Reinforcing bars

Fibre-reinforced polymers (FRP) rebars, usually made of an epoxy matrix reinforced with carbon or aramide fibres, have also been proposed both as prestressing wires and reinforcement. Nevertheless, they are not discussed here, because these applications are still in the experimental phase and there is a lack of experience on their durability. In fact, while they are not affected by electrochemical corrosion typical of metals, they are not immune to other types of degradation. FRP are also used in the form of laminates or sheets as externally bonded reinforcement in the rehabilitation of damaged structures this application will be addressed in Chapter 19. 

Before discussing corrosion-resistant reinforcement, mention will be made of the requirements for reinforcing bars and the characteristics of normal carbon-steel reinforcement 

Reinforcing bars must fulfil several requirements regarding strength, ductility, bond to concrete, weldability, etc. To achieve suitable anchorage when embedded in concrete, ribbed bars are normally used. Mechanical properties that are relevant for reinforcing bars are elastic modulus, strength and ductility. For instance, European standards that deal with reinforcement for concrete structures generally pre- 

Luca Bertolini, Bernhard Elsener, Pietro Pedeferri, Rob P. Polder Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30800-8 

Steel Alphanumeric B500A B500B B500A B500B B500A B500B 

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At core B2 a transverse crack going through from the surface down to the duct was found, as well as some weak Fig. 6 Drill core B2 reinforcement bars. Because both the crack and the bars could... 

A number of individual radar scans have been joined together Variations in cover depth give rise to variations in arrival time for the wave reflected from the reinforcing bars... 

The ribs on the reinforcing are distinguishable and can be measured, along with the diameter of the reinforcing with an estimated accuracy of about 10%. The upper part of the film is slightly overexposed causing less clear definition of the reinforcing bars. 

Radiography provides the only means of reliably detecting voids in pre-stressed cable ducts or of detecting loss of section or fracture of eables inside the duets. The maximum thiekness of eonerete whieh ean be radiographed for confident loeation of voids inside ducts is of course dependant on a number of variables, e g. amount of reinforcing bars, size of void in duet etc... 

Constmction and dem olition (C D) debris is a potentiahy large source of recyclables. However, as of 1995, generation rates and ferrous scrap content were not weh estabUshed and estimates were highly variable. Eerrous materials in C D debris are typicahy reinforcing bars, wire mesh, and stmctural steel. Some of the scrap is sold for recycling once concrete is effectively removed and the scrap is sized to specification (17). 

Corrosion of metal reinforcement bars has caused concrete to fall off the corners of cooling towers. 

Corrosion usually results in a leak or failure of a support because a vessel or support gets too thin. It is then not strong enough to withstand the pressure or load. However, rust can cause failure in another way. It occupies about seven times the volume of the steel from which it was formed. V/hen rust occurs between two plates that have been bolted or riveted together, a high pressure develops. This can force the plates apart or even break the bolts or rivets (see Section 9.1.2 g). Corrosion of the reinforcement bars in concrete can cause the concrete to crack and break away. 

Normally concrete is reinforced with plain carbon steel, but under conditions where rapid carbonation can occur or there is a risk of chloride contamination, corrosion-protected or more corrosion-resistant reinforcing steels may be necessary. Currently there are three reinforcing bars which have enhanced corrosion resistance ... 

A conductive polymer electrode has been designed specifically for the cathodic protection of steel reinforcing bars in concrete and is marketed under the trade name Ferex . The anode consists of a 16 AWG stranded copper conductor surrounded by a carbon-loaded polymeric coating similar to that used on the Anodeflex system ) to provide a nominal anode diameter of 8 mm The manufacturer claims that at the maximum recommended current density of 0 08 Am the anode life in concrete will be 32 years with a proportionately longer life at lower current densities. 

Conductive paints (resins) have recently been used for the cathodic protection of steel reinforcing bars in concrete, but they are always used in conjunction with a primary anode material, e.g. platinised-niobium or platinised-titanium wire or a conductive polymer rod. 

Hayfield, P. C. S. and Warne, M. A., Titanium Based Mesh Anodes in the Cathodic Protection of Concrete Reinforcing Bars , presented at UK Corrosion, Brighton (1988)... 

