CCS GROUP

Compaction is the process which expels entrapped air from freshly placed concrete and packs the aggregate particles together so as to increase the density of concrete. It increases significantly the ultimate strength of concrete and enhances the bond with reinforcement. It also increases the abrasion resistance and general durability of the concrete, decreases the permeability and helps to minimise its shrinkage-and-creep characteristics.

Proper compaction also ensures that the formwork is completely filled – ie there are no pockets of honeycombed material – and that the required finish is obtained on vertical surfaces.

When first placed in the form, normal concretes, excluding those with very low or very high slumps, will contain between 5% and 20% by volume of entrapped air. The aggregate particles, although coated with mortar, tend to arch against one

another and are prevented from slumping or consolidating by internal friction.

Efflorescence is the formation of salt deposits, usually white, on or near the surface of concrete causing a change in appearance. Apart from the discolouration, efflorescence is generally harmless. This is best described as being 'a skin trouble and not a deep-seated disease'. Primary efflorescence is efflorescence occurring during hardening of the concrete.

Secondary efflorescence is the efflorescence resulting from the weathering of the hardened concrete.

Cryptoflorescence is the deposition of salt within the pores of concrete below the exposed surface. The force of crystallisation growth may cause some fretting. Light coloured concrete shows the deposit much less than darker coloured concrete. With time, efflorescence becomes less extensive. Efflorescence is most obvious in the winter but may be observed throughout the year after a heavy rain and a drop in temperature.

Drying shrinkage has been ineffectively used by some design engineers as a means of controlling cracking in slab construction. The AS 1012.13 drying shrinkage test measures the shortening of small concrete prisms due to drying from a water-saturated condition at the age of 7-days for various periods of drying with a standard drying period of 56 days. It therefore measures a later-age shrinkage.

There are significant differences between the mechanisms and magnitude of early shrinkage and the standard drying

shrinkage. It is the early-age shrinkage that influences cracking.

Estimating the concrete volume is a simple process and basically involves multiplying the length, width and depth (all in metres) of a concrete element to arrive at an estimated volume of concrete in cubic metres. For example the estimated volume of concrete for a garage floor slab 7.5 m long, 3.5 m wide and 0.1 m thick (100 mm), is calculated as follows:

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Estimated Volume = 7.5 x 3.5 x 0.1 = 2.63 m3

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Larger projects are estimated in the same way, by breaking often complex shapes and buildings into basic elements, calculating the volume in each element and adding them together to arrive at a total volume estimate for the project.

Concrete is one of the most widely used construction materials. Anyone who uses concrete or is responsible for managing or supervising its use should be aware that it may be a hazard. Professional advice should be obtained for appropriate risk management strategies.

The basic function of residential pavements such as driveways, paths and patios is to provide safe, easy access onto or around a property. Concrete is typically used to not only provide a durable paving surface, but one which can also incorporate a wide range of decorative finishes to complement the design and landscaping of the residence.

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While providing a concrete pavement on the ground is relatively straightforward, there are many aspects of residential pavement design and construction that need to be considered in order to produce a finished product that will satisfy both the functional and aesthetic requirements demanded by home owners.

Knowledge of the shrinkage characteristics of concrete is a necessary starting point in the design of structures for crack control. Such knowledge will enable the designer to estimate the probable shrinkage movement in reinforced or prestressed concrete and the appropriate steps can be taken in design to accommodate this movement.

This Data Sheet reviews the factors affecting drying shrinkage of concrete seeking to put in perspective their varying influences enabling practical and effective controls to be placed on drying shrinkage...

Plastic settlement cracks are so-called because they form while the concrete is still plastic, ie has not set. The settling concrete is restrained and cracks form at the surface. They may become visible very early, ie while finishing is proceeding, but are often not noticed until some hours after placement. They are distinguished from plastic shrinkage cracks by their distinct pattern which typically mirrors the pattern of the restraining elements such as the reinforcement.

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The cracks occur while the concrete is plastic and frequently while bleed water is still rising and covers the surface. They tend to roughly follow the restraining element, eg reinforcing bars, or changes in the concrete section. They can be quite wide at the surface, tend to extend only to the reinforcement or other restraining element and taper in width to that location. In exposed situations this may increase the risk of corrosion of the reinforcement and pose a threat to durability. Cracks may develop further, due to subsequent drying shrinkage, leading to possible cracking through the full depth of the concrete member.

No-fines concrete is a concrete containing little or no fine aggregate. The coarse aggregate should preferably be a single-size material (nominal maximum sizes 10 mm and 20 mm being the most common). However, blended aggregates (10 and 7 mm; and 20 and 14 mm) have been found to perform satisfactorily. Because it is characterised by uniformly distributed voids, it is not suitable for reinforced or prestressed concrete construction.