Pavement friction testers

The first European friction workshop is taking place now (during the last week of May 2017). Manufacturers of equipment are having the chance to show their units to colleagues and have results compared. This is very interesting, at least to me, as I consider friction testing one of the most complex of the pavement evaluation procedures. In addition to being related to users safety, therefore of tremendous importance, friction testing is at the same time a challenge more difficult to get right than deflection testing or roughness testing. Getting some friction numbers is not hard, the challenge comes if you want to provide accurate, responsible results.nbsp;Here I’m taking the risk and sharing my view on friction testing devices, and I will dare to mention my preferred one. Keep reading. Pavement testing It′s common knowledge among pavement engineers that in order to characterize pavement condition it is necessary to determine the “state of affairs” both structurally and functionally. A pavement might look really nice and comfortable but it won′t last a week if its structure is poor. Or it might be a strong one and last forever but be virtually impassable and hated by the users. The two types of condition are not necessarily related. For structural testing pavement engineers use a deflectometer to measure ... deflections! (duhh!). There are also other devices like radar (which is becoming great), and a few more. For functional testing we may evaluate smoothness, texture, use microphones (for tire-pavement noise), and even quatify reflected solar energy (albedo). For smoothness we use profilometers, and from them we first get .. yeap, you′re right, the profile. We get the elevation profile of the pavement surface. Or instead of profilometer we may use roughness meters if we allow a little less accurate readings. So, before you get tired, let me summarize here by just saying that there are several tools to get an accurate picture of a pavement true engineering condition. Friction. The difficult one. And then it comes pavement friction.nbsp;It is hard in part because we are dealing with a “coefficient of friction” and not with a defined magnitude that could be easily contrasted. The coefficient of friction, represented by the greek letter “mu”, is (bear with me) the ratio between the horizontal force (at the interface) and the vertical force applied to the tire. The horizontal force is the result of the interaction between the tire and the pavement, not a property of the tire or the road surface individually. Intuitive part It is very intuitive to realize that if we change the type of tire, then we get different friction results. However, even if we test with the same exact tire and the same horizontal force is exerted at the interface, the coefficient that we would be reporting depends on the vertical force, so we would need to know and hopefully fix that as well. Easy? Well, if the pavement is relatively smooth, a testing device can be made with a weight over the test tire, thus fixing the vertical force, right?nbsp;Wrong. Pavements are not smooth, and the contraption will be bouncing. So, sadly, the force changes, and it should be measured, not assumed. Well, but let′s assume for now that we get that solved. Not free. Not free rolling, I mean. Consider now that a free rolling wheel is not what we want to use, because the accidents happen when the brakes are applied or when the tire is skidding laterally off the road. Therefore, if we are making a friction testing apparatus, besides having a standard test tire and a standard vertical force, it needs to have a standard drag method. We could place the tire at an angle with respect to the road direction, so there is some drag on the tire that simulates vehicles skidding off the road on a curved stretch. Or we could alternatively have that tire rolling not at an angle but instead force it to be locked and therefore not rotating, to simulate fully applied brakes. Or we could partially lock the rotation, to simulate what the anti-lock braking system (ABS) do. Either way, we would need to agree on the angle if we use a test tire at an angle (let′s pick something from 90° down to a couples of degrees) or agree on the percentage of rotation that is locked (let′s say a single percent number between 100% locked down to almost free rotation).nbsp;Ok, so we need standardized tire, known vertical force, and an agreed-upon drag system. Is that all? Not enought. Dry or wet Well, think now whether you want the pavement dry or wet at the time of testing? because it matters a lot. Wet? How wet? Wet with just water or should we try oil or ... ). Or how about we let the client decide when to test with his or her friction tester.nbsp;Not really. If the purpose is to audit the operator of infrastructure, we want to define test conditions carefully. It is not that we think our contractors or concessionaires or public officers would pick their preferred conditions. No, we know they wouldn′t select their best conditions to report friction results. An over-conscientious engineer may wish to test on the worst conditions (for example when there is ice on the road), but that will cost him his job as the results are certainly going to be below spec. Perhaps there should be a condition that everyone could replicate to make the test more reproducible?nbsp; Speed Does it affect? Unfortunately yes, so there is one more parameter to decide on, as a friction testing industry. Standard tire Getting back to the standard test tire, should we pick the most common tire on your fleet? Or an old style tire that would represent a worst case scenario? Should it be different for different regions, or a worldwide standard to allow for country to country fair comparison? Ahh, let′s use tires made with rubber compounds that are less susceptible to temperature effect on the stiffness of the tire. We are engineering a solution, aren′t we? And tire size, perhaps the bigger the tire, the more repeatable results we get?. Mmmh, but it should somehow represent the typical tires bought by the users we want to protect. A smaller tire might be easier to standardize and make pavement friction testers less expensive. Ah, and once you start using it, keep it rotating the same direction. Airport pavements? Right. Given that the pavement surface materials are the same, it makes sense to have the same well-defined friction tester apparatus to test both airport and road