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Composites: Part B 38 (2007) 429–436

Strengthening of a reinforced concrete bridge with externallybonded steel reinforced polymer (SRP)

Alexis Lopez *, Nestore Galati, Tarek Alkhrdaji, Antonio Nanni

University of Missouri-Rolla, Center of Infrastructure Engineering, 223 ERI Rolla, MO 65409, United States

Received 12 March 2006; accepted 14 September 2006Available online 17 November 2006

Abstract

This paper introduces steel reinforced polymer (SRP) as a way to strengthen civil structures. This technique consists of cords formedby interwoven high-strength steel wires embedded within a polymeric resin. A comprehensive study addressing analysis, design, instal-lation, load rating and monitoring of a bridge strengthened with this technology is reported. The SRP system was easily installed, dem-onstrating its similarity to more traditional fiber reinforced polymer (FRP) strengthening techniques. Load tests were performed toevaluate the bridge structural behavior prior and after the strengthening in accordance with AASHTO specifications, showing thatthe rehabilitation performed as predicted and therefore created the possibility to remove the bridge load posting.� 2006 Elsevier Ltd. All rights reserved.

Keywords: D. Non-destructive testing; Steel reinforced polymer

1. Introduction

The availability of many design guides [1–4] and the num-ber of projects demonstrate that the use of fiber reinforcedpolymer (FRP) materials to strengthen civil structures isbecoming a mainstream technique [5]. Therefore, nowadays,the term ‘‘emerging material’’ applied to FRP compositesassumes the meaning ‘‘material for which field validation isnecessary’’ instead of ‘‘material of recent application’’.

The majority of FRP applications for civil structures arerelated to carbon FRP (CFRP) composites [6–8]. CFRPhas been proven to be a strong, non-corrosive, of-rapid-execution, and long-term durability composite system [9].A drawback of CFRP composites is related to their cost,which constitutes the motivation to develop other equallystronger and potentially lower-cost composite systems[10–12]. Steel reinforced polymer (SRP) systems weredeveloped using interwoven high-strength steel cordsassembled in a tape form as an alternative to the carbonfiber fabrics (Fig. 1) [13]. These tapes could also be impreg-

1359-8368/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compositesb.2006.09.003

* Corresponding author. Tel.: +1 573 341 4560.E-mail address: aalkv5@umr.edu (A. Lopez).

nated by cementicious grout in place of polymeric resin, toallow for other advantages such as fire resistance.

The SRP laminate consists of the steel cord tape andepoxy resin. Studies in the University of Missouri-Rollawere undertaken to characterize the material properties[10,11]. Mechanical properties of the laminates to be usedfor design (i.e., guaranteed values related to the net steelarea), are shown in Table 1. Unlike steel materials, thestrain–stress curve of SRP is linear elastic up to the failure.SRP tapes are made of high carbon steel cords with an adhe-sion optimized coating. Two steel tapes different in densitiesand morphology were used for this project. A first high-den-sity cord tape, used for flexural reinforcement, containing 23cords per in. (0.9 cords per mm), each cords formed by threestraight wire filaments wrapped by another two filaments ata high twist angle, for a strength per cord of 0.346 kip(1.54 kN); and a medium-density cord tape, U-wrappedon the girders for shear reinforcement, containing 12 cordsper in. (0.47 cords per mm), each cords formed by threestraight wire filaments wrapped by another filament at lowertwist angle, for a strength per cord of 0.302 kip (1.34 kN).

This paper presents the first field application of SRPmaterials to strengthen an under-reinforced concrete bridge

Fig. 2. Lateral view of bridge P0962.Fig. 1. Fabric made of steel cords.

430 A. Lopez et al. / Composites: Part B 38 (2007) 429–436

located in Missouri. This structure was the test bed forthree strengthening techniques, namely: SRP laminatesmanual lay-up, carbon FRP laminates manual lay-up andnear-surface mounted (NSM) CFRP bars. Analysis,design, constructability, load testing and load rating ofthe portion of the bridge strengthened with SRP are pre-sented here.

