Prepackaged grouts offer a higher grade of protection
In the past half-century, innovations in architectural and structural bridge design and construction have produced some of the world’s most spectacular man-made structures. Segmental bridges have resulted in fast, more cost-efficient bridge building, as well as more durable bridges. Increasingly, post-tensioning has been utilized to produce longer, stronger, safer bridges. Aesthetically and functionally, these technologies offer us the option to bridge what were once thought to be impossible expanses.
However, in the last few decades, the grouting used in these structures has become the focus of extensive study. Several states conducting inspections of grouting in segmental bridges have reported the occurrence of voids at anchorages and along tendons. In some of these cases, the grouting technology has been found to be unsatisfactory.
Given the critical issue of public safety and the economic burden of new construction and bridge deficiency repairs, industry response has been appropriate and aggressive. Many improvements in the materials, design and installation of cementitious grouts have resulted from these investigations. In particular, high-performance prepackaged grouts have been formulated that offer state-of-the-art protection for stressed and steel tendons, as well as many other desirable qualities.
In general, post-tensioning in U.S. bridges is carried out through internal or external tendons. The internal tendons in precast or cast-in-place cantilever construction are tendons that are typically installed in the top and bottom slabs of the box. After the tendons are stressed and anchored, the duct is filled with grout (in bonded post-tensioning). External tendons in precast segmental span-by-span bridges are multi-strand tendons that are grouted in polyethylene ducts inside the box and external to the concrete.
Grout serves a critical function in post-tensioned applications, primarily providing protection against corrosion to the prestressing steel. In this role, grout acts as the final stronghold against corrosion by providing an alkaline environment around the tendons and anchorages. Any compromise to this protective shield, including violations induced by incompatible materials, can leave the steel vulnerable to corrosion. Once corrosion of the steel is initiated, it becomes a self-fulfilling process. Post-tensioned structures are more highly susceptible to corrosion in marine and chloride-aggressive environments, such as those locations where usage of deicing salts is high.
Secondarily, (in bonded post-tensioned applications) grout develops a bond between the prestressing steel and concrete, transferring tensile stresses to the concrete along the length of the bonded tendon. Obviously, loss of tensioning could prove catastrophic.
Stop the bleed
Often, progress can only be witnessed from a historical perspective. In general, early grouts were formed mainly from portland cement and water, usually to a maximum .45 water-cement ratio. Sometimes anti-bleed or expansive admixtures or both were used, particularly in vertical applications with multiple lifts. A common problem associated with these grouts is the segregation of water from the mixture. Essentially, due to differences in weight or the filtering action of the strands, water from the grout separates and rises, while the solid materials settle to the bottom of the duct.
The term “bleed water” refers to this water that typically accumulates at the top of the duct. This occurrence can eventually create an air void when the water is later reabsorbed into the grout. It is possible for bleed water to become trapped in the duct causing an air void to occur in various locations creating what is called a “bleed lens.” The higher the vertical rise, the higher the pressure and the more bleed water. Ultimately, these voids can permit the entrance of air, water and chlorides into the duct, which may in turn cause corrosion of the steel tendons.
From 1992 to 1996 the use of post-tensioned bridges was banned in the United Kingdom, due in part to inadequate grouting. This action was predicated on the 1967 failure of the Bickton Meadows footbridge and the Ynys-Y-Gwas Bridge collapse in 1985. To date in the U.S. significant damage has been limited to a small percentage of post-tensioned bridges, and no structural failure attributable to corrosion of post-tensioning tendons has been reported. However, these incidents of corrosion-related distress have prompted numerous studies.
Obviously, improvements in grouting practices and the quality of corrosion protection materials were needed to provide the long-term durability that is essential in the industry. A major materials technology research project conducted in 1999 at the University of Texas evaluated some of the grouting deficiency issues and resulted in the development of high-performance grouts for post-tensioned structures. Additionally, interim improvements to grouting practices was recommended by the American Segmental Bridge Institute Grouting Committee in 2000.
While corrosion protection should be designed as a system (rather than a sole source), cable grout is the last level of protection for the strand. From a durability standpoint, one of the most important components of a grouting operation is the use of a high-performance grout.
Several proprietary prepackaged grouts are available that meet the industry requirements. These are categorized as Class C grouts and are considered suitable for both aggressive and non-aggressive environments. Depending on the formula, they may or may not exhibit thixotropic (fluid when agitated, gel-like at rest) characteristics. Because variations exist between manufacturers, grout material testing should be verified for compliance with the Class C (prepackaged) grout requirements detailed in the Post-Tensioning Institute guide specification (summarized below) including:
In general, the design of prepackaged grout enables its usage in both new construction and repairs. Several formulas currently on the market meet the requirements of all industry specifications. These grouts can be placed at very pumpable consistencies for long working times (two hours at 90ºF with no loss of flow). Some also are ideal for vacuum installation methods.
