UIUC engineers make rail tracks safer, longer-lasting

Trains Magazine
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  • Total thickness: The total depth of the entire pavement structure (from the top surface to the subgrade) is substantial, with total thicknesses cited between 68 cm and 75 cm (so a. minimum of 26.75") for some Autobahns.
    The deep, multi-layered, and freeze-resistant construction, especially the use of a substantial reinforced concrete base layer, is a key factor in the Autobahn’s exceptional durability and longevity under heavy, high-speed traffic.

I wonder about the quality of cement used in each country, whether for roads or railroad ties.
My. Impression on DB ROWs was of some concrete ties having been in place for many years

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Not all ground is stable - railroads have their issues at scattered locations across their networks and it would be folly to think that highways and Interstates don’t. Many locations require continual repairs, not because of shoddy work but because the ground is so unstable that every effort t stabilize it fails.

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I have read in years past, Germany (think auto bahn) #1 pay great attention to sub-grade. Before concrete is placed, what has been done to insure a stable base. i.e. if (IF don’t quote my example) a sub-base USA is 5’ over there they have 10’. More care/concern for end result.
Next would be the details that were taught in the excellent video Chuck referenced.
The DOT guys have radioactive equiped detectors which check compaction. I have no idea how it works, BUT I do know this. IF the DOT dude has one in his vehicle by law he must have a placard warning of radioactive material on board. Conversely should the material be unloaded and the placard NOT REMOVED, a fine ensues here also. Imagine a truck in a wreck and has the placard displayed, and nutin’ can be found in their frantic search for what Ain’t There. regards mike endmrw1105251247

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In recent years, DB has increasingly encountered problems with longitudinal cracks in the head area of the sleepers.

These are partly caused by splitting stresses resulting from the introduction of prestressing force into the concrete. Today, the B70 sleeper is manufactured with four prestressing wires without any additional non-prestressed reinforcement such as stirrups or longitudinal bars. Tensile forces from the transfer of shear forces must be absorbed by the concrete. To this end, the prestressing force was apparently increased, which led to problems in the area where the prestressing force is introduced.
Photo: https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcRjaHW0vKYOFBKsHQdAgC8EHXn7ix3IbC27lA&s

There is now discussion about using thicker prestressing wires without changing the prestressing force. This will reduce the load on the introduction area.

With regard to concrete quality, AREMA requires a minimum of 5000 psi, which corresponds to European concrete C35/45. Concrete up to 7000 psi is used, which corresponds to European concrete B55/60, the standard for German sleepers.

The fact that German sleepers do not show cracks on the top in the middle or under the rail support on the bottom suggests that the design according to EN 13230 determines the moments well.

Cracks on the top of American sleepers indicate, according to the linked comparison, that AREMA underestimates the moments in the middle of the sleeper.

For me, it is not the concrete, but the incomparable design methods, whereby it is completely unclear to me how AREMA arrived at its tables and graphs.
Regards, Volker

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The radioactive sources check compaction by measuring the density of the compacted soil. Getting good compaction depends in part on the soil having the optimum moisture content along with the various mechanical means used for compaction.

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While there may be ‘good compaction’ to allow the next construction process to proceed - doesn’t compaction continue as the mass of the next surface weighs into the construction - the further question - once compacted can specific points lose compaction over time?

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That is a fairly significant difference in concrete specs and in AREMA engineering compared to those used in Germany. And correct me if I am wrong but in US, heavier railcars and trucks.

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Indicates to me that they are ballasting equally across the width of the tie, not just preferentially a distance either side of the rail seat. Many older designs avoided this by necking the ‘tie’ part between the parts that transmit rail load to the ballast, in some cases actually reducing it to a metal girder or pipe between concrete weight-bearing sections – the ultimate, perhaps, in a ‘non-centerbindable’ crosstie.

If the tie is then exposed to HAL or unanticipated levels of repeated shock, it will experience accelerated settling of one or both areas under the rail seat… while the center portion of the tie acts as a kind of defective fulcrum. I would not be surprised to see cross-cracks in the top face of such a tie.

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I vaguely recall that European highway building uses ponding (?) of water over the setting cement.

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“Good compaction” implies that the soil is either at maximum density or very close to it. This doesn’t mean the soil underneath the fully compacted soil is itself fully compacted. The process of compaction is done on a relatively thin “lift” to ensure the compaction is accomplished for the entire thickness of the recently placed lift.

I seem to recall that RR civil engineers figured a new fill would require about 5 years to fully settle under traffic.

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Cracks on the upper side are initially a sign that the tensile stresses can no longer be absorbed, i.e., the associated moment is too large. One probable cause is the one you described. However, the condition of complete bedding can also develop gradually during operation.

