Bond Behavior of Reinforcing Steel in Ultra-High Performance Concrete

CHAPTER 1. INTRODUCTION

GENERAL

The use of accelerated bridge construction (ABC) techniques continues to grow as owners across the country look for construction solutions that reduce impacts on the users of the infrastructure. In ABC construction, one common technique used is prefabricated bridge elements and systems (PBES). In this practice, bridge elements are prefabricated offsite, then assembled and connected onsite during an expedited construction timeframe. The use of prefabricated bridge elements necessitates the use of field-applied connections between these elements. Field-cast concrete or other cementitious material connections have been deployed for decades by State departments of transportation (DOTs). However, decades of experience has led to the recognition that the field-cast connections often prove to be susceptible to degradation that can lead to substandard performance of the overall bridge system.

Connection systems for PBES are selected based on a variety of considerations. Critical properties of connection systems can include the rate of mechanical property development within the connection, the dimensional stability of the field-cast material, the durability of the field-cast material, and the ease of construction of the overall system.⁽¹⁾ This document reports on research conducted through the Structural Concrete Research Program at the Federal Highway Administration Turner-Fairbank Highway Research Center. In this research project, connection details using different grout materials, including traditional non-shrinkage grout, epoxy grout, Ultra-High Performance Concrete (UHPC), and magnesium grout, are being evaluated. This report mainly focuses UHPC materials.

UHPC is a relatively new class of cementitious composite materials. Since 2000, when UHPC became commercially available in the United States, a series of research projects has demonstrated the capabilities of the material. A handful of State DOTs have deployed UHPC components within their infrastructure, and many more are actively considering the use of UHPC. Many State DOTs, bridge design firms, and construction firms have expressed their interest of using UHPC in bridge construction, especially for field-cast connections deployed in the construction of PBES structures. As late 2013, 32 bridges in the United States have been constructed using field-cast UHPC connections.⁽²⁾

As opposed to conventional grouted connections which frequently contain complex reinforcement configurations, UHPC connections often involve much simpler reinforcement configurations such as the lap splicing of straight lengths of reinforcement. A few specific connection details, such as those discussed inBehavior of Field-Cast Ultra-High Performance Concrete Bridge Deck Connections Under Cyclic and Static Structural Loading and Development of a Field-Cast Ultra-High Performance Concrete Composite Connection Detail for Precast Concrete Bridge Decks, have been rigorously tested at service and ultimate performance limits.^(3,4) The advanced material properties of UHPC provide a potential to develop a simple and robust connection system for prefabricated bridge elements. The research project discussed in this document investigated the bond performance between deformed reinforcing bar and UHPC. One objective of the research is to facilitate the development of design guidelines for using field-cast UHPC in innovative connection details.

ULTRA-HIGH PERFORMANCE CONCRETE

Advances in the science of concrete materials have led to the development of a new class of advanced cementitious materials, namely UHPC. These concretes tend to contain high cementitious materials contents and very low water-to-cementitious materials ratio, and to exhibit high compressive and tensile strengths. The discrete steel fiber reinforcement included in UHPC allows the concrete to maintain tensile capacity beyond cracking of the cementitious matrix. UHPC has been defined as follows:

UHPC is a cementitious composite material composed of an optimized gradation of granular constituents, a water-to-cementitious materials ratio less than 0.25, and a high percentage of discontinuous internal fiber reinforcement. The mechanical properties of UHPC include compressive strength greater than 21.7 ksi (150 MPa) and sustained post-cracking tensile strength greater than 0.72 ksi (5 MPa). UHPC has a discontinuous pore structure that reduces liquid ingress, significantly enhancing durability compared to conventional concrete.^(2,5)

A typical field-cast UHPC material properties are presented in Table 1, which represents average values for a number of test parameters relevant to the use of UHPC as obtained from independent testing of a commercially available product.⁽⁶⁾ This research published by the Federal Highway Administration in 2006 investigated a number of material properties of UHPC. The research analyzed both mechanical- and durability-based behaviors of UHPC to assess its potential for use in future highway and bridge construction projects. It should be noted that the UHPC investigated in that study was designed for precast applications with accelerated curing and thus exhibited a reduced compressive strength under field casting and curing applications as compared to current UHPC products.

