For an initial period after mixing, the fluidity or consistence of a paste of cement and water appears to remain relatively constant. In fact, a small but gradual loss of fluidity occurs, which can be partially recovered on remixing. At a time called the initial set, normally between two and four hours after mixing at […]
We have seen that, at any stage of hydration, the HCP consists of: a residue of unhydrated cement, at the centre of the original grains the hydrates (the gel), chiefly calcium silicates (C-S-H) but also some calcium aluminates, sulphoaluminates and ferrites, which have a complex fibrous form and contain the gel pores, which are between
It is a good idea to begin the design of the foundations of a building at the same time as architectural design work commences. Subsurface conditions beneath a site can strongly influence fundamental decisions about a building—its location on the site, its size and shape, its weight, and the required degree of flexibility of its
The bonds that we have just described in previous articles can occur between atoms in gases, liquids and solids and to a large extent are responsible for their many and varied properties. Although we hope construction materials do not change state whilst in service, we are very much concerned with such changes during their manufacture,
Most of the materials that we use are not pure but consist of a mixture of one or more constituents. Each of the three material states of gases, liquids and solids may consist of a mixture of different components, e.g. in alloys of two metals. These components are called phases, with each phase being homogeneous.
Pure metals in the ‘as-cast’ condition after slow cooling are generally soft, have low yield stresses and are very ductile; this is a consequence of the ease of dislocation movement or slip. They are therefore unsuitable for use as a structural material, with perhaps the most critical property for design being the low yield stress.
Many construction materials, particularly metals and some ceramics, consist of small crystals or grains within which the atoms are packed in regular, repeating, three-dimensional patterns giving a longrange order. The grains are ‘glued’ together at the grain boundaries; we will consider the importance of these later, but first we will discuss the possible arrangements of
Having produced and formed a metal, it is necessary to ensure that it performs well during service. A major consideration in this is corrosion. This involves loss of material from the metal’s surface and can be divided into two processes: dry oxidation and wet corrosion. Oxidation and Corrosion of Metals Dry Oxidation The earth’s atmosphere
Corrosion Prevention and Control by Design At ambient temperatures significant corrosion occurs only if moisture is present. Thus, surfaces should be exposed as little as possible to moisture and arranged so that they dry out quickly after wetting. In practice all surfaces are at risk, vertical surfaces suffer ‘run off’, flat surfaces retain moisture on
Heat Treatment of Steel Properties of steel can be altered by applying a variety of heat treatments. For example, steel can be hardened or softened by using heat treatment; the response of steel to heat treatment depends upon its alloy composition. Common heat treatments employed for steel include annealing, normalizing, hardening, and tempering. The basic