The EN 13230 standard distinguishes between two load cases:

Load case 1 (max. tensile stress on the underside)

Load case 2 (max. tensile stress on the top)

A 2015 study by four authors at the University of Illinois entitled FLEXURAL ANALYSIS OF PRESTRESSED CONCRETE MONOBLOCK SLEEPERS FOR HEAVY-HAUL APPLICATIONS: METHODOLOGIES AND SENSITIVITY TO SUPPORT CONDITIONS showed that other bedding distributions can deliver even greater moments.
https://railtec.illinois.edu/wp/wp-content/uploads/pdf-archive/IHHA2015_3637_Wolf_et_al-(DWC-comments).pdf

The first part of the study compares the calculation specifications of American, Australian, and European standards. This part is difficult to understand, as figures and graphics from these standards are repeatedly cited without being shown. There are considerable differences.

In the second part, the influence of the bedding on the moments is examined using half a threshold as an elastically supported beam. If I understand correctly, the bedding of beam sections A to I is varied section by section between 0%, 25%, 50%, 75%, and 100% of the rail load. The remaining load is distributed evenly across all other sections.

The results can be found on page 6 in Table 2. Areas highlighted in gray exceed the maximum values of the standards. New design formulas are developed from this.

My opinion: An interesting study that shows the sensitivity of the system to changes in the bedding. Which of the load cases examined are realistic remains open.

Quote from the study:

Conclusions
The flexural behavior of a concrete sleeper is highly dependent on the sleeper support conditions. Current design recommendations make different assumptions for these sleeper support conditions, which leads to different recommended design bending moments. The parametric study presented shows the high level of sensitivity of the center bending moment as a function of changing support conditions. Design bending moments at the sleeper center can be exceeded under small shifts in distribution of the ballast reaction. This demonstrates that frequent tamping to keep the ballast reaction concentrated under the rail seats can prevent very high center negative bending moments that cause cracking. This high sensitivity also suggests that current design recommendations for center bending moment may need to be increased. Sleeper span-to-depth ratios indicate that for rail seat positive bending the sleeper behaves as a deep beam, transferring load through a compressive field. As such, reductions in design bending moments for positive bending at the rail seat for both the AREMA and AS recommendations may be warranted. At the very least, treating the rail seat load as a point load is overly conservative. The assumption that the rail seat load acts over the entire width of the rail base is used in the proposed equations. For center negative bending the span-to-depth ratio is greater and the sleeper experiences closer to true flexure. The support condition assumptions used in current design recommendations did not correspond closely with the support conditions measured in field testing, which suggests that current support condition assumptions may need to be modified to more closely match field conditions.

Regards, Volker

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I don’t know if it is still used. Most highways are built at a surface slope of at least 2% to transport rain water to the sides. Water spray yes but ponding?

Today mostly membrane compounds are used, I think.
Regards, Volker

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Water spray used to be common to keep cement work moist as it set. Straw used before membrane.

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Two questions:

  1. European highways IIRC do much thicker/deeper subbase? One can have “perfect” compaction on a 5’ thick layer of subbase, but a 10’ subbase compacted would seemingly be better.
  2. RR center of crosstie: If NOT ballasted as much as under the rail…did I read this might INCREASE the risk to tie bending in the middle? endmrw1106251016
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Question 1: https://forum.trains.com/t/uiuc-engineers-make-rail-tracks-safer-longer-lasting/418046/38?u=volker
The hydraulically bound base layer must achieve a compressive strength of 2200 psi.
Question 2: Ballasting like in Load case 1 is preferred to limit the bending moments in the middle of the tie.

With a bedding evenly distributed over the entire length of the sleeper (load case 2), greater moments occur in the middle of the sleeper.

The study linked in post #51 shows that there are bedding distributions that deliver greater moments than evenly distributed bedding.
Regards, Volker

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You da man to answer, you do live there (Germany) correct? What about MY statement (IIRC) the work there on sub base for the best concrete highways is much more robust than here in the USA? Yes/No?
Thanks for 2200 psi info, but again back to the above question.
As far as the tie and ballast placement, Load case 1. Perfect, makes sense
Appreciate all of your contibutions. Regards mke endmrw1106251435

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Look at the post I linked regarding your question 1. It shows the German requirements for Autobahnen.
It shows just one eample. Here is the complete table the example was taken from:
https://beton.wiki/images/thumb/8/8b/RSTO.jpg/2560px-RSTO.jpg

Sorry, I don’t know the US requirements so someone else needs to chime in.
Regards, Volker

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