LITERATURE REVIEW ON BOND STRENGTH BETWEEN DEFORMED REINFORCING STEEL AND UHPC

Limited research has investigated the bond performance of deformed reinforcing bar in UHPC. New York State Department of Transportation performed pullout tests on reinforcing bar embedded in 15.7 in. (400 mm) diameter UHPC cylinders.⁽⁵⁾ The No. 4, 5, and 6 bars were embedded 2.9, 3.9, and 4. inches (75, 100, and 125 mm) into the UHPC, respectively, and all fractured before bond failure.

Fehling et al. ⁽⁷⁾ performed pullout tests on 0.47 in. (12 mm) diameter bars with varied concrete cover and embedment length in UHPC. Their test results indicated that increasing concrete cover and increasing embedment length each resulted in an increase on the bond strength. For specimens with a concrete cover of 1.5d_s (d_s: diameter of reinforcing steel), an embedded length of 8d_s would yield the bar with a yield strength of approximately 80 ksi (551 MPa). For specimens with a concrete cover of 2.5 d_s, an embedded length of 5d_s would yield the bar.

Table 1. Typical field-cast UHPC material properties.

Material Characteristic	Average Result
Density	2,480 kg/m3	(155 lb/ft3)
Compressive Strength (ASTM C39; 28-day strength)	126 MPa	(18.3 ksi)
Modulus of Elasticity (ASTM C469; 28-day modulus)	42.7 GPa	(6200 ksi)
Split Cylinder Cracking Strength (ASTM C496)	9.0 MPa	(1.3 ksi)
Prism Flexure Cracking Strength (ASTM C1018; 305-mm (12-in.) span)	9.0 MPa	(1.3 ksi)
Mortar Briquette Cracking Strength (AASHTO T132)	6.2 MPa	(0.9 ksi)
Direct Tension Cracking Strength (Axial tensile load)	5.5-6.9 MPa	(0.8-1.0 ksi)
Prism Flexural Tensile Toughness (ASTM C1018; 305-mm (12-in.) span)	I30 = 48
Long-Term Creep Coefficient (ASTM C512; 77 MPa (11.2 ksi) load)	0.78
Long-Term Shrinkage (ASTM C157; initial reading after set)	555 microstrain
Total Shrinkage (Embedded vibrating wire gage)	790 microstrain
Coefficient of Thermal Expansion (AASHTO TP60-00)	14.7 x10-6 mm/mm/°C (8.2 x10-6 in./in./°F)
Chlorid Ion Penetrability (ASTM C1202; 28-day test)	360 coulombs
Chloride Ion Permeability (AASHTO T259; 12.7-mm (0.5-in.) depth)	< 0.06 kg/m3	(< 0.10 lb/yd3)
Scaling Resistance (ASTM C672)	No Scaling
Abrasion Resistance (ASTM C944 2x weight; ground surface)	0.73 grams lost	(0.026 oz. lost)
Freeze-Thaw Resistance (ASTM C666A; 600 cycles)	RDM = 112%
Alkali-Silica Reaction (ASTM C1260; tested for 28 days)	Innocuous

Graybeal and Swenty ⁽⁸⁾ conducted pullout tests on No.4 reinforcing steel embedded into 6 in. (152 mm) cubes. Two types of UHPC were tested, with the test configuration including a 3 in. (76 mm) debond length and a 3 in. (76 mm) bond length along the centerline of the cube. One UHPC formulation resulted in bar yield before the ultimate pullout failure. The other UHPC formulation resulted in bar rupture before bond failure.

Pullout tests were also performed by Holschemacher et al. ^(9,10) and they observed that the bond strength and stiffness increases with testing ages.

OBJECTIVE

The objective of this research is to extensively evaluate the factors that affect bond strength between deformed reinforcing bar and UHPC and make recommendations for designs using reinforcing steel in UHPC.

OUTLINE OF REPORT

The research discussed herein investigated the parameters that could affect bond strength between deformed bar and UHPC. Direct tension pullout tests were conducted and more than 200 tests were included in this report. The parameters, including the structural characteristics like the embedment length, concrete side cover, bar spacing, bar size, and bar type and the materials properties like UHPC compressive strength and bar yielding strength, were considered in the study.

This report is divided into five chapters. Chapter 1 provides the introduction and the objective of the research. The background information about UHPC materials and the previous research on bond strength between deformed bar and UHPC are also included in Chapter 1. Then the experimental tests setup and the tests results are presented in Chapter 2 and Chapter 3, respectively. Design recommendations for using reinforcing steel in UHPC are presented in Chapter 4. At the end, Chapter 5 summarizes the conclusions based on this research.