investigation of closure- strip details for connecting

75PCI Journal | Summer 2011

Editor’s quick points

■ Two potential prefabricated systems were investigated: prefab-ricated composite slab-on-girder beam elements connected by cast-in-place concrete closure strips, and full-depth prefabri-cated concrete deck panels laid across new or existing girders and connected with cast-in-place closure segments.

■ This paper presents the findings of a series of laboratory tests on 2/3-scale models of the closure strip system to study the serviceability and ultimate state behavior of the joint detail.

■ U-shaped, L-shaped, and welded straight reinforcing bars used as connection details in the closure strips performed well.

Investigation of closure-strip details for connecting prefabricated deck systemsAlexander Au, Clifford Lam, and Bala Tharmabala

The Ministry of Transportation of Ontario (MTO) recently conducted a laboratory research project1 to develop a work-able prefabricated bridge system for use in Ontario. Based on the common types of bridges in MTO’s bridge invento-ry, two types of prefabricated bridge systems were selected for development (Fig. 1):

• System A consisted of fabricating precast concrete T-shaped girders, installing them at the site, and cast-ing closure strips to complete the bridge superstruc-ture.

• System B consisted of installing precast concrete sec-tions of full-depth deck slabs at the site on top of steel or prestressed concrete girders and placing closure strips over the girders and between adjacent panels to complete the superstructure.

One of the main challenges inherent in these two sys-tems is to develop a simple yet durable detail for the joint between precast concrete components. Connection details can affect the constructability, performance, and long-term serviceability of the superstructure.2–4 Therefore, MTO decided to adopt a joint detail that would involve minimal in-place concreting to provide the desirable monolithic connection between the precast concrete elements.

From a structural standpoint, there are concerns about the performance of cast-in-place concrete closure strip connec-tions, in particular their durability and integrity in transfer-ring loads to adjoining girders. To address these concerns, an experimental program was conducted on 2/3-scale mod-els of the closure pour connections to study the following:

Summer 2011 | PCI Journal76

carry vertical loads by arching action due to transverse slab confinement and restraint.5 This paper describes the labora-tory tests that were conducted to study the performance of the different connection details and the results of these tests.

Description of closure strip models

General details

Two-thirds scale models of the standard 225-mm-thick (9 in.) deck slab used in Ontario bridges were used as test specimens in this study. The models consisted of rectan-gular concrete slabs measuring 1800 mm (72 in.) long, 600 mm (24 in.) wide, and 150 mm (6 in.) thick (Fig. 2). A 300-mm-wide (12 in.) middle section of the slab was cast separately to model the closure-strip connection. The slab was reinforced with top and bottom layers of steel reinforcement consisting of 10-mm-diameter (0.375 in.) steel bars spaced at 150 mm (6 in.) center to center in both transverse (4 bars) and longitudinal (12 bars) directions. All bars had a nominal concrete cover of 15 mm (0.6 in.).

To study the load transfer characteristics of the connection steel details, different systems of providing continuity of reinforcement in the closure strips were considered. The test models included the following:

• behavior under static load

• long-term performance of cold joints at the interface between the prefabricated section and the closure joint concrete under repetitive cyclic loading

• ultimate strength capacity of the connection system.

To study the load-transfer characteristics of the connection steel details, different systems of providing continuity of reinforcement in the closure strips were considered. The following reinforcement lapping systems were used:

• U-shaped bars

• L-shaped bars

• welded straight bars

• straight bars with both normal lap lengths and reduced lap lengths that were confined by steel spiral/stirrup reinforcement

The test models were tested as simply supported flexural members and loaded by a point load at the center of the specimens. This test arrangement provided conservative re-sults because real deck slabs in slab-on-girder deck systems

Figure 1. The two precast concrete bridge systems considered for regular use.

Prefabricated slab-on-steel

girder

Prefabricated slab-on-

prestressed concrete

girder

Cast-in-place

closure stripconcrete

Cast-in-place

closure stripconcrete

Prefabricated full-depth deck slab

Prefabricated bridge system A

Prefabricated bridge system B


first and left to cure for a minimum of 28 days before the closure strip was placed to complete the test model. Some specimens were also cast monolithically as controls and for the study of reinforcement lapping details. Figure 4 shows some pictures of the fabrication of the test models.

Materials

The materials used in the test models had the following properties:

• Ordinary portland cement concrete with a nominal 28-day strength of 30 MPa (4350 psi) was used in both precast concrete member sections and closure strips.

• Proprietary high-early-strength concrete was used in some closure strips. This material has a nominal 1-day compressive strength of 25 MPa (3600 psi) and 28-day strength of 45 MPa (6500 psi).

• Reinforcing steel bars (Grade 400 [58 ksi]) that were 10 mm (0.375 in.) in diameter with a minimum yield strength of 400 MPa (58 ksi) were used as primary concrete slab reinforcement.

