Vibration Or Compaction Of The Concrete

Another way to devalue the quality of your concrete that you have forked out good cash for, is to not compact it correctly.  Concrete when mixed in the bowl of the mixer entraps a certain amount of air.
In the process of getting it out of the truck and into position on the job, there is inevitably a certain amount of more air trapped in the mix.  Apart from some special concrete mixes, this air is not wanted.  We remove the air, to compact the mix back to it’s basic ingredients by vibration. This liquefies the mix and allows the air bubbles to rise to the surface.

Screeding concrete
Placing concrete – Savvas works around the outer edge with a 3M. long aluminium screed, (he has a selection of different screeds) leveling a strip about 400mm wide to work from, using the top of the formwork as his guide.  He then uses those strips as guides, to screed off the main area  He does not actually screed off the top of the forms.   Charley works with him leveling and raking away the excess.
Petrol engined concrete vibrator
Placing concrete – By far the most commonly used concrete vibrator on house building sites.  Petrol engine driven 1 1/4″ flexible shaft.
vibrating concrete
Placing concrete – keep the poker vertical and slowly dip it in and out.
  • For the job pictured above, a 100 thick driveway, the vibration was done by hand.
  • From time to time as the pour progressed one of the guys would grab a hammer and tap the formwork, setting up a vibration along the timber form.  When this is done, you can actually see bubbles rising to the surface.  Apart from the quality of the concrete aspect of this, there is also a visual aspect.  If you don’t do it, you get unsightly air holes on the edge of the concrete when the formwork is stripped.
  • When the concrete is being placed and levelled the guys on the shovel and the rake work the mix up and down to compact it.
  • The guy on the screed again tamps the surface up and down, as he starts levelling it.
  • On thin slabs on the ground that are more critical than a residential driveway, like road surfaces or tilt slab panels, it is common to use large powered mechanical screeds that vibrate the surface and the edge forms consistently.
  • For deeper concrete, like in foundations or walls it is essential to use a powered vibrator.  Then the concrete is vibrated on the inside, rather than external vibration mentioned before.
  • These consist of a motor driving a flexible shaft with a steel vibrator head on it.  Commonly called pokers.
  • They range from small 20mm or 25mm models, electric motor driven, to the normal 30mm petrol engined machine.  Larger models still are compressed air driven.
  • When using a poker, keep the thing vertical and drop it in and out every say 150mm.
  • Don’t lie the poker flat and drag it, that tends to segregate the concrete and leave weak areas.  Don’t try to use the poker to shift the concrete, let the guys on the shovel or rake do that.
  • It seems very satisfying to wedge the poker between the rebar, and spread the vibration through the reo steel. Don’t do it.  This can move the steel from it’s design position.
  • Same again, don’t wedge it between the formwork and the rebar.   It seems like a good Idea when the whole thing is shaking, but you could be dislodging bar chairs that are providing the correct concrete over for the steel.  You can mar the face of the formply with poker scars.
  • Pre-cast makers of things like concrete blocks, pavers, and garden ornaments use vibrating tables.
  • On large construction jobs engineers design purpose made bolt on external vibrators for steel formwork.

So, to recap this section, no matter how you do it, one way or another vibrate that concrete!

Getting The Finish Height.

On small jobs it is common to set up the top of the forms at the exact finish height, and then screed off them to get the concrete height.  Doing it this way if you use timber pegs to hold the forms, you can saw them off flush with the top so that they are not in the way of the screed.

In this way, you can set up something like a 3M wide driveway, and using a straight length of timber about 4M long, two men can work together at each end first tamping the concrete in an up and down motion, to compact it and then working in a sideways sawing motion screed the slab from the edges.

Three or four metres is about the limit that you can do it like that, so you have to set up temporary screeding rails for wider pours.

screeding a house slab
Placing concrete – In this shot of a larger slab, the boss is to my right with a dumpy level, when required the boys smooth a small patch and he checks it for level.  A couple of patches are joined with a screed, to give the strips shown.  Note! the guy with the yellow helmet on the right is the concrete pump operator.  He is holding the remote control behind his back.
getting spot levels
Placing concrete – The guy with is back to me is using a builder’s auto level to give spot heights to the guy in the light blue shirt with the staff.  Note this shot shows a crap method of forming up the edges, relying on pickets and the timber braces against them with no way of stopping them slipping.  The guy had string lines on the forms and he needed them as he spent most of his time frigging about keeping the shutters straight.

A lot easier way is to get spot levels with a laser level or a dumpy level. (I don’t really mean a dumpy level, I mean a builder’s auto level, but old habits die hard).  This is a lot easier, but demands a fair bit of skill from the guys on the screeds.

  • The formwork is set up as close to level as possible, to give the guys a visual reference when pouring.
  • The boss takes a reading all round and finds the lowest spot.  He works from that height.
  • The pegs and odd bits of formwork can be higher than the finish levels.
  • The first section of concrete is poured raked out and vibrated.
  • A guy with a staff and a steel float levels and smooths a small patch and they check it with the dumpy.
  • He adjusts it up or down, and then carries on and does another say 3M away.
  • The bloke on the screed levels between the two then they repeat the process at 3M parallel.
  • Then using the leveled strips as guides the rest is screeded by hand and by eye.

Control Joints

Pressed metal control joint  with rubber cap
Placing concrete – A metal control joint that causes a weak line in the slab, so the concrete will crack under, but the top will have a straight, 10mm wide rubber insert.
dropping a metal joint into the wet concrete
Placing concrete – Savvas laying a metal control joint into the freshly screeded concrete.
foam isolation joint between exist house wall and new concrete
Placing concrete – a foam isolation joint fixed to the plastered surface of the house slab/wall. Fixed with masonry nails.  The mesh will be lifted by hand as the concrete is poured.

We have to have joints in concrete. They are there to stop random cracking.  As concrete dries out it shrinks, so it will crack. So one of the main reasons for putting in a control joint is to, guess what?   Control where the crack will be, rather than leave it to the concrete to decide.  Sometimes in spite of all our best efforts the stuff cracks.

Quite a while ago I built a house on a large triangular plan, and it had cut-outs in the slab with acute angled internal corners.  (Like the inside corner of a letter “V”).   I had put heaps of extra rebar across those internal corners, to try to stop cracking.  The day before the pour the owner ask myself and the three concreters if it would crack at those awkward places.  We looked at each other and all we could say was “probably not”, “maybe yes maybe no”. that sort of stuff.   I worked it out later that between the four of us we had something like 120 years of concreting experience and yet we couldn’t come up with a yes or no answer.

Joints weaken the concrete in places we choose, and so the concrete cracks where we want it to crack and not randomly.  On the driveway job pictured the boys used a few different types of joints.

  • A 12mm thick by 100mm deep bitumen impregnated foam strip, around the whole perimeter of the existing house slab, where the new concrete paths abut it.  This material is commonly called an expansion joint.  Or in this case it is used as an isolation joint That is, it separates two distinct parts of the building, and allows each them to move without affecting the other.
  • At the end of a days work just a plain butt joint.  On more critical jobs, key joints are sometimes used.
  • Every 4M. or 5M. in the driveway, and say every 1.5M. in the footpath, a metal strip about 35 deep with a rubber cap is set into the wet concrete.