r/civilengineering • u/Turbulent-Set-2167 Municipal Engineer • 1d ago
What are the metal trapezoids for?
So Caltrans is doing some highway work. Could someone educate me on the name and purpose of the metal trapezoids and the thicker, longitudinal bars?
Ty in advance
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u/nforrest CA PE - Civil 1d ago
The thinner trapezoidal part is called a basket and it's there to hold the dowels (the thicker part) in the right place. These dowels are there to transfer load fro one panel to the next as heavy vehicles pass over the soon-to-be concrete pavement. They're purple because they're epoxy coated to prolong their lifespan before they start rusting.
Further reading: https://rosap.ntl.bts.gov/view/dot/23728
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u/Assasin_Zer0 1d ago
As the others have said above.
The large dowel is the important part here. The rebar cage is just to hold the dowel in the right place. Both ends will most likely get greased to allow them to slide and the bar itself should stop vertical deflection.
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u/Kojo907 1d ago
The thicker metal bars are dowels, they are placed at construction joints to keep the slabs together. The "trapezoids" are rebar chairs that raise the dowels to the correct elevation. Dowels are typically placed at the middle of the slab.
These are especially important wherever you have vehicular traffic since the wheels tend to damage the joints. See Chapter 6 of ACI 360R - Guide to Design of Slabs on Ground
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u/ArcticEngineer 1d ago
Because of the subreddit I think it's important to add that, at least in my neck of the woods in Canada, these are also sprayed with something akin to form release oil so that they don't limit the lateral contraction forces but still allow vertical load between adjacent panels. In my case it was due to the large forces aircraft wheels would unevenly exert while taxiing along concrete aprons.
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u/mrthekicker2 1d ago
Dowel baskets placed at the contraction joint. They reinforce the joint to allow for load transfer between the two sides.. without them you lose some concrete strength between the joint
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u/bothtypesoffirefly PE transportation 1d ago
I’m not in Ca, I have a question for the engineers there: are they using fiber reinforcement or going to add rebar before they pour?
Edit: we use asphalt except on bridge decks, we’re in the mountains.
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u/tgrrdr PE 1d ago
Some concrete pavement in California is reinforced (CRCP) but most of what I've seen placed is just plain concrete with no reinforcement (JPCP).
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u/bothtypesoffirefly PE transportation 1d ago
I totally believe you but holy shit I find that questionable. I guess you don’t have to worry about run time on slab on grade but still.
Maybe they have enough traffic it’s cheaper and they know they’re going to have to rip it out regularly anyway? But then why not use fiber. I would think the reinforcement would be good when you have earthquakes. They’ve got to be using a very different mix than we do in the mid-Atlantic.
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u/tgrrdr PE 1d ago
I don't know what they use in the mountains, I'll see if I can find design guidance.
Supposedly in the temperate area where I live the life cycle cost analysis favors concrete over HMA. "Recently" we've had some jobs with precast concrete slabs and I've also seen several CRCP projects. I'll see if I can find real numbers for upcoming projects but I'm not sure how the data is segregated.
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u/poldim 1d ago
ELI5: If it’s a single monolithic pour, it doesn’t make sense to me. Are these going to be poured in square-ish segments such that the dowels are at a split between two pours?
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u/etlr3d 1d ago
Yes. Supporting the dowels so they wind up in the middle of the section of otherwise unreinforced concrete (maybe some fibers in the mix tho. Many airport aprons and other high load applications are done like this.
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u/Frenchie1507 1d ago
They likely just haven’t installed the rest of the reinforcement yet. There is no way a slab that size has no rebar grid within it, especially if they’re expecting it to transfer load from one slab to another.
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u/mulligan72 9h ago
Google PCP
might have to be more specific and search for plain concrete pavement
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1d ago
[deleted]
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u/hattie29 1d ago
No. These aren't expansion joints. These are dowel rods. They help transfer the load of a vehicle from one panel to the next.
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u/Icy-Palpitation-2522 1d ago
Contraction joints possibly.
Edit. Without a sleeve I wanna change my answer to expansion joint
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u/jaywaykil 1d ago
Definitely smooth-bar reinforced contraction joints. The concrete is poured continuously along the road, then after it sets they'll cut a partial depth notch above the bars with a saw. This causes the concrete to crack in a straight line instead of randomly.
The straight saw cut/crack can be sealed to keep water from soaking the subgrade. The smooth bars prevent the (now semi-isolated) adjacent slabs from moving up and down.
An expansion joint would require a full-depth formed joint with a gap that is filled with solid but compresible material.
Contraction joints are every few meters like shown here. Expansion joints are much more spaced out.
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u/Icy-Palpitation-2522 1d ago
Yes I knew I was wrong with expansion joints but wasn't going to edit a 2nd time ha
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u/Substantial-Sector60 1d ago
So is the concrete placed monolithically, then joints sawn at the dowel locations, but only to the depth of the dowels?
I am puzzled.
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u/cursingbulldog 1d ago
Yes, so the one thing about concrete is that it is going to crack no matter what. So after they place the concrete a crew comes in while the concrete is still hardening and cut joint lines with a saw an inch or two deep every 12-15’ or so. Now once the concrete fully hardens it will crack where we
This gives the concrete a weak spot so it cracks where we want. Downside is water can now seep through the cracks weakening the sub grade, so we add a sealant to keep water out.
The other downside is as heavy trucks drive by all the load is on one panel before suddenly shifting to the other. This can rather break the concrete if it’s too weak or shift the sub base around causing one panel to sink and the other to lift. To fix this dowels are added that transfer the vertical loads so both panels share the load and as a vehicle drives over the joint.
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u/Pwnedzored 1d ago
Yes. Joints are sawn. Creates a weak spot where cracks form when there concrete contracts.
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u/smcsherry 1d ago
Dowel bar baskets. They help make sure the panels move together.
If you have ever seen the little piano keys on a concrete highway, usually in groupings of 6, it’s because these were retrofitted into old concrete to improve its service life. https://en.wikipedia.org/wiki/Dowel_bar_retrofit