Mysteries of concreting exposed

We are asked all sorts of questions about our area of expertise. Here’s a selection of our most frequently asked questions with answers that should add clarity.
Concrete needs to be cured properly to achieve the desired properties for its intended use. The process relies on adequate moisture, temperature, and time. To illustrate: depending on the application, relative humidity in the concrete should be maintained at greater than 80%, at temperatures over 10 degrees Celsius and typically, for between 3 and 14 days. That’s how concrete cures to achieve the required properties.

Blowholes, sometimes called bug holes, are small cavities that appear when the formwork is stripped away. They are unavoidable using regular, impermeable forms. Prevention is best. Avoid stickiness against the form or use permeable forms, which can greatly reduce, if not eliminate, blowholes.

Crazing is where fine random surface cracks lead to irregular hexagonal patterns on the concrete surface. Avoid using too wet a mix, overworking the surface, using driers to remove water, or delaying the curing process.

Dusting of the surface can occur when the concrete specification is not right for abrasion resistance, or when finishing or curing is done improperly. Dusting can be avoided with correct specification, compacting the surface and finishing properly in a timely fashion.

Honeycombing is where the mortar fails to fill spaces between the coarse aggregate. Best avoided by making sure there’s enough fines to fill the voids, by ensuring the integrity of the formwork, and that the mix is fully compacted and placed. Patching can be done to repair but avoiding honeycombing altogether is better.

No. Concrete is made of aggregates (sand, gravel or crushed stone) and a paste, made from Cement and water. The cement is portland cement, a generic type, not a brand of cement, used in virtually all concrete. Aggregates comprise 60–75% of the total volume of concrete.

Concrete reaches 65% strength in 7 days, 90% strength in 14 days and 99% strength in 28 days. It gets harder still, after that.

Concrete with steel bars (a.k.a. reo or rebars) within it. This is a very tough building product. It can even withstand fire.

Concrete can be reinforced with a number of materials, but steel is good because it contracts and expands roughly in tandem with the concrete, meaning problems with concrete cracking are avoided.

The sheer strength of reinforced concrete is why it has found use in pipes, bridges, skyscrapers and swimming pools.

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