STATE-OF-THE-ART: New Reinforced Concrete (RC) Structures

TU1207 (2014) STATE-OF-THE-ART: New Reinforced Concrete (RC) Structures, Working Group 2, COST Action TU1207 - Next Generation Design Guidelines for Composites in Construction

Disclaimer: this is a draft version. Both the table of contents and the content of the documents included below are subject to change. New sections will be added when additional documents are made available.
Any comments or suggestions, please contact the chair or co-chair of Working Group 2

1. Serviceability Limit States

1.1 Serviceability limit states

A. Bilotta and E. Nigro

The term Fiber Reinforced Polymers (FRP) indicates a wide range of composite materials, consisting of a polymeric organic matrix, which is impregnated with continuous fiber reinforcement. The technology can be used to produce bars with high mechanical properties. In the fields of civil engineering, the use of FRP bars to reinforce the concrete was demonstrated particularly advantageous for the construction of civil or industrial buildings as well as bridge decks (Rizkalla and Nanni, 2003) mainly because FRP can ensure adequate durability of the structures. Moreover, a further specific property of fiber-reinforced composites, such as magnetic transparency, can be particularly useful when magnetic disturbance is required to be avoided: for instance in the construction of hospital rooms where advanced diagnostic equipment for Magnetic Resonance Imaging (MRI) is used (fib bulletin no. 40, 2007). Other possible applications, which appear as very promising and attractive, are related to provisional structures and tunnel coatings (M. Schürch and P. Jost, 2006). In particular, for tunneling projects the main purpose of a ground containment wall in concrete (namely soft eye walls) is allowing both the entrance and removal of the Tunnel Boring Machine (TBM). By using concrete reinforced with GFRP bars in the soft-eye region, the cutting operations are simplified due to the low transversal resistance of GFRP materials compared to steel.

2. Short and Long Term Deflections

2.1 Tension stiffening effect in FRP RC tensile members

M. Baena , L. Torres , A. Turon

Suitable modelling of reinforced concrete (RC) cracking and, particularly, post-cracking behaviour, is the most important and difficult task of the deformation analysis. Due to the interaction between reinforcement and concrete, the intact tensioned concrete between adjacent cracks is able to sustain certain level of tensile stresses and contributes to the stiffness of the RC element. This tension stiffening (TS) effect is of high importance in deformation analysis (crack formation, crack spacing and crack widths) and should be accounted for in design practice, especially at service loads.

2.2 Long-term deflections

C.Miàs, L. Torres, M. Guadagnini

The long-term increase in deflection is a function of member geometry, load characteristics (magnitude and duration of sustained load, and age of concrete at the time of loading), and material properties (elastic modulus of concrete and FRP reinforcement, creep and shrinkage of concrete) (ACI 440.1R-06). Therefore, properties of the FRP bars have a significant influence on the long-term deflections of FRP reinforced concrete (RC) beams. Due to lower stiffness of FRP bars compared to steel, under the same conditions (concrete class, dimensions and area of reinforcement), the neutral axis depth of FRP RC cracked sections are lower than those of steel RC. Consequently, the sectional curvature and the tensioned area are larger, while the compressed area is smaller, and therefore, larger deformations are expected. However, in terms of time-dependent behaviour, the relative curvature increment associated with creep and shrinkage is lower than for conventional steel RC due to the smaller compressed area of concrete, and therefore lower time-dependent deflections are expected in FRP RC.

2.3 Removing the shrinkage effect from short-term moment-curvature and tension-stiffening relations

V. Gribniak, G. Kaklauskas

Shrinkage is important, although most frequently neglected, effect related to the constitutive modelling of reinforced concrete (RC). In the general practice, shrinkage along with creep is taken into account in pre-stress loss and/or long-term deformation analysis. However, even at the first loading, free shrinkage strain of concrete may be of magnitude that exceeds the cracking strain. Due to restraining action of a stiff reinforcement, shrinkage induces tension stresses in the concrete that might significantly reduce the crack resistance and, consequently, increase deformations of RC members. Kaklauskas et al. (2009), Kaklauskas & Gribniak (2011) and Gribniak et al. (2013a, 2013b) indicated that the short-term test data of shrunk RC elements might be essentially distorted ignoring the shrinkage effect.

3. Fire and Elevated Temperatures

3.1 Experimental results on FRP RC members exposed to fire

A. Bilotta and E. Nigro

Fiber-reinforced polymer (FRP) materials have several important characteristics, such as high strength-to- weight ratios and resistance to corrosion, which are advantageous in the construction field. Recent progress in research and technology of FRPs have led to reduced material costs and increased confidence in the use of polymers for a variety of civil engineering applications, as testified by many examples worldwide. Recent studies carried out at by Keller et al. (2005,2006) and Correia et al. (2010) on the fire response of GFRP pultruded profiles showed that FRP profiles can be used also in fire situation. On the other hand, several building codes (CAN/CSA 806-02, 2002; ACI 440.1R-04, 2003; CNR-DT203, 2006) are now available for the design of concrete structures reinforced with fiber reinforced polymer (FRP) bars in place of traditional steel reinforcement, even if few provisions and no calculation model have been suggested that take account of fire conditions.

3.2 Theoretical models for FRP RC members exposed to fire

A. Bilotta and E. Nigro

The vulnerability of organic polymers to high temperatures is probably the biggest drawback for the fiber- reinforced polymer (FRP) bars. Although many examples of FRP reinforced concrete (RC) structures are available worldwide, they are often structures for which fire is not a significant design condition. Though fire is an event that cannot be ignored for many civil structures, as well as parking lots and industrial structures, many international codes generally provide suggestions for design in normal temperature conditions of concrete structures reinforced with (FRP) bars in place of traditional steel reinforcement (CNR-DT 203/2006, 2006; CSA S806-02, 2002; ACI 440.1R-06, 2006; fib Task Group 9.3, 2007) However, few provisions and no calculation models are suggested that take account of fire conditions.