1217

SP-230—69

Bonded and Mechanically Fastened FRPStrengthening Systems: A Case Study

by A. Lopez, A. Rizzo, N. Galati, and A. Nanni

Synopsis:Synopsis:Synopsis:Synopsis:Synopsis: This paper describes a case study addressing analysis, design, installation,load rating, load testing, numerical verification and monitoring of the strengthening ofan old deficient bridge under a field validation research project. The strengthening wasperformed by means of two composite technologies: the common manual lay-up CFRPsystem and a mechanically fastened FRP (MF-FRP) system. The later consisted of pre-cured FRP laminates having high transverse bearing strength (i.e., bidirectional fibers)attached to the structural element by means of wedge anchors. Although considered atemporary solution, the technique is attractive because it requires minimum surfacepreparation. The bridge will be monitored for 5 years to evaluate the long-termperformance of the techniques. In this sense, load tests were performed to evaluatethe bridge structural behavior prior to and after the strengthening, showing that therehabilitation performed as predicted by numerical analysis and therefore created thepossibility to remove the bridge load posting.

Keywords: bridge; load rating; load testing; manual lay-up CFRPlaminates; mechanically fastened CFRP; strengthening

1218 Lopez et al.Alexis Lopez is a PhD Candidate in Civil Engineering at the University of Missouri-

Rolla. He received his BS in Civil Engineering and his Master in Structural Engineering

from the University of Los Andes, Venezuela. His research interests include analysis and

design of reinforced concrete strengthened with composite material.

Andrea Rizzo is a PhD Candidate in Civil Engineering at the University of Lecce-

Italy where he also received his B.Sc. in Materials Engineering. He obtained his M.Sc.

degree in Engineering Mechanics at the University of Missouri-Rolla. His research

interests include analysis and design of RC structures.

Nestore Galati is a research engineer at the University of Missouri – Rolla. He

obtained is PhD in Civil Engineering at the University of Lecce- Italy where he also

received his B.Sc. in Materials Engineering. He obtained his M.Sc. degree in

Engineering Mechanics at the University of Missouri-Rolla. His research has

concentrated on the area of retrofitting of masonry and upgrade and in-situ load testing of

RC structures.

Antonio Nanni (FACI) is the V & M Jones Professor of Civil Engineering at the

University of Missouri-Rolla. He is the founding Chair of ACI Committee 440, Fiber-

Reinforced Polymer Reinforcement, and is the current Chair of ACI Committee 437,

Strength Evaluation of Existing Concrete Structures. He is an active member in technical

committees of ASCE (Fellow), ASTM, and TMS.

INTRODUCTION

Strengthening of existing reinforced concrete (RC) and prestressed concrete (PC)

structures may become necessary as a result of many factors, such as inadequate design

or construction, deterioration due to corrosion of the embedded reinforcement, need for

increase in structural capacity, the adding of ductility and seismic retrofit.1

In the last two decades, the use of fiber-reinforced polymer (FRP) materials for

external strengthening of structures has emerged as one of the most exciting and

promising technologies in materials and structural engineering and has been gaining

increasing popularity in the civil engineering community.2-4

This success is a result of

well-documented benefits: low-cost, no traffic disruption, rapid execution and anticipated

long-term durability.5

Moreover, composite materials are attractive because of their

intrinsic properties: high strength-to-weight ratio, corrosion resistivity, electromagnetic

neutrality and high energy absorption capacity. International design guide availability 6-9

and many projects confirms fiber reinforced polymer (FRP) materials as a mainstream

technique.10

The cost impact of having old deficient bridges load posted, imposing limits on heavy

vehicles in farm-to-market roads justify the interest of Departments of Transportation

(DOT) to develop specifications11

for strengthening techniques. For these aforementioned

reasons, field validation of composite technology at a large scale is necessary.

FRPRCS-7 1219In order to make these technologies available to bridge owners and practitioners, the

University of Missouri-Rolla (UMR) and MoDOT joined in a cooperative research

project to perform validation through field testing. From an initial list of hundreds of

structurally deficient bridges, five load-posted reinforced concrete (RC) bridges,

geographically spread over the state, were selected to be strengthened using five

variations of composite strengthening techniques.12

One of the objectives of this project

was to create a comprehensive and authoritative series of documents (specifications and

guides) on the technology of composite strengthening of bridge structures.

