Not MofW literate to the level of mudchicken -
The worst element of concrete ties has always been their loss of integrity when damaged in minor wheel off the rail derailments that create cracking and spalling, generally requiring the replacement of the tie.
Wood ties in similar derailment end up with some marks from the wheel flange and are not in need of being replaced.
Few are. Miss his expert knowledge.
I saw Prof. Andrawes present his research at this fallâs AREMA conference. Itâs interesting stuff for sure, but based on what I remember and based on the abstract of the published research paper, I think that the linked news article misstates the point behind the research. It would not help repair ties that have been physically damaged, and probably wouldnât help them survive a derailment.
Concrete ties, like many precast concrete object, are pre-tensioned. Iâm going to spend a minute explaining what that means and why itâs done. If you already know about that feel free to skip ahead to the good stuff.
To pre-tension precast concrete members, you set thin, very strong pieces of steel bar or wire (âstrandsâ) in the forms. You pull them very tight, and you anchor them against against a solid object before the concrete is poured. Think of them like springs that you stretch out and then hold in the stretched-out position. Then the concrete is poured and bonds to the strand as it cures. Once the concrete is hard, the strands are cut loose from the anchors. Like a spring, the strands try to snap back into their natural, unstretched position. But the canât shrink because they are bonded to the concrete, and concrete doesnât compress very easily. So youâre left with a situation where the wires are pulling in on the concrete, compressing it, while the concrete pushes out on the strands, keeping them stretched.
There are two related reasons why this is a good thing. The first has to with the fact that concrete cracks and fails quickly if you try to stretch it. Imagine you build a simple beam by laying a ruler a ruler across a gap between two tables. When you put a weight on ruler in the middle of the gap, the ruler âbowsâ and takes on a curved, smile shape. The way this happens is that the top side of the ruler gets shorter and the bottom side gets longer, and the resulting internal forces balance out the weight of the load and keep everything in balance. With wood, that works fine. But if your ruler beam is made of plain concrete, the stretching on the bottom side will cause cracks, which will propagate up and cause the whole thing to break.
Now suppose you pretension the beam as I described above, so that the concrete is constantly being squeezed by the steel strand. When you bend that beam, the strands on the bottom side get a little bit longer, but they are still SHORTER than their natural, stress-free state. That means that the strands are still squeezing the concrete, just a little bit less than they were before. The concrete does get longer, but itâs still shorter than its natural stress-free state, so it doesnât crack.
The other reason why postentioning is good is that all the little micro cracks that form in concrete as it cures, as the temperature changes, etc. are squeezed closed by the force of the strand, so that water and other material canât easily work its way into those cracks and cause more damage.
The reason why concrete ties tend to get ruined in a derailment is that a.) concrete is still a brittle material, and will still crack if you hit it with a short, shop shock, and b.) if the crack extends down to the level of the strand, the strand can tear away from the concrete and snap like a rubber band. Per the FRA, if the surface is damaged enough that you can see the strand, your tie is not doing its job.
THE GOOD STUFF:
The shape memory alloys (SMAâs) that Prof. Andrawes is studying simply offer an alternative method for pretension the concrete. Theyâre pretty neat things. With a regular steel wire, if you stretch it a little bit, it snaps back into shape like a spring. If you stretch it too much, though, it stays stretched out and doesnât snap back. With a wire made of SMAâs, you can stretch it so far out that it doesnât snap back, but then if you come back later and heat it up, it will gradually shrink back to its original length. Or at least, it will try to. If you hold onto the ends while you heat it up, you can keep it from shrinking back, but youâll have to keep pulling on it to do so. Once you let go it will still spring back.
So: you put strands made of pre-stretched SMA into your concrete, pour the concrete, and wait for it to cure and bond with the strands. Then, you heat up the strand, by using a magnetic field to induce an electrical current in the strand as if it were a frying pan on an induction range. When it heats up, the strand tries to shrink. But just like with conventional pretensioning, the concrete keeps it stretched, and therefore the strand keeps the concrete squeezed.
There are two potential advantages right off the bat for using SMA strands. The first is that you no longer have to have the strand tied to some external anchor while the concrete is being poured and cured. You do all the stretching before you even put it in the forms. That means that the strand can be entirely internal, and you can put it at any spot, in any direction, without having to worry about youâre going to pull on it. In theory this gives you more flexibility to squeeze the concrete in just the right ways to keep it from stretching and cracking under load.
