Understanding concrete joints

We use concrete joints in concrete flatwork (and indeed in concrete masonry and brickwork) for very good reasons. The correct placement of joints in concrete will maintain the appearance and long life of your slabs and pavements.

The reason for joints is that concrete moves.

an unsightly crack in stencil concrete
Concrete joints - This concrete driveway is about 6M long. It was built with no control joints! It has a joint now, eh!

A good long lasting concrete job depends on many factors, and the correct planning, choice and positioning of the joints is a crucial part. But with the best joints in the world your concrete will still crack if you neglect the other important aspects like sub-grade preparation etc. etc.

Concrete moves in two ways.

1.) Shrinkage from water loss.

The three main constituents of concrete are aggregate, cement and water. When the water dries out the concrete shrinks. Not a large percentage in a well designed mix but all concrete when it dries shrinks.

This shrinkage happens mostly in the time immediately after the concrete is poured, but it continues on in a slowly diminishing rate for years. It is common for shrinkage cracks to appear a long time after the event. The pressures in the concrete builds up until a crack appears.

Our best hope is to keep shrinkage to a minimum, but knowing that it will occur we design control joints in concrete. The control joint is there for one reason only, to anticipate shrinkage and to provide a nice straight line for it to occur rather than the unsightly mess on the right.

Other good concrete practices can minimise shrinkage, the correct placement of rebar and curing concrete correctly can reduce the shrinkage but it can never stop it entirely.

2.) Thermal Movement.

The daily and seasonal cycles of warm and cold provides movement to all building materials. Hot material expands and then it contracts as it cools.

I have a nice patio area with a corrugated iron roof. Quite often I am stirred out of a lunchtime nap by a series of loud cracks from the roof. It is the metal expanding against the grip of the screws. In the evening it shrinks to the same noises.

In concrete this movement is very small. It is so small that we can't detect it with the naked eye, but concrete being what it is the force of even this small change in size can be extremely powerful and potentially destructive.

The types of concrete joints.

Because concrete moves in two ways, we similarly have two distinct types of concrete joints.

  1. Concrete joints that allow movement. This during the whole life of the material due to thermal expansion and contraction forces.
    • Expansion joints Used in larger jobs with long runs of concrete. These should be set at a maximum of every 15M.
    • Isolation Joints. Used where new paving or flatwork touches fixed structures such as buildings, manholes or columns.
  2. Concrete joints that control cracking. Due to shrinkage as the concrete dries. Used at a maximum of every 3M. These have different names.
    • Control joints
    • Weakened plane joints
    • Sawn joints

Every concrete joint is one or the other of these two types.

Construction joints.

Many large companies pour concrete in continuous pours, they work shifts and huge projects are completed without any stop. The rest of us like to get a bit of sleep and leisure now and again, so during the course of a job we halt construction and have a bit of a break. When we stop the work and then restart it we form construction joints between each days work.

On a well planned job these construction joints may well occur at an expansion joint, but they can be made at a control joint also.

One other joint that has a name but it should never happen.

Cold Joints or unplanned concrete joints.

Imagine this scenario, you are half way through a pour and the next truck is delayed, or the concrete pump breaks down. (been there, done that, more than once!)
Good concrete relies on being poured continuously, that is with no breaks in the process. If the supply of fresh concrete is interrupted and the concrete edge goes off, then there is a danger of creating a cold joint. This creates a very rough random area that has two sections that are not properly bonded together. Avoid this at all costs. Make a temporary construction joint. Pour the rest of the concrete in another bay, whatever, but do not try to join a random line of partially set concrete to fresh concrete and then try to hide the joint.

Concrete should be poured steadily from one end of the work to the other keeping the fluid edge.

Always have a backup plan, do a "what if" check beforehand so that you won't be short of tools or materials in the event of something going wrong. Remember Murphy's Law.

cracks in concrete
Concrete joints - There is an isolation joint around the gray exposed aggregate slab,(the thicker black joint material) which is good, but in the red stenciled concrete it was obvious that it was going to crack at the change in direction. A small piece of metal control joint would have looked so much better.

Concrete joints - A short digression.

The term expansion joint is often used under the mistaken impression that concrete once poured expands to larger than its original size. It never does this. Any thermal expansion is almost always less than the original shrinkage that happens to freshly poured concrete. In a perfect world once the concrete has dried out leaving shrinkage gaps between the reasonably spaced panels, then there would be no need for expansion joints.

