FAQs - Concrete Questions
How long does concrete last before going hard?
- Australian Standards suggest concrete should be utilised within 90 minutes of leaving the concrete plant.
Should I place steel reinforcement in my concrete slab?
- Concrete is strong in compression (squashing) and weak in tension (stretching). The introduction of steel (reinforcement) in concrete improves the strength of concrete in tension. In a slab-on-ground, reinforcement increases the tensile strength and helps control the width of shrinkage cracks. If soil conditions are known to be poor, eg expansive clay soils, consult a structural engineer.
How many bar chairs do I need?
- Bar chairs are placed underneath your sheets of reo mesh to ‘sit the sheets of mesh’ up off the slab floor. With the majority of residential slabs being 100mm deep the most popular size of bar chairs is 50/65 mm bar chairs. The concrete when poured then binds around the mesh for maximum tensility and strength.
Does the concrete come ready mixed, or do I have to mix it myself?
- Concrete is a mixture of sand, soils, aggregates (i.e. rock or stone), water and cement. Concrete trucks deliver the concrete already pre mixed to your site. Simply prepare your slab and await the truck.
Can I receive concrete on weekends?
- Yes. Saturday is an extremely popular day for individual home owners to pour concrete.
What aggregate or soil should I lay under my concrete slab?
- Most concreters recommend laying crusher dust under the slab to smooth out the surface and provide a fill that allows the concrete to stabilise in an even manner. Once again, ring the plant for a price on the delivery of this to your slab.
How do I make my concrete harden?
- Concrete hardens as a result of hydration that is the chemical reaction between cement and water. Hydration occurs only if water is available and if the concretes temperature stays within a suitable range. After placing concrete, the concrete surface needs to be kept moist for a period of time to permit the hydration process.
What is meant by ‘curing’ concrete?
- The curing period for concrete is approximately 28 days after placing conventional concrete. New concrete can be wet with soaking hessian, sprinklers or covered with wet hessian or plastic sheets or can be watered with commercially available curing compounds which seal in moisture. Add only small amounts of water where possible. During winter, request the plant adds a ’winter mix’ to your delivery. This ensures the concrete will harden faster in wet and cold conditions.
Can I have ‘coloured’ concrete?
- Yes! We have vast selection of colours to choose from. Simply request to look at our colour range and the plant will mix your concrete to that colour on the day prior to delivery.
What is exposed aggregate and can I have it supplied with my concrete?
- Many homeowners request exposed aggregate as they can see the rock or stones within the finish of the concrete. This provides an ascetically more appealing finish on driveways or paths where there is exposure to the public. Whilst slightly more expensive than standard concrete it is a popular finish. We can supply this exposed aggregate finish for around your pool, driveway or home, as and when required.
Why does concrete crack?
- Like all other materials, concrete will slightly change in volume when it dries out. This change in volume brings about tensile stresses with the concrete which causes it to crack. This is the reason that contractors put joints in concrete pavements and slabs to allow the concrete to crack in a neat, straight line at the joint, allowing it to move when the volume of the concrete change due to shrinkage. The prevailing weather conditions may also contribute to cracking with the risk most likely on warm days, dry days with low humidity and/or windy days.
How can I prevent my concrete cracking?
- Prepare the site properly, including where control joints will be. Ensure correct placement of steel reinforcement. Ensure that formwork is level, firm and properly fixed in place. Place the concrete carefully and ensure thorough compaction of the concrete during placing. Avoid, if possible, executing major concreting projects on hot days with low humidity or windy days. If necessary, start early; avoid concreting in the middle of the day. Make sure the work is adequately prepared because there will be less time to place, compact, finish and cure concrete in these weather conditions.
- Mud flats crack when they dry out in the sun as the soil stretches and expands in the heat, as does your concrete slab. Finishing the concrete correctly from initial screening to final finish by properly positioning and constructing control joints will enable the concrete to adequately cure.
What causes surface dusting on my concrete slab?
- Concrete surface dusting is typically caused by finishing the concrete surface too early, while bleed water is still rising to the surface. Thus working bleed water back into the concrete weakens the concrete surface resulting in dusting of the hardened concrete. Generally, repairing dusting floors is not difficult. IF the problem is not severe, the surface can be repaired by applying a chemical surface hardener. In severe cases it may be necessary to grind the floor to remove the weak surface layer and apply a bonded topping.
- When the surface of your concrete continues dusting and scuffing after laying, a concrete sealing product can be purchased. Once applied the sealing product works to return your slab to a dust free state.
How can I remove oil and grease stains from my concrete slab?
- Oil and grease stains can be difficult to remove completely because they penetrate the concrete surface rapidly. If an oil spill occurs, stop it spreading by encircling with sand, dirt or sawdust. Soak up as much surface oil or grease as possible with an absorbent cloth or powder. Cover residue stain with a poultice made of 1 part lime, 2 parts mineral turpentine. Spread a 5mm layer of the paste over the stained area ensuring a margin of 50 to 100mm around edges. Cover with plastic sheeting and leave for 24 hours. Remove cover and scrape off the powder. It may be necessary to repeat this process again with a day or so. Scrub with warm water and detergent then rinse with clean water at the end of the treatment.
What are ‘waffle pods’ and why are they used?
- This is where the concrete is sitting on (and around) a series of boxes (or pods) set out in a grid pattern. Think of a waffle pod as a raft lying flat above your soil. The advantage of your waffle pod slab is that is lying above the surface, as opposed to traditional foundations, which are dug into the earth. The traditional slab can crack and move within the soil as heat, humidity and wet weather cause the earth to expand and contract. Conversely the waffle pod slab sits above the earth and is therefore less susceptible to changing weather conditions and its effect on the soil below.
How do I lay ‘waffle pods’ in my slab?
- Remove topsoil and level site
- Lay sub floor pipes, electrical conduits etc.
- Lay crushed rock (i.e. about 2mm diameter and about 19mm deep)
- Put up formwork and line with sheets of that black bui8lders plastic
- Place waffle pods in place with about 100mm gaps between. They are clipped together so nothing can go wrong here
- Place reinforcement bars in gaps between waffle pods. The clips have a specially made groove to sit the reinforcement in.
- Place the mesh on ‘bar chairs’ over entire area
- Fill area with concrete
Why do people choose ‘waffle pods’ over a normal slab?
- There are minimal earth works required as there are no trenches to dig
- In the middle of winter any trenches quickly fill with water and it can take months before the concrete can be poured
- Waffle Pods bring house further out of the ground. In an area that is we the higher the better.
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