Cracks in PT Wedges: Is this an Issue?
WRITTEN BY KLINE senior project engineer, joseph dubois
Cracks are always a bad thing, right? They reduce both the aesthetic appeal and the structural integrity of whatever has been cracked. The problem is that cracks are nearly impossible to avoid, which begs the question: Which cracks are okay and which cracks are unacceptable? Post-tensioning (PT) does not escape this question, so we will address the following:
· The Situation
· Post-tensioning Wedge Design
· Understanding Acceptable vs Unacceptable PT Wedge Cracks
Setting the Stage, er, Wedge!
It is a sunny afternoon and today’s task is stressing the tendons of the first floor, ensuring the specified force is met along with setting the wedges properly. You know both of these factors are critical to the structural integrity of the building, so you carefully check the gauge – 7,000 psi: check! Next, you remove the jack and inspect the wedges. The wedges are fully seated in the anchor and the tendon is securely gripped, but wait, there is a hairline crack on the left wedge! The crack seems so small that you are not too concerned, but you have never seen this before and want to make sure the connection is sound.
PT Wedge Purpose and Construction
To properly address the scenario above, we must take a look at the specific purpose of PT wedges, along with their construction. The ultimate goal of PT wedges is to securely anchor post-tensioned tendons/strands to the surrounding concrete (through the anchorage), and maintain the tension force throughout the life of the building. This requires PT wedges to be strong and durable. The Federal Highway Administration Post-Tensioning Tendon Installation and Grouting Manual (FHWA-NHI-13-026) states that all PT wedges must have “internal serrated teeth for gripping the strand,” be made of “case-hardened low carbon or alloy steel.” The reason for requiring a hardened case is to ensure that the serrated teeth firmly grip the tendon without deforming or allowing the tendon to slip. Notice that only the outer “case” of the PT wedge is hardened; this is because the wedge must have a “ductile core, in order to … conform to the irregularity between the strand and wedge hole” (FHWA-NHI-13-026). To summarize, PT wedges have a soft interior to conform between irregular surfaces and a hard exterior with gripping teeth to retain the force in PT tendons.
Myth or Truth?
Knowing what PT wedges are designed for and how they are constructed, let us take another look at the crack you found. There are three main reasons why a PT wedge might crack: First, the tendon surface is irregular which forces the wedge to bend and conform when the tendon is stressed. Second, hardening the steel in the wedge case makes the steel brittle. Third, the wedge is defective or damaged. Due to the need for deformation to enable the wedge to properly grip the PT strand, the first two reasons for cracking are not a concern when the crack is limited to the surface. This is affirmed by PTI’s Field Manual for Unbonded Single Strand Tendons (PTI M10.3-00) which states that “hairline cracks may appear in the case-hardened surface of wedges …” and that they “… do not affect the integrity of the post-tensioning system.” However, what is not acceptable is if the ductile core of the wedge also cracks, breaking the wedge into multiple pieces (FHWA-NHI-13-026). This can happen both from manufacturer defects and from installation errors, such as unevenly seated wedges, highly corroded wedges, skewed wedges and/or anchors, and improperly placed stressing jacks. Not acceptable are wedges that have been broken into pieces signifying not only hard surfaces but also brittle cores. Horizontal or inclined breaks are also considered unacceptable.
So most cracks observed in PT wedges are not bad, and now you know how to tell the difference! Wedges cracked completely through should be considered defective, but wedges with radial surface cracks are structurally sound.
In general, radial micro cracks should not affect the wedge capability. As long as the strands are not slipping, Re-wedging is not necessary.
If you are a contractor or inspector, it is essential to share this information with your team and colleagues to dispel the myth surrounding cracks in post-tensioned wedges. If you have other questions or concerns about post-tensioning, check out our blog on the "Top 10 Common Post-Tensioning Myths Debunked," and do not hesitate to reach out to us for expert advice and assistance.