This paper describes the analysis and design of one of these five bridges, called

bridge Y0298, in Pulaski County, Missouri. It specifies the fundamental design and

constructability issues regarding composite reinforcement. Five different strengthening

techniques were used in the entire program12

but only two were used for bridge Y-0298:

manual lay-up carbon FRP laminates and mechanically fastened hybrid glass/carbon FRP

laminates. The use of the later technique is justified by the fact that it requires minimum

surface preparation: the load transfer mechanism to and from the composite laminate is

provided by fasteners. This new strengthening technique consisting of fastening a

composite plate to the concrete elements by means of closely spaced powder-actuated

pins was recently developed13

at the University of Wisconsin – Madison (UW). The

installation of MF-FRP system has proven to be fast and easy, and requires unskilled

labor and only common hand tools. In addition, the surface preparation can be reduced to

the removal of sizeable protrusions such as form lines. The efficiency of this rapid-repair

strengthening system was demonstrated by rehabilitating an existing bridge and testing to

failure two slabs cut from the structure.14

However, this technique may damage the

concrete substrate since the pins are shot into the element: in the presence of hard

aggregates, the cracking around the holes becomes unacceptable. A modification of this

strengthening method was developed at University of Missouri – Rolla (UMR) in order to

overcome the disadvantages of the original system. The new strengthening technique

utilizes wedge anchors driven or hammered into the concrete instead of nails shot in it.

As field validation, three off-system bridges were strengthened using the MF-FRP

system.15

BRIDGE DESCRIPTION AND OBJECTIVES

Figure 1 shows a side view of bridge Y-0298 in Pulaski County, Missouri, built in 1937.

It was load posted to 8.15 tonnes with a speed restriction to 24 km/h. The structure has

two equally-spaced spans (3.7 m perpendicular to the supporting walls), each of them

consisting of a solid RC deck of 178 mm. For design purpose, each span was considered

simply supported on RC vertical walls. The total bridge length is 6.4 m and the total

width of the deck is 7.3 m. The concrete at the East slab was in poor condition in areas

close to the edges of the bridge due to water filtering from the top of the deck (see Figure

2). A plan view of the bridge is shown in Figure 3a.

At the onset of the project, existing material properties were validated in field by

extracting two concrete cores and steel bar samples. The compressive strength of the

concrete was found to be 27.6 MPa. The steel reinforcement consists of two grids: 19 mm

1220 Lopez et al.and 25 mm diameter rebar in the top and in the bottom of the deck, respectively. Steel

yield strength, fy, was 276 MPa. Figure 3b reports the details of the cross section.

The objective of the retrofitting was to increase the flexural capacity of the structure

in order to remove the existing load posting in five years, based on the performance

throughout the time under a monitoring program. For both bridge spans, manual lay-up

CFRP laminates were used for the flexural strengthening of the deck. Due to the high cost

of repairing the concrete, a system consisting of mechanically-fastened FRP laminates

was chosen to strengthen some areas in poor condition mentioned before (see Figure 2).

Unlike traditional bonded methods, where adhesion is used to transfer the load to and

from the reinforcement, the MF-FRP technique requires minimum surface preparation

because the mechanism of load transfer is provided by fasteners.

Manual lay-up and MF-FRP strengthening systems were used in order to maximize

the benefits-to-costs ratio. The manual lay-up consisted of dry CFRP fiber fabric plies

(see Figure 4a) that were impregnated with epoxy resin and adhered to a prepared

concrete surface. Table 1 summarizes the mechanical properties of the CFRP laminate.

The MF-FRP laminates system consisted of pre-cured FRP strips (see Figure 4b),

having high transverse bearing strength attached to the concrete surface using concrete

wedge anchors. The laminate is a glass and carbon hybrid pultruded strip embedded in a

vinyl ester resin. Its thickness and width are 3.175 mm and 101.6 mm, respectively. Table

1 summarizes the mechanical properties of the pre-cured laminate.16

BRIDGE ANALYSIS AND EXTERNAL REINFORCEMENT DESIGN

Load configuration and bridge analysis were consistent with AASHTO

Specifications.17

The HS20-44 (MS18) truck was used as load for bridge analysis.

