Dimensional Stability of Grout-Type Materials Used As Connections for Prefabricated Bridge Elements

CHAPTER 1. INTRODUCTION, OBJECTIVES, AND APPROACH

INTRODUCTION

Accelerated bridge construction (ABC) consists of innovative bridge construction methods that are used in a safe and cost-effective manner to minimize the inconveniences to the travelling public while delivering a superior finished product. This technology has become common over the last two decades in the United States, and many States are considering ABC on more typical projects.

The use of prefabricated bridge elements (PBEs) facilitates ABC. These elements are typically produced in a controlled environment that facilitates high production quality. The most critical field construction process for prefabricated subassemblies is the completion of the connections between elements. In general, connections must be robust, durable, and efficient. One common connection method involves the use of field-cast concretes or grouts in the interstitial spaces between the prefabricated components. As would be expected, some PBE connection details have been linked to constructability and serviceability problems within the deployed systems. Many times these issues have been attributed to less-than-desirable performance of the field-cast grouts that can be used in the connections.

Grout-type materials, especially cement-based grouts, can provide ease of placement as well as rapid strength development when used to connect PBEs. However, they have also shown dimensional instability due to the rapid rate of (inherent) shrinkage and the presence of expansive agents to try to counteract most of that shrinkage. While the ASTM C1107 Standard Specification for Non-Shrink Packaged Dry, Hydraulic-Cement Grout test method describes how cement-based grouts are to be tested, this specification focuses on ensuring that the materials achieve a minimum strength and that the expansion achieved is below a maximum limit.⁽¹⁾ However, the specification lacks a clear presentation of shrinkage limits and does not speak to potential incompatibilities with the surrounding materials (i.e., prefabricated concrete substrate) that can have negative effects on the performance of the material. This research investigated the dimensional stability performance of a variety of different grout-type material categories that may be used in concrete infrastructure connections.

RESEARCH OBJECTIVES

Grout-type materials have been extensively used in the construction industry. Their performance in terms of high workability and rapid strength development is well known and accepted by the end-users. Different types are currently available (e.g., cement-based, epoxy-based, etc.). However, rapid strength gain typically leads to rapid volume changes (e.g., expansion and/or contraction), especially in cement-based grouts, which are the most commonly used type of grout. In fact, there have been concerns about the lack of dimensional stability for this type of material. The main objective of the research effort reported herein was to better understand how these materials perform in terms of dimensional stability, especially at early ages, for their use in connections between prefabricated concrete elements. Once the dimensional stability was evaluated, means to partially mitigate most of the volume changes (especially shrinkage) were investigated. While one strategy consisted of providing internal curing (IC) through the use of prewetted lightweight aggregates (LWAs), other non-traditional grouts, such as an ultra-high performance concrete (UHPC), were also investigated as potential strategies to enhance the dimensional stability of grout-type materials.

In summary, the goals of the overall report are as follows:

1. Assess the dimensional stability of common grout-type materials using standardized tests.
   
   
2. Discuss the appropriateness of existing dimensional stability test methods.
   
   
3. Introduce cost-effective shrinkage mitigation strategies. This is done by: (1) laying the groundwork for including IC in cement-based grouts and (2) investigating the dimensional stability performance of a UHPC.

RESEARCH APPROACH

The research described in this report mainly focuses on the evaluation of the dimensional stability of commercially used grout-type materials that can potentially be used for connecting prefabricated concrete bridge elements. A selection of 11 different materials was made, including cementitious-based, epoxy-based, and magnesium phosphate-based grouts. A UHPC was also included in the research. Following the guidelines described in the ASTM C1107 test method, the grout performance in terms of initial fresh workability, compressive strength, and dimensional stability was assessed.⁽¹⁾ However, the test methods used for evaluating dimensional stability described in this standard specification have the inconvenience of considering several parameters simultaneously (e.g., thermal expansion, chemical expansion, chemical shrinkage, autogenous shrinkage, plastic shrinkage, settlement, etc.), thus providing a qualitative approach that is only useful for comparative purposes. To more completely assess this variety of parameters, volume changes must be assessed from a fundamental point of view by measuring pure expansion/shrinkage deformations. As such, additional tests to evaluate the dimensional stability of the grouts were carried out. These are described in ASTM C157, Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete, and ASTM C1698, Standard Test Method for Autogenous Strain of Cement Paste and Mortar, test methods.^(2,3) Other standardized tests were used in order to further characterize these materials. Finally, and given the fact that these grouts commonly exhibit shrinkage, this research also included additional tests focused on partial shrinkage mitigation by including IC in some of the cement-based grouts.

RESEARCH SIGNIFICANCE

While the use of grout-type materials as connections between prefabricated concrete elements in bridges has been shown as a promising technique to facilitate ABC, the fact that they are designed with a low water-to-solids ratio (w/s) makes them prone to early-age shrinkage. Shrinkage under restraint can cause the development of tensile stresses within the grout, leading to premature cracking when the tensile strength of the material is still low, or can cause stresses at the interface leading to loss of bond between the grout and the concrete substrate. In this report, several types of commonly used prepackaged grouts were selected for assessment of their dimensional stability at both early and later ages. In some of the cementitious grouts, the inclusion of IC through the addition of prewetted fine LWAs was evaluated. The IC technology in concrete has been broadly studied within the last decade, and its implementation in the concrete mixture design procedure is well defined at this point. In this study, IC is included in cement-based grouts, and the challenges encountered to do so are discussed. This method is proposed to improve curing conditions, since most of the grouts are poured in either sealed locations or points of difficult access for providing external (or conventional) curing. The results and conclusions are expected to provide guidance to designers and end-users in selecting the right grout-type material for use in connections between prefabricated concrete elements in bridges and other concrete structures.

OUTLINE OF REPORT

The report is divided into five chapters and an appendix. Chapters 1 and 2 provide an introduction and literature review. Chapter 3 presents a detailed description of the experimental program followed in the study. Chapter 4 presents the results and an in-depth discussion of the results. Finally, chapter 5 provides the main conclusions and recommendations taken from the study. An appendix is also included that contains some of the main material properties taken from the manufacturers' product data sheets.