Most of these products fall within the range specified for fluid grouts in ASTM-C953, >3 and <12 hours. (Note: ASTM C-191, “Standard Test Method for Time of Setting of Hydraulic Cement by Vicat Needle” is an appropriate test method for stiffer paste mixes per the referenced guide specification, C4.4.1).
Fluidity and pumpability, based on thixotropic and non-thixotropic properties, are described by efflux time in accordance with ASTM C-939 and the modified version of this specification. Again, there are prepackaged grouts that clearly meet these requirements. Note that in situ performance may vary from Cement Concrete Research Laboratory certification depending on actual ambient and material temperature at time of placement. Thixotropic grouts are preferred because they offer higher bleed resistance and reduced segregation.
Given that corrosion is more likely to occur in aggressive environments, the risk associated with bleed water is potentially greater under these adverse conditions. It is recommended that Class C (also B and D) grout meet the requirements of modified ASTM-C-940 and the Schupack Pressure bleed test. (Note: Some states have more stringent requirements concerning acceptable bleed water values. It is always advisable to check the manufacturer’s data to be certain that the product complies completely with the specification and was tested according to the required procedures).
Prehardening early volume change and hardened height change characteristics also are indicative of corrosion resistance. Materials that produce early height changes will usually reduce the density of the grout, and that may reduce the resistance to chloride ion penetration. Prepackaged grout designs have achieved laboratory results within the acceptable limits. This consists of a vertical height change of 0.0% to less than +0.1% at 24 hours and no greater than +0.2% at 28 days per ASTM C-1090.
Historically, grout formulations included aluminum powders as expansive agents. Aluminum powder has been linked to failure due to embrittlement of steel tendons. Today’s formulations are based on a different chemistry and do not utilize aluminum powders.
In terms of chloride permeability, some prepackaged grouts evidence better results than others. The product must meet ASTM-C-1202 or the modified requirement (less than 2500 Coulombs after six hours using 30v rather than the standard 60v). Note that the lower the coulomb ratings, the better the resistance to chloride ion penetration.
Lastly, testing in accordance with the Accelerated Corrosion Test has shown some high-performance grouts to provide better corrosion resistance than the .45 water-cement control sample. Though this test is considered reliable by most, it is not unilaterally accepted as an industry standard. The wet density test is another way to verify the water/cement ratio of prepackaged grout. According to the referenced Guide Specification for Grouting of Post-Tensioned Structures published and developed by the Post-Tensioning Institute, at minimum, only two mud balance tests are required per day, or if there is an apparent change in the characteristics of the grout.
Class C grouts have been extremely well received and sanctioned by many industry agencies and professional authorities. In addition to the performance advantages described, prepackaged grouts feature other additional benefits.
It should be stated that in every industry, continuing education is at the heart of all progress. The American Segmental Bridge Institute (ASBI) plays a critical role toward this end. ASBI now holds certification training programs. These courses provide the training that is needed to understand and implement grouting specifications for post-tensioned structures.
Both certifications are based on successful completion of the ASBI Training Certification program. Individuals who complete the course and document three years of experience in grouted post-tensioned structures are awarded an ASBI Certified Grouting Technician certificate. Via the same course, an ASBI Grouting Training Certification is issued in the absence of the three years of industry experience.
This course has now been accredited and provisioned by some state DOTs, including Florida and Georgia. Essentially, this requires all grouting operations in post-tensioned structures to be conducted under the direction of an ASBI Certified Grouting Technician (or a program equivalency) in accordance with specification. The authors advocate that this training is necessary in all applications and with all classes of grout.
The use of high-performance prepackaged grouts is only one piece of a much larger puzzle. It is not intended that all corrosion problems be blamed on grouting, nor is it implied that new grout technology can cure all problems associated with corrosion in post-tensioned structures. Certainly, other elements play a part in corrosion. Deterioration processes (fatigue, freeze-thaw cycles, etc.), poor quality concrete, workmanship, design, structural contaminants or other construction details have contributed to the overall problem. These issues are extremely relevant to the recent concerns.
Trade organizations and associations, as well as government and university agencies, manufacturers and individual experts, have worked relentlessly to improve all aspects of grouting operations. Much valuable testing and research has been conducted and the findings have served to optimize grouting materials, design, quality assurance and control, equipment and construction techniques.
By now, most are familiar with the aforementioned Guide Specification for Grouting of Post-Tensioned Structures. This document is recognized as the industry standard for cementitious grouts detailing the minimum requirements of selection, design and installation.