RSS FEEDS
 
Figure 1 is a diagram featuring two schematics, comparing the old TxDOT deck configuration (on the left) to the new configuration on the right 

Fig. 1. Reinforcing bar-to-grout bond failure during fatigue loading in lab experiments carried out at Federal Highway Administration’s Turner Fairbanks Research Center.
            

Dimensional stability of cementitious grouts in prefabricated bridge connections – Ensuring good performance
Igor De la Varga, FHWA, Zachary B. Haber, FHWA, Benjamin A. Graybeal, FHWA

Prefabricated Bridge Elements and Connections

Accelerated bridge construction (ABC) has become common for both new and replacement bridge construction. The majority of ABC projects rely on using prefabricated bridge elements and systems (PBES) to meet tight construction windows. Prefabricated bridge systems commonly depend on field-cast cementitious grouted connections between elements for structural continuity. Thus, the mechanical properties of these grouts directly impact the short- and long-term performance of prefabricated bridge structures. Currently, a broad-scope research project on the performance of field-cast cementitious grouts and their use in PBES connections is being conducted at the FHWA Turner-Fairbank Highway Research Center (TFHRC). This article presents one of the focus points of the aforementioned project: evaluation of dimensional stability of typical non-shrink cementitious grouts that may be used in PBES connections. Results from a series of material-level tests on the dimensional stability properties of non-shrink grouts are discussed. The implication of dimensional stability on the system-level behavior of PBES connections is also discussed along with some possible strategies for mitigating poor dimensional stability.

Material-Level Behavior: Evaluation of Dimensional Stability

The research study mainly focuses on the evaluation of the dimensional stability of commercially-available non-shrink cementitious grouts that can be used for connecting prefabricated concrete bridge elements1. Some of the results of four cement-based grouts, named as C1, C2, C3, and C4, are presented in this article. In this study volume changes have been assessed from a fundamental point of view, measuring pure expansion/shrinkage deformations in both sealed (i.e., autogenous) and drying conditions. This was done using both ASTM C16982 (Figure 2-a) and ASTM C1573 the latter for long-term results (Figure 2-b). The curing condition of field-cast grout materials is important because some PBES connections will be completely sealed, while others will be partially exposed to the environment (drying) .

Figure 2 graphically depicts the results of dimensional stability tests, by plotting strain as a function of time. Figure 2-a shows autogenous (sealed) deformations measured from time of set via ASTM C1698, and Figure 2-b long-term deformations in both sealed and drying conditions measured from the age of 1 d via ASTM C157 
Fig. 2. Dimensional stability test results: (a) Autogenous (sealed) deformations measured from time of set via ASTM C1698, and (b) long-term deformations in both sealed and drying conditions measured from the age of 1 d via ASTM C157             

As observed in Figure 2-a, and despite being designed as “non-shrink”, the grouts show autogenous shrinkage at some point, either preceded by a flat region (C1, C4) or by an initial expansion (C2, C3) during the first hours. Long-term shrinkage results (Figure 2-b) show a considerable amount of autogenous shrinkage (about 500-600 µε) for C1, C3, and C4. The large expansion observed in C2 in Figure 2-a helps in reducing most of its final shrinkage (about 200 µε). However, it has been stated that it is the rate of shrinkage (i.e., the slope of the autogenous shrinkage response) what makes the material more prone to shrinkage cracking, rather than the total shrinkage. Drying shrinkage is at least 1000 µε greater than sealed shrinkage in all cases, due to the drying effect of the capillary pores.

