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

Chapter 5. Conclusions and Recommendations

SUMMARY

The research presented in this report focuses on addressing performance concerns related to dimensional stability (primarily early age shrinkage) of commonly used grouts. Some grouts, many of which are classified as "non-shrink grouts," have been observed to display significant dimensional instability when deployed in connection details during bridge construction projects. ASTM C1107, the test method commonly used to assess the dimensional stability of these grouts, has been observed to deliver an incomplete picture of the overall performance.⁽¹⁾ This research demonstrates the types of performance that can be expected from these types of grouts, the shortcomings of the commonly used test methods, and alternative test methods that may better demonstrate real-world performance.

The general approach followed in this research was to first evaluate the dimensional stability of 11 commercially available grout-type materials following the guidelines described in the ASTMC1107 test method.⁽¹⁾ After an initial evaluation, it was observed that the tests methods used for evaluating dimensional stability described in this standard specification consider 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, measuring pure expansion/shrinkage deformations over time. As such, additional tests to evaluate the dimensional stability of the grouts were used (e.g., ASTM C157 and ASTM C1698).^(2,3) Finally, given the fact that these grouts commonly exhibit shrinkage, this research also included additional tests focused on the partial shrinkage mitigation by including IC in some of the cement-based grouts.

CONCLUSIONS AND RECOMMENDATIONS

Based on the results obtained, the following conclusions can be drawn:

• Most of the cement-based grouts evaluated in this research (G1-G4) seemed to perform well in terms of dimensional stability when tested in accordance with ASTM C1107.⁽¹⁾ However, separate testing to assess autogenous and drying deformations (shrinkage and expansion) show that behaviors not captured by the test method can result in a lack of dimensional stability. These grouts tended to undergo expansion during the first day or two, followed by shrinkage, especially in drying conditions. It has been demonstrated that this expansion is typically dominated by the formation of ettringite phases. The subsequent shrinkage is due mainly to the conversion of the ettringite phase into monosulfate or monocarbonate phases, a reaction that involves chemical shrinkage. Part of the expansive behavior of some of these grouts is also attributed to the formation of gas within the matrix. Given that ASTM C1107 is a standard for defining nonshrink grouts, it is relevant to raise awareness of the specific types of shrinkage and expansion that are captured by the standard.⁽¹⁾
  
  
• In general terms, the other types of tested materials, including epoxy, magnesium phosphate, and fly-ash-based, showed improved performance compared with the cementitious grouts based on the lower autogenous and drying deformations obtained. These are G5, E1, E2, F1, M1, and M2. Although commonly marketed as repair materials, these materials can be used as grouts, according to the materials' suppliers, due to their high initial flowability and early-age strength development. These materials barely showed shrinkage or dimensional instability, which makes them proper materials for applications where dimensional stability is a strong requirement. Note that limited results were obtained for M2 due to the very short setting times (about 5 min). Also, the ASTM test methods used in this research for assessing volume changes in cementitious grouts can present some difficulties when testing some non-cement-based grouts such as the ones used in this research.
  
  
• IC (through the use of prewetted LWAs) seemed to mitigate most of the autogenous shrinkage and reduced the drying shrinkage in the two cement-based grouts tested by half. This might be a good option for cementitious grouts, considering that other benefits might be provided (increased strength and durability, reduced transport, more compliant material with a lower cracking propensity, etc.). The implementation of the IC technology as a grout extension can be helpful not only in reducing shrinkage but also in improving curing conditions in some locations where conventional (external) curing is difficult or impossible to implement. This technology would also provide some robustness to the surface preparation (in terms of moisture content) of the precast (or existing) concrete elements because prewetted LWA may also serve as additional reservoirs if water is drawn from the grout into the substrate.
  
  
• Both ASTM C827 and ASTM C1090 test methods present a series of shortcomings that should be further considered when evaluating dimensional stability of grout-type materials, the main one being that both methods consider the simultaneous occurrence of several parameters that affect dimensional stability (e.g., surface settlement).^(39,40) This allows for a qualitative performance comparison, rather than for a quantitative assessment of shrinkage and expansion propensity. The experimental details in the test also present shortcomings. For instance, the current version of the ASTM C827 test method is time-consuming and prone to human error. As an alternative, a different setup was used in the presented research study. Other method limitations were previously described in the report.
  
  
• Material cost should be considered in the evaluation. While non-cement-based grouts are typically more expensive than cement-based grouts (up to four times more expensive, in some cases), they show higher strength development and fewer volume changes. Cement-based grouts are widely used due to their lower cost. If cement-based grouts are extended with prewetted LWA to include IC, then their initial cost could decrease while their performance could benefit.

The authors of the study recommend the use of modified and alternative ASTM test methods to better characterize the dimensional stability performance of grout-type materials. As shown in the study, some grout materials may seem to fulfill the volume stability requirements defined by the ASTM C1107 test method.⁽¹⁾ However, further evaluation shows a considerable amount of shrinkage in most of the grouts tested. The misinterpretation of performance could lead to an improper use of these materials, which would have an impact in the general performance of these materials when used as connections for PBEs.

In addition, IC is recommended as a convenient strategy to reduce shrinkage deformations and, consequently, shrinkage cracking. The inclusion of IC in pre-bagged grout materials could easily be implemented in the field as a grout extension or even as part of the premix material. This would also facilitate curing operations, especially in difficult-to-access locations.

ONGOING AND FUTURE RESEARCH

The assessment of the dimensional stability is an early step in evaluating the propensity for shrinkage and thermal cracking of these materials. Other tests need to be performed where a certain degree of restraint is provided so that the material would indeed crack, thus evaluating the cracking resistance of the material. In this regard, the commonly known dual ring test would be a good candidate for evaluating the autogenous and thermal cracking capacity of these materials because some of them (especially the repair materials) release a high amount of heat during their reaction that might cause not only shrinkage stresses but also thermal stresses.⁽⁴⁷⁾ In addition, other material properties that have influence on the cracking resistance should also be assessed, including tensile strength, elastic modulus, and tensile creep.

It is also important to mention that dimensional instability will theoretically have an influence on the bond performance of these materials. This is of special importance in the type of applications where these materials are expected to be used (i.e., connections for prefabricated concrete elements). Research is currently underway to investigate the correlation between dimensional stability and bonding of grouts to precast concrete.

Finally, research is also needed to further optimize the IC design in grout-type materials. For instance, an overdose of prewetted LWA in the system could be detrimental from the durability point of view. The addition of LWA has also been observed to potentially reduce the fresh flowability of cement-based grouts. As an alternative, SAP might be an option worthy of evaluation.