Material of the Month: Concrete
Maria Felice examines the properties of this ubiquitous material.
Concrete has been used since ancient times and a beautiful example of this is the Pantheon in Rome. Future generations will have no problem finding 21st Century examples of concrete – an incredible 10km3 is used each year and this is increasing.
The terms concrete and cement are sometimes used interchangeably but refer to two separate materials. Concrete is a ceramic-matrix composite consisting of aggregate, a cement matrix and water. The aggregate particles provide the strength and the cement provides the workability. In ancient times, a mixture of powdered lime and volcanic ash – pozzolana – was used to make cement. This remained the case for almost two millennia until the development of Portland cement by Joseph Aspdin in Leeds in 1824. This cement, which is by far the most common type used today, was named after the natural stone found on the British island of Portland, which it resembles. Portland cement consists of clay and limestone, which are crushed and fired in a kiln. The constituents are similar to cement used in ancient times but the firing step is novel. The material obtained from the kiln is clinker, to which a small amount of gypsum is often added to retard the setting process. The resulting mixture is cement. This is then added to water and aggregate. Portland cement is referred to as hydraulic cement because it hardens by hydration. Not by drying as one might think – water participates in a chemical bonding reaction. An important foundation for Aspdin’s development was John Smeaton’s work in the 18th Century. He determined the compositional requirements for cement to be hydraulic and designed the third Eddystone Lighthouse in England, sometimes referred to as Smeaton’s Tower.
Portland cement is produced to have a specific surface area of approximately 300m2/kg and grain sizes of 20–30μm. A reduced nomenclature is used for cement chemistry where lime (CaO) is C, alumina (Al2O3) is A, silica (SiO2) is S and water is H. The chief ingredients of Portland cement are di- and tri-calcium silicates (C2S and C3S), and tri-calcium aluminate (C3A). When cement is added to water, the first reaction to occur is the hydration of C3A, which causes the cement to set. Next the C2S and C3S react with water and cause the cement to harden. This reaction takes up to 100 days to complete. Due to this lengthy reaction time, the strength of concrete is often given as a 28-day strength value.
Aggregates make up 60–80% of the volume of concrete and reduce its cost. The different sized particles, such as sand and stone, result in dense packing and good interfacial contact. It is essential that the cement coats all the surfaces of the aggregate particles for satisfactory results to be achieved.
Everyone has heard of reinforced concrete, but few know of its origins. The Romans used bronze bars in concrete with little success, due to the difference in thermal expansion of bronze and concrete. In the 1840s and 1850s, much progress was made in this field. Frenchman Joseph Monier made gardening pots and similar objects using concrete and a steel mesh. Steel has similar thermal expansion to concrete and is much more effective. Joseph Louis Lambot, also from France, made a boat using a similar reinforced material. William Wilkinson, a plasterer from Newcastle, made a reinforced concrete floor and showed an understanding of the constructional principles of reinforced concrete. François Coignet designed the first actual building made with reinforced concrete in France. Nowadays, reinforcement is very popular and, since the tensile strength of concrete is at least 10 times less than its compressive strength, steel pre-stressed with a tensile load is often used. This transfers a residual compressive load into concrete.
Portland concrete is the most widely used construction material on Earth. The 10km3/year statistic previously mentioned can be compared to 1.3km3 of timber and 0.1km3 of steel used annually in construction. The energy expenditure and CO2 output related to concrete production is surprisingly low in comparison with other construction materials. It is the sheer amount of concrete that is produced that causes concern, and this is why more sustainable constituent materials and techniques are being sought.
Concrete production accounts for about 6% of CO2 emissions globally and 80% of this is from cement. In turn, 60% of the CO2 released by cement production is from the chemical breakdown of calcium carbonate. The remaining CO2 is produced in the heating process, although the efficiency of this has greatly improved over the years.
A number of ways have been suggested to reduce the amount of calcium carbonate that is broken down. These include reducing the amount of calcium oxide in the clinker or reducing the amount of clinker in the cement, by using supplementary cementitious materials such as fly ash. The latter is better for ensuring the quality and safety of concrete. However, the availability of materials must be considered carefully. Substitute materials located thousands of kilometres away negate the benefits of replacing clinker in carbon-heavy transportation.
The use of waste as part of the aggregate in concrete is another option being considered. One technique is to use shredded car tyres. Around one billion car tyres reach the end of their useful life annually, and usually end up stockpiled as whole tyres or shredded and landfilled. Studies have shown that the strength of concrete is slightly reduced when rubber is used. However, this concrete is recommended for use in zones with severe earthquake risk and for systems subject to severe dynamic actions, such as railway sleepers, as well as for non-loadbearing purposes, such as noise reduction barriers.
Biotech concrete is another innovative idea. Sealers are often added to concrete to reduce permeability and increase durability. They are commonly organic polymers with some level of toxicity. A more eco-friendly idea being proposed is the use of bacteria and the biomineralisation process, whereby bacteria create a local microenvironment that favours the precipitation of certain minerals. Bacteria from the genus Bacillus are suitable for this and can be used on the surface of the concrete or within it. They are ureolytic bacteria and can break down urea into ammonium, carbon and CO2. In the presence of calcium, calcium carbonate is precipitated, which can seal gaps. A possible downside to this technique is the release of ammonia, but, as mentioned, this technique is still in early development stages.
Research is ongoing to ensure that the environmental impact of concrete structures is minimised, not only by changing the techniques and constituents to reduce the impact of concrete production, but also by designing concrete structures that will have longer, safer lives.
This article has been updated since it was first published on 1 September 2013.