Reinforcement that survives the site

Why the position matters so much

Concrete is strong in compression and weak in tension, roughly a tenth as strong. Steel is the opposite. Reinforced concrete works by putting the steel exactly where the tension is, and leaving the concrete to handle the compression.

That is why the position of a bar is not a detail. A bar 40mm out of place in a 150mm slab has lost a meaningful share of its effective depth, and the capacity of the section falls with it. The bars are not there to make the concrete generally stronger. They are there because a calculation says tension will occur in that plane, at that point.

Concrete does not fail where the steel is. It fails where the steel was supposed to be.

What the bars are

Reinforcement supplied in Kenya is generally to BS 4449, and the grade in normal use is B500, meaning a characteristic yield strength of 500 newtons per square millimetre. Older drawings may refer to Y bars for high yield deformed bar and R bars for plain round mild steel.

Cover, the most commonly ignored dimension on site

Cover is the thickness of concrete between the outside face of the element and the nearest steel. It does two jobs. It keeps oxygen and moisture away from the bar so it does not corrode, and it gives the bar enough concrete around it to develop bond and to resist fire.

Steel that rusts expands to several times its original volume. That expansion cracks the concrete off from the inside, exposing more steel, which rusts faster. It is a process that accelerates once it starts, and inadequate cover is what starts it.

The values above are typical. The figure for your building is on the drawing, it was chosen for the exposure conditions of that element, and it is not negotiable on site.

Laps, anchorage and where bars may be joined

Bars come in fixed lengths, usually 12m, so joins are unavoidable. A lap transfers force from one bar to the next through the concrete around them, which means it needs a length to work over and it needs to be in a place where the force it is carrying is low.

• Lap length is a multiple of bar diameter, commonly in the region of 40 to 50 times for grade 500 steel in normal concrete, and it is stated on the drawing. Never estimate it
• In a simply supported beam or slab, the bottom steel carries the most tension at midspan, so bottom bars are lapped near the supports, not in the middle
• Over a support in a continuous beam, the top steel is in tension, so top bars are lapped near midspan
• Laps should be staggered so that not every bar is joined at the same section
• A lap that is simply two bar ends resting near each other with a twist of binding wire is not a lap. They have to overlap by the full specified length

Links and stirrups

The small rectangular bars wrapped around the main steel in a column or beam are links, or stirrups. They are not spacers. In a beam they resist shear, which is what causes the diagonal cracking near supports. In a column they hold the main bars in position and stop them buckling outwards under load, and they confine the concrete inside, which is a large part of what keeps a column standing when it is overloaded.

• Spacing is on the drawing and is usually closer near the top and bottom of a column and near the supports of a beam, because that is where the forces concentrate
• Links must be closed, with hooks bent to 135 degrees where specified, turned inwards into the core of the concrete. A link closed with a 90 degree hook can spring open under load, which is the point at which the column loses its confinement
• Every link should be tied to the main bars. Links that slide down the cage during a pour leave a section of column with no confinement at all
• Do not open links out to make placing concrete easier and then bend them back. They are worked steel and bending them twice weakens them

The six errors that actually cause failures

In roughly the order of how often I find them on Kenyan residential sites, and how much they matter.

1. Top steel in a cantilever pushed down

A balcony, a canopy or a projecting slab is held up by the steel in the top of the slab, near the surface, running back into the building. Nothing else is holding it. If the steel fixers walk on it, or if it was never lifted onto chairs in the first place, it ends up in the bottom of the slab where it does nothing at all.

This is the single most dangerous reinforcement error in residential construction, because a cantilever with its top steel in the wrong place has almost no capacity, and it fails suddenly rather than gradually. Balcony collapses very often come back to this.

2. Cover blocks missing, or stones used instead

Bars resting on the formwork have no cover at all. They will rust, and the concrete will spall off within a few years. On a soffit it shows first as rust staining, then as a piece of concrete falling away with steel visible behind it.

3. Laps in the wrong place, or too short

Discussed above. Invisible after the pour, and it reduces the capacity of the element at the point where it was most needed.

4. Column starter bars in the wrong position

Starter bars are cast into the foundation or slab and the column is built off them. If they are set out wrong, someone bends them sideways to line up with the column above. A bar with a sharp kink in it pushes outwards under load at exactly that point, and it can burst the cover off the side of the column. Where starters are out of position the fix is a designed one, not a hammer.

5. Slab mesh or bars flattened during the pour

Wheelbarrows run over the reinforcement, chairs are too far apart, or nobody is assigned to lift the steel back as concrete is placed. The steel ends up sitting at the bottom of a slab that needed it at the top, or vice versa. A person walking behind the pour with a hook, doing nothing else, is a worthwhile job on any slab.

6. Bars omitted because the bar schedule was short

It happens more often than anyone would like, usually near the end of a pour when the steel runs out and the concrete has been ordered. There is only one correct response, which is to stop and get more steel.

The ten minute check before any pour

You do not need to be an engineer to do most of this. It needs the drawing, ten minutes and a willingness to hold up the pour.

1. 1Count the main bars in a beam or column and compare with the drawing. Not the general note, the actual section
2. 2Check the bar diameter. Y12 and Y16 are easy to tell apart once you have compared them side by side
3. 3Look under the cage. Are there cover blocks, are they the right thickness, and are they at sensible spacing rather than three in the whole beam
4. 4In a slab, check whether the top steel is genuinely at the top, sitting on chairs, and whether those chairs are close enough that it stays there
5. 5For anything cantilevered, satisfy yourself that the top steel runs back into the building far enough, and that it is tied so it cannot be walked down
6. 6Walk the length of the steel looking for laps. Are they where the drawing puts them, and do they overlap by the specified length
7. 7Check the link spacing in columns, particularly near the top and bottom
8. 8Check that links are closed and hooked inwards
9. 9Make sure the cage is tied enough to keep its shape when concrete lands on it
10. 10Look for anything left inside the formwork: timber offcuts, wire, a water bottle. All of them become voids

A note on what this is