Our new paper, Creep of Prestressing Steels in fire, is now in press in the journal Fire and Materials (you can access it here; an open access version is to come). It is a great piece about prestressing steels used in Post-tensioned structures. We show how high- temperature creep is effected by the chemical composition of the steel. While it may seem obvious to some, the effect this can have for prestressing steel in high temperatures seen in real fires is astounding.
Consider several different prestressing steels, all behaving the same in ambient service temperatures, but when stressed and heated (700 MPa and 427C) as if they were in a fire, one fails after 5 hours, were the others fail below 1.5 hours. All likely due to the chemical composition from the production process of the steel (see the above figure). These tests are described in the paper (that and about 80 others). The same trends shows the same effect in simple strength tests at high temperature.
The paper also describes the use of digital image correlation to measure the strain at high high temperature. An innovated technique, but leading to important insights about true areas and strains as the steel fails in high temperature. A compiled video of a creep test is provided below which illustrates how quickly tertiary (third stage) creep takes effect before failure.
The above video shows the prestressing steel coated in a black and white speckle texture pattern. Image correlation is a powerful technique for measuring high temperature strains. Its a very simple process described below
Teaching the undergrad students about emerging materials this week. To make our labs more educational this year (with massive amounts of extra safety protocols added), we conducted several concrete compression tests on conventional concrete wrapped in a carbon fiber reinforced polymer (frp). The students are able to compare their normal conventional concrete compression cylinder tests to compression tests of concrete cylinders with frp wrap.
A big thanks to Sika canada for supplying us these frp and adhesive materials, helping with training, and above all making this lab an awesome addition to our undergrad class this year! You can see one of the many tests below;
The above video shows one of the concrete compression tests with frp wrap (the best bit is near the end). A sacrificial camera was used to record failure in a safe way (an old Sony Cybershot from 2006). The cylinder failed with a compressive stress above 65 MPa (around 500 or so KN), whereas the average normal compressive failure strength of this concrete mix (without frp) was about 40 MPa.
The students were asked at the end of the lab to discuss the mechanisms and effects of ‘confinement’ using frp after observing the lab.
We did a very ad hoc style image correlation test using a steel stub column in the undergrad labs. Though not really an accurate way to adhere image correlation paint though, however; it was a good illustration of observing yielding and buckling effects on the column as the test progressed. Images in this video were taken at 1 second intervals, and the video represents 100s of image put together. Images were taken using a Canon 5d Mark 3.