Concrete is often overlooked as a potential pavement surface, mostly because of its lack of colour or interest, although the decorative patterned concrete is becoming more popular and cementitious overlays offer a huge range of colours, patterns and textures. Plain concrete can be very useful in areas where a decorative look is not required, such as caravan hardstandings, shed and garage bases or dog kennels. Modern concrete technology has led to the development of fibre reinforced concretes that create high-strength pavements without the need for steel reinforcement mesh.
Given good ground, mass concrete can be placed directly onto a damp-proof membrane over the prepared sub-grade. Concrete paths should be 75-100mm thick, whereas drives, garage bases or hardstandings should be at least 100mm thick.
The purpose of the dpm is two-fold: Firstly, to protect the underside of the slab from attack by dampness and aggressive salts (such as chlorides) or other chemicals in the sub-base or sub-grade, and damaging the concrete, unseen, from beneath. Any 'laps' required in the membrane should overlap by at least 350mm, and preferably be taped down to prevent ingress of ground water or egress of mix water.
Reinforcement in concrete is used to ensure that the cracking which always occurs does not threaten the structural integrity of the concrete. Movement joints (expansion joints, contraction joints, isolation joints, crack control joints, etc.) are used with concrete slabs to protect the slab itself from cracking, to accommodate the inevitable movement of the slab and to prevent excessive forces being transmitted to adjacent structures. A shovel or a strong rake is used to roughly level out the wet concrete, and then a straight-edged timber can be used to tamp down the concrete to the correct level.
Vibrating poker units can be used to expel most of the unwanted air pockets, and so-called 'Bug Rollers' are useful for making sure any hard aggregate is pushed into the concrete, although this is more important on PIC than plain concrete.
For larger projects, Hire Centres can supply Vibrating Beam Screeds and Roller Strikers that are dragged over the roughly levelled out concrete, vibrating it to eliminate air pockets, pushing surplus concrete in front of the beam, and levelling, all in one operation. A reasonably smooth surface can be obtained by repeatedly tamping wet concrete, but for a finer finish, the surface should be smoothed using a steel float trowel. Judging exactly when to commence floating, either with hand tools or with a power float, is an art in itself, and is gauged from years of experience; too soon and the trowel marks will never disappear, too late and the concrete might not 'rub-up' to a close finish. A perfectly smooth concrete surface can become quite slippy, especially outdoors in wet or icy conditions. On light-use applications, such as driveways, garage bases and the like, there is a tendency to leave the concrete, once the finishing has been done, to harden and cure at its own pace. Temperature control is only a serious issue when there is a risk of the ambient temperature falling below 4°C.
In winter, freshly poured concrete may be protected with hessian sacking, straw or polystyrene to insulate the slab overnight. Formwork and shuttering can be removed from a concrete slab after 24 hours, although it is often left for 3 days unless there is a need to examine the edges.
The keeled garage slab cross-section is provided as an example of a typical detail and NOT a recommendation. Pre-fabricated sectional garages can often be erected on a simple 150mm slab, such as that shown above; this keeled slab is the type of raft foundation that may be used with a single brick construction garage.
Whilst not the most attractive type of pavement, a well laid concrete area should give many years of maintenance free service. Ballytherm Polyisocyanurate insulation is suitable for a variety of floor construction types, such as precast concrete, suspended timber and ground bearing concrete. Every floor is going to carry the dead load of the people, furniture and possibly heavy equipment occupying the building. Improved standards of thermal insulation within building elements have focused attention upon the amount of heat lost at junctions between elements where there is no continuity of insulation or there are exposed heat paths. The thermal performance of any given floor is calculated using the dimensions of the floor, the insulation and other construction materials used to build the floor. In order to have complete thermal insulation, the construction must focus on the heat that can be lost between flooring elements, such as at doorjambs where one floor meets another. Building Regulations and Building Standards set targets for the overall energy efficiency of buildings, as well as setting limiting U-values for building elements. The U-value tables on this website indicate the maximum area-weighted U-values permitted by local regulations.


