All-composite superstructures for accelerated bridge construction

Recently, a main concern in bridge engineering had been the rehabilitation and reconstruction of deteriorated bridge members. Accelerated bridge
construction (ABC) can overcome the shortcomings of traditional construction methods, which are typically labor-intensive and time-consuming.
ABC can minimize disruption to public communities and save indirect costs
associated with detouring traff c. Concrete members prefabricated in factory are often utilized in ABC, where the connections of prefabricated concrete members have been extensively studied in recent years

As alternatives to
prefabricated concrete members, bridge superstructures entirely made of
advanced composites or f ber-reinforced polymer (FRP) are also potential
solutions to ABC, due to the following advantages of the materials:


Light weight – FRP superstructures weigh about 25% or less of a conventional reinforced concrete superstructure. Therefore, FRP composites reduce the dead load of a superstructure signif cantly. Reduction of dead
load can increase the rating factor when capacity rating of a bridge is
performed. Since capacity rating of a bridge is proportional to the rating
factor, the load carrying capacity of a superstructure reconstructed or
rehabilitated with FRP composites may be re-rated to its original design
load.



Durability – Corrosion of steel reinforcement in concrete is the leading
cause of deterioration of conventional concrete superstructures. Steel
reinforcement is susceptible to oxidation when exposed to deicing salt
and other chlorides. Compared to conventional materials, FRP composites provide high resistance to corrosion. Therefore, less maintenance
may be required for FRP superstructures.


Rapid Installation – Factory fabrication and modular construction
of FRP superstructures allows rapid installation on construction sites.
Installation of FRP superstructures in bridges with high traffc volume
can substantially reduce the construction time of superstructures to half
when compared to the labor-intensive and time-consuming construction process of a conventional cast-in-place concrete superstructure. As
a result, disruption to communities, and indirect costs associated with
detouring traff c, can be minimized by FRP superstructures.


High Specifc Stiffness/Strength – Advanced composites offer the advantages of high stiffness-to-weight ratios (specif c stiffness) and strengthto-weight ratios (specif c strength) when compared to conventional construction materials. Therefore, they can provide good combinations of
strength/stiffness and weight for bridge superstructures.



Lower Life-cycle Cost – A life-cycle cost usually consists of an initial
construction cost and maintenance cost. Since FRP superstructures are
expected to have good long-term durability and require little maintenance, they can be more cost-effective in service life than bridge superstructures with reinforced concrete

Structural analysis and design

When designing all-composite bridge superstructures, designers can tailor
FRP materials and structural conf gurations of superstructures to meet
design requirements. As an example, the design of a two-lane FRP superstructure is discussed in this section. The FRP superstructure is now in
service in South Korea, and has a sandwich construction (Ji
et al., 2010).
Corrugated cores are sandwiched between top facing and bottom facing.
The sandwich construction in this study can reduce weight and provide
signifcant stiffness in the selected direction. The discussion in this section includes design requirements, material selection, preliminary design
based on design requirements, and detailed design of the cross-section.

Design requirements

The design of FRP superstructures is stiffness-oriented (Federal Highway
Administration, 2011). Although no offcial specifcations are available for
the defl ection limit of FRP superstructures, general design guidelines for
conventional bridge superstructures are provided by the AASHTO LRFD
Bridge Design Specif cations (AASHTO, 2010). The AASHTO’s requirement for the stiffness of bridge superstructures with conventional materials is that the maximum defl ection due to vehicular live loads should not
exceed 1/800 of the bridge span length.
The criterion above was used to design the FRP superstructure in this
chapter. The FRP superstructure in this chapter was expected to be in service in South Korea. As a result, the design vehicular live load in this study

6.1 DB-24 truck load and tire contact areas.

was consistent with Ministry of Construction and Transportation (MOCT)
(2000a). As for the strength requirements, the maximum strain under service
load should be limited to 20% of the ultimate strain. Laminate failure prediction was based on the f rst-ply failure criterion.
For a simply-supported bridge with span less than 44 m, the standard
design truck load DB-24 produces higher moment and defl ection than the
design lane load (MOCT, 2000a). A DB-24 truck load is approximately
1.3 times heavier than the HS-20 truck load specif ed in AASHTO LRFD
Bridge Design Specif cations (2010). One standard design truck load DB-24
with tire contact areas, as shown in Fig. 6.1 (MOCT, 2000b), was assumed to
be the design live load. The weight of a DB-24 truck is 423 kN.

Materials and lamina design

Superstructures with FRP composites including the one in this study can
be characterized at three levels: the lamina level (a single ply), the laminate
level (multiple plies), and the structural level (Altenbach
et al., 2004). This
section focuses on the materials and lamina design. The design at the latter
two levels will be addressed in the following sections.
The primary constituents of FRP composites are f bers and resin matrices.
In FRP bridge superstructures, glass f bers are mainly used as fber reinforcement, and polyester or vinyl ester is the most widely used resin matrix.
Their combinations can be the most cost-effective in current practice. In this
study, E-glass f bers and vinyl ester resin were chosen as the main constituents for the all-FRP superstructure for ABC. Their material properties are
given in Table 6.1 (Ji
et al., 2010).

Properties of constituent materials

Table 6.1 Properties of constituent materials
Material
E (GPa) G (GPa) υ ρ (kN/m3)
E-glass fiber 72.4 27.6 0.22 25.4
Vinyl ester resin 3.91 1.38 0.37 12.4
E is the Young’s modulus; G is the shear modulus; υ is the Poisson’s ratios; ρ is
the density.

To design a superstructure with FRP composites, it is necessary to know
the mechanical properties of a single ply or a lamina made of fber reinforcement and resin matrix. The properties of a lamina with unidirectional
fbers can be predicted from properties of f bers and resin matrix according to micro-mechanics models based on several simple rules of mixtures.
Most calculations about the properties of a unidirectional lamina depend on
the volume fraction of f bers and resin matrix. The f ber and matrix volume
fractions,
Vf and Vm, respectively, for a unidirectional lamina based on their
weight fractions and densities are calculated in Equation 

where w
f and wm are the weight fractions of f bers and resin matrix, ρf and ρm
are the densities of f bers and resin matrix.
The mechanical properties of a unidirectional lamina with known fber
and matrix properties and their volume fractions can be calculated by simple rules of mixtures as shown in Equations

where η0 is the eff ciency factor of f bers – for unidirectional fbers, η0 is
equal to 1.0;
Ef and Em are the Young’s moduli of f bers and resin matrix,
respectively;
Gf and Gm are the shear moduli of f bers and resin
matrix, respectively, and;
υf and υm are the Poisson’s ratios of f bers and resin
matrix, respectively.

 

 

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