Fig. 3. Cross-section of the bridge.

2. Bridge description

Fig. 2 shows a lateral view of bridge P-0962 in DallasCounty, Missouri. The bridge was built in 1956 and carriesan average daily traffic of 350 vehicles. It was load postedto 18 tons (16.3 tons in SI units) with a speed restriction to15 mph (24 km/h). The superstructure consists of threesimply supported 42.5 ft (12.95 m) long spans having askew angle of 15�. Central supports consist of reinforcedconcrete (RC) bents, each supported by two RC rectangu-lar piers. The bridge is 23.75 ft (7.24 m) wide and carriesone lane of traffic in each direction. The superstructurecross-section consists of three cast-in-place RC girders,equally spaced, supporting a 6 in. (152 mm) deep RC deck(see Fig. 3).

For two of the three spans of the bridge, strengtheningconsisted of CFRP manual lay-up laminates used as flex-ural reinforcement for deck, girders and bents. Addition-ally, CFRP bars installed as near-surface mountedreinforcement on the central girder. For the third span,deck and girders were strengthened with SRP. A detaileddescription of the strengthening layout can be found else-where [14].

Fig. 4. Truck loads HS20-44 and 3S2.

3. Bridge analysis and SRP Reinforcement design

The bridge analysis was carried out according toAASHTO Specifications [15]. The HS20-44 (MS18) and

Table 1Properties of SRP constituent materials

Materiala Ultimate tensile strength,f �fu ksi (MPa)

Ultimate sin./in. (mm

Resin 4.4 [30] 0.015SRP-flexural 460 [3170] 0.016SRP-shear 345 [2390] 0.017

a Values reported by the manufacturer.

3S2 [16] trucks were used as load for bridge analysis, withthe former being the controlling design truck. Fig. 4 showsschematic drawing of the footprint for the two mentionedtrucks. The HS20-44 (MS18) truck has a front axle loadof 8.0 kip (35.6 kN); a second axle load of 32.0 kip(142 kN), located 14.0 ft (4.3 m) behind the drive axle with,and a rear axle load of 32.0 kip (142 kN), positioned at var-iable distances, ranging between 14.0 and 30.0 ft (4.3–9 m).As prescribed by AASTHO, the ‘‘lane load condition’’ wasalso considered, but did not control the design. For designpurposes, the maximum internal forces (i.e., bendingmoment and shear) induced by a train of trucks movingfrom one side to the other of the bridge were considered.

train e�fu/mm)

Tensile modulus,Ef ksi (GPa)

Nominal thicknessSRP tape in (mm)

551 [3.8] –28,750 [197] 0.0173 [0.44]20,300 [139] 0.0105 [0.26]

A. Lopez et al. / Composites: Part B 38 (2007) 429–436 431

Bridge material properties and geometries were based ondrawings obtained by the Missouri Department of Trans-portation (MoDOT) and verified in situ, based on visualand non-destructive testing (NDT) evaluation. Concretecores were drilled from the deck and then tested, showingas average compressive strength, f 0c , of 6845 psi (47 MPa).Steel yield strength, fy, was assumed to be 40 ksi(276 MPa), as prescribed by AASHTO [15] for bridges ofthat age since no sample could be obtained on site.

A design approach similar to the one used for of FRP lam-inates installed by manual lay-up was followed [1]. The flex-ural capacity (see Fig. 5) is estimated based on forceequilibrium, strain compatibility and constitutive laws ofmaterials up to failure. Whitney’s rectangular stress blockwas used to simulate the behavior of the compressed con-crete. The maximum deformation of concrete in compres-sion was taken as 0.003. No slip between SRP andsubstrate was assumed. Initial strains in the concrete at thetime of SPR installation,eib, was determined on the assump-tion of a cracked section subjected to the dead load.Such strain was subtracted from the one in the SRPstrengthening.

