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Concrete plays an essential part in civil engineering with most structures using it in one way or the other. Its evolution dates back to the Roman Empire as it was widely used in most Roman structures, thereby moving from stone and brick construction. However, since then the face of construction has radically changed with buildings built over tens of floors regularly and tunnels built underwater using pre-cast concrete. Hence, the array of construction materials available, including concrete, have evolved significantly since and improved.
Concrete plays a very important role in the UK economy. It contributes to almost 10% of the overall economic growth and employs a significant amount of labour. According to Sustainable Concrete (2010), the UK exported 535,000 tonnes of ready-mixed concrete, which was worth £9million to the economy.
This literature review will focuses on concrete, particularly traditional and pervious concrete. It will then go on to contrast their cost, availability, performance, maintenance and sustainability in an UK context.
Traditional concrete is the most basic form of concrete and is very easily available for consumers. According to Popovics, S. (1992, pp. 1), concrete is composed of three to four basic ingredients. These include:
* Hydraulic cement, otherwise known as Portland cement
* Mineral aggregates
Hydraulic cement is one of the key constituents to forming concrete and greatly impacts the strength of the resulting mixture. Schwartz (1993, pp. 91) states "ratio of Portland cement in concrete directly affects the strength and cost of the concrete. The more cement in a mix, the stronger and more expensive it is".
Mineral aggregates typically consist of gravelled stones or stone-like solids. The purpose of aggregates is to significantly reduce the required content of cement, the costliest component within concrete. Additionally, it helps a constructor minimise the creep, or deformation caused over a long time, caused by the resulting mixture. Popovics, S. (1992, pp. 275) state that aggregates occupies roughly three-fourths of the total concrete volume.
By adding water to this mix, the loose mixture of cement and mineral aggregates gel into a thicker and more solid mix. Schwartz (1993, pp. 91) states "water/cement ratios may go from high-strength concrete, to 8.5 to 1 for a low-strength mix". By adding too little of the water, one can risk making the concrete too weak. Thus, based on the concrete's usage and cost constraints the mixture of the underlying constituents can be varied.
It is possible to engineer the mixture's property based on its intended use by adding admixtures. Perkins (1997, pp. 22) defines admixtures as a chemical compound that is added in small proportion to the concrete mixture to produce a desired characteristic. The types of admixtures used are accelerators, set retarders and water reducers. For example, the purpose of accelerators is to increase the initial reaction between the cement and the water, whereas the set retarders do the exact reverse. According to Perkins (1997, pp. 22), UK is well behind Continental Europe, USA and other developed markets in terms of extent of usage of admixtures within concrete.
For decades, concrete has been one of the preferred construction materials owing to its excellent technical properties. One of its main traits is its high compressive strength due to which it is used in applications such as columns. However, its tensile strength is roughly one-tenths of its compressive strength. Thus, in order to improve its tensile strength it is reinforced with steel, which creates a strong bond with concrete.
When loaded over a long period, traditional concrete can prove to be susceptible failure and in some case even eventual failure. From a UK context, concrete's performance against cold and wet weather would be crucial. Pigeon et al (1995, pp 33) state that concrete can freeze in its saturated state and cause tensile stress within the material due to the formation of ice crystals in the pores of the concrete. However, if the concrete is not cured or reinforced effectively then it could cause its performance to weaken and, under worst cases, eventual failure.
Typically, the strength of concrete is measured by its compressive strength, which is its strongest feature. According to Abeles et al (2003, pp. 21), strength of concrete increases with age, with the rate of increase dependent on the quality of cement and aggregates used. The content and cost of the concrete can be varied depending on its application. Commercially, the strength of concrete is quoted based on its weight and the compressive strength it would attain after 28 days. For example, C20 concrete would stand for a normal weight concrete and a compressive strength of 20 N/mm2. NRMCA (2003) quote that concrete's compressive strength can be varied from 2500 psi, or 17 Mpa, for residential concrete to 4000 psi, or 28 Mpa, upwards for commercial structures. The variation in strength is primarily due to water-cement ratio, admixtures and curing process among others.
Availability of concrete for commercial purposes typically depends on the constituents and the location of the building works. For example, admixtures work best when introduced immediately after the wetting of the cement and might require the mixing to be done on-site rather than in a factory. Additionally, the location of quarries and sourcing of materials (Contract Journal, 2008) is crucial in understanding availability and the distribution pipeline. Most UK concrete companies are fully capable of sourcing all components of concrete themselves. For example, Brett Group, one of UK's leading construction groups, provides customers with a wide range of options from type of aggregates to the type of job.
