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What does 28 -day strength mean?

Concrete hardens and gains strength as it hydrates. The hydration process continues over a long period of time. It happens rapidly at first and slows down as time goes by. To measure the ultimate strength of concrete would require a wait of several years. This would be impractical, so a time period of 28 days was selected by specification writing authorities as the age that all concrete should be tested. At this age, a substantial percentage of the hydration has taken place.  He 28-day strength is commonly measured on concrete cubes (100mm or 150mm side length) that are removed from the moulds and cured in water at 23⁰C until testing.

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What are the recommended strengths for the various uses for concrete?

Source: Cement and Concrete Institute

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Portland cement is hydraulic cement which means that it sets and hardens due to a chemical reaction with water. Consequently, it will harden under water.

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Stains can be removed from concrete with dry or mechanical methods, or by wet methods using chemicals or water.

Common dry methods include sandblasting, flame cleaning and shot-blasting, grinding, scabbing and scouring. Steel-wire brushes should be used with care because they can leave metal particles on the surface that later may rust and stain the concrete.

Wet methods involve the application of water or specific chemicals according to the nature of the stain. The chemical treatment either dissolves the staining substance so it can be blotted up from the surface of the concrete or bleaches the staining substance so it will not show.

To remove blood stains, for example, wet the stains with water and cover them with a layer of sodium peroxide powder; let stand for a few minutes, rinse with water and scrub vigorously. Follow with the application of a 5% solution of vinegar to neutralize any remaining sodium peroxide.

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How do you protect a concrete surface from aggressive materials like acids?

Many materials have no effect on concrete. However, there are some aggressive materials, such as most acids, that can have a deteriorating effect on concrete. The first line of defence against chemical attack is to use quality concrete with maximum chemical resistance, followed by the application of protective treatments to keep corrosive substances from contacting the concrete. Principles and practices that improve the chemical resistance of concrete include using a low water-cement ratio, selecting a suitable cement type (such as sulphate-resistant cement to prevent sulphate attack), using suitable aggregates, water and air entrainment. A large number of chemical formulations are available as sealers and coatings to protect concrete from a variety of environments; detailed recommendations should be requested from manufacturers, formulators or material suppliers.

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Alkali-silica reactivity (ASR) is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolans, admixtures and mixing water. External sources of alkali from soil, de-icers and industrial processes can also contribute to reactivity. The reaction forms an alkali-silica gel that swells as it draws water from the surrounding cement paste, thereby inducing pressure, expansion and cracking of the aggregate and surrounding paste. This often results in map-pattern cracks, sometimes referred to as alligator pattern cracking. ASR can be avoided through 1) proper aggregate selection, 2)minimisation of cement content, 3) use of blended cements, 4) use of proper pozzolanic materials and 5) contaminant-free mixing water.

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The timing of removal of formwork will depend on the rate of strength gain of the concrete:

1. Vertical surfaces can be struck when the concrete is strong enough to resist the adhesion on the concrete/form face surface. 
2. Soffits cannot be struck until the concrete has gained sufficient strength for the element to support its self-weight and construction loads for all or part of the structural span. It is possible to speed up the removal of much soffit formwork by removing a small part of the formwork and then installing props to this area. Another area of formwork can then be removed and props installed. This sequence can be repeated until all the formwork has been removed and the soffit is fully propped.

When removing formwork it is important to avoid damage to the surface of the concrete and particularly to edges and corners. Following removal of the formwork, all exposed surfaces must be covered with polythene or wet hessian to prevent moisture loss for about 5 days, so that the concrete is sufficiently cured.

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No-fines concrete (NFC) consists of large, coarse aggregate (typically 19 mm stone)and cement paste. Aggregate particles are each covered with a thin layer of cement paste and are in point-to-point contact with each other. At each point of contact the paste forms a small fillet; in the hardened state these fillets hold the particles together and give strength to the concrete.

Typical application s for NFC include: drainage layers; backfill between walls and excavated ground; porous dams; roof screeds (below the waterproofing system); paving (foot traffic for quick drainage), etc.

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