AASTHO prescribed a “lane load condition”, but the maximum internal forces induced

by a single HS20-44 (MS18) truck controlled the design. Table 2 summarized the as-

built, the demand and the strengthening moment capacities using manual lay-up

laminates.

The design of the strengthening systems was made in compliance with ACI 4406

where applicable. The MF-FRP strengthening was computed according to the procedure

suggested by Lamanna.13

For manual lay-up laminates, the flexural capacity is estimated based on force

equilibrium, strain compatibility and constitutive laws of materials up to failure. An

equivalent rectangular stress block was used to simulate the behavior of the compressed

concrete. The maximum compressive concrete strain was taken as 0.003. No slip between

bonded FRP laminates and substrate was assumed. Initial strains in the concrete at the

time of bonded FPR installation, εib

, was determined on the assumption of a cracked

section subjected to the dead load. Such strain was subtracted from the one in the bonded

FRP strengthening.

FRPRCS-7 1221In order to determine the CFRP design strength,

fuf , the value of the guaranteed

ultimate tensile strength, *

fuf , was reduced for the FRP to account for environmental

exposure, using a knock down factor, E

C . Being linear elastic material, FRP design

strain,fu

ε , is proportional to fuf by the tensile modulus.

For flexural design, the strain level developed at the ultimate-limit state in the FRP is

also reduced by a bond-dependent coefficientm

k , to account for the possible

delamination from the concrete substrate. The effective strain in the FRP reinforcement,

εfe

, is then restricted by the following equation:

*

fufe m fu m E

k k Cε ε ε≤ = (1)

where fu

ε is the design rupture strain of FRP, which is equal to the guaranteed

ultimate tensile strain, *

fu

ε , reduced by the knock down factor E

C . The factorm

k , is

function of the stiffness of the FRP laminate (ACI Committee 440 2002, Eq. 9-2) and is

intended to guarantee a value of maximum strain, εfe

, at which the bonding forces can be

sustained by the concrete substrate.

The nominal flexural capacity was computed accounting for the contribution of the

CFRP as shown in the following equations:

n

1 1

s s f fe

M

c c

A f d A f h

2 2

=

β β − +Ψ −

(2)

where As and A

f represent the steel reinforcing bars and FRP cross sectional areas,

respectively, and fs and f

fe their respective stress levels; d and h represents the

reinforcement depth and cross section height; β1c is the depth of the equivalent

rectangular block. ψ is an additional reduction factor to account for the novelty of the

technology 6

having a value of 0.85.

No shear strengthening was provided to the slab since the value of the as-built shear

capacity was higher than the new loads demand.

For a check at service level, limitations prescribed by ACI 4406

for FRP strengthened

members adopt a tensile stress in the internal steel reinforcement to be limited to 80 % of

the yielding strength. Also, the level of stress in the FRP reinforcement was limited to

55% of fuf under sustained load and cyclic service load to avoid time-dependent rupture.

A different approach was used for the load analysis of the major deteriorated areas.

Since the continuity of the deck over the central wall was conservatively neglected, the

deck was modeled as a slab simply supported between two consecutive supports. The

design value was determined from the truck design condition when the rear axle of the

1222 Lopez et al.two trucks is in the middle of the span close to the guardrails, resulting in Mu = 55 KN.m

per m. Due to corrosion of the bars at both sides of the bridge, the area of steel was

assumed to be half of the provided amount. In addition, the MF-FRP strips were placed

parallel to the guardrail. Thus, with the chosen pattern of the bolts (see Figure 5), the

ultimate moment of the strengthened slab resultedn

Mφ = 55.7 KN.m per m. Note that the

MF-FRP strengthening system was verified at ultimate and service conditions, according

to the procedure suggested by Rizzo.16 In fact, the flexural capacity is smaller for

sections closer to the supports since the laminates are anchored by a smaller number of

fasteners: thus, at ultimate conditions, it was checked for each section that the design

moment was smaller than the capacity of the strengthened section. On the other hand,

since under service conditions the load carried by each fastener is a function of the

loading configuration as well as of the fasteners distribution, it was verified that the load

carried by each fastener, at service load condition, were smaller than the maximum

bearing load in order to avoid jeopardizing the structural behavior of the strengthened

element.