System-Level Behavior: Consequences of Poor Dimensional Stability

Some of the possible consequences of poor dimensional stability on the system-level behavior of PBES connections with different field-cast cementitious grouts were observed in a recent and related study at the TFHRC. A series of precast deck panel connection tests were carried out to advance the understanding of deck-level connections under realistic performance demands.4 A number of parameters frequently considered during the design of these connections were investigated including three different cementitious grouts; the C2 grout shown in Figure 2 was one of the included grouts. Prior to testing, a number of deck panels with C2 grout exhibited significant shrinkage cracking in the grouted connection region (Figure 3-a). Deck panels were subjected to low- and high-level fatigue loading and then monotonic loading until failure. Upon application of load, preexisting shrinkage cracks grew and propagated continuously during fatigue cycles which resulted in bond deterioration between reinforcing bars and the grout material. In many cases, these deck panels failed during flexural loading due to bond failure of the reinforcement, which occurred prior to reinforcing bar yielding (Figure 3-b). This research also suggests that the bond strength between precast concrete and field-cast grout could be compromised in the presence of excessive shrinkage; research is currently underway to investigate the correlation between dimensional stability and bonding of grout to precast concrete. Shrinkage cracking in connection zones could also lead to infiltration of corrosive agents and loss of flexural stiffness. On the other hand, materials that are excessively expansive could introduce forces into the bridge system not accounted for in the design, which could cause unexpected structural damage .

Figure 3 consists of two side by side photographs. The photograph on the left, 3-a, shows shrinkage cracking on a precast concrete deck using C2 grout. The photograph on the right, 3-b, shows deck failure under the fatigue inducing machinery 
Fig. 3. Observations from deck panel connection tests using C2 grout: (a) Shrinkage cracking observed prior to testing, and (b) Reinforcing bar-to-grout bond failure during fatigue loading.
            

Strategies for Mitigating Dimensional Stability Issues

Different strategies are available for mitigating issues related to dimensional stability such as excessive shrinkage, which was observed in the grouts discussed in this paper. In this study, two different strategies were evaluated: internal curing (IC) and the use of a fiber reinforced ultra-high performance concrete (UHPC). Non-shrink cementitious grouts are often pre-packaged and can be extended using small aggregate for volumetrically large pours. In this study, pre-wetted lightweight aggregates (LWA) were added to two of the previously tested grout systems to provide IC. As observed in Figure 4, IC helps to mitigate most of the autogenous shrinkage in both C3 and C4 grouts, as well as it reduces drying shrinkage by half. The autogenous shrinkage reduction is a result of prolonged internal saturation (i.e., higher internal humidity). This will have a direct effect on the stress developed in the material, as the size of the pores that are being emptied during hydration is larger than that of grouts without IC. The partial drying shrinkage reduction would presumably correspond to two different reasons: 1) mitigation of autogenous (or internal) drying, and 2) extension in the time it takes to reach equilibrium with the local drying environment as it may take longer to empty out the same-sized pores in the system with IC versus the system without IC. Drying shrinkage in the UHPC material was an order of magnitude lower than the cement-based grouts without IC. Low shrinkage in UHPC material can in part be attributed to the presence of a high volume of steel fiber reinforcement in the mix.

Figure 4 graphically depicts the results of dimensional stability improvement strategies, by plotting strain as a function of time. Shrinkage reduction in sealed (4-a) and drying conditions (4-b) using IC or an UHPC material via ASTM C157 
Fig. 4. Results from tests with using dimensional stability improvement strategies: Shrinkage reduction in (a) sealed and (b) drying conditions using IC or an UHPC material via ASTM C157.
            

For more information, the authors may be contacted at the following: Igor De la Varga, Concrete Materials Engineer, SES Group, FHWA Turner-Fairbank Highway Research Center, igor.delavarga.ctr@dot.gov, Zachary B. Haber, Research Bridge Engineer, PSI, Inc., FHWA Turner-Fairbank Highway Research Center, zachary.haber.ctr@dot.gov, Benjamin A. Graybeal, Team Leader – Bridge and Foundation Engineering Research, FHWA Turner-Fairbank Highway Research Center, benjamin.graybeal@dot.gov. 

References

  1. De la Varga, I., Graybeal, B. “Dimensional stability of prebagged cementitious grouts for prefabricated bridge element connections,” Proceedings. 2014 National Accelerated Bridge Construction Conference, Miami, FL, USA, December 4-5, 2014.
  2. ASTM C1698-09. “Standard Test Method for Autogenous Strain of Cement Paste and Mortar”
  3. ASTM C157-08. “Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete”
  4. Yuan, J., Graybeal, B., Haber, Z. B. “Field-cast connections for prefabricated bridge elements,” Proceedings. 2014 National Accelerated Bridge Construction Conference, Miami, FL, USA, December 4-5, 2014 .