外文翻译
Materials and Structures ? RILEM 2010 10.1617/s11527-010-9700-y Original Article
Impact of crack width on bond: confined and unconfined rebar
David W. Law1 , Denglei Tang2, Thomas K. C. Molyneaux3 and Rebecca Gravina3
(1) School of the Built Environment, Heriot Watt University, Edinburgh, EH14 4AS, UK (2) VicRoads, Melbourne, VIC, Australia
(3) School of Civil, Environmental and Chemical Engineering, RMIT University,
Melbourne, VIC, 3000, Australia David W. Law
Email: D.W.Law@hw.ac.uk
Received: 14 January 2010 Accepted: 14 December 2010 Published online: 23 December 2010
Abstract
This paper reports the results of a research project comparing the effect of surface crack width and degree of corrosion on the bond strength of confined and unconfined deformed 12 and 16 mm mild steel reinforcing bars. The corrosion was induced by chloride
contamination of the concrete and an applied DC current. The principal parameters investigated were confinement of the
reinforcement, the cover depth, bar diameter, degree of corrosion and the surface crack width. The results indicated that potential relationship between the crack width and the bond strength. The results also showed an increase in bond strength at the point where
1
外文翻译
initial surface cracking was observed for bars with confining
stirrups. No such increase was observed with unconfined specimens. Keywords Bond - Corrosion - Rebar - Cover - Crack width - Concrete
1 Introduction
The corrosion of steel reinforcement is a major cause of the deterioration of reinforced concrete structures throughout the world. In uncorroded structures the bond between the steel reinforcement and the concrete ensures that reinforced concrete acts in a composite manner. However, when corrosion of the steel occurs this composite performance is adversely affected. This is due to the formation of corrosion products on the steel surface, which affect the bond between the steel and the concrete.
The deterioration of reinforced concrete is characterized by a general or localized loss of section on the reinforcing bars and the formation of expansive corrosion products. This deterioration can affect structures in a number of ways; the production of expansive products creates tensile stresses within the concrete, which can result in cracking and spalling of the concrete cover. This cracking can lead to accelerated ingress of the aggressive agents causing further corrosion. It can also result in a loss of strength and stiffness of the concrete cover. The corrosion products can also affect the bond strength between the concrete and the reinforcing steel. Finally the corrosion reduces the cross section of the reinforcing steel, which can affect the ductility of the steel and the load bearing capacity, which can ultimately impact upon the serviceability of the structure and the structural capacity [12, 25]. Previous research has investigated the impact of corrosion on bond [2–5, 7, 12, 20, 23–25, 27, 29], with a number of models being proposed [4, 6, 9, 10, 18, 19, 24, 29]. The majority of this research has focused on the relationship between the level of corrosion (mass
2
外文翻译
loss of steel) or the current density degree (corrosion current applied in accelerated testing) and crack width, or on the
relationship between bond strength and level of corrosion. Other research has investigated the mechanical behaviour of corroded steel [1, 11] and the friction characteristics [13]. However, little research has focused on the relationship between crack width and bond [23, 26, 28], a parameter that can be measured with relative ease on actual structures.
The corrosion of the reinforcing steel results in the formation of iron oxides which occupy a larger volume than that of the parent metal. This expansion creates tensile stresses within the surrounding concrete, eventually leading to cracking of the cover concrete. Once cracking occurs there is a loss of confining force from the concrete. This suggests that the loss of bond capacity could be related to the longitudinal crack width [12]. However, the use of confinement within the concrete can counteract this loss of bond capacity to a certain degree. Research to date has primarily involved specimens with confinement. This paper reports a study comparing the loss of bond of specimens with and without confinement.
2 Experimental investigation
2.1 Specimens
Beam end specimens [28] were selected for this study. This type of eccentric pullout or ‘beam end’ type specimen uses a bonded length representative of the anchorage zone of a typical simply supported beam. Specimens of rectangular cross section were cast with a longitudinal reinforcing bar in each corner, Fig. 1. An 80 mm plastic tube was provided at the bar underneath the transverse reaction to ensure that the bond strength was not enhanced due to a (transverse) compressive force acting on the bar over this length.
3
外文翻译
Fig. 1 Beam end specimen
Deformed rebar of 12 and 16 mm diameter with cover of three times bar diameter were investigated. Duplicate sets of confined and unconfined specimens were tested. The confined specimens had three sets of 6 mm stainless steel stirrups equally spaced from the plastic tube, at 75 mm centres.
This represents four groups of specimens with a combination of different bar diameter and with/without confinement. The specimens were selected in order to investigate the influence of bar size, confinement and crack width on bond strength.
4
外文翻译
2.2 Materials
The mix design is shown, Table 1. The cement was Type I Portland cement, the aggregate was basalt with specific gravity 2.99. The coarse and fine aggregate were prepared in accordance with AS 1141-2000. Mixing was undertaken in accordance with AS 1012.2-1994. Specimens were cured for 28 days under wet hessian before testing.
Table 1 Concrete mix design MateriCement w/c Sand al 10 mm washed aggregate 7 mm washed Salt aggregate Slump Quanti381 kg/0.4517 kg/463 kg/463 kg/18.84 kg140 ± 25 mty m3 9 m3 m3 m3 /m3 m In order to compare bond strength for the different concrete compressive strengths, Eq. 1 is used to normalize bond strength for non-corroded specimens as has been used by other researcher [8].
where is the bond strength for grade 40 concrete, τ exptl is the experimental bond strength and f c is the experimental compressive strength.
(1)
The tensile strength of the Φ12 and Φ16 mm steel bars was nominally 500 MPa, which equates to a failure load of 56.5 and 100.5 kN, respectively.
2.3 Experiment methodology
Accelerated corrosion has been used by a number of authors to replicate the corrosion of the reinforcing steel happening in the natural environment [2, 3, 5, 6, 10, 18, 20, 24, 27, 28, 30]. These have involved experiments using impressed currents or artificial weathering with wet/dry cycles and elevated temperatures to reduce the time until corrosion, while maintaining deterioration mechanisms
5