Figure 6 shows the layout of the CFRP fabrics and MF-FRP pre-cured laminates.

More details about the design of the strengthening can be found in Nanni.18

INSTALLATION OF EXTERNAL REINFORCEMENT

Surface preparation

Because the installation of manual lay-up CFRP fabrics

are adhesion critical

applications,11,18

proper bond was provided by establishing a cleaned sound substrate.

Holes and voids on the surface to be strengthened were filled, finished and textured to

establish a suitable surface profile. Thereafter, abrasive sandblasting was used to clean

the concrete and ensure a concrete surface roughness equivalent to a Concrete Surface

Profile number 3, as defined by the International Concrete Repair Institute.19

No surface

preparation was required for the MF-CFRP strips.

Reinforcement installation

Manual lay-up -- Dry fiber fabric plies were impregnated with epoxy resin and

adhered to a prepared concrete surface. First, the epoxy saturant was applied to the

prepared concrete surface; after that, the first ply was adhered to the surface manually

and saturated by rolling; and, finally, a second saturant layer was applied over the ply to

complete the impregnation. The procedure was repeated for the second plies. Figure 7

shows the pre-cut fiber fabrics impregnated during the installation.

MF-CFPP -- Figure 8 shows some steps of the application of the MF-FRP system.

Strips were cut to length according to the design specifications and spread with epoxy. It

should be noted that the addition of resin was not considered in the design, allowing

increasing the safety factors of the structure. Since the substrate presented severe

unevenness, FRP laminates were not predrilled. The position of the anchors was

optimized by drilling strips and concrete substrate in the same operation. In order to

avoid drilling holes in steel reinforcement, the concrete surface was checked by scanning

FRPRCS-7 1223with a rebar locator. Holes were filled with adhesive epoxy to fill all the gaps between

fasteners and concrete, and between fasteners and laminate.

LOAD RATING

Load rating was performed to obtain the safe load carrying capacity of the bridge after

the strengthening. It was performed at two load levels, namely operating and inventory

rating,20

that defines the maximum allowable load to avoid damage on the bridge and the

load level that the bridge can carry daily, respectively.

Load rating was calculated for four different trucks, as mandated by MoDOT.21

For

this load condition, if the stress levels produced by trucks (i.e, maximum shear and

maximum moment) configuration are exceeded, load posting may be required. According

to the rating, the bridge does not need to be load posting for any single of multi axle

vehicles.

FIELD VALIDATION AND FEM ANALYSIS

In order to validate the behavior of the bridge prior to and after strengthening, in-situ

load tests were performed before and immediately after the strengthening of the bridge,

with the trucks indicated in table 3. The loading configurations were chosen to maximize

the stresses and deflections at mid-span of deck panels. During each stop, the truck was

stationary for at least two minutes before proceeding to the next location in order to allow

stable readings. Deflections in the mid-span were measured using a non-contact

monitoring method;22

while strains in the strengthening material were monitored by

means of strain gages. Figure 9a shows: targets (i.e., reflecting prisms) attached

transversally to the deck of one span of the bridge, the surveying equipment theodolite

and the loading trucks. Figure 9b shows the strain gages attached to the strips.

A FEM analysis model was developed in order to interpret the experimental data. For

this purpose, a commercially available finite element program23

was used. Solid elements

were chosen to model the concrete and the FRP laminates.

The concrete was assumed to be isotropic and linear elastic, since the applied load

was relatively low with respect to the ultimate load condition: its modulus of elasticity of

the concrete was based on the measured compressive strength of the cores according to

the standard equation AASHTO17

Article 8.7.1: Ec

= 24.8 GPa. The concrete Poisson’s

ratio was set to 0.19.