However, it is even more dependent on its usage. For use in residential repairs, it is usually procured from local stores by components and mixed on-site, whereas if it is for a larger project then it would be pre-cast and delivered by the manufacturers. For example, the construction of Eurotunnel was made possible by use of pre-cast concrete supports embedded on a concrete track.
Maintenance of traditional concrete is crucial to provide long-term serviceability for users. Failure to implement an adequate control process can result in expensive repairs. In some cases, cracks can start to appear on the concrete as load starts to increase over time. This would necessitate usage of sealants, which act to bridge the cracks with an adhesive material.
In the UK, some of the common failures of concrete (St. Astier, 2009) are due to:
* Poor maintenance & incorrect diagnosis
* Inefficient waterproofing capabilities causing excessive penetration of moisture
* Excessive carbonation or chloride levels
Traditional concrete is a resource and energy dependent material, as it requires material from large quarries and factories to prepare the concrete. The UK construction industry currently uses 400 million tonnes of resources per year, of which 10% are unused product going straight to a landfill (Sustainable Concrete, 2010). Additionally, each year structures that are beyond repair are demolished and taken to the landfills. All this combined, yield a large wastage of resources. However, instead of letting this waste go unused it is possible to re-cycle them by crushing them and using them as aggregates within the concrete mixture.
According to the Concrete Centre (2010), it is their vision to ensure that "by 2012, the UK concrete industry will be recognized as the leader in sustainable construction, by taking a dynamic role in delivering a sustainable built environment in a manner that is profitable, socially responsible and functions within environmental limits".
This section of the report will look at pervious concrete and its composition. Following this, the durability and drawbacks of this concrete will be discussed.
Traditional concrete, despite its many advantages, does not perform well in precipitous environment. Pervious concrete allows for air and water to flow freely through the mixture. This limits the runoff from its surface and enhances drainage features. SE Cement (2008) defines pervious concrete as a mixture of Portland cement, water, coarse aggregate and almost no sand. The key feature of this mixture is that it has 15-25% void within its volume. The air pockets within the concrete allow for water to seep through the structure and perform as normal. It is also necessary to ensure that the coarse aggregates are not too loose and get carried away with water. Thus, the mixing and placing process has to be carefully carried out such that the water and Portland cement are coagulated well to form a thick paste around the aggregates. This is crucial to maintain a well-connected system of voids to ensure effective drainage of water. Due to this porous nature, the pervious concrete is quite lightweight at about ????????????
Pervious concrete is particularly useful in parking lots, riverbanks and areas that are highly precipitous as it limits the interference of water and easy drainage of water. According to Limbachiya (2009, pp.554), the use of pervious concrete dates back to 1852. Its use became further widespread following the Second World War and increased construction activity.
According to Brandt (2009, pp. 63), the porous nature of pervious concrete makes it less strong than traditional concrete. Even then, compressive strength of 50 MPa can be reached with small size aggregates and usage of the right admixtures. The reduced compressive strength has led to its restrictive use such as roadways with low to medium traffic. However, SE Cement (2010) estimates that for most applications compressive strengths of 3.5 to 27.5 MPa will suffice.
Despite its strong performance in precipitous climate, there are questions that remain to be answered regarding its durability in colder climates. Traditional concrete is subject to cracking through the normal thaw and freeze cycle that occurs in colder areas of the country. Such conditions have been simulated to test the performance of pervious concrete and it was found by Delatte (2008, pp. 118) that the durability of pervious concrete was poor when the void system was filled with water. They have also indicated that the durability was improved when the moisture in the voids was drained and the cement paste air-entrained.
The mixing process and labour costs for pervious concrete are far more involved than that of traditional concrete. Thus, Limbachiya (2009, pp. 556) expects the initial costs of pervious concrete to be higher than those for traditional concrete because of the thicker installed size of pervious concrete. However, it is worth noting that the additional investment made is for a particular purpose, i.e. the porous structure of the concrete. Thus, the extra cost should be weighed against this advantage gained over its lifetime. Since pervious concrete would need to be customised for the region and application by customers, most manufacturing companies use a large distribution system to minimize delays in batching for customers.
Limbachiya (2009, pp. 556) states that maintenance of pervious concrete pavements is a highly debatable subject. They proceed to state that structures not maintained well continue to perform well, although not at the initial infiltration rates which is its key objective. However, one key concern within pervious concrete is to prevent any clogging of void structures within it, as this would fail one of its objectives, i.e. to be porous. Typically, proper maintenance of pervious concrete pavements includes vacuum sweeping or power washing. Pressure washing of a clogged pervious concrete pavement has restored it to 80-90% of its original permeability (Pervious Concrete, 2009).
