will allow for the depth of the slab plus for a gravel base of at least 100mm.
  3. Check the excavated surface for soft areas. Typical soft areas can be; water logged, topsoil, and badly backfilled trenches.
  4. Excavate any soft areas and backfill with gravel well compacted in 100mm layers.
  5. Lay the gravel base to the required level making sure its well compacted, preferably with a vibrating plate compactor.
  6. Keep people and equipment off the area and make sure surface water is not allowed to flow across the area.
  7. Cover with a polythene layer. You can normally buy this off a roll by the m at a building suppliers for small jobs.

 

For more posts on on getting your paths and driveways correct see Concreting

Concrete – Cracking

One of the inevitable facts about concrete slabs is that there will be some cracking.

One of the sayings in concrete design is “ All concrete cracks…..the purpose of reinforcement is to hold the broken bits together.”

So what does this mean when you are having a new house built?

Surface Cracking – All Concrete

Hairline surface cracking is not unusual particularly in steel power floated house slabs. It can be caused by

    • Excess water in the mix,
    • Too much work when finishing the surface
    • Or drying out too quickly when curing.

Although the cracks don’t look very good they shouldn’t affect any subsequent flooring such as tiles.

Full Depth Cracks – House Slabs

House slabs are normally designed as a single slab with reinforcement so that any cracks that occurs will be prevented from opening too far.

Cracks of up to 2mm wide are considered acceptable in a floor slab. If you should get any cracks this wide and want a tiled surface its important to make sure a flexible tile adhesive is used.

Full depth Cracking – Driveways and Paths

Good design of driveways and paths is based on making sure that cracks are encouraged to occur, but only at the location of constructed joints.

This is done by making sure these joints are weaker than the main slab.

For construction details and joint spacing see: Concrete Joints 1

The Standards say “cracks other than at the joints are unacceptable if more than 1.5mm”, although most people say that any crack is unacceptable.

More Information on Standards

See pages 16-19 of the Guide to Standards and Tolerances 2007 available here Victorian Building Authority

 

For  posts on on getting your paths and driveways correct see Concreting

 

Concreting – Bar Chairs

No we are not talking about seats in a pub.

Bar Chairs are used to make sure the steel reinforcement in concrete ‘sits’ in the right place.

Here is a typical plastic ‘chair”

And here is a different type.

Getting the steel reinforcement in place is very important in making sure you get the best quality slab. . . .and the only way to make sure is to ‘sit’ the reinforcement mesh, or bars, on these chairs.

The chairs need to be 800mm apart unless the bars in the mesh are greater than7mm diameter.

The reinforcement  in paths and driveways should be a minimum of 40mm from the top and bottom of the slab. (This typically means around the middle of the slab)

Some concreters, especially the one who give you a cheap price, will tell you you don’t need bar chairs as they can pull the reinforcement up to the right position with special hooks while they are pouring the concrete*.

This is completely wrong.

Below is a picture of the underside of a concrete slab that had to be dug up due to excessive cracking after using this money saving** ‘technique’.

You can clearly see the rusting reinforcement that was on the underside of the slab was doing nothing to stop the slab cracking.

To ensure you get the best slab make sure there are bar chairs in place while the concrete is being poured…………. I have been told its not unknown to put the chairs in place for a pre-concrete inspection and then take them out before the pour!

 

*Footnote 1: Many concretors disagree with me. . . but in America where they lay huge amounts of concrete roads they have developed special machines to pull the steel up into place. . . .In 300mm thick slabs they have found that the steel can be 75mm out of place (Federal Highways Administration).

So how can workers correctly “hook and pull” mesh reinforcement accuratly While Standing On The Reinforcement?

**Footnote 2: The actual cost of the chairs works out at around $1/m2 of concrete!

For more posts on on getting your paths and driveways correct see Concreting

 

Concrete – Reinforcement Mesh

Purpose of Steel Mesh Reinforcement

A lot of people believe that the steel mesh adds substantially to the strength of concrete paving.

This is a myth!

The truth is large areas of concrete paving will crack.

What well thought out reinforcement steel does is:

  • Limits cracking where you don’t want it (in the middle of slabs)
  • If some cracking does occur it holds the cracks tightly closed.
  • A reduced amount of steel across joints helps to make sure that cracks do occur at the joints (where you want it to crack).

Sizes and Types of Mesh Reinforcement

A single full sheet of mesh is 6m by 2.4m.

Many local suppliers will sell you half a sheet, or less, for small jobs if you plan on doing it yourself.

These are the common types of mesh that you can buy in Australia for paving work, with my opinions on what mesh to choose.