To ensure that people inside of these buildings are protected against Radon, construction in at-risk Radon locations must have protective measures against it. For more information about Ballytherm underfloor insulation and its properties, talk to one of our insulation experts! If you have got a plan to work to it will give you the position of the shed in relation to two of your boundary lines, unless you are on a huge block and it gives you a position off the house or something else. Remove the lines and the pegs and dig a shovel width trench around the marked perimeter about 100-150 deep, throwing the spoil outside.
We want this thicker edge to stiffen up the slab where we will be putting in hold down bolts or other fixings. If the site has a slope, say it falls 150 from corner to corner, you may want to level the area with clean fill sand, as sand is cheaper than concrete.
Nail them together in the trench and drag them into position, measuring to the fence lines and trimming the trench as needed. Get odds and ends of packers, bits of brick, cement sheet etc to hold the timber in a level position, using a 900 or 1200 long spirit level to check. What you are aiming for is to get the top of the forms 100 higher than the natural ground level at it's highest point.
Another way is to get someone to hold a heavy sledge hammer against the form to keep it solid while you are nailing. Run around the edge of the forms with a shovel and put some soil or sand down the back of them, firming it up by pressing it in with your foot, to stiffen the forms up more.
A couple of points, if the garden shed concrete slab is to be used for something a little better than a simple storage shed (I know of a few that later got turned in home offices etc. Before the concrete is poured go around the forms and mark where the door is and the HD bolts will go. Then when the concrete is poured ( see my other concrete pages) and just starting to firm up you push in and jiggle the bolts a touch to compact the concrete around them.
At the same time that you drop in the HD bolts run an edging tool to the concrete at the doorway.
If you are building a steel shed yourself or if it is a kit, put something between the steel and the concrete to protect the steel.
Make sure that the outside cladding runs down at least 25 past the concrete edge to weather it. Turning Safe Search OFF may display content intended for mature audiences.You must be at least 18 years old to continue. Please enter at least one email addressYou are trying to send out more invites than you have remaining. Your local ready mixed concrete supplier will advise on the most suitable type and strength of concrete for your project. For heavier applications, or on bad ground, it may be advisable to construct a sub-base of compacted granular material or lean-mix concrete beneath the actual concrete slab, or to lay a sub-base over a geo-textile. To calculate the required amount, simply multiply length in metres by width in metres by depth in metres to calculate a volume in cubic metres. Tamping helps eliminate air pockets being trapped in the body of the concrete and also helps to push the hard aggregate into the concrete, bringing sufficient of the matrix to the surface to make smoothing (floating) easier. 3 or 4 passes with a beam screed is usually sufficient to give an acceptable finished level, with a tolerance of ±6-10mm. This is best done when the concrete has started to stiffen as part of the hardening process, as the float trowel leaves fewer trowel marks. It is recommended that once the surface has been smoothed with the float trowel, light brush strokes be made over the surface, transverse to the direction of travel, to improve traction once the concrete hardens. It is usually checked after 10-12 hours, just in case any immediate repairs or 'rubbing-up' is required, and while this may be satisfactory for small jobs, larger slabs, especially those where the final strength is critical, require careful management of the critical first stages of curing if they are to meet their design targets.
It is unusual for concrete to be poured in harsh weather conditions, that is, when the ambient temperature is below 4°C and falling, unless there are extenuating circumstances. Care must be taken when striking the formwork to avoid damaging the concrete which will still be 'green', as we call it in the trade.


Although reinforcing fabric mesh is not essential in concrete slabs, it does strengthen the concrete and is something we always recommend. The pages within this section provide design guidance, design details, installation instructions and advice on sitework. Where structural considerations make that impossible, for example when the wall insulation is inside a cavity, thermal bridging may be avoided by continuing the floor insulation vertically at the perimeter of the floor so it overlaps the wall insulation.
To ensure that these thermal bridges do not exist is to make sure the insulation of one floor seamlessly meets that of the next. However, it may be necessary to achieve U-values lower than those shown, in order to attain the whole-building energy efficiency target.
By combining plain concrete with a decorative edging, such as a brick edging, it can be made to look quite attractive.
On soft or unreliable ground conditions, a sub-base will help spread the load of a mass concrete slab, but in such conditions it's best to obtain a professional opinion from a civil or structural engineer. Concrete slabs intended for exceptional loads, such as commercial yards, lorry parks etc., will be at least 200mm thick and should be specifically designed as they will probably require a sub-base of at least 100mm thickness, and steel reinforcing mesh or a fibre-reinforced concrete. Builders' merchants normally carry 'visqueen' or PIFA 1200 as an off-the-shelf product, as it is much used throughout the construction industry. Ready mixed concrete companies usually deliver in multiples of half-cubic metres, so "round-up" your calculated quantity accordingly.
The ready mixed concrete supplier will understand these references, and the delivery wagon usually carries extra water that can be mixed into the concrete before pouring, if the mixture is too stiff.
Hand floats, also known as Bull Floats, are fine for small areas where a near-perfect finish is not essential. Additional space heating may be provided in such situations to keep the ambient temperature above the 4° threshold.
In most cases, a concrete slab should be kept completely free from all traffic for the first 48 hours. Fibre reinforcement may be used in place of, or in addition to, the steel mesh in the plain slabs. The high compressive strength of Ballytherm makes it capable of being used in those constructions. How thick Ballytherm Insulation must be to meet these values is available in Table 4 (Below Ground Bearing Slabs), Table 5 (Above GroundBearing Slabs), Table 6 (Above Pre-Cast Concrete Floors) and Table 7 (Suspended Floors).  First, calculate the perimeter of the floor divided by the area of the floor and then find the corresponding entry in the tables. Bring Ballytherm Insulation up slightly into the walls to protect against gaps at the base.
This occurs naturally in the soil and can make its way through under floor insulation into buildings.
If the area of the paving is troubled with weeds, it may be necessary to treat the excavated sub-grade with a general weedkiller such as Sodium Chlorate, but it is unlikely any weed will be able to penetrate the upper layers.
However, bear in mind that adding further water to the prepared mix can affect the cured strength of the finished concrete and may invalidate the strength guarantee of the supplier, leaving the contractor responsible for any remedial work.
Long-reach floats can be used on larger areas, and for that perfectly smooth finish, a power float is the tool for the job. After that, foot traffic can be allowed onto the slab, with care, but it should not be open to heavy foot traffic or slow-moving vehicles until 5 days after pouring.
The 100mm thick slab normally has a single sheet of mesh, while 150mm and thicker slabs will have two layers of mesh, "top and bottom", each with at least 50mm of cover. It is sometimes more convenient to use the same board thickness as specified for the rest of the floor. These are awkward, heavy machines, available from Hire Centres, but are not recommended for use by DIY'ers. For heavy usage, such as highways, runways, ports or commercial yards, it may be 10 days before the slab is ready to take traffic.



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