TECHBRIEF Influence of Differential

Deflection on Staged

Construction Deck-Level

Connections

FHWA Publication No.: FHWA-HRT-12-055

NTIS Accession No. of the report covered in this TechBrief: 

PB2012-111528

FHWA Contact: Ben Graybeal, HRDI-40, (202) 493-3122, 

benjamin.graybeal@dot.gov

This document is a technical summary of the unpublished 

Federal Highway Administration (FHWA) report, Influence 

of Differential Deflection on Staged Construction DeckLevel Connections, available through the National Technical 

Information Service (NTIS) at www.ntis.gov.

Objective

The objective of this research project was to determine the 

impact of differential movement across a staged construction 

connection. Specifically, the research investigated the bond 

strength of reinforcing bars embedded within freshly cast 

connection grouts as impacted by differential movement of 

the rebar with respect to the embedment material.

Introduction

Rapid construction methods help prevent traffic delays and 

minimize inconveniences to the traveling public. Many new 

methods have been investigated and implemented using precast concrete subassemblies on bridges. These methods have 

shown great promise, partly because precast components 

can be produced with high quality control in precast plants, 

resulting in superior products that allow for expedited construction schedules. Owners continue to investigate and advance 

their respective bridge programs through the use of precast 

products such as precast bulb tees, full-depth precast bridge 

decks, and box beams.

The most critical field construction process for precast subassemblies is the completion of the connections. Long-term 

performance problems have developed in connections on past 

projects. These performance problems have been attributed 

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.tfhrc.gov/research/
2

to a variety of causes, including construction 

techniques, materials, and poor designs. Much 

attention has been placed on creating better 

connections between the components. 

One area of concern is the process of completing

connections while portions of the bridge remain 

under traffic loads. This is frequently referred 

to as staged construction. The traffic loading 

causes deflections on portions of the bridge, 

potentially resulting in significant differential 

deflections across the connections that join construction stages. For connections that contain 

discrete reinforcement, degraded performance 

may result from differential deflection-induced 

bond degradation between the reinforcement 

and the embedment material. A conceptual 

illustration of deck-level differential deflection 

across a staged construction connection is 

shown in figure 1.

This research effort studied the impact of differential deflections across staged construction 

connections. The size, frequency, and duration 

of the deflections are all factors of concern. 

Many types of grout materials have been proposed for use in these field-cast connections, 

and differential deflections may affect each one 

differently. The goal is to understand how the 

bond between the rebar and the grout material 

changes due to movements that occur during 

the curing process.

Summary of Approach

The testing program focused on the pull-out 

behavior of steel reinforcing bars cast into 

grouts that might be used in prefabricated 

bridge element connections. The implemented 

test method, which included 6-inch cube 

pull-out specimens with #4 (#13 metric (M)) 

rebar, was built on the ASTM C234-91a standard.(1) Differential deflections were imparted 

that ranged from 0.1 to 0.005 inches (0.254 to 

0.0127 cm) of linear bar movement perpendicular to the bar axis. The deflections were 

applied to the cube molds while the rebar was 

fixed in place. The deflections were imparted 

periodically at 30-s intervals and continued 

from casting until the final set of grout. 

A range of different grout materials were 

engaged in this research program. These 

included standard conventional grouts, deck 

concretes, ultra-high performance concretes 

(UHPCs), epoxy grout, magnesium phosphate 

grout, and cable grout. After the final set, the 

cubes were allowed to cure until approximately 

24 h after casting, at which point they were 

tested. A series of control cubes were also cast 

and tested for pull-out resistance. These control 

cubes remained stationary during curing. 

The pull-out tests were completed on sets of 

six specimens, with three having been deflected 

during curing and three serving as the nondeflected control specimens. A bond breaker 

between the bar and the grout allowed the test 

to assess only the bond of the bar to the grout 

in the portion of the specimen between 3 and 

6 inches (76 and 152 mm) from the location 

where the bar entered the specimen. The 

pull-out specimen geometry is illustrated in 

figure 2. The uniaxial tensile load was applied 

Figure 1. Differential deflection at a closure pour.

Concrete Deck

Stage 2—Dead Load

Closure Pour

Reinforcing Bars 

in the Closure Pour

Differential Deflection

Concrete Deck

Stage 1—Dead + Live Load
3

to the bar, and the force was resisted by the 

face of the grout cube through which the bar 

passed. The pull-out response was captured, 

and comparisons were made between the sets 

of specimens. Primary comparisons focused on 

the impact of deflection amplitude and type of 

grout on the overall pull-out performance. 

Test Results

From the perspective of the pure bond strength, 

deflecting the rebar prior to the final set of the 

embedment material can have a detrimental 

effect on the bond. When the rebar deflected 

0.05 inches (1.27 mm) or more, reduced 

bond capacity was observed. Deflections of 

0.01 inches (0.25 mm) or less created only small 

changes in the bond strength.

