Concrete and heavy construction materials Frequently Asked Questions (FAQ)

• What are the constituent parts of concrete?

• What is concrete used for?

• Worldwide, how many tones of concrete are used per person every year?

• What does the heavy materials industry contribute to the Australian economy?

• What are the extractive, cement and concrete industries are collectively known as?

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

• Why does concrete crack?

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Q: What are the constituent parts of concrete?
A: sand, aggregate and cement

Q: What is concrete used for?
A: Building roads, footpaths, schools, hospitals, homes and workplaces

Q: Worldwide, how many tones of concrete are used per person every year?
A: 3 tonnes. In fact, next to water, it is the most consumed material on the planet.

Q: What does the heavy materials industry contribute to the Australian economy?
A: Worth $6 billion in revenues to the economy and contributes $11.7* billion to GDP

Q: What are the extractive, cement and concrete industries are collectively known as?
A: The heavy construction materials industry

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.

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.

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.

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.

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.