pavements. OK. And the problem is? …nbsp;the problem is …nbsp;there are several well-defined pieces of equipment, and they are different. That′s been a big problem. It means that when you are reporting friction values, at least the name of the testing apparatus needs to be included. Mu values from equipment A are different from Mu values from equipment B. As bad as this may sound, big efforts have been made over many years to come up with reliable pavement friction testing. Great people have pushed and they keep on pushing for solutions. Another chapter in this effort is taking place in France this week. the problem is …nbsp;there are several well-defined pieces of equipment, and they are different. That′s been a big problem. My preferred friction tester I′ve seen many units, big and small. Beautiful and ugly.nbsp;Some of the questions above have led to standardized parameters and this has allowed us to get valuable results and, most likely (no exaggeration here), save lives through pavement friction specifications.nbsp; For instance, airport pavements are tested with a 1mm thick water film, or tested without applied water during storms and icy conditions. Two typical speeds are used when testing airports, 65 and 95 Km/h. Road pavements are tested typically (but not always) with 0.5mm of water on them. Speed is also somewhat variable on road testing, being 50 Km/h a good value. Plain water is used to wet the pavement, but it is applied with normalized nozzles … but not always. Locked-wheel testers, that required sample testing, are being replaced by partially blocked wheels (known in the industry as “fix-slip”) to allow for continuous testing. In fact the aviation industry calls them CFME for Continuous Friction Measuring Equipment.nbsp;The percent slip on the testing wheel is not a standard value, 10 to 20% is typical, although some manufactures leaves this open to the client, which in my opinion creates problems. Anyway, if you have read this far, you probable wanted to know which is my preferred pavement friction tester. Here is a picture of it. It′s called the Traction Watcher One. Easy: The number ONE device (in my humble personal opinion), called the TWO (which stands for Traction Watcher One), offered by pavmnts, the company that sells them in Latin America. Buy one of these now, 1, 2 amp; 3!!! In Latin America the practice of testing pavement friction is diverse and incipient. There are some SCRIM units, GripTesters and Mu meters. Theses are all commercial names of devices used for the same purpose that the TWO friction tester. The SCRIM measures with one test tire, at a 20° angle.nbsp;The Griptester measures with one test tire, fixed slip at 15%.nbsp;The Mu meter uses two test tires, both inclined at 7.5°.nbsp;The SCRIM is a full size truck whereas the GripTester and the Mu meter are small size carts or trolleys (call them what you want, in Spanish carritos works well), towed by a vehicle. The road authority in Chile operated a SCRIM for several years, and is still the reference device, at least in paper, as operators try to bring it back to a working condition. Then some GripTesters were bought, but they are yet to be used in a regular basis. The road authority in Argentina bought some SCRIMs a few years ago, and I′m sure they have tried to use them (as far as I know, not much success, but I could be wrong), and they are in the process now to move to smaller, more practical devices. Colombia has seen the use of Mu meters, but I′m told that there is a standard that points to locked wheel testers, like in the US (as far as I know, there has never been one of those in the country). There are GripTesters in Brazil.nbsp;Mexico′s standards call for Mu meters. An occasional GripTester or Mu meter makes its way to Peru. And that′s about it. With these devices one can get friction numbers, meet standards, and get by. But the SCRIM is big and expensive. The tire at an angle provides sideway friction level, but the accidents are more related to lack of breaking friction in the direction of traffic. And it does not have an absolute calibration procedure (therefore, believe it or not, the average of several units becomes the reference). The GripTester is towed behind a vehicle, but because it is lightweight, the vertical force is sometimes too low to provide good results. And it′s prone to accidents itself (I have heard first-hand stories of units falling from the truck that transported them, or getting disengaged from the vehicle hitch during testing and ending up where they shouldn′t be). The water dispensing part is also a problem. And on top, the manufacturer demands that the calibration must be done at the factory, in Europe. And it is easy to imagine that most clients don′t like this. And calibrating once a year does not happen. The Mu meter is also a lightweight cart that bounces, and it has two test tires (therefore more expensive to replace when they wear out). Correlation with other CFMEs seems to be a problem too, although it has a long history in airports. A 1986 publication indicated: “Although the Mu meter has been used for pavement evaluation, most highway agencies have abandoned it, because it was developed for runway friction determination and is inappropriate for highways (The Wet-Pavement Traction Measurement: A State-of-the-Art Review, 1986). The T.W.O. friction tester The T.W.O. is not towed behind a vehicle, but mounted directly to it. This way it gets the full reaction from the vehicle and is therefore able to maintain the vertical force within a narrow range. It is very portable, which is extremely good for those who need to test several airports. This is the case of consultants that work in different countries (like pavmnts), but also for the military, the aeronautical authorities, and even private operators who are responsible for maintaining pavements in a network of airports.nbsp;The newer technologies in the T.W.O. friction tester makes it more efficient than other units. The FAA (Federal Aviation Administration) approved it in 2010, and in Scandinavia it has been used in roads for years. And they do get months of slippery roads each year! There are more specs and advantages to share. Drop me an email if you want to know more about this friction tester, or contact directly the manufacturer. I′m sure we will see more units of this friction tester sold in my region of the world by pavmnts. Look for them soon in runways and highways in Latin America. Thanks for reading.