In order to determine the SRP design strength, ffu, thevalue of the guaranteed ultimate tensile strength, f �fu, wasreduced for the SRP to account for environmental consid-eration, using the same knock down factor, CE, suggestedfor carbon fiber. Such assumption seemed to be reasonablesince the steel cords used for this application have a brasscoating inhibiting corrosion. Being linear elastic material,SRP design strain, efu, is proportional to ffu by the tensilemodulus.

For flexural design, the strain level developed at the ulti-mate-limit state in the SRP is also reduced by a bond-dependent coefficient km, to account for the possible delam-ination from the concrete substrate. The effective strain inthe SRP reinforcement, efe, is then restricted by the follow-ing equation:

efe 6 kmefu ¼ kmCEe�fu ð1Þ

Fig. 5. Internal strain and stress distribution for a rect

where efu is the design rupture strain of SRP, which is equalto the guaranteed ultimate tensile strain, e�fu, reduced by theknock down factor CE accounting for environmental con-ditions [1]. The factor km, is function of the stiffness ofthe SRP laminate (Eq. 9-2 in Ref. [1]) and it guarantees avalue of maximum strain, efe, at which the bonding forcescan be sustained by the concrete substrate.

For shear design, the effective strain in the SRP U-wraps, efe (i.e., reduced strain level developed at the ulti-mate-limit state to account for delamination from theconcrete) is restricted by a bond-reduction coefficient kv,as is shown by the following equation:

efe ¼ kvefu 6 0:004 ð2Þwhere efu is the design rupture strain of SRP U-Wraps. Thefactor kv, is function of the concrete strength, type of wrap-ping scheme and the stiffness of the SRP U-wrap laminate(Eq. 10-7 in Ref. [1]) and it guarantees a value of maximumstrain, efe, at which the bonding forces can be sustained bythe concrete substrate. Table 2 shows design values ob-tained for the most demanding sections of the internal gird-ers in flexure and shear.

The nominal flexural and shear capacities were com-puted accounting for the contribution of the SRP as shownin the following equations:

Mn ¼ Asfs d � b1c2

� �þWAfffe h� b1c

2

� �ð3Þ

V n ¼ V c þ V s þWV f ¼ 2ffiffiffiffif 0c

pbd þ Avfyd

sþW

Afvffed f

sf

� �

ð4Þ

where As and Af represent the steel reinforcing bars andSRP cross-sectional areas, respectively, and fs and ffe theirrespective stress levels; d and h represents the reinforcementdepth and cross-section height; b1c is the depth of theequivalent rectangular block (see Fig. 5). w is an additionalreduction factor to account for the novelty of the technol-ogy, assuming a value of 0.85 [1].

angular section under flexural at ultimate stage [1].

Table 2Design values for flexure and shear SRP reinforcement

Material Design values

Ultimate Service

Km or kv SRP effectivestrain efe in./in.(mm/mm)a

SRP effectivestress ffe

a

ksi (MPa)

Steel stressfs ksi(MPa)

Neutral axisdepth b1c

in (mm)

Tension steelstress fs

ksi (GPa)

SRP stressff ksi (GPa)

SRP flexural 0.900 0.012 350 [2412] 39.8 [274] 1.758 [45] 29.4 [200] 37 [255]SRP-shear 0.148 0.002 40.6 [277] N/A N/A N/A N/A

a Most demanding sections: using three plies in flexural (mid span) and three plies in shear (support).

432 A. Lopez et al. / Composites: Part B 38 (2007) 429–436

The terms Vc, Vs, and Vf in Eq. (3) represent the contri-bution to shear capacity of concrete, internal steel stirrupsand SRP U-wraps, respectively. The expression for theterm Vf is determined by calculating the force resultingfrom the tensile stress in the SRP across an assumed shearcrack pattern. bw is the cross-section width and df is SRPshear reinforcement depth (see Fig. 6); Av and Afv representthe areas of internal steel stirrups and SRP U-wraps,respectively; and s and sf their respective spacing; ffe isthe tensile stress in the shear SRP reinforcement, which isproportional to the strain in this reinforcement at ultimatelevel, efe.