In order to take into account the presence of cracks and holes in the deck, as a result

of a parametric analysis, the modulus of elasticity was conveniently reduced in the

elements corresponding to the unevenness as shown in Figure 10. Depth and width of

cracks and holes was chosen according to the data collected during the in-situ inspection

(Figure 11). The guardrails were modeled with a full section with a reduced stiffness of

the web to take into account their actual perforated geometry. Different elements shapes

were used to optimize the model and decrease the computation time reducing the number

1224 Lopez et al.of the elements in the “secondary” parts of the model, such as the guardrails.

The steel reinforcement was spread over two layers in accordance with Figure 3b: its

modulus of the elasticity and Poisson’s ratio were assumed as 200GPa and 0.3

respectively.

The connections between the FRP laminates and the concrete surface were modeled

as rigid, neglecting any form of non-linearity due to a potential initial non-perfect

engagement of the strengthening (Figure 11). Elasticity modulus and Poisson’s ratio of

FRP laminates were assumed to be 62 GPa and 0.3, respectively.

The bridge was vertically restrained at the three supports while the longitudinal

displacement was fixed to zero at the central wall only: the continuity of the slab over the

central abutment was maintained despite the model used for design purpose. The loads

were assumed as uniformly distributed over 508 x 254 mm areas as specified in

AASHTO17

Section 4.3.30. Such loads were applied at the top of the deck simulating the

truck wheel prints. The uniform load was concentrated at the nodes corresponding to the

truck wheel print and each force was determined by dividing the total load for the number

of nodes.

Figure 12 compares the deflection in the middle of the two spans prior to the

strengthening for the load condition specified in same figure. A similar comparison is

shown in Figure 13 after the bridge has been strengthened. Note that figures 12 and 13

correspond to two different load cases. These cases intend to demonstrate the efficiency

of the MF-FRP strengthening by placing the truck in positions that produce the maximum

strain in the strengthening. Efficiency The model gave satisfactory results showing a

good match in deflections between experimental and analytical data: the difference found

in the negative moment region deflections can be ascribed to the unknown position of the

steel reinforcement in the top of the section and to the impossibility to model all the

damaged areas of the deck, that change the overall structure response to the applied loads.

Eventually, the bridge performed well in terms of overall deflection: in fact, the

maximum deflection measured during the load test was below the allowable deflection

prescribed by AASHTO17

Section 8.9.3 (max

4 mm

800

L

δ ≤ = ).

The maximum strain readings on the FRP strengthening were between 100 and 125 µε ,

indicating a satisfactory performance of the MF-FRP laminates. Figure 14 reports the

reading of the strain gages for the load condition specified in the same figure. The graph

shows the good match in strains between experimental and analytical results.

CONCLUSIONS

The rational choice of the two different strengthening systems resulted in an optimal

solution in terms structural performances. The FEM analysis show good agreement with

the experimental data. Nevertheless, it must be pointed that the static load test was carry

FRPRCS-7 1225out at service loads and no comparison is possible with ultimate load, in which the

effectiveness of the strengthening design is based.

Both composite systems were easily installed. Although considered a temporary

solution, the mechanically fastened FRP system technique is attractive because requires

minimum surface preparation: since the load transfer mechanism to and from the

composite laminate is provided by fasteners. This system proved to be a feasible solution

for the strengthening of the major damaged areas of the bridge. Cost effectiveness is high

because high capacity was given to the bridge with a rapid and economical strengthening.

The slab was considered as simply supported for design purposes and continuous in

the FEM analysis. For design, based at ultimate capacity, the worse case scenario is that

in which the steel at the support is completely corroded and therefore it can not carry the

moment. Therefore the strengthening was based on the conservative assumption of

simply supported conditions. This assumption is justified by the deteriorate state of

concrete and top steel reinforcement. On the other hand, FEM analysis is based on

service load at which static load test is performed, and where continuity can not be

ignored.

The load rating showed that the strengthening systems increased the flexural

capacities of the bridge. Based on this analysis, and the good agreement between the

experimental data of load test and the FEM analysis, a recommendation to remove the

load posting was made. Static load test showed an initial good performance of the

strengthened bridge. The long-term performance will be monitored throughout five years

with semiannual load tests.