Type

Description

Comment

SL52

5mm dia bars @ 200mm spacing

For garden paths (max joint spacing 2m)

SL62

6mm dia bars @ 200mm spacing

For plain paths or patios (max joint spacing 3m)

SL72

6.75mm dia bars @ 200mm spacing

Caravan or Boat parking areas

SL82

7.6mm dia bars @ 200mm spacing

Front Driveway minimum for plain concrete

SL92

8.6mm dia bars @ 200mm spacing

Front Driveway best for patterned concrete

RL81

7.6mm dia bars @ 100mm spacing

Polished concrete floors

There are many more types of mesh but they are more specialised, the above will be fine for typical concrete slabs around your home.

Depth of Cover

The minimum cover from the top and bottom surfaces of the slab should be 30mm.

This will prevent the steel rusting once the concrete is placed.

To make sure you get this cover see this link: ‘Bar Chairs‘.

Overlap

When you need to join two pieces of mesh the overlap should be 2 squares.

Do Not overlap mesh across the site where a joint is going to be. That will stop the crack forming at the joint.

Rust

Don’t worry about a bit of surface rust on the mesh, although you should remove any rust flakes. The roughness of the corroded area help the concrete grip the reinforcement.

The alkali nature of the concrete will stop any further rust, as long as the 30mm cover is maintained.

 

See this link for advice on the thickness of concrete slabs: Concrete Thickness

For more posts on on getting your paths and driveways correct see Concreting

 

Liquid Limestone

You might have heard of Liquid Limestone as an alternative paving material…….But what is it?

Really its just a different type of concrete.

It is much more common in West Australia than other states.

The differences between conventional concrete and Liquid Limestone are:

  • Instead of standard Portland ‘Grey’ Cement it uses White Cement.
  • It uses crushed limestone rather than other types of rock gravel and sand.
  • Quite often a plasticiser is added. This means the mixture can be poured without having to add too much water.

Various patterns can be applied to the surface as the concrete sets. (see above photo)

As well as the standard limestone appearance the supplier can add various pigments. If you want a strong colour I think you would be better off  just going for coloured concrete.

Because it can be laid in large slabs like concrete there are less joints than in conventional brick or concrete slab paving. (There will still need to be some joints. For joint spacing see: Concrete Joints 1)

With the lighter colour it can be cooler underfoot than other pavements.

To maintain its appearance liquid limestone will need to be sealed around a week after laying.

Thanks to Concept Concrete WA. for these two great examples of  Liquid Limestone Paving

For  posts on on getting your paths and driveways correct see Concreting

 

Plain Concrete Finishes

Basically there are three different common finishes for plain concrete driveways paths etc.

Brushed

This is probably the simplest finish if you are going to do some concreting yourself.

You compact the concrete and ‘strike it off with a straight edge.

A stiff brush is then drawn over the concrete to form a textured surface.

The texture means this is a good anti slip surface for paths and driveways.

 

Steel Float

This finish requires a bit more skill than a brushed finish.

It done by ‘polishing’ the surface with a steel float to give a very flat smooth surface.

With no texture its not recommended for slopes and can be slippy when wet.

It is good for garage floors as it makes it easier to turn the wheel at slow speed and is easier to clean

Wood Float

A wood float finish is finished in a similar way to steel float but the wood float gives a texture between brushed and steel float finishing.

The texture is generally achieved with sweeping circular strokes as shown in the photograph.

This effect takes a bit of practice, so if you want to try this yourself its worth practising in an out of the way place before trying in a high profile area.

Concrete Edging

Whichever finish you choose the other decision you need to make is whether to edge it like this photograph.

Most concreters in Perth, and other parts of Australia, will normally include edging as part of their overall job.

If you want to do the work yourself you will need to buy a special edging tool.

Different Driveway

I see a fair bit of discussion in forums about whether to go for a plain concrete driveway of the additional expanse of an exposed aggregate surface.

While I was in Japan last year I saw this concrete driveway which was a plain slab with black stones pressed into the concrete at irregular internals.

Minimalist but distinctive.

I think it looks very effective.

Do you like the idea?

Driveway Slope

If your vehicle is a specially adapted off-roader like this Land Rover then driveway slope is not going to be an issue.

For the rest of us it is something you should be aware of.

If you have lowered your suspension, installed a body kit, or have a sports car then it could be critical.

Maximum and Minimum Slopes

 

For proper drainage of the driveway you will need a minimum gradient of 1 in 100 (1%, or 10 mm per metre)

This could be either along the driveway or cross fall.

Local Authorities rules on maximum slopes do vary (so check) but typical gradients are:

  • Public areas 1 in 20 (5%, or 50mm per metre)  in the public footpath area or
  • Within the property boundary 1 in 4 (25%, or 250mm per metre) .

Transitions

At the top of slopes there is a risk of the underside of the vehicle grounding.

At the bottom of the slope the front or rear of the vehicle can ground.

For standard vehicles a change in gradient of 12.5% is typically the point where problems can start to occur. (For lowered vehicles it may be much less than 10%)

A typical way to minimise the risk is to have a transition section of around 2m long between the two gradients.

For example

To go from a Flat slab (gradient 0%) to a gradient of 15% a transition section would be 7.5% ( [0% + 15%] / 2 )