Fig. 6. Illustration of dimensional variables used in shear strengthening [1].

Fig. 7. Flexural SRP strengthening layout detail and moments for internal girdemand, and flexural capacities after strengthening.

Fig. 7a shows the SRP layout for flexural strengthen-ing (half member) for interior girders obtained followingthe procedure for analysis and design previouslydescribed. Fig. 7b shows the envelope of momentdemand, the moment capacity before the strengthening(as built) and strengthened moment capacity for the inter-nal girders (half member). Note that the demand is acurve line (as uniformly distributed load) because corre-sponds to the envelope of maximum internal forces (i.e.,bending moment and shear) induced by a train of trucksmoving from one side to the other of the bridge. Simi-larly, Fig. 8 shows the SRP layout for shear strengthen-ing (half member) for interior girders and the envelopeof demand for shear, the shear capacity before thestrengthening (as built) and strengthened shear capacityfor the internal girders (half member). U-wraps were usedin the mid span to anchor the external flexural reinforce-ment, so the value of Km was taken up to 0.9 since bothcover delamination and SRP debonding are effectivelyprevented.

For a check at service level, the same limitations pre-scribed by ACI 440 for FRP strengthened members were

ders. (a) Bottom view of flexural strengthening (half member), (b) flexural

Fig. 8. Shear SRP Strengthening layout detail and shear for internal girders. (a) Lateral view of shear strengthening (US customary 1 ft = 0.305 m),(b) shear demand, and shear capacities after strengthening.

A. Lopez et al. / Composites: Part B 38 (2007) 429–436 433

adopted. Therefore, the tensile stress in the internal steelreinforcement was limited to 80% of the yielding strength.Also, the level of stress in the SRP reinforcement was lim-ited to 55% of ffu under sustained load and cyclic serviceload to avoid time-dependent rupture.

4. Installation of SRP

4.1. Surface preparation

The installation by manual lay-up of SRP laminates isvery similar to the one utilized for FRP fabrics [17,18].For adhesion-critical application, load transfer in any com-posite system depends on the quality of bond between thelaminate and concrete substrate [19]. To guarantee proper

Fig. 9. Surface preparation. (a) Abrasive sandblasting

bond, a clean and sound substrate is necessary. Unsoundconcrete was removed and patched. Holes and voids onthe surface to be strengthened were filled, finished and tex-tured to establish a suitable surface profile. Concrete sur-face irregularities and sharp angles were removed andsmoothed to less than 0.04 in. (1 mm) to avoid localizedstress concentration and SRP separation. Rounding ofthe corners is not required for SRP because the steel cordis typically bent mechanically with equipment to make90� sharp bents with no damage to the steel.

As shown in Fig. 9a, abrasive sandblasting was used toclean the concrete and ensure a concrete surface roughnessequivalent to a Concrete Surface Profile number 3, asdefined by the International Concrete Repair Institute [20].To avoid pollution impact over soil and water, particles,

surface preparation, (b) environmental protection.

434 A. Lopez et al. / Composites: Part B 38 (2007) 429–436

dust, cement, paint and other contaminants were containedby enclosing the bridge in plastic sheeting as shown inFig. 9b.

Fig. 10. Cutting the cord tape.

5. Reinforcement installation

Cord strips were cut to length according to the designwith commercial handheld electric shears, as shown inFig. 10. A polymeric resin was roller-applied to the concreteto prime and seal the surface pores (Fig. 11). Properlyaligned steel strips were installed as flexural reinforcementby the manual lay-up method on deck and girders(Fig. 12), pressed mechanically onto the resin applied tothe concrete surface with enough force to impregnate them.For a complete impregnation, the strips were squeezed outby rolling, and a second epoxy resin application was per-formed (Fig. 13). This process was carefully planned to becompleted within the time limits of the resin pot life, allow-ing enough working time for the rolling of the laminates. U-

Fig. 11. Application of epoxy resin. (a)

Fig. 12. Installation of SRP by manual la

Fig. 13. Full impregnation of SRP. (a) Squeezing o

Wraps, installed after the curing of flexural strengthening,were divided in two L-shape parts (see Fig. 14a) and fullyoverlapped over the bottom side of the girders (seeFig. 14b). After curing, a topcoat was applied to the affectedsurfaces to provide cosmetic finish and environmentalprotection.

