6505-11 - 1 Progress in Concrete Technology Challenge:...

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Unformatted text preview: 1 Progress in Concrete Technology Challenge: Compared to steel, normal concrete has a low strength/weight ratio. Concrete with a higher strength/weight ratio is needed for tall buildings and long-span bridges. Solution: To increase the strength/weight ratio, reduce the weight by using lightweight concrete or increase the strength by using high-strength concrete . Challenge: For placement of concrete in heavily reinforced structures, high workability is needed. Solution: Use high-workability or flowing concrete . Challenge: Drying shrinkage of normal concrete frequently leads to cracking. Solution: Use shrinkage compensating concrete . Challenge: Normal concrete has low toughness and low resistance to impact. Solution: Use fiber-reinforced concrete . Challenge: Low permeability and high chemical resistance are needed for durability of concrete. Solution: Use polymer concrete . Challenge: Control of temperature rise in mass concrete use for dams and other large structures is needed to reduce the chance of cracking of the concrete. Solution: Use techniques for precooling of concrete materials or roller-compacted concrete . Challenge: High density material is needed for use as radiation shielding in nuclear power plants. Solution: Use heavyweight concrete . 2 Lightweight Aggregate Concrete Structural Lightweight Concrete • Definition : Concrete having a 28-day compressive strength greater than 2500 psi (17 MPa) and an air- dried unit weight not greater than 115 lb/ft 3 (1850 kg/m 3 ). • Composition : Similar to normal concrete except that it is made with lightweight aggregates or combination of lightweight and normal-weight aggregates. All- lightweight concretes use both lightweight coarse and lightweight fine aggregates. Sanded- lightweight concretes used natural sand and lightweight coarse aggregates. • Lightweight aggregates used : expanded clays, shales, slates and slags. Strength Requirements for Structural Lightweight Concrete as specified in ASTM C 330 3 Structural Lightweight Concrete (continued) • Problem with high slump : For fresh concrete with high consistency, the lightweight aggregate tends to segregate and float to the surface. It is often necessary to limit the maximum slump and to entrain air to lower the mixing water requirement. ACI 213 recommends a maximum slump of 100 mm (4 in.). Air entrainment is generally between 4.5 and 9%. • Workability required : Placing, compacting, and finishing a lightweight concrete requires less effort. A lightweight concrete with a slump of 50 to 75 mm (2 to 3 in.) has similar workability as a normal concrete with a slump of 100 to 125 mm (4 to 5 in.) Structural Lightweight Concrete (continued) • Strength : – Compressive strength is usually related to cement content at a given slump rather than to the w/c ratio....
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This note was uploaded on 05/26/2011 for the course CGN 6505 taught by Professor Mang during the Spring '11 term at University of Florida.

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6505-11 - 1 Progress in Concrete Technology Challenge:...

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