ACKNOWLEDGMENTS

This research and demonstration project was financially supported by the Missouri

Department of Transportation (MoDOT) and the University Transportation Center on

Advanced Materials and NDT Technologies at the University of Missouri-Rolla. The

material systems for the entire project were provided by the members of the NSF

Industry/University Cooperative Research Center-Reparation of Building and Bridges

with Composite (RB2

C). The contractor and designer for the project were Structural

Preservation System, Chicago, IL, and Co-Force America, Rolla, MO.

REFERENCES

1. De Lorenzis, L., “Strengthening of RC Structures with Near-Surface Mounted

FRP Rods,” PhD Thesis, University of Lecce, Lecce, Italy, 2002.

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Proceedings of the International Workshop July 20-21, 2001, Capri, Italy, 304 pp.

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Construction,” Proceedings, the Global Outlook for Carbon Fiber 2000, Intertech, San

Antonio, Texas, 6 pp. 2000.

1226 Lopez et al.4. Nanni, A., “FRP Reinforcement for Bridge Structure,” Proceedings, Structural

Engineering Conference, University of Kansas, Lawrence, Kansas, 5 pp. 2000.

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Bulletin No. 14, Lausanne, Switzerland, 130 pp. 2001.

10. Bakis, C. E.; Bank, L. C.; Brown, V. L.; Cosenza, E.; Davalos, J. F.; Lesko, J. J.;

Machida, A.; Rizkalla, S. H; Triantafillou, T. C., “Fiber-Reinforced Polymer Composite

for Construction—State-of-the-Art Review,” Journal of Composites for Construction,

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11. National Cooperative Highway Research Program (NCHRP) “Bonded Repair and

Retrofit of Concrete Structures Using FRP Composite,” Transportation Research Board

National Academy, Washington, D.C., 89 pp.

12. Lopez, A.; Nanni, A., “Validation at a Large Scale of FRP Technology for Bridges

Strengthening.” Concrete International – ACI. 2005. Accepted

13. Lamanna, A.J., “Flexural Strengthening of Reinforced Concrete Beams with

Mechanically Fastened Fiber Reinforced Polymer Strips,” PhD Thesis, University of

Wisconsin – Madison. 2002.

14. Borowicz, D.T.; Bank, L.C.; Nanni, A.; Arora, D.; Deza, U.; Rizzo, A., “Ultimate

Load Testing and Performance of Bridge with Fiber Reinforced Composite Materials and

Powder-Actuated Fasteners,” Proceedings of 83rd Annual Transportation Research Board

Meeting, CD-ROM, Washington, D.C. 2004.

15. Bank, L.; Nanni, A.; Rizzo, A.; Arora, D.; Borowicz, D., “Concrete Bridges Gain

Strength with Mechanically Fastened, Pultruded FRP Strips,” Composites Fabrication

Magazine, September, 2004, pp. 32-40.

FRPRCS-7 122716. Rizzo, A., “Application in Off-System Bridges of Mechanically Fastened FRP

(MF-FRP) Pre-Cured Laminates,” M.Sc. Thesis, University of Missouri-Rolla. 2005.

17. AASHTO, “Standard Specifications for Highway Bridges”, 17th Edition, Published

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1228 Lopez et al.

Figure 1. Side View of the Bridge Y-0298 in Pulaski County, MO

FRPRCS-7 1229

Figure 2. Areas of Deck with Major Concrete Deterioration on both Sides of the Bridge

Figure 3. Geometry of the Bridge

Figure 4. FRP Materials Used in Strengthening System

1230 Lopez et al.

Figure 5. MF-FRP System for the Strengthening of the Major Deteriorated Areas

Figure 6. Strengthening Layout of the Bridge Y-0298

FRPRCS-7 1231

Figure 7. Installation of Manual Lay-up laminates

Figure 8. Application of the MF-FRP Strengthening System

1232 Lopez et al.

Figure 9. Instrumentation to Monitor Displacements and Strains

Figure 10. FEM Model for the Bridge Y-0298

FRPRCS-7 1233

Figure 11. Composite Reinforcement in FEM Model

Figure 12. Experimental and Analytical Displacement prior to Strengthening

Figure 13. Experimental and Analytical Displacement after Strengthening

1234 Lopez et al.

Figure 14. Strain in the MF-FRP Laminates