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10.03.2014, admin  
Category: Modern Shed

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. 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.
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.
Whilst not the most attractive type of pavement, a well laid concrete area should give many years of maintenance free service. 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. 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. 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. 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. 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. In most cases, a concrete slab should be kept completely free from all traffic for the first 48 hours. 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. 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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