• Reinforcing steel rods that were 6-mm-(0.25 in.) in diameter with a nominal yield strength of 400 MPa (58 ksi) were used in spiral and stirrup reinforcements for reinforcement lap confinement.

• U-shaped bars: closed loop bars anchored by trans-verse straight bars at corners

• L-shaped bars: spliced bars with 90 deg bends at the ends and anchored by transverse straight bars at corners

• Welded straight bars: spliced bars welded together to provide continuity

In addition, test slabs were also fabricated to study the effect of different reinforcement lapping details for splicing straight bars. The reinforcement lapping systems that were investigat-ed included straight bars spliced using the full code-specified lap length and also a reduced (50%) lap length (Fig. 3). In the latter case, the effect of confining the reduced lap length splices by steel spiral and stirrup reinforcements was also studied (Fig. 3). The diameter of the steel spiral was 42 mm (1.67 in.) with a pitch of 24 mm (0.94 in.).

Control specimens (without closure strips) were also cast (Fig. 4) and tested for comparison. A total of thirty 2/3-scale closure pour joints were fabricated and tested in this study. Table 1 lists test specimen details.

Test model construction

The closure strip models were constructed in two stages. The sections modeling the precast concrete parts were cast

Figure 2. Details of a typical closure-strip test model. Note: All dimensions are in millimeters; all reinforcing bars are 10 mm in diameter. 1 mm = 0.0394 in.

600

150

057057

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Section X

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300

150

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150(typical)

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1800

(Closure strip)

Initial precast concrete segment


Scope of tests and details of specimens

Test objectives

The objectives of the test program were to study the fol-lowing:

• the effectiveness of different reinforcing-bar connec-tion details to provide continuity of reinforcement in the closure-strip connections

• the effect of repetitive cyclic load on the closure-strip details under both serviceability and ultimate limit states

• the effect of using a high-early-strength concrete in the closure strip to accelerate prefabricated bridge construction

Instrumentation

Electrical-resistance strain gauges and linear variable dif-ferential transducers were installed on the soffit of selected test specimens to monitor the strains in the concrete and the vertical deflections at the centers of the slabs. When used, the strain gauges were installed at midspan and across one of the cold joints. Figure 5 shows two strain gauges that were glued in the middle of the closure strip and across one of the cold joints. The vertical displace-ment transducer monitored the stiffness characteristics of the closure-strip connection system. All instrumentation devices were connected to an electronic data acquisition system (IOTECH) for monitoring and data recording.

Figure 3. Continuity of steel reinforcement in closure strips was provided by these connection details in the test specimens.

Straight bars with full-lap-length detail

Half-lap-length bars confined by steel spirals

Straight bars with half-lap-length splice detail

Half-lap-length bars confined by vertical stirrups


• the performance of different lap details for splicing straight bars

A total of seven slab specimens, representative of the type of closure-strip reinforcement, reinforcement lap detail, and closure-strip concrete, were first subjected to a minimum of 3 million cycles of a pulsating load that had a peak amplitude of 18 kN (4.0 kip). They were then tested for ultimate strength while the remaining specimens were subjected only to strength tests to assess the effect of cyclic loading on the connection systems.

Summary of test specimens

Table 1 shows the main details of all specimens tested in this study. A total of 30 specimens were fabricated. They are grouped as follows:

• control specimens (placed monolithically) using either normal concrete or a proprietary high-early-strength concrete

• closure-strip models (placed in two stages) with nor-mal concrete in both stages

• closure-strip models with normal concrete in the initial placement and a proprietary high-early-strength con-crete in the closure strip

• monolithic specimens with normal concrete and differ-ent reinforcement lapping details

Properties of the materials used are given earlier in the “Materials” section.

Experimental setup and testing procedure

Test arrangement

Figure 6 shows the experimental setup used for testing the deck-slab models. Steel rollers supported the specimens at 100 mm (4 in.) from both ends while a hydraulic jack system applied a concentrated load at the center of the test specimens.

Cyclic load tests

The load was applied as a concentrated load through a 100 mm (4 in.) square pad at the center of the test specimen to simulate the footprint of a typical wheel load (Fig. 7). A total of seven specimens were subjected to cyclic load tests (Table 1).

Load modeling The design truck load in the Canadian Highway Bridge Design Code (CHBDC)6 is a 625 kN (141 kip) truck consisting of five axles spaced over a length

of 18 m (59 ft). The maximum single wheel load is 87.5 kN (19.7 kip). With a live load factor αL at the fatigue limit state (FLS) of 1.0 and a dynamic load allowance (DLA) of 0.4, the factored wheel load at FLS is 122.5 kN (27.5 kip). Because a geometric scale factor of 2/3 was used in the slab model, the scale factor for applied test loads can be assumed to be 4/9. Thus, the scaled design wheel load for fatigue test requirements is 54.4 kN (12.2 kip) to be applied on an effective slab width corresponding to the girder spac-ing. For the test specimens used in this study, the scaled FLS load was estimated to be 18 kN (4.0 kip). Under this

Figure 4. Fabrication of test slabs.

