A Guide to Concrete and Concrete Mixtures
September 15, 2013Though a familiar part of our 21st century environment, concrete has been used for construction since ancient times. Roman forms of concrete, using hydrated lime and volcanic ash, were markedly superior to the predominantly sand and lime mixes used by other civilisations.
Mixing
The characteristic strength and density of concrete is related to the proportions of the three principal ingredients: water, cement and aggregate. The aggregate component is generally a type of gravel and/or small stone mixture which gives a rough or smooth texture to the finished concrete, reflecting the fine or coarse nature of the aggregate mixture. Water is generally regarded as a critical ingredient: too much produces ‘wet’, weaker concrete, too little produces a ‘dry’, unviable mix. Though not the most accurate method, concrete ingredients are traditionally mixed by volume using simple ratios. Commercial concrete products, including more specialised types and formulas, are generally mixed by weight.
Domestic concrete
The traditional 3: 2: 1 ratio of aggregate, sand, and cement produces a general purpose mix suitable for paths and floors. Where even greater strength or waterproofing qualities are required for ponds, slabs or structures, a ratio of 5: 3: 1 is often applied. Increased strength and volume for foundations and other large-volume projects is achieved by adding greater amounts of aggregate, which is often termed ‘ballast’. Dry ‘ready-mix’ bags of pre-mixed concrete are widely available. Often different mixes are available for different projects. Component costs are relatively high, but these products are usually very cost-effective, reducing working time and eliminating waste.
Additives
- Various chemical preparations are available to increase, or alter, the standard properties of mixed concrete. Some of the more common additives used include:
- Waterproofer – to enhance the ability of the hardened concrete to resist water penetration.
- Dustproofer/surface hardener – to reduce the likelihood of finished surfaces crumbling and dusting.
- Frostproofer/rapid hardener – to both accelerate hardening and improve curing at lower temperatures.
- Waterproofer and retarder – used for rendering to allow larger areas to be worked in one operation.
Modern industrial concretes
- A wide variety of concrete products have been developed in modern times both to add special features and to meet the demands of specialist applications. Here are four examples in common use:
- Shotcrete requires compressed air to ‘shoot’ concrete into large structures. This type of application needs no ‘forming’ (framework) to shape the concrete. Thus it is an ideal method for installation and repair work on bridges and dams. Because shotcrete can be applied quickly with little preparation, it is also very useful for tunnelling applications and stabilisation of rock and soil surfaces.
- Pervious concrete contains a honeycomb matrix of holes to permit the movement of both air and water through the concrete. Unlike standard concrete, its free-draining properties allow rainwater to permeate the surface thus reducing the impact of construction work on groundwater levels. In some instances pervious concrete can eliminate the need for surface-water drainage.
- Glass concrete, which uses recycled glass as an aggregate, gives a unique texture and aesthetic appeal. Studies have shown the inclusion of glass aggregate adds greater durability and strength whilst also improving thermal insulation properties.
- Stamped concrete is an architectural concrete using textured moulds to stamp designs and finishes on the concrete surface which is then sealed to produce an attractive hard-wearing finish.
For more information about concrete mixtures, or to discuss your concrete delivery and mixing needs with a qualified professional, get in touch with the team at Roadmaster Concrete Mixers today.