The second advantage is that you build the stresses between the strands and the concrete gradually as you heat the strands. With conventional pretensioning, when you cut the strands off the anchors, you release all the built-up tension all at once, and the strands snaps back violently. From a manufacturing point of view, a certain number of ties crack visibly during this tension-releasing step and have to be thrown out. From a quality point of view, you are potentially inducing extra unseen cracks that could weaken the structure in use. Avoiding this sudden, violent force by using SMAâs should improve yields and quality.
But none of these features really make for a self-healing tie or anything like that. Just like with conventional pretensioning, if the strand gets damaged or unbonded from the concrete, then it canât be fixed. At least, thatâs my understanding. There may be some situations where the properties of the SMA could be used to repair a damaged tie, but I canât really see what they would be - and I donât think those repairs could be done in the track.
Dan
As a crew carrier, once I recovered a crew that âcouldnât winâ. Scenario: 1. They passed HBD, all clear. 2. Conductor saw fire (hot box). 3. The city of Rector is their location. 4. They chose NOT to bail air and have a pile up downtown. 5. Slow stop caused tremendous tie damage. 6. Wheel axle melted from hotbox. 7. Car had to be set outâŚHOS got the crewâŚI transported. The crew got chewed out due to tie damage. 4 x Dâs. Darned if you Do and Darned if you Donât. regards mike endmrw1101252053
One thing I have noticed among a number of US videos where concrete ties can be seen in use is that the Main Track switches and crossovers are all done in wood ties.
In my area, where there are concrete ties, the main track switches also have concrete switch ties.
I noticed that their cross section of a tie in ballast isnât correct. The ballast is tamped to support the tie under the rail, not in the middle of the tie.
Jeff
After these ties are in place, would it then be a part of regular maintenance routine to run a special train that applies an AC magnetic field to the ties? If so, I guess it could be combined with a track inspection car.
I have to admit that if they think repairing damage to centerbound concrete ties is a selling point, theyâll have surprises in store.
When did the cost of Nitinol come down to where it could be included in tendons or in wire reinforcement?
Doesnât seem to ou would want something analogous tohelp much with the cracked-tie problem, except that in theory you could stretch the tie lengthwise hydraulically, tremie or epoxy-bond the cracked area(s), and do the directed induction heating after cure. Be interesting to see if that could be done cost-effectively with safety.
The equipment to do periodic âmaintenanceâ heating from a moving car is interesting to contemplate. The field has to be strong comparatively deep in the tie, and would have to be sustained while precluding spalling or other damage from moisture that has wicked into the cracks. I suspect youâd want something analogous to the Shotweld process where time and intensity are modulated to deposit specific energy into the right parts of each tie. You wouldnât do that easily at the required generated field strength from a train moving quickly; more likely the head would slide on the same kind of carriage as in some modern track equipment, indexed to spend the correct dwell tie by tie while the maintenance train itself runs at controlled speed.
I would think most of the heating from applying an AC magnetic field to the tie would be concentrated in the âSMAâ sections and not the concrete. This would most likely mean using a relatively low frequency - think skin effect. Generating the required field doesnât sound like it would that big of a deal as one advantage of using frequencies of 100Hz or lower is the ability to use electrical steel to help shape the magnetic field.
I was also wondering about the cost of nitinol, which has some really weird properties in where fatigue resistance increases with increasing tensile stress. As for the name, it is derived from NIckel TItanium Naval Ordinance Laboratory.
FWIW, work at Purdue suggests that mixing pieces of steel shaped like staples does wonders for the tensile strength of concrete.
Weâve proposed a version of that in structural Shotcrete for many years. The version proposed by AVG used short chopped lengths of reinforcement (in a chloride-resistant stainless): we proposed both deforming and bending the pieces for better 3D intermeshing while still allowing âshootingâ to place without significant segregation.
Are you talking about steel fiber reinforced concrete? If so, there is one drawback: it cannot achieve the same tensile strength as post-tensioning.
Here is a video showing the production of post-tensioned concrete slepers in Germany. The tension is not transferred to the concrete abruptly, but gradually by loosening the nuts.