Concrete placed initially to model precast concrete member sections

Closure strip ready for concrete

Closure strip concrete in place

Concrete placed monolithically


Table 1. Full details of specimens tested in this study

Sample identification

Reinforcement continuity details

Precast concrete segment Closure stripsLoad cycles

appliedConcrete type

Concrete strength, MPa

Concrete type

Concrete strength, MPa

Lap length, mm

C1 Continuous bar Normal 45.5 n.a. n.a. n.a. 0

C2 Continuous bar Normal 45.5 n.a. n.a. n.a. 0

C3 Continuous bar HESC 48.9 n.a. n.a. n.a. 0

C4 Continuous bar HESC 48.9 n.a. n.a. n.a. 0

ANH1 U-shaped bar Normal 45.5 HESC 48.9 250 0

ANH2 U-shaped bar Normal 45.5 HESC 48.9 250 0

ANN1 U-shaped bar Normal 45.5 Normal 30.3 250 0

ANN2 U-shaped bar Normal 45.5 Normal 30.3 250 2.9 million

AN1 U-shaped bar Normal 45.5 n.a. n.a. 250 0

AN2 U-shaped bar Normal 45.5 n.a. n.a. 250 0

BNH1 L-shaped bar Normal 45.5 HESC 48.9 250 0

BNH2 L-shaped bar Normal 45.5 HESC 48.9 250 0

BNN1 L-shaped bar Normal 45.5 Normal 30.3 250 2.8 million

BNN2 L-shaped bar Normal 45.5 Normal 30.3 250 0

BN1 L-shaped bar Normal 45.5 n.a. n.a. 250 0

BN2 L-shaped bar Normal 45.5 n.a. n.a. 250 0

CNH1 Welded straight bar Normal 45.5 HESC 48.9 250 0

CNH2 Welded straight bar Normal 45.5 HESC 48.9 250 3.1 million

CNN1 Welded straight bar Normal 45.5 Normal 30.3 250 0

CNN2 Welded straight bar Normal 45.5 Normal 30.3 250 0

CN1 Welded straight bar Normal 45.5 n.a. n.a. 250 0

CN2 Welded straight bar Normal 45.5 n.a. n.a. 250 0

W-260A Straight bar full-lap-length splice Normal 31.4 n.a. n.a. 260 3.7 million

W-260B Straight bar full-lap-length splice Normal 31.4 n.a. n.a. 260 0

WA-130A Straight bar half-lap-length splice Normal 31.4 n.a. n.a. 130 0

WA-130B Straight bar half-lap-length splice Normal 31.4 n.a. n.a. 130 1.2 million*

WB-130AStraight bar half lap length confined by stirrups

Normal 31.4 n.a. n.a. 130 3.1 million

WB-130BStraight bar half lap length confined by stirrups

Normal 31.4 n.a. n.a. 130 0

WC-130AStraight bar half lap length confined by steel spiral

Normal 31.4 n.a. n.a. 130 0

WC-130BStraight bar half lap length confined by steel spiral

Normal 31.4 n.a. n.a. 130 3.1 million

*Sample WA-130B failed during cyclic loading test after approximately 1.2 million load cycles.Note: HESC = high-early-strength concrete; n.a. = not applicable. 1 mm = 0.0394 in.; 1 MPa = 145 psi.


applied load, the theoretical stress at the soffit of the test specimens was close to the nominal cracking stress of the concrete used in the slabs.

Test procedure The test specimens were subjected to a cyclic sinusoidal load with a peak amplitude of 18 kN (4.0 kip) applied at the center of the test specimens at a frequen-cy of 1 Hz. A total of approximately 3 million load cycles were applied on each specimen.

Behavioral tests To investigate the effect of the cyclic loading, behavioral tests were performed on the test slabs at specific stages during the cyclic load testing, typically after every 1 million load cycles. In these tests, the slab was load-ed statically in equal load increments up to the maximum

Figure 5. Instrumentation of specimens.

Vertical

displacement

transducer

Strain gauge across

cold joint

Strain gauge at middle

of closure strip

Figure 6. General arrangement for testing specimens

Test specimen setup

Hydraulic jack system applied load on specimen

Figure 7. Simulation of a single wheel load.


fine cracks would typically widen gradually as the cyclic loading progressed. No other cracks were observed in any of the cyclic load tests.

Concrete strain behavior

Figure 8 shows the load-concrete strain relationships obtained at midspan and cold joint during initial loading of a typical closure strip (that is, before the start of the cyclic load test). The test specimen is identified as CNH2 (Table 1). The different runs, representing complete loading-un-loading cycles, were obtained by increasing the maximum load in each successive run until the maximum applied cyclic load level of 18 kN (4 kip) was reached.

The relationships were characterized by the following observations:

• Behavior was linear, particularly during the ascending part of the load cycle.