Flexural reinforcement, (b) U-wrap.

y-up procedure. (a) Girder, (b) deck.

f strip by rolling, (b) second resin application.

Fig. 14. U-wraps for shear reinforcement. (a) L-shapes before installation, (b) overlapping L-shapes.

Fig. 15. Static load test. (a) Surveying equipment, prisms and load trucks, (b) transversal deflections before and after strengthening (1 in. = 25.4 mm;1 ft = 305 mm).

A. Lopez et al. / Composites: Part B 38 (2007) 429–436 435

6. Load rating

The safe load-carrying capacity of the bridge after thestrengthening was determined through load rating. Loadrating is performed at two load levels. The first one, oper-ating rating, defines the maximum allowable load to avoiddamage on the bridge. The second rating, inventory rat-ing, defines load level that the bridge can carry daily[21]. Load posting is established using 86% of the operat-ing rating.

Load rating was calculated for four different truck types,as mandated by MoDOT [16]. For each of these four differ-ent load conditions, the maximum shear and maximummoment are computed for each element of the bridge,i.e., deck, girders and bents. For the SRP-strengthenedspan, rating was controlled by shear capacities in the exte-rior girders, specifically 37.1 and 27.3 tons (33.6 and24.7 tons in SI units) for operating and inventory ratingtype, respectively, resulting in a 26.3 tons (23.8 tons in SIunits) load posting.

7. Long-term performance

To experimentally evaluate the load-carrying capacityof the bridge, non-destructive load tests were conducted.In situ load tests were carried out before and immediately

after the strengthening of the bridge. These tests arerepeated every six months and will continue over a periodof five years. Deflections were measured at several loca-tions using a non-contact monitoring method [22].Fig. 15a shows: targets (i.e., reflecting prisms) attachedlongitudinally to the girders of one span of the bridge,the surveying equipment theodolite and the loadingtrucks. Typical results of the load tests in this instance,before and after strengthening, are shown in Fig. 15bfor the SRP-strengthened span. As expected, a marginaldecrease in deflection after the application of the SRPreinforcement can be seen. This represents an initial goodperformance, although the long-term performance will bemonitored throughout the next years. It is believed thatheavier traffic will not cause extensive cracking of thestrengthened members and affect the bridge long-termdurability. Nevertheless, there is no hard evidence at handsince load posting has not yet been removed on thebridge.

8. Conclusions

Strengthening with SRP of a bridge superstructure wasproven to be a reliable and relatively easy-to-install tech-nique. SRP is still not included in the ACI 440 design

436 A. Lopez et al. / Composites: Part B 38 (2007) 429–436

guidelines for composite strengthening, but the procedureused for FRP appears to be equally applicable. The prepa-ration of a suitable substrate surface and the installation ofthe material are similar to those used for FRP manual lay-up, although no rounding of the corners is requiredbecause the steel cord is typically bent mechanically withequipment to make 90� sharp bents. The load ratingshowed that the SRP strengthening increased the flexuraland shear capacities of the superstructure. Therefore a rec-ommendation to remove the load posting was made. Staticload test showed an initial good performance of the SRP-strengthened span. The long-term performance will bemonitored throughout five years with semiannual loadtests.

Acknowledgements

This research and demonstration project was financiallysupported by the Missouri Department of Transportation(MoDOT) and the University Transportation Center onAdvanced Materials and NDT Technologies at the Univer-sity of Missouri-Rolla. The material systems for the entireproject were provided by the members of the NSF Indus-try/University Cooperative Research Center-Reparationof Building and Bridges with Composite (RB2C). The con-tractor and designer for the project were Structural Preser-vation System, Chicago, IL, and Co-Force America, Rolla,MO.

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