Regards, Volker
Edit: Exchanged the video link with the English language version of the same video.
Fiber-reinforced concrete has only been intended to provide the same benefit as the original type of âbeton reinforceeâ with rods or mats, on a finer scale. It does not substitute for prestressing, and it does not substitute for posttensioning.
The question arises whether using SMA in comparatively long-strand reinforcement could provide an analogue of (comparatively light) three-dimensional posttensioning if it were heated as described. That would be for âself-healingâ of various types of cracking of the cured concrete more than restoring structural tension resistance. It does not substitute for use of SMA as, or in parallel with, transverse posttensioning as described in the paper.
When I look at the fully automated production of post-tensioned concrete ties in Germany the new research looks to me like the answer to a question that would never had been asked in Germany.
BTW as a civil engineer with 35 years in structural design I know the limitations of fiber-reinforced concrete. I just wasnât sure of anybody else.
Regards, Volker
I know you know. It was for everyone else here.
Engineers know not to allow ties to become centerbound in use â particularly concrete ties with reduced center-section width.
The questions I would raise:
What are typical German freight car weights and wheel/axle loadings compared to US?
Can the concrete ties using the existing German system work here?
Given more frequent derailments here, can the German ties withstand?
What about spauling?
Thatâs definitely a practice that varies by railroad. Most US Class 1âs have come to the conclusion that concrete ties are good for sharp curves, but there is no industry-wide consensus their use in turnouts.
Some of the arguments for:
1.) They hold gauge well in an area where that tends to be a problem
2.) They COULD have a longer life if everything works out right.
Some of the arguments against:
1.) Every single tie in the turnout is unique. Being able to replace a small number of damaged ties requires access to a large stock of spares.
1â.) And ties can get destroyed in a relatively minor derailment.
2.) If not properly cushioned or damped, concrete degrades quickly under impact loads (which are highest at turnouts as wheels transition from one rail to another). They also transmit more of those impact loads to the ballast, causing it to degrade faster. There are various softer components inserted into the load path to cusion those impacts, but they tend to wear out over time. So the ties may last longer, but some of the components may have relatively short lives.
3.) They are frickinâ heavy. Main track turnouts are shipped to the field in panels. In the field the panels are layed out and welded together next to the track, then pulled into the track in a single piece. Every step of that process requires bigger equipment and has more risk of damage when things weigh more.
Dan
The strand is supposed to maintain the tension for the life of the tie. Heating it up again wouldnât change anything.
If something has caused the tension in the strand to decrease, heating it up might somehow be able to pull things back together. But for that to happen there is an underlying problem somewhere else in the tie. Depending what that problem is, restoring the tension might help fix it, or it might buy you some extended life, or it might accomplish nothing. So there might be some opportunities to use the SMA for repairs, but it wouldnât be used for âroutine maintenanceâ.
Dan
German axle loads are up to 24 tons so the would need to get re-designed for the use on US railroads what apperently has happened. The production video was selected to show that there are no shock loads on the ties when the pretensioned steel rods are detached from its anchor. I havenât found information on production in the US so I canât confirm this for the US.
There is the iron ore line between Kiruna, Sweden and Narvik, Norway that allows axle loads of 33 tons and uses concrete ties also. The choose to put pads under the ties to dampen the contact with the ballast:
https://www.getzner.com/media/7927/download/Case%20Study%20Heavy%20Haul%20Line%20Malmbanan%20EN.pdf?v=1
Cracks and spalling can never be completely avoided.
In order to be able to derive appropriate measures, DB Netz assesses cracks in prestressed concrete sleepers in accordance with guideline (Ril) 821. 2018 [4] according to decreasing defect levels (FS):
FS1, loss of load-bearing capacity
FS2, crack width > 0.5 mm or > 1.0 mm
FS3, crack width ⤠0.5 mm or ⤠1.0 mm.
For the inspection of concrete cracks that occur, the assessment is carried out in accordance with
Ril 821.2003Z05 and repairs are carried out in accordance with Ril 824.0101Z01.
The Sleeper Crack Grouting method is one way that has been developed for the treatment of cracks, which can also be used to repair derailment damage. The material penetrates the crack to a depth of approximately 10 times the crack width and seals it.
https://www.rhomberg-sersa.com/en/services/products-sleeper-crack-grouting
Regards, Volker