• Concrete strains measured in the middle of the closure strip were generally close to the values predicted by theory.

• Strains measured across the cold joint were signifi-cantly higher than the strains recorded at midspan despite the lower bending moment at the joint. For example, a maximum strain of about 1235 με was measured at the cold joint compared with a peak strain of only 71 με at the middle of the closure strip.

load level used in the dynamic test and then decreased in similar steps to zero. The structural response (strains, verti-cal displacements, or both) of the test model was monitored for every load step. Data were generally obtained for at least two load cycles.

Ultimate load tests

The ultimate load test was conducted using a concentrated load representing a single wheel (Fig. 6). It was applied on a 100 mm (4 in.) square patch to simulate the footprint of a typical wheel load (Fig. 7).

Test procedure The ultimate load test was performed by applying an increasing load at the center of the slab until the slab failed. The deflection at the center of the test speci-men was monitored to generate load-deflection curves. To study the post-elastic behavior of the deck slab, a number of load cycles were performed, with the maximum load increased for each successive cycle. A number of loading and unloading cycles, each with increasing load level, were thus obtained before failure.

Cyclic load test results

Slab cracking behavior

All test specimens that were subjected to cyclic load tests were closely examined for cracks after every 1 million load cycles. It was observed that the first hairline cracks invariably formed at the cold joint between the precast (first stage) concrete and the closure strip. These initially

Figure 8. Load-strain relationships at middle of closure-strip and cold-joint locations for test model CNH2 prior to cyclic load test. Note: 1 kN = 0.225 kip.

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0 200 400 600 800 1000 1200 1400 1600

App

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Longitudinal strain, με

Midspan - Run 1

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Theory

Cold Joint - Run 1

Run 2

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Strain across cold joint

Concrete strain at midspan


• Similar and repeatable deflection patterns were obtained at the different stages during the cyclic load test.

• The slope of the load-deflection relationship under-went a small but distinct decrease in magnitude as the cyclic load test progressed, indicating a slight reduction in slab stiffness as a result of the repetitive loading.

Longitudinal concrete strain Figure 10 shows the load–concrete strain relationships at the center of the slab soffit of specimen CNH2 at different stages during the cyclic loading test.

The relationships were characterized by the following:

• linear behavior, particularly during the ascending part of the load cycles

• distinct increase in the slope of the load-strain graphs during cyclic loading, leading to lower incremental concrete strains in the closure strip compared with corresponding values observed prior to the start of cyclic loading

These strains represent incremental values because the da-tum for the strain gauge measurements was reset for each behavioral test. Nevertheless, the change in slope of the load-strain graph suggests that cracks formed at the cold joints during the initial loading runs had a major effect on

• Hysteretic losses were observed at both locations, caused by different loading and unloading paths as the loading reached the maximum applied load of 18 kN (4.0 kip).

Permanent strains were recorded by the strain gauge at the cold joint by the end of run 4, which coincided with the development of visible hairline cracks there. Based on a concrete cracking stress of 0.4 f

cl (where fcl is the

concrete compressive strength) given by CHBDC, the theoretical cracking strain was estimated to be about 93 με, higher than the maximum strain measured at midspan but considerably smaller than that at the cold joint. Permanent cracking was therefore inevitable at the cold joint under the test load applied.

Behavioral test results

Static load tests were performed on the slab specimens at regular intervals during the cyclic loading. Following is a discussion of the results of these tests.

Slab deflection Figure 9 shows load-deflection rela-tionships at the middle of the closure strip of specimen CNH2 at different stages during cyclic loading, typically after about every 1 million cycles.

The following observations can be made:

• The vertical displacement generally varied linearly with applied load.

Figure 9. Load-displacement relationships obtained at the middle of the closure strip of specimen CNH2 at different stages during cyclic loading, typically after about every 1 million load cycles. Note: 1 mm = 0.0394 in.; 1 kN = 0.225 kip.


Figure 10. Load–concrete strain relationships obtained at the center of the slab soffit of specimen CNH2 at different stages during cyclic loading. Note: 1 kN = 0.225 kip.

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Run 2

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1.35M - Run 1

Run 2

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2.2 M - Run 1

Run 2

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3.1 M - Run 1

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Run 3

Prior to cyclic loading

During cyclic loading

Figure 11. Load-deflection relationships obtained at midspan of slab specimens that illustrate the effect of closure strips on flexural behavior. Note: 1 mm = 0.0394 in.; 1 kN = 0.225 kip.

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Vertical displacement, mm

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With closure strip

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Specimen ANN1: U-bar connection Specimen BNN2: L-bar connection

Specimen CNH1: welded bar connection All specimens


the flexural curvature at the soffit of the slab, affecting the load effects and strains in the closure strip.

