THE CIVIL ENGINEER

History

The Assyrians and Babylonians used clay as cement in their concrete. The Egyptians used lime and gypsum cement. In the Roman Empire, concrete made from quicklime, pozzolanic ash / pozzolana and an aggregate made from pumice was very similar to modern Portland cement concrete. In 1756, the British engineer John Smeaton pioneered the use of Portland cement in concrete, using pebbles and powdered brick as aggregate. In modern times the use of recycled materials as concrete ingredients is gaining popularity because of increasingly stringent environmental legislation. The most conspicuous of these is fly ash, a by product of coal fired power plants. This has a significant impact by reducing the amount of quarrying and landfill space required.

The properties of concrete have been altered since Roman and Egyptian times, when it was discovered that adding volcanic ash to the mix allowed it to set under water. Similarly, the Romans knew that adding horse hair made concrete less liable to shrink while it hardened, and adding blood made it more frost resistant. In modern times researchers have added other materials to create concrete that is extremely strong, and even concrete that can conduct electricity.

Composition

The composition of concrete is determined initially during mixing and finally during placing of fresh concrete. The type of structure being built as well as the method of construction determine how the concrete is placed and therefore the composition of the concrete mix (the mix design).

Cement

Main article: Portland cement

Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar and plaster. English engineer Joseph Aspdin patented Portland cement in 1824, and it was named after the limestone cliffs on the Isle of Portland in England because its color is similar to the stone quarried there. It consists of a mixture of oxides of calcium, silicon and aluminium. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay, and grinding this product (called clinker) with a source of sulfate (most commonly gypsum). When mixed with water, the resulting powder will become a hydrated solid over time.

High temperature applications, such as masonry ovens and the like, generally require the use of a refractory cement; concretes based on Portland cement can be damaged or destroyed by elevated temperatures, but refractory concretes are better able to withstand such conditions.

Water

Potable water can be used for manufacturing concrete. The w/c ratio (mass ratio of water to cement) is the key factor that determines the strength of concrete. A lower w/c ratio will yield a concrete which is stronger, while a higher w/c ratio yields a concrete with a lower strength. ^[2] Cement paste is the material formed by combination of water and cementitious materials; that part of the concrete which is not aggregate or reinforcing. The workability or consistency is affected by the water content, the amount of cement paste in the overall mix and the physical characteristics (maximum size, shape and grading) of the aggregates.

Aggregates

The water and cement paste hardens and develops strength over time. In order to ensure an economical and practical solution, both fine and coarse aggregates are utilised to make up the bulk of the concrete mixture. Sand, natural gravel and crushed stone are mainly used for this purpose. However, it is increasingly common for recycled aggregates (from construction, demolition and excavation waste) to be used as partial replacements of natural aggregates, whilst a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.

Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative “exposed aggregate” finish, popular among landscape designers.

Admixtures

Admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. In normal use, admixture dosages are less than 5% by mass of cement, and are added to the concrete at the time of batching / mixing. The most common types of admixtures are:

• Accelerators speed up the hydration (hardening) of the concrete.
• Retarders slow the hydration of concrete.
• Air-entrainers add and distribute tiny air bubbles in the concrete, which will reduce damage during freeze-thaw cycles.
• Plasticizers can be used to increase the workability of plastic or “fresh” concrete, allowing it be placed more easily, with less consolidating effort. Superplasticisers allow a properly designed concrete to flow in place even around congested reinforcing bars, see self consolidating concrete described below. Alternatively, they can be used to reduce the water content of a concrete (water reducers) while maintaining workability. This improves its strength and durability characteristics.
• Pigments can be used to change the colour of concrete, for aesthetics.

Additions

Additions are very fine inorganic materials that usually have pozzolanic or latent hydraulic properties. They are added at the concrete mixer to improve the properties of concrete. The term is not used when the materials are added at the factory as constituents of blended cements.

• Fly ash: A by product of coal fired elgenerating plants, it is used to partially replace Portland cement (by up to 60% by mass). The properties of fly ash depend on the type of coal burnt. In general, silicious fly ash is pozzolanic, while calcareous fly ash has latent hydraulic properties.
• Ground granulated blast furnace slag (ggbs): A by product of steel production, is used to partially replace Portland cement (by up to 80% by mass). It has latent hydraulic properties.
• Silica fume: A byproduct of the production of silicon and ferrosilicon alloys. Silica fume is similar to fly ash, but has a particle size 100 times smaller. This results in a higher surface to volume ratio and a much faster pozzolanic reaction. Silica fume is used to increase strength and durability of concrete, but generally requires the use of superplasticizers for workability.