Concrete Slab procedures

 

 

THE PROCESS IN POURING YOUR CONCRETE SLAB.

There are several stages involved when pouring a Concrete slab, whether it’s a small slab or a large one it doesn’t matter, pouring concrete is a process that you follow every time,  and so here I wanted to explain what these stages are. The actual step by step procedure is covered in the How to Concrete section but here I have laid out the processes in detail.

TAP THE FORMWORK.

Once the concrete is placed inside the area and up against the form work boards and before you do anything else, just give the boards a tap with a hammer. This will give you a nice edge to the Concrete Slab as tapping them will slurry up that edge.

CONCRETE PADS

These pads are created when the concrete has been placed inside the formwork and you have started to spread it out. These pads are made to the height you want your concrete mix and determine the level of the rest of the slab.

They are first created all along the inside edge of the formwork and in the wider slabs you will need to create a long pad further in to the inside length of your screed, this enables you to screed over your pads working the concrete to their levels.

SCREEDING.

This is a term that describes the initial levelling of the concrete, this can either be done using your boards (only if they have been placed in at the exact level) and with a good straight piece of timber, work on top of your formwork in a sawing motion from side to side spreading the concrete. Ensuring there are no low lying areas of concrete along the board.

You may also achieve this by using a Concreters tool called a screed. You will have to be working in the concrete for this so wear a good pair of wellington boots. Don’t worry, you will be able to step on the steel once it’s lifted and there is concrete underneath it.

To achieve a good sealed surface with your concrete using screeds or wood will depend upon the Concrete Mix itself. you really need the correct ratio of materials, it’s the fine sand that will give you that creamy slurry.

FLOATING.

Floating is a term used for the tools we will use after screeding the concrete, it helps bring the slurry, (which is the sand/cement part of your mix) to the surface. This will enable you to obtain a good finish without the stones interfering.

When using this method to create you pads, I have found it best to use the wood float, it cuts into the concrete better enabling you to determine the correct level easier and when you are finishing the job with what’s called a float finish I will use Coning Trowel or sometimes called the steel float.

To finish with a float it is usual to just float the tool over the surface in a figure of Eight pattern, not pushing too hard and not done too soon when the concrete is not ready. To know when the best time is usually it’s when the surface is still moist to touch, but there is no or very little movement in the concrete mix when the float rides over it.

EDGING.

The edging although it has to be done initially after the screeding and floating is also a finishing tool, being the very last tool and procedure you will need to do for your job. The reason we do it straight after the screeding is because you will need to shape the stones that are on the edge to the mould of the edger, this helps you later on when finishing.

When finishing with the edger, you will do this at the same time as your finishing surface. So if you are finishing with a float, as a section is done with the float, you will edge that section and continue this process all around your formwork.

BULL FLOATING.

This is a tool that is mainly use by the pro’s, but if you intend to do a lot of Concreting then it’s probably worth you getting one. It’s basically a float on a telescopic handle and by twisting the handle clockwise and anti-clockwise you lift the leading edge up. Always try to use it in the opposite direction to your screeding and you can fix all the little imperfections, as you push the float over the surface of your concrete, keeping the handle as low as you can and twisting so the furthest edge is slightly lifted and then when you reach the other side of your slab and you need to pull it back, twist the handle the other way hence lifting the leading edge every time.

It’s important not to do this too soon otherwise the concrete is too soft and you will just create more grooves in the surface which will have to be fixed later.

It’s a bit of an art but with practice can make your finished slab look a lot better, when starting I advise people to just work it in about a meter from the edge, that way it is easier to fix if a mistake is made.

 

 

EXPANSION JOINT.

This is a joint that is prearranged as to the line before the concrete is placed in, as there should also be a break in the steel. This is a rather controversial method, some Engineers want you to have no break and some do. I prefer to have a break, so when I mark the line either by a peg or some other method I will cut the steel 50mm (2”) either side of that mark, then when the concrete is in place I know exactly where to place that joint.

These are placed in when the concrete has been screeded at specified lines that will divide the slab into a section no more than 12.0 m2.

There are several methods used depending on peoples opinion. The original method was to place lengths of a mastic type product that allows the concrete to expand and contract in the different weather conditions without causing the slab to crack or buckle. This is a product usually sold in 2.4m (8’) lengths and varying widths depending on the thickness of your slab.

I just use a piece of long angle which I knock into the mix keeping a straight line, this separates the concrete making it easier to get the joint in. I will then float the concrete either side and run an initial edger along the joint also each side as well, then continue screeding to the next one.

Another method is to cut the slab up into sections after it has started to set, usually within a couple of days after pouring. This practice has become very popular now as it is much quicker when laying the concrete. There is often enough going on with the job during that process as it is, without having to worry about the joints.

However, I have my concerns with its effectiveness as an expansion joint. These saw cuts can fill with dirt quit easily with the dirt forming hardening in the joint, there defeating the purpose. You will often see cracks in the concrete either side of a saw cut, these can be cause by the saw cut not being deep enough or the cut has filled with dirt and is as good as if it wasn’t there.

I would rather you learn how to concrete the way that will be most effective and you can decide which way you want to do this armed with all the information …

DUMMY JOINT.

This is a tool that will create a mould in the surface that looks like a  joint in the concrete. It is not used quite as much now a days but it is still available. You are able to buy one that will also fit the bull float handle for those wider areas, as well as the hand one.

It is important, like the edger to do an initial moulding with this toll after the screeding process to deal with the stones, making it easier when finishing, and like the edger is used then at the time of putting the finished surface on the concrete slab.

If I am going to cut the slab then I will make it more official as an expansion joint, with cutting the steel, marking where it’s going to be and using the dummy and cut in the centre groove part of the mould.

This will make the concrete slab look more professional and cutting deep into the slab making it a bit more of an effective break. Still using the proper expansion joint material for the specified area

BROOM FINISH.

This simply is what it says, a broom finish on the concrete. It’s quite popular as it leaves very fine lines on the surface, providing just enough grip to safely walk on the level, not really recommended for slopes as not only will it wear easier leaving the concrete smooth but will not provide enough grip.

It is best done with the brooms specially designed for that purpose which will attach to the same handle as the bull float, you will have a lot more control over the finish. But if you aren’t doing a lot of it just a good fine house broom will do the job, just remember to overlap the edge with whatever broom you are using and shake off the slurry that collects in the broom after each run.

STEEL TROWEL.

The steel trowel is also mainly used in the finishing process but can be used at the time of the initial edging to help flatten out the mark left by the back edge of the edger.

It’s a trowel that needs the leading edge lifted slightly in each direction to prevent it from digging in to the surface.

It is used to finish concrete slabs that need to be made smooth so they can be easily swept clean, for example garage or shed slabs. But this finish will provide no grip on the surface to walk on, and can be quite slippery when wet.

It is best to finish with this tool like the float, when the surface of the concrete is still moist to touch but there is either no or very little movement in the concrete.

There is also a trowel that will fit on the same handle as the bull float, this is called a walking trowel, as you can achieve the same finish standing up and walking around the edge of your formwork, again twisting the handle and lifting the leading edge of the trowel with which ever direction you are moving. When you are cranking up you skills and you are well on the way to learning how to concrete properly then a lot of these extra tools are worth purchasing.

 

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