Rebar an abbreviation of reinforcing bar, referring to reinforced concrete. 

Finally, it is necessary to point out that although a particular method of corrosion control may be quite effective for the structure under consideration it can introduce unforeseen corrosion hazards elsewhere. Perhaps the best example is provided by cathodic protection in which stray currents (interaction) result in the corrosion of an adjacent unprotected structure or of steel-reinforcement bars embedded in concrete a further hazard is when the cathodically protected steel is fastened with high-strength steel bolts, since cathodic protection of the tatter could result in hydrogen absorption and hydrogen cracking. 

It is important to note material such as those plastics or wood that are weak in either tension or compression will also be basically weak in shear. For example, concrete is weak in shear because of its lack of strength in tension. Reinforced bars in the concrete are incorporated to prevent diagonal tension cracking and strengthen concrete beams. Similar action occurs with RPs using fiber filament structures. 

Specifically, the improved solidification (cementation) technology involves the use of (a) a special dry powder admixture for the generation of a nonsoluble crystalline formation deep within the pores and capillary tracts of the concrete—a crystalline structure that permanently seals the concrete against the penetration or movement of water and other hazardous liquids from any direction (b) special nonmetal reinforced bars for enhancing the concrete block s tensile and compressive strengths and (c) a unique chemical crystallization treatment for the waterproofing and protection of the concrete block s surface. 

Direct Shear. For type I cross-sections (0 < 2°) the concrete between the flexural reinforcement Is capable of resisting direct shear. However, because cracking at the support yield line reduces the shear capacity, diagonal bars must be provided to at least resist the shear capacity of the concrete, v. For type II and III cross-sections (0 > 2 ), with little or no concrete shear resistance, The diagonal reinforcing bars must be designed to resist the entire shear load at the support. 

TABLE 5.A.2 Dynamic Increase Factors (DIF) for Reinforcing Bars, Concrete, and Masonry... 

Typical uses include the production of non-dispersible underwater concrete and reduction of the accumulation of bleed water in mass concrete placed in deep forms. Consequently, AWAs are useful in mass concrete work because they prevent the formation of laitance on the surface of the concrete and thereby reduce the excessive cleaning between successive lifts. The admixtures also reduce the voids formed under horizontal reinforcing bars. Therefore, bond to steel increases and potential corrosion problems are reduced. The admixtures are also used in conjunction with WRAs in oil-well cementing grouts to reduce pipeline friction and rapid water loss and grouting of pre- and post-tensioned concrete ducts [47]. New valves and control devices under development in Europe and Japan used in conjunction with AWA will likely advance the field on underwater concrete. 

Virmani, Y.P., Clear, K.C. and Pasko, T.J. (1983). Time to Corrosion of Reinforced Steel in Concrete Slabs, Vol. 5, Calcium Nitrite Admixtures or Epoxy Coated Reinforced Bars as Corrosion Protective Systems, FHWA-RD-83-012, FHWA, US Department of Transportation, 71. 

Large sizes Concrete tunnel with coated reinforcement bar. 

Reinforced Concrete. Because of the low tensile strength of concrete, steel-reinforcing bars are usually incorporated in structures when con-... 

Crete is stressed in tension. Although the tensile strength of concrete-polymer is higher by a factor of 4 than that of concrete, it is still relatively low compared with that of steel. Standard tests on reinforcing bars in which a 1-inch diameter steel rod is pulled out of a 6-inch diameter concrete cylinder indicated that the force needed to pull the bar from a concrete-polymer cylinder was three times the force required for a cylinder of ordinary concrete. This greatly improved bond strength between concrete-polymer and steel is important in concrete structural design. 

AIRFORMED CERASHELL is based on the concept of air-supported forming, used successfully for the past 25 years with steel reinforced concrete structures. An airform, or balloon, is designed to inflate to a specific size and shape. Polyurethane foam is sprayed onto the inside surface of the balloon, with concrete as the interior coating. Steel reinforcing bars are pinned to the foam, prior to spraying the con-... 

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