Effect of closure strips

Figure 11 shows the effect of closure strips on the load-deflection relationships for three individual test specimens that represent the different reinforcement connection de-tails in the closure strips. These three specimens were not subjected to any cyclic loading tests. In addition, a similar graph, which included all specimens tested in this study, was plotted to show the global effect of closure strips. The relationships were typically characterized by the following:

• The control (monolithic) specimens exhibited a distinct linear load-deflection relationship up to an applied load of about 30 kN (6.7 kip), after which the slope showed a marked decrease as the slab stiffness underwent a significant change.

• The closure-strip specimens exhibited a somewhat linear behavior up to a load of about 10 kN (2.2 kip) before undergoing a gentle and progressive change in slope as the flexural stiffness gradually decreased.

• The monolithic specimens exhibited a steeper initial load-deflection slope, leading to lower initial deflec-tions than the closure-strip specimens.

• The closure strips had generally less of an effect on the load-deflection relationships after the applied load exceeded about 35 kN (7.9 kip).

The difference in the initial load-deflection slopes for spec-imens cast with and without closure strips indicates that closure strips have a distinct effect on the overall stiffness of the test specimens, with closure-strip specimens show-ing larger initial deflections than the controls. This behav-ioral characteristic is amplified in these specimens because

they were tested as simply supported flexural members. It is unlikely that this behavior will have the same effect in real bridge decks, in which vertical loads are carried by arching action due to slab confinement and restraint.

Effect of closure-strip concrete

Figure 12 compares the load-deflection relationships obtained when a high-early-strength concrete is used in the closure strip. The results shown apply to closure strips with L-shaped and welded reinforcement connection details. The high early concrete strength does not have a major effect on the overall behavior of the test specimens in the long term, with observed displacements that were, in general, marginally higher for models with early-strength concrete in the closure strip. This shows that the high-early-strength concrete tends to give specimens a slightly lower flexural stiffness when used in closure strips than its normal concrete counterpart. One possible explanation is increased shrink-age of the early-strength concrete, leading to larger or more numerous shrinkage cracks at the cold joints.

Effect of reinforcement connection details in closure strip

Figure 13 shows a comparison of the load-deflection rela-tionships obtained with different reinforcement connection details that were used to provide continuity of reinforce-ment in the closure-strip specimens. As described earlier, three different reinforcement details were used to provide continuity of steel reinforcement in the closure strips:

• U-shaped bars

• L-shaped bars

• welded straight bars

Figure 12. Load-deflection relationships at midspan of slab specimens illustrating the effect of two types of closure-strip concrete. Note: 1 mm = 0.0394 in.; 1 kN = 0.225 kip.

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L-shaped bar Welded straight bar


All three reinforcement connection details provided gener-ally similar performance in terms of overall flexural stiff-ness, particularly under applied loads of up to 20 kN (4.5 kip). For higher loads, the welded-bar detail tends to give a stiffer flexural system with smaller deflections, especially if the specimen has not been subjected to cyclic loading.

Reinforcement lapping systems for straight bars

Use of straight bars to provide continuity between precast concrete deck panels requires the reinforcing bars to be spliced with a specified lap length in the closure strip. De-pending on the bar size, this could translate into relatively large closure pours, which would slow down construction. As a result, this study was expanded to investigate the per-formance of different reinforcement lapping systems when straight bars are used to provide continuity between precast concrete deck components. Four different scenarios were considered in which the following lap details were used:

• standard lap length per CHBDC

• reduced (half) lap length

• half lap length confined by two vertical stirrups

• half lap length confined in steel spiral (Fig. 3)

The last two options were intended to examine the effect of lateral confinement on the lap splice. Eight slab specimens were constructed for this study and were cast monolithical-ly to isolate the effect of closure strips. Half of the speci-mens were subjected to cyclic loading, and all were tested to failure afterward to assess their relative strengths.

Effect of lap lengths

Cyclic Loading Figures 14 and 15 show the effect of cyclic loading on the load-deflection relationships obtained from the behavioral tests for the different reinforcement lapping details described previously. The deflections shown in these figures represent incremental values, as a new datum is used for deflection measurements after each incre-mental load cycle. The following observations are made:

• Standard lap lengths provided the best performance in terms of overall slab behavior, with deflections that remained fairly constant over the course of the cyclic load test and peaked at about 0.6 mm (0.024 in.) after more than 3 million load cycles.

• Half-lap-length splices provided the worst results, with the specimen failing in flexure after about 1.2 million load cycles and a maximum deflection of ap-proximately 1.2 mm (0.047 in.) after 1.1 million load cycles.

• When confined by vertical stirrups, the half-lap-length specimen endured the full cyclic load test, though the slab showed a significant decrease in flexural stiffness early on, with a maximum measured deflection of about 1.0 mm (0.039 in.) after 1.1 million load cycles, with fairly constant deflection thereafter.

• When confined inside steel spirals, the half-lap-length specimens showed a gradual decrease in flexural stiff-ness over the course of the cyclic load test but ended up with a similar maximum deflection of about 1.1 mm (0.043 in.).

