Since the late 1980s fiber-reinforced polymer (FRP) sheets or wraps have been used to replace corrosion-vulnerable steel plates in repair applications. FRP sheets offer the advantages of light weight, high strength, low cost, constructability, and durability (non-corrosiveness). Despite the expensive cost relative to glass fibers (GF), carbon fibers (CF) and carbon FRP sheets/plates have been primarily used for repair and retrofit. This is mainly due to the fact that CF has a high elastic modulus and high ultimate strength (see Fig. 1). GF is popular, particularly for column jacketing (confining) retrofit, as it only costs about 5–10 % as much as CF. GF has much less ultimate stress and very low elastic modulus (only about a quarter of that of steel), but very large ductility (Fig. 1). It is noted that aramid fibers (AF) have both very large ductility and relatively high elastic modulus (Fig. 1); however, because AF is as costly as CF, little economic advantage may be gained from the use of AF.
Brittleness is a major drawback of all these fibers (CF, AF and GF), since they have no yielding point and associated nonlinear behavior (Fig. 1). To improve the ductility of the fibers, a number of composite material science investigations have been conducted on hybrid fibrous composites (e.g., Bunsell and Harris 1974; Phillips 1976; Aveston and Sillwood 1976; Marom et al. 1978; Chou and Kelly 1980; Manders and Bader 1981; Miwa and Horiba 1994; Pan and Postle 1996). Applications of hybrid FRP composites, such as hybrid FRP bars and sheets combined with concrete, have been studied by several researchers (e.g., Nanni et al. 1994; Harris et al. 1998; Grace et al. 2002). The primary purpose of these civil engineering applications was to achieve “pseudo-ductility” similar to the ductile response of nonlinear steel materials. Pseudo-ductility can be defined in this paper as when, after the first fiber failure (first drop in load), the load carrying capacity is recovered or improved as the remaining fibers elongate. Pseudo-ductility is desirable because clear sound warning is produced during the first fiber failure, which indicates distress and the possible impending failure of structures.
The secondary purpose of hybrid FRP composites in civil engineering applications might be to actively utilize the so-called “hybrid effects.” Marom et al. (1978) defined the hybrid effects as the deviation of the behavior of a hybrid composite from the rule of mixtures, while Manders and Bader (1981) simply defined it as the difference in behavior between a fiber in a hybrid composite and in a non-hybrid composite. Both positive and negative hybrid effects are possible; the effects are deemed positive when mechanical properties are above the prediction based on the rule of mixtures and vice versa for negative effects. It is extremely difficult to theoretically predict the hybrid effects and mechanical properties of hybrid fibrous composites, which are known to depend on the volumetric ratio of each fiber component, bonding property between the components, and elastic moduli of the fibers or their ratio (Pan and Postle 1996). This is mainly due to the unavoidable uncertainty of the bonding property. Furthermore, the size effect is involved. In civil engineering applications, hybrid FRP sheets or plates consisting of fiber rovings (strands) would be practical and feasible. A high-strength CF roving is typically made of about 12,000 filaments (12 K) or multiples of 12,000 filaments (e.g., 24 or 48 K), while an E-GF roving is made of 1,200, 2,200 tex or multiple of 2,200 tex, where 1 tex is 1,000 m/g. Figure 2 shows microscopic cross-sectional views of CF and GF rovings taken using a scanning electron microscopy of the University of Oklahoma. Thus, some findings from previous research on a micro-composite or a composite made of a fraction of different fibers embedded in the composite matrix (i.e., in the fiber roving or strand) may not be applicable to the hybrid FRP sheets that are focused on infrastructure repair or other civil engineering applications.
When the hybrid carbon-glass FRP sheet is subjected to tension, the CF with high elastic modulus and low ultimate strain ruptures first. The GF, with lower elastic modulus and higher ultimate strain, then takes over and resists the load. As noted, if the stress at GF rupture is equal to or higher than that at CF rupture, which depends on a volume ratio of (GF/CF) (e.g., Manders and Bader 1981), the pseudo-ductility can be obtained. Hybrid effects are additionally expected to be gained, such that it is possible to enhance (first) failure stress (or strain) beyond that predicted from the rule of mixtures, given Eq. (1) below:
$$ E_{HF} = E_{CF} \left( {\frac{{V_{CF} }}{{V_{HF} }}} \right) + E_{GF} \left( {\frac{{V_{GF} }}{{V_{HF} }}} \right) $$
(1)
where E
HF
is the weighted mean elastic modulus of a carbon-glass hybrid composite; E
CF
and E
GF
are the elastic moduli of CF and GF, respectively; V
CF
and V
GF
are the CF and GF volumes, respectively; and V
HF
is the combined CF and GF volume or the volume of the hybrid composites.
Manders and Bader (1981) reported that the increase in strain at CF rupture in sandwich laminated hybrids would be about 50 % of that of single CF, and Aveston and Sillwood (1976) also experimentally confirmed that the strain at CF rupture of hybrid carbon–glass–epoxy composites could be increased up to about 0.01. Furthermore, Miwa and Horiba (1994) suggested the empirical rule of “hybrid” mixtures as:
$$ f_{u\_C\_HF} = f_{u\_CF} \left( {\frac{{V_{CF} }}{{V_{HF} }}} \right) + f_{u\_GF} \left( {\frac{{V_{GF} }}{{V_{HF} }}} \right) $$
(2)
where f
u_C_HF
is the mean stress of a carbon-glass hybrid composite at CF rupture and f
u_CF
and f
u_GF
are the ultimate stresses of CF and GF ruptures, respectively.
However, researchers (e.g., Manders and Bader 1981; Miwa and Horiba 1994) did not reach any definite conclusion on the ultimate stress of hybrid carbon-glass composites at GF rupture. Pan and Postle (1996) reported that due to the cross-coupling effects between the different fibers, a positive hybrid effect would be expected at the first fiber rupture, whereas a negative effect would be expected at the second fiber rupture; however, this appears to be the case only for the first fiber embedded in the matrix or a postulate without examination of an optimal ratio of two different fibers. It is not appropriate to apply the shear lag model (Cox 1952) to the case of interest, since the hybrid sheet may have a substantially different degree of interfacial shear stress as in the case of a short-fiber embedded in the matrix. The increased or decreased strain (or stress) at GF rupture of the hybrid composites, particularly hybrid FRP sheets that are common in civil engineering applications, have not been well studied. A continuous FRP sheet consisting of fiber rovings may have a moderate level of frictional coupling between GF and CF rovings and behave very differently than the micro-composites with a high level of frictional coupling.
This study consists of: (1) material test programs for identification of hybrid effects in the carbon-glass FRP sheets; (2) development of design models for stress–strain relationships with and without consideration of the hybrid effects; and (3) structural member-level verification of the hybrid effects.