Structures such as shopping malls and buildings have roadways and parking lots around for vehicle use. If these pavements are built using traditional concrete, then they risk flooding under heavy rain. Thus, zoning regulations necessitate controls such as retention ponds, which essentially collect the excess water on the pavements from the rain. This requires extra capital investment and resources for setting up. However, if instead of traditional concrete, pervious concrete is used to create the pavements, then the excess water on the surface percolates through the voids in the concrete into the soil underneath. This eliminates ponding on the pavement and preserves more land, capital and resources for alternate use.
However, the basic configuration of pervious concrete allows all liquids to go through the voids. A more environment friendly version of pervious concrete was developed in the UK, in 1999, wherein surface water is allowed to pass through to a specially engineered sub-base while oils and other water pollutants are retained (Concrete Products, 1999). This ensures that the water table under the pavement does not get contaminated.
Within the UK, a unique standard for drainage technology called Concrete Block Permeable Paving (CBPP) has been set-up. According to Paving (2010), this code has been championed by all the major pre-cast concrete paving manufacturers in the UK. Such developments will be positive for the progress of the sustainability within UK's pavement community and minimise damage to the environment from constructions.
This literature review has analysed the features of two popular variants of concrete - traditional and pervious. In particular, it has discussed the basic composition, durability, availability & costs, and finally sustainability. It has been idenitified that both types of concrete have their advantages and drawbacks. Sustainability remains a very important subject within the construction industry in the UK, with the government imposing landfill and aggregate taxes to discourage resource-intensive manufacturers. Additionally, manufacturers are paying increasing attention towards climate change in order to provide protection to the pavements over the life cycle rather than repairing or replacing the structure frequently.
Concrete repairs have been a major issue for the UK economy. This has been further accentuated by the rapid expansion of the UK construction industry, which contributes to roughly 10% of the GDP. According to Mays (1992, pp XI), approximately £500 million is being spent annual on concrete repairs in the UK. This clearly stresses the fact that it is essential to understand the context of concrete's application, as it is better to install the right type of concrete rather than to repair and re-install at a later stage.
References used for literature review
1. Popovics, S. (1992), Concrete materials: properties, specifications, and testing, 2nd Edition, Noyes Publication, New Jersey
2. Schwartz, M. (1993), Basic Engineering for Builders, Craftsman Book Company, USA
3. Perkins, P.H., (1997), Repair, protection and waterproofing of concrete structures, 3rd Edition, Chapman & Hall, UK
4. Pigeon, M., Pleau, R., (1995), Durability of concrete in cold climates, 1st Edition, Chapman & Hall, UK
5. St. Astier (2009), "Concrete Repairs FAQ's", http://www.st-astier.co.uk/concrete-repair-coatings/concrete-repair-faqs (Date viewed, 24 Jan 2010)
6. Contract Journal (2008), "The market for concrete products is ever-changing", 9 July 2008, http://www.contractjournal.com/Articles/2008/07/09/60162/the-market-for-concrete-products-is-ever-changing.html (Date viewed, 27 Jan 2010)
7. Sustainable Concrete (2009), "Sustainable Concrete", http://www.sustainableconcrete.org.uk/main.asp?page=4 (Date viewed, 26 Jan 2010)
8. Abeles, P.W., Bardhan-Roy, B.K., (2003), Prestressed concrete designer's handbook, 3rd Edition, Spon Press, London
10. Brandt, A. M., (2009), Cement Based Composites: Materials, Mechanical Properties, and Performance, 2nd Edition, Taylor & Francis, UK
11. Mays, G., (2001), Durability of concrete structures: investigation, repair, protection,2nd Edition, Spon Press, London
12. Southeast Cement (2010), "What is pervious concrete?", Portland Cement Association,
http://www.secement.org/pervious_concrete.htm (Date viewed, 27 Jan 2010)
13.Concrete Products (1999), Porous concrete promotes 'greener' water system, 1 December 1999, http://concreteproducts.com/mag/concrete_porous_concrete_promotes/
14. Pervious Pavements (2009), "Inspection and Maintenance", 2009, http://www.perviouspavement.org/inspection%20and%20maintenance.htm (Date viewed, 26 Jan 2010)
15. Limbachiya, M.C., (2009), Excellence in Concrete Construction Through Innovation, Taylor & Francis Group, UK
16. Delatte, N., (2008), Concrete Pavement, Design, Construction and Performance, 1st Edition, Taylor & Francis Group, UK
17. Paving (2009), "Block Paving", http://www.paving.org.uk/block_paving.php (Date viewed, 27 Jan 2010)