Figure 15 shows the load-deflection relationships at the end of the cyclic load test for the two best-performing reinforcement lap details (that is, the standard lap length

Figure 13. Load-deflection relationships at midspan of slab specimens, illustrating the effect of different types of reinforcing-bar connection systems and combination of cyclic loading and reinforcing-bar connection details on flexural behavior. Note: 1 mm = 0.0394 in.; 1 kN = 0.225 kip.

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Before cyclic loading After 3 million load cycles


• Specimens that were not subjected to cyclic loading exhibited a stiffer flexural behavior during the initial elastic load range.

and the half lap length confined inside a steel spiral). The relationships are characterized by the following features:

• All specimens initially behaved in a linear elastic manner.

Figure 14. Load-deflection relationships at midspan of slab specimens illustrating the effect of different types of reinforcing-bar lapping systems for straight bars and cyclic loading on overall flexural behavior. Note: 1 mm = 0.0394 in.; 1 kN = 0.225 kip.

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Reinforcing bars lapped with standard lap length Reinforcing bars lapped with half of standard lap length

Reinforcing bars lapped with half of standard lap length, confined inside steel spiral

Reinforcing bars lapped with half of standard lap length, confined by vertical stirrups

Figure 15. Load-deflection relationships at midspan of slab specimens illustrating the effect of cyclic loading on the flexural behavior of specimens with standard lap length and half lap length confined with steel spiral. Note: 1 mm = 0.0394 in.; 1 kN = 0.225 kip.

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Reinforcing bars lapped with standard lap length Reinforcing bars lapped with half of standard lap length,confined with steel spirals


• The difference in flexural stiffness due to cyclic load-ing in the initial elastic load range was significantly less for the standard lap length.

• In the postelastic load range, the specimens that were cyclically loaded exhibited greater stiffness (less de-flection) than those that were not cyclic loaded.

• The lap detail has a major effect on overall slab behavior.

Effect of lap confinement

Figure 16 illustrates the effect of lap confinement and cyclic loading on the load-deflection behavior of the test specimens. For specimens not subjected to cyclic loading, the unconfined standard-lap detail and the confined half-lap-length detail exhibited similar flexural stiffness proper-

ties during the initial elastic load range, after which the load-deflection relationships underwent significant changes with the confined lap detail giving much larger deflections than the unconfined standard lap detail. For specimens that were subjected to cyclic loading, the confined half-lap-length detail initially showed a significant drop in flexural stiffness (larger deflection) of the test specimen. However, as the load was increased beyond the 40 kN (9.0 kip) level, the load-deflection behavior tended to be similar for both details.

Comparison of reinforcement lapping details

Figure 17 illustrates the relative performance of the dif-ferent reinforcement lapping systems investigated in this study. To isolate the effect of cyclic loading, the results are presented separately for specimens subjected to cyclic

Figure 16. Load-deflection relationships at midspan of slab specimens illustrating the effect of lap confinement and cyclic loading on overall flexural behavior. Note: 1 mm = 0.0394 in.; 1 kN = 0.225 kip.

0

10

20

30

40

50

0.0 1.0 2.0 3.0

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, kN


Standard lap length

Half-lap length in coil

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20

30

40

50

0.0 1.0 2.0 3.0

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Standard lap length


Without cyclic loading With cyclic loading

Figure 17. Load-deflection relationships at midspan of slab specimens illustrating the performance of different reinforcing-bar lapping details. Note: 1 mm = 0.0394 in.; 1 kN = 0.225 kip.

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Continuous bar (no lap detail)

Half-lap length (unconfined)


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Without cyclic loading With cyclic loading


with increasing load level, were obtained before failure of the sample was achieved in the final run (Table 2). The deflection of the sample at the loading location was moni-tored to generate load-deflection curves. The failure mode and crack pattern of all the test specimens were recorded during the strength tests.

Failure crack pattern

For models with closure strips, cracking invariably started at the cold joints and grew steadily until slab failure oc-curred (Fig. 18). Additional transverse cracks would also form at the soffit of the slab before the specimen failed in flexure. For specimens cast monolithically, failure was initiated by cracks that typically formed within a 750-mm-wide (30 in.) area at the midspan of the specimen and gen-erally coincided with the location of the bottom transverse steel reinforcement.

Figure 19 shows the load-deflection relationships repre-senting the multiple loading and unloading cycles obtained during the ultimate-load test of a typical closure strip specimen (ANN2). The specimen initially behaved in a linear elastic manner for applied loads of up to about 30 kN (6.7 kip). Thereafter, a distinct change in slope of the load-deflection curves can be observed, indicating the tran-sition to the post-elastic behavior when deflections started to increase more rapidly with increasing load.

