Revisions to Accelerated Corrosion Test Method for Post-Tensioning Grout

Along with requirements related to placement, post-tensioning grouts should confer a minimum degree of corrosion protection to the embedded prestressing strand. In the U.S., this degree is currently being measured with the use of an anodic polarization test procedure known as the accelerated corrosion test (ACT). Two conditions for approval of a specific grout are currently in use: a) an ACT time-to-corrosion result greater than or equal to one from a 0.45 water-cement ratio (w/c) standard mixture (only portland cement and water) tested under the same conditions; and b) a minimum result of 1000 hours average ACT time-to-corrosion. Various specifications use different combinations of these two approval conditions. The ACT test procedure has been criticized because it does not currently require that the applied voltage account for the variations in the resistance of the grout (IR drop) when selecting the appropriate applied voltage. Grout resistance can vary significantly with the addition of pozzolans and admixtures, perhaps adversely affecting the results of the test. The research reported in this paper investigated different post-tensioning grout mixtures in ACT tests with varying levels of admixtures and the option of compensation for the effects produced by resistivities that are inherent to electrochemical systems. Testing confirmed that variation in the type of test is significantly affected by variation in these resistivities, allowing grouts of potentially lower quality to pass the test when IR compensation is not available in the equipment used to conduct the tests.

Keywords: admixture; grout; post-tensioning; test.

INTRODUCTION

The period between the years of 1992 and 1996 was significant for grouted post-tensioned structures. In that period, the UK temporarily banned new post-tensioned bridges because of the collapse of the Bickton Meadows and Ynys-y-Gwas bridges (occurring in 1967 and 1985, respectively) due to tendon corrosion. These two bridges were considerably different from typical construction in the U.S., but severe corrosion-related problems were also identified more recently in some U.S. bridges located in Florida (the Niles Channel Bridge, the Mid-Bay Bridge, the Sunshine Skyway Bridge, and the I-95/I-595 Sawgrass Interchange bridges), renewing engineers and researchers' concerns about grouted post-tensioning tendons. Although it is known that only a very small portion of post-tensioned bridges have shown corrosion problems, the system is inherently difficult to inspect and critical problems may go undetected. Consequently, development of good inspection procedures and tools as well as improved design and construction methods are necessary to ensure a durable post-tensioning system.

In grouted post-tensioned bridges, the last line of defense for the steel strand against corrosion is the post-tensioning grout, that is, the mixture of portland cement and water that embeds the strands (sand, mineral, and chemical admixtures may also be present). Adequate flowability, reduced shrinkage and settlement, and minimal bleeding are necessary characteristics that, along with good workmanship, will ensure that the tendon ducts are filled adequately with grout. Another important characteristic is the ability of the grout to protect the tendon from corrosion.1 The first attempt to measure the degree of corrosion protection of grouts was made by Lankard et al.2 using a variation of the potentiostatic polarization method,3 which is a common electrochemical technique. The test was named the accelerated corrosion test (ACT) and was later further evaluated and refined by Hamilton,4 Koester,5 Schokker,6 and Pacheco.7

One drawback of the standard ACT test is that grouts may be treated differently depending on their ohmic resistance. Any electrochemical system inherently provides a certain degree of electrical resistance that is variable according to the characteristics of each system. This is true even for pure electronic circuits that do not involve any aqueous phases. Naturally, in these circuits, the amount of resistivity is usually negligible because metallic phases are excellent electric conductors. In electrochemical circuits, however, the errors generated in measurement may be significant (particularly in concrete or grout systems) because the applied voltage is reduced considerably in the electrolyte. This drop is called the IR drop of the system and may be compensated for in corrosion testing when the proper equipment is used.

The ACT has been incorporated by the Post-Tensioning Institute (PTI) into its Guide Specifications for Grouting of Post-Tensioned Structures.8 According to these recommendations, an appropriate grout to be used in bonded posttensioning tendons must have better ACT results than that of a 0.45 water-cement ratio (w/c) neat grout. In addition, the commentary of the PTI specification mentions 1000 hours average time-to-corrosion in an ACT test as an acceptable minimum value. This value of 1000 hours has also been adopted by several specifications including those of the Florida Department of Transportation. Unfortunately, no guidance is given on two important aspects of this test procedure: 1) how to adequately account for the IR drop on the time-to-corrosion; and 2) how to account for the slow reactivity of some pozzolans compared to portland cement.