Cement and concrete as an engineering material: An historic appraisal and case study analysis
Introduction
There is a general lack of understanding of the difference between cement and concrete, with the two terms often used interchangeably. However, cement is actually an ingredient of concrete and can be considered the ‘glue’ that binds aggregates together to form concrete. Therefore, concrete is basically a mixture of aggregates and paste – the aggregates being sand and gravel or crushed stone; the paste being water and Portland cement. Portland cement is not a brand name it is the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel. Cement will constitute 10 to 15 percent of the concrete mix by volume and, through a process of hydration the cement and water harden and bind the aggregates into a rocklike mass. This hardening process will continue for years implying that concrete will get stronger as it gets older.
Varying the mix of cement, sand and aggregate used in a concrete blend enables its use in a range of applications. Construction of a typical family home will require 14 tonnes of cement, a kilometre of motorway will contain as much as 2,500 tonnes of cement, and a building can be made to last for 100 years. Products can be designed, coloured and shaped to accommodate a variety of environmental conditions, architectural requirements and to withstand a wide range of loads, stresses and impacts.
Today, second only to water, concrete is the most consumed material, with three tonnes per year used for every person on earth [1]. Twice as much concrete is used in construction as all other building materials combined. There is little doubt that concrete will remain in use as a construction material well into the future. However, with such extensive use of the material, discovery of any shortcoming or problem associated with concrete or reinforced concrete structures will become a matter of considerable public concern – both from a safety perspective and associated costs of rectification. Accordingly, this paper will initially review the historical development of cements and concrete and will then focus on the mechanical response of concrete and reinforced concrete to its working environment. At appropriate points within the narrative, case study input will be used to illustrate or highlight salient themes.
Section snippets
Cement and concrete
‘Cement’ is a generic term that can be applied to all binder materials. From earliest times, builders have used binders in conjunction with rock and stone to form more stable structures. Simple mud was employed as a binder, and is still in use in parts of the world today. In the days of early civilisations of Egypt, Greece and Rome, a lime cement was made by a process of ‘burning limestone’ to give Quicklime [2], [3]. When mixed with water, quicklime formed slaked lime (calcium hydroxide) and,
The age-strength relationship of concrete
Strength of concrete is generally given as a compressive value, expressed in mega pascals (MPa) at an age of 28 days. However, other test ages can also be quoted, therefore it is important to recognise the relationship between the 28-day strength and other test ages. Seven-day strengths are often estimated to be about 75% of the 28-day strength and 56-day and 90-day strengths are about 10–15% greater than 28-day strengths as shown in Fig. 4.
The compressive strength that a concrete can achieve
Reinforcing concrete
Without the benefit of additional reinforcing, concrete will act as an inherently brittle material, exhibiting identical structural limitations as that of quarried stone. Both materials are hard and brittle, with the compressive strength of concrete being 10 times higher than that of its tensile strength. So, concrete is a strong material when compressed, but shearing forces or moderate tensile force will cause it to crack or buckle. The answer to these material shortcomings is to give concrete
Pre and post-tensioning
In conventional reinforced concrete, the high tensile strength of a chosen rebar is combined with the exceptional compressive strength of concrete to form a structural material that is strong in both compression and tension. Designers use a system of pre-stressing as a way to reinforce concrete by introducing compressive stresses in an element or member prior to it entering service. The principle behind pre-stressing concrete is that compressive stresses can be induced by high-strength steel
Fibre reinforcement and fires
Firestopping and fireproofing products can be ablative in nature. In this context, use of the term ‘ablative’ can signify endothermic materials, or merely materials that are sacrificial and become “spent” over time spent while exposed to fire. The latter version has also been used to describe silicone fire-stop products, which, by themselves, are sacrificial. In other words, given sufficient time under fire or heat conditions, these products actually char away, crumble and disappear. The idea
Shotcrete
Shotcrete has become an important component of modern tunnelling technology, underground mining, slope and rock consolidation, repair of concrete structures and artificial rock structures (Fig. 22). There has been a rapid technological process development over the last 20 years, with wet and dry spraying methods vying for process dominance.
In a dry shotcrete process, all ingredients except water and sometimes liquid accelerators, are mixed in the dry state and the mix is conveyed by an air
Concrete repair
Concrete structures are routinely exposed to freeze/thaw cycling, abrasion, chemical spillage, and thermal cycling. When subjected to such aggressive environments, concrete surfaces will degrade. This degradation may advance to a state where the structure is rendered unserviceable. Nowadays, it is possible to undertake repairs that will restore the surface to a satisfactory operational standard. There are many causes of concrete failure and many methods available for the repair of failures.
The versatility of concrete
An appreciation of the structure, mix variation and bonding will allow an understanding of service behaviour of concrete. Furthermore, awareness of the nature of concrete reinforcing, pre-stressing and post-tensioning will provide an insight to load bearing capacity and service performance. Structural integrity can be maintained by an appreciation of environmental (corrosion) issues that may compromise the natural protection of steel in concrete. However, to appreciate the versatility of
Designing for every eventuality??
As a parting thought, it may be impossible to design for every eventuality. A backhoe weighing 8 tons was being transported on top of a flatbed trailer. The combination was heading east on Interstate 70 near Hays, Kansas, USA. When the shovel arm hit the overpass, it sliced almost halfway through the reinforced deck (Fig. 26). The overpass deck was constructed from commercial-grade concrete, reinforced with 37 mm diameter steel rebar spaced at 15 cm intervals in a criss-cross pattern layered in 30
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