Integration and innovation By definition, outsourcing implies that the firm has once produced with its own employees what it now buys over the market. Instead, the pattern found in construction is characterized by subcontracting of services that frequently lack a history of having been produced in-house (Costantino and Pietroforte 2002, 2004). On the … Read more
There are a huge number of uses: • Railway Sleepers; • Communications poles; • Pre-tensioned precast “hollowcore” slabs; • Pre-tensioned Precast Double T units – for very long spans (e.g., 16 m span for car parks); • Pre-tensioned precast inverted T beam for short-span bridges; • Pre-tensioned precast PSC piles; • Pre-tensioned precast portal frame … Read more
There are two methods of prestressing: • Pre-tensioning: Apply prestress to steel strands before casting concrete; • Post-tensioning: Apply prestress to steel tendons after casting concrete. Pre-tensioning This is the most common form for precast sections. In Stage 1 the wires or strands are stressed; in Stage 2 the concrete is cast around the stressed … Read more
Concrete The main factors for concrete used in PSC are: • Ordinary portland cement-based concrete is used but strength usually greater than 50 N/mm2; • A high early strength is required to enable quicker application of prestress; • A larger elastic modulus is needed to reduce the shortening of the member; • A mix that … Read more
The classic everyday example of prestressing is this: A row of books can be lifted by squeezing the ends together: The structural explanation is that the row of books has zero tensile capacity. Therefore the ‘beam’ of books cannot even carry its self weight. To overcome this we provide an external initial stress (the prestress) which compresses the books together. Now they can only separate if the tensile stress induced by the self weight … Read more
The main advantages of prestressed concrete (PSC) are: Smaller Section Sizes Since PSC uses the whole concrete section, the second moment of area is bigger and so the section is stiffer: Smaller Deflections The larger second moment of area greatly reduces deflections for a given section size. Increased Spans The smaller section size reduces self weight. Hence a given section can span further with prestressed concrete than it can with ordinary reinforced concrete. Durability Since the entire section remains in compression, no cracking of the concrete can occur and hence there is little penetration of the cover. This greatly improves the long-term durability of structures, especially bridges and also means that concrete tanks can be made as watertight as steel tanks, with far greater durability.
Which this paper aims to summarise.Although foamed concrete was used some 2000 years ago it wasn’t until recently that modern foamed concrete began to be developed. In 1987 a full scale trial on the use of foamed concrete for trench reinstatement was conducted in the UK, which led to a further increase in use in … Read more