  • Take a series of paving slabs that have joints at the end of each days pour. The process of normal drying out provides a gap between the slabs. Small as they are these gaps can handle any thermal expansion and contraction.
  • When the slabs are at their coldest (with large gaps between panels) normal traffic, wind, water etc. causes small particles of sand or grit to fall or get washed into the opened gaps.
  • At the next expansion cycle (hot) this sand gets crushed and compacted, allowing more sand to enter on the next cold cycle.
  • This repeating cycle continues until all those gaps are full of a densely packed mixture that is as solid as the concrete itself.
  • Sooner or later the concrete wants to expand but it has nowhere left to go.
  • Expansion and Isolation joints allow movement but still seal the gaps.

Placement of concrete joints.

  • Always follow the guidelines for maximum spacing. If in doubt use closer spacing than recommended. This is particularly important on decorative concrete surfaces.
  • For slabs wider than footpaths the longest side of any section should be no longer than 1.5 times the width of the shorter side.
  • Put a joint at any change of direction. Always.
  • Pay particular attention to re-entrant corners.
  • Never place a joint at an acute angle to the concrete edge. Always make a turn with the joint to arrive at right angles to the edge. That is don't leave sections with pointed ends, they always crack. The trick is to set a line square off the sloping edge. This averages the angle.
  • In the sketch below I have drawn curves instead of a straight line, but the principle is the same.

a crack at a re-entrant corner
Concrete joints - The crack here in this ground concrete feature panel should have been anticipated. All internal corners or re-entrant corners as we call them want to crack. Here the design makes it hard to put a control joint there. In a case like this (and the previous photo) more reinforcing should have been placed across the anticipated line of the crack. It is so easy to do and there would have been heaps of scrap mesh lying around.

concrete joints layout
Concrete joints - A sketch plan of a house to illustrate some of the points in the text.

In the silly little plan above I have not tried to fit things in that would make it easy. I just sketched it out and then worked the joint lines out after, which is what the contractor has to do when faced with a weird plan.

Some of the sizes go a touch over the recommended 3000, but in the end it is up to the individual. If you are trying to skimp on concrete depth or reo mesh maybe because you are mixing on site or you only have a lightish mesh available, then tighten up the joints.
However if I was doing a 130 thick slab with N30 concrete and F72 mesh then it would be OK.

A tangent is a line that just touches an arc at one spot. The lines that I have drawn and denoted TL are square off them.

As far as re-entrant corners go it it just about impossible to do a job like this without them. The problems always arise because of people being completely ignorant of what is going on. Now that you know about half the battle is won. Just be aware of them and do the best compromise that you can. I have put in a couple in the sketch. The "TM" refers to trench mesh, which is used in trenches for foundations and has in general larger bars closer together than standard fabric used on driveways. I have used it whenever it was handy, but don't go out an buy stuff specially. Any spare reo bars will do.

The trick is to imagine two hands trying to pull the concrete apart at that spot and so just put in some rebar to stop them.

An important point to note when planning a concrete pour to say a driveway slab is the use of a plastic sheet under the concrete.
It can hardly be called a vapour barrier any more, being outside but it still performs a couple of useful functions.

  1. Concreters love it because it stops the moisture soaking into the subgrade and the mix drying too soon. They need the time to work the concrete.
  2. What most of them don't realize is that the plastic underlay is useful in providing a barrier between the subgrade and the concrete allowing the slab to expand and contract without having to overcome the grip of the uneven subgrade.
  3. If plastic underlay is not used then a sand layer is recommended. In addition wet the area down before placing concrete to slow down the tendency for the concrete to dry out too soon.

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When I were a lad we used to say "there's only two sorts of concrete. Concrete that's cracked and concrete that's gunna crack".
Concrete slabs as floors or pavings. Laid flat as opposed to concrete walls.
Murphy's Law
Anything that can go wrong will go wrong.



Please Note! The information on this site is offered as a guide only!  When we are talking about areas where building regulations or safety regulations could exist,the information here could be wrong for your area.  It could be out of date!  Regulations breed faster than rabbits!
You must check your own local conditions.
Copyright © Bill Bradley 2007-2012. All rights reserved.
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