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FREQUENTLY ASKED QUESTIONS

What is reinforcement and why should I use it in my slab?
What is curing and how do I cure my new concrete slab?
Why does concrete crack?
How much sand/cement/stone/water do I need for a typical concrete mix for my driveway?
Why has the surface of my concrete floor gone dusty? How do I fix it?
How do I remove oil stains from my concrete driveway?
How do I remove white desposits from my brickwall (efflorescence)?


Q:

What is reinforcement and why should I use it in my slab?

A:

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.

More Infomation
Concrete Basics
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Q:

What is curing and how do I cure my new concrete slab?

A:

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 concrete's 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. this period is referred to as the curing period and is usually 5-7 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.

More Infomation
Curing of Concrete
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Q:

Why does concrete crack?

A:

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.

To control the incedence of concrete cracks, it is important:
  • To 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.
  • To place the concrete carefully and ensure thorough compaction of the concrete during placing.
  • To 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.
  • To finish the concrete correctly from inital screeding to final finish including properly positioned and constructed control joints.
  • To adequately cure the concrete.


  • More Infomation
    OnSite - Plastic shrinkage cracks
    Plastic Settlement Cracking
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    Q:

    How much sand/cement/stone/water do I need for a typical concrete mix for my driveway?

    A:

    For a passenger vehicle driveway and footpath a Grade 20 (20 MPa) concrete is usually used. For a more heavy duty application eg truck driveway a Grade 25 (25 MPa) concrete should be used. The following indicates the quantity required of each material to make up to one cubic metre of Grade 20 or Grade 25 concrete.

    Volume of Concrete (m3)
    Grade 200.20.40.60.81
    Cement (40Kg bags)23578
    Sand (m3)0.10.20.30.40.5
    20mm Coarse Aggregate (m3)0.20.40.60.81.0
    Water (litre)50-6070-80140-150180-200220-230
    Grade 25
    Cement (40 Kg bags)24689
    Sand (m3)0.10.20.30.40.5
    20mm Coarse Aggregate (m3)0.20.40.50.70.8
    Water (litre)50-6070-80140-150180-200200-220


    This is a guide only. Less water may be required due to inherent dampness of materials particularly sand.

    Remember:

  • Cement should be Type GP (General Purpose Portland Cement) or Type GB (General Purpose Blended Cement).
  • Sand should be concreting sand or sometimes refined to be sharp sand NOT brikies sand or plasters sand.
  • Care in adding water is the key to a successful mix.

    More Infomation
    Assessing Concrete Volumes
    OnSite - Ordering Premixed Concrete
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    Q:

    Why has the surface of my concrete floor gone dusty? How do I fix it?

    A:

    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 harden concrete. Generally, repairing dusting floors is difficult, however, 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.

    More Infomation
    OnSite - Dusting concrete surfaces
    Flaking Floors
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    Q:

    How do I remove oil stains from my concrete driveway?

    A:

    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 to 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 within a day or so. Scrub with warm water and detergent then rinse with clean water at the end of the treatment.

    More Infomation
    Removing Stains from Concrete
    Cleaning Concrete
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    Q:

    How do I remove white desposits from my brickwall (efflorescence)?

    A:

    Efflorescence is caused by water entering the wall and dissolving salt that could be present in the mortar or in the bricks or in both into a solution which is brought to the surface. The water evaporates leaving these white deposits on the surface of the wall. Dry brushing is the first method of removing efflorescence. If this is not effective, water washing with or without pressure can be used. If this is not effective chemical cleaning is the last alternative. Chemical cleaning involves acid treatment to dissolve the white deposits. Acid treatments can often cause etching of the masonry especially concrete masonry. Thus fading, streaking and changing colour of the wall may result if chemical cleaning is not carried out correctly and by experienced operator. After cleaning if efflorescence reappears, the source of water pentration of the wall must be investigate and addressed before any recleaning.

    More Infomation
    Efflorescence Datasheet
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