Tests aimed at assessing the impact of varying 

the rate of deflection (i.e., the deflection frequency) did not demonstrate any appreciable

difference in performance. Deflection frequencies of 2 and 5 Hz were assessed.

The test method proved to be an appropriate 

mechanism through which to assess the impact 

of differential deflection during staged construction on the bond performance of a rebar 

to field-cast grout. The specimen geometry

and loading protocols allowed the majority of 

test specimens to fail in a pull-out mode, thus 

providing an indication of the pull-out resistance of the bond. The lone exceptions to this 

observation were the epoxy grout, UHPC, 

and magnesium phosphate grout when subjected to 0.01 inches (0.25 mm) of differential 

deflection. The magnesium phosphate grout 

exhibited splitting failures. This type of failure 

is the result of a low tensile strength in relation to the local bond interaction between the 

bar and the grout. For materials of this type, 

modification of the testing protocols may be 

necessary. The epoxy grout and UHPC produced rebar tensile failures. For specimens 

producing this type of failure, either a shorter 

bond length or earlier testing at a lower 

compressive strength may be appropriate.

The companion static tests provided an indication of the relative bond strength of each 

grouting material. The epoxy grout and UHPC 

displayed the highest static bond strengths. 

With 3-inch (76-mm) embedment, #4 (#13M) 

rebar cast into these grouts ruptured at the 

rebar ultimate tensile capacity. The compressive 

strengths of these grouts at the time of pull-out

testing were approximately 12 and 14 ksi (83

and 96 MPa), respectively. Identical tests on 

conventional grouts resulted in pull-out mode 

failures at less than half of the rebar rupture load.

Conclusions

Differential deflection across a staged construction field-cast connection can have a detrimental

impact on the performance of the connection. 

This study investigated the bond of #4 (#13M) 

reinforcing bars and found that differential 

deflections equal to or greater than 0.05 inches 

(1.27 mm) caused a reduction in the bond 

strength. Not surprisingly, differential deflections seemed to ream a hole in the embedment 

material around the reinforcing bar, thus reducing the bond capacity. Although not studied in 

this project, movements along more than one 

axis may have an even greater impact on the 

bond performance of the reinforcement.

The grouts investigated in this study displayed 

a range of bond strengths. Grouts with higher 

Figure 2. Pull-out test specimen geometry.

Plan

Elevation

6 inches

6 inches

3 inches

3 inches

18 inches

1/2 inch

#4 (#13M) rebar 

Concrete

 Block

Foam Insulation 

(1.5-inch outside 

diameter)

1 inch = 25.4 mm
4

Researchers—This study was directed by Ben Graybeal at FHWA’s Turner-Fairbank Highway Research 

Center. Additional information can be gained by contacting him at (202) 493-3122 or in the FHWA 

Office of Infrastructure Research and Development located at 6300 Georgetown Pike, McLean, VA 

22101-2296.

Distribution—The unpublished report covered in this TechBrief is being distributed through the 

National Technical Information Service at www.ntis.gov.

Availability—This TechBrief may be obtained from the FHWA Product Distribution Center by email to 

report.center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at http://www.fhwa.dot.

gov/research.

Key Words—Prefabricated bridge elements and systems (PBES), Staged construction, Differential deflection, Reinforcing bar bond, Connection details, Pull-out resistance.

Notice—This document is disseminated under the sponsorship of the U.S. Department of 

Transportation in the interest of information exchange. The U.S. Government assumes no liability for 

the use of the information contained in this document. The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ names appear in this TechBrief only because they 

are considered essential to the objective of the document.

Quality Assurance Statement—The Federal Highway Administration provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. 

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bond strengths may provide appropriate performance even after being subjected to differential deflections. The overall system performance 

is highly dependent on the specific details of 

the connection design and the reserve capacity 

provided therein.

Differential deflections are highly dependent 

on a bridge’s structural configuration, the lateral load distribution system, and the locations 

of the applied loadings. Individuals associated with projects of this type are advised to 

carefully consider the staging process and the 

impacts that it might have on the structural 

performance of the bridge system. Reducing 

structural live loadings, increasing the stiffness 

of the superstructure load distribution system, 

and/or detailing the connections to exhibit 

reserve capacity may be appropriate remedies 

to address this issue.

Reference

1. ASTM C234-91a. (1991). ”Standard Test 

Method for Comparing Concretes on the 

Basis of the Bond Developed with Reinforcing Steel,” ASTM Book of Standards, 

ASTM International, West Conshokocken, PA.

august 2012 FHWA-HRT-12-055

HRDI-40/08-12(450)E