Table 2 lists the failure loads obtained from all the ulti-mate load tests. Because the specimens were not cast from the same concrete batch, the concrete strengths would vary among specimens, which would affect the flexural strengths to some extent. To isolate this effect, the ultimate failure loads were normalized with respect to the individ-ual theoretical flexural strengths (Fig. 20). The following observations can be made:

load tests and those that were not. It can be concluded from the load-deflection relationships that for specimens not subjected to cyclic loading, the different lap details exhibited similar flexural stiffness properties, particularly during the initial elastic range, after which the load-de-flection relationships underwent significant changes with the standard-lap-length detail providing the highest overall flexural stiffness. The standard-lap detail provided the best load-carrying capability, followed by the half lap length confined in steel spiral and the unconfined half lap length Both standard-lap detail and continuous-bar detail (without lapped splice) exhibited similar overall flexural behavior, confirming the effectiveness of the standard-lap detail.

For specimens subjected to cyclic load tests, the following observations are made:

• All specimens with reduced lap lengths exhibited similar flexural stiffness properties during the initial elastic-load range, while the standard-lap-length detail gave the highest flexural stiffness.

• The standard-lap-length detail and the half lap length confined in a steel spiral detail gave generally similar load-deflection characteristics, particularly in the pos-telastic range, while the remaining reduced-lap-length detail that was successfully tested (half lap length con-fined by vertical stirrups) exhibited reduced stiffness and reduced ultimate-load-carrying capability.

Ultimate load test results

All test specimens, including those subjected to cyclic load tests, were subsequently tested for ultimate flexural strength. The ultimate-load test was conducted by apply-ing an increasing load at the center of the slab until the slab failed. To study the post-elastic behavior of the test specimens, a number of loading and unloading cycles, each

Figure 18. Typical failure crack patterns developed during ultimate strength test of specimen BNH2.

Side view of closure strip Bottom view of slab soffit


any effects in actual bridge decks are not expected to be significant, as the test specimens in this study car-ried load in flexure as simply supported spans while bridge decks carry vertical loads by arching action between girders.

• Reduced lap length in conjunction with a steel spiral for lap confinement can be as effective as a full-lap-length splice detail.

• Reduced-lap length bars with no lap confinement or reduced-lap-length bars confined by vertical stirrups are unsuitable as a splice detail for straight bars.

Acknowledgments

The authors would like to thank the technical staff of the Bridge Office Structural Laboratory for constructing the test specimens, installing the electronic instrumentation, setting up the load tests, and acquiring the test data. These hard working technicians include Leon Pena and Howard Sahsuvar. Their excellent and dedicated efforts are grate-fully acknowledged.

The authors would also like to express their gratitude to King Packaged Material Co. for providing concrete mate-rial for the test specimens.

• All three connection details for providing reinforce-ment continuity in the closure strips satisfied the strength criteria after being subjected to extensive cyclic loading.

• If straight lapped bars are used, a reduced lap length can be used provided that the lapped splice area is con-fined in a steel spiral.

• Two of the straight-bar-lapping systems (that is, un-confined half-lap-length bars and half-lap-length bars confined by vertical stirrups) failed to meet calculated strength predictions and are unsuitable as a splice detail for straight bars.

Conclusion

Based on the results of this experimental study, the follow-ing conclusions can be drawn:

• U-shaped bars, L-shaped bars, and welded straight bars are equally effective in providing reinforcement continuity in closure strips from both fatigue and ulti-mate strength standpoints.

• The use of high-early-strength concrete in closure strips does not have a significant effect on their long-term structural behavior.

• Closure strips result in increased deflections in test slab specimens as compared with monolithic speci-mens, but do not affect ultimate strength. However,

Figure 19. Load-deflection relationships at midspan of test slab ANN2 during loading/unloading test cycles up to failure. Note: 1 mm = 0.0394 in.; 1 kN = 0.225 kip.

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Table 2. Ultimate load test results for all test specimens

Sample identificationReinforcement

connection detailClosure strip Load cycles applied Failure load, kN

C1 Continuous bar No 0 72.8




ANH1 U-shaped bar Yes 0 78.2

ANH2 U-shaped bar Yes 0 74.7

ANN1 U-shaped bar Yes 0 68.3

ANN2 U-shaped bar Yes 2.9 million 73.8

AN1 U-shaped bar No 0 81.7

AN2 U-shaped bar No 0 75.1

BNH1 L-shaped bar Yes 0 64.5

BNH2 L-shaped bar Yes 0 80.5

BNN1 L-shaped bar Yes 2.8 million 66.2

BNN2 L-shaped bar Yes 0 65.6

BN1 L-shaped bar No 0 74.9

BN2 L-shaped bar No 0 81.7

CNH1 Welded bar Yes 0 65.8

CNH2 Welded bar Yes 3.1 million 64.5

CNN1 Welded bar Yes 0 79.8

CNN2 Welded bar Yes 0 65.8

CN1 Welded bar No 0 76.7

CN2 Welded bar No 0 63.1

W-260A Full lap length No 3.7 million 56.0

W-260B Full lap length No 0 57.8

WA-130A Half lap length No 0 38.0

WA-130B Half lap length No 1.2 million* 17.6

WB-130AHalf lap length confined by vertical stirrups

No 3.1 million 39.1

WB-130BHalf lap length confined by vertical stirrups

No 0Broke accidentally during test set-up

WC-130AHalf lap length confined by spiral

No 0 60.6

WC-130BHalf lap length confined by spiral

No 3.1 million 56.3

*WA-130B failed during cyclic loading after about 1.2 million load cycles.Note: 1 kN = 0.225 kip.


References

1. Au, A., C. Lam, and B. Tharmabala. 2008. Investiga-tion of Prefabricated Bridge Systems Using Reduced-Scale Models. PCI Journal, V. 53, No. 6 (November–December): pp 67–95.

2. Harryson, P. 2003. High Performance Joints for Concrete Bridge Applications. Structural Engineering International, V. 13, No. 1 (January): pp 69–75.

3. Ralls, M. L., M. D. Hyzak, R. D. Medlock, and L. M. Wolf. 2004. Prefabricated Bridges—Current U.S. Practice and Issues. FHWA/AASHTIO. Paper present-ed at the 2nd National Prefabricated Bridge Elements and Systems Workshop, New Jersey.

4. Shah, B. N., K. Sannah, M. R. Kianoush, S. Tu, and C. Lam. 2007. Experimental Study on Prefabricated Concrete Bridge Girder-to-Girder Intermittent Bolted Connections System. Journal of Bridge Engineering, V. 12, No. 5 (September–October ): pp 570–584.

5. Bakht, B. 1996. Revisiting Arching in Deck Slabs. Canadian Journal of Civil Engineering, V. 23, No. 4 (August): pp. 973–981.

6. CSA International. 2000. Canadian Highway Bridge Design Code. CAN/CSA-S6-00. Toronto, ON, Canada: CSA International.

Notation

αL = live load factor

fcl = concrete compressive strength

Figure 20. Failure loads from ultimate strength tests of all specimens normalized by theoretical flexural load capacities.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

C1

C2

C3

C4

AN

H1

AN

H2

AN

N1

AN

N2

AN

1

AN

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N2

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W-260A

W-260B

WA

-130A

WA

-130B

WB

-130A

WB

-130B

WC

-130A

WC

-130B

Nor

mal

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mat

e lo

ad

Sample identification

C

C

C

C

Controls U-shaped bars L-shaped bars Welded bars

C C

Straight lapped bars

C - Cyclic loaded

C


About the authors

Alexander Au, PhD, P.Eng, is an associate research engineer in the Bridge Office of the Ministry of Transportation of Ontario in St. Catharines, ON, Canada.

Clifford Lam, PhD, P.Eng, is the head of bridge research in the Bridge Office of the Ministry of Transportation of Ontario in St. Catharines.

Bala Tharmabala, PhD, P.Eng, is the manager of the Bridge Office of the Ministry of Transportation of Ontario in St. Catharines.

Synopsis

In 2001 the Bridge Office of the Ministry of Trans-portation of Ontario initiated a research project to investigate the use of precast bridge technology as an innovative approach to bridge construction and rehabilitation. Two potential precast systems were investigated: precast composite slab-on-girder beam elements connected with cast-in-place concrete closure strips, and full-depth precast concrete deck slab panels laid across new or existing girders and connected with cast-in-place concrete closure segments.

One of the main issues inherent in these precast systems is the presence of cold joints created by the closure pours and their potential effect on the overall deck system behavior. In addition, there is a need to develop effective connection details between the pre-cast elements to provide continuity of reinforcement in

the closure strips so that load sharing between girders is not compromised.

This paper presents the findings of a series of labora-tory tests performed on 2/3-scale models of the closure-strip system to study the serviceability and ultimate state behavior of the joint detail. Several types of reinforcement lapping systems were considered for the steel reinforcement in the closure strips: U-shaped bars, L-shaped bars, welded straight bars, and straight bars with full lap splice length as well as splices with reduced lap length but confined by steel spirals or stir-rups.

Scale models of the closure-strip connections were subjected to extensive cyclic loading and then tested to failure. Overall, the test results demonstrated the excel-lent performance of U-shaped, L-shaped, and welded straight reinforcing bars as connection details in the closure strips. The use of high-early-strength concrete in the closure pour was shown to have little effect on long-term performance, while a shortened lap length confined in a steel spiral was shown to be as effective as the conventional full-lap-length splice detail.

Keywords

Closure strip, connection details, cyclic load test, prefabricated bridge system, reduced-scale models, ultimate load test.

Review policy

This paper was reviewed in accordance with the Precast/Prestressed Concrete Institute’s peer-review process.

Reader comments

Please address any reader comments to [email protected] or Precast/Prestressed Concrete Institute, c/o PCI Journal, 200 W. Adams St., Suite 2100, Chicago, IL 60606. J

investigation of closure- strip details for connecting

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