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rigid pavement design
4

rigid pavement design

rigid pavement design

(OP)
Hello
This is the first time i am designing rigid pavement for a container terminal project and i am not really experienced with the subject so please bare with me .

My questions are as follows :
1- I am using Westergaard modified method with a concrete of 28 MPa compressive strength and a slab thickness of 20 cm (6 in ) and a poisson ratio of 0.25 and taking only the front wheels of a truck which i believe delivers 6 t of live load . However even after double checking units and stuff i am always getting negative tensile stress in corner loading condition of about -0.09 MPa is that normal ?
Edit: i believe it only means there is a cantilever situation so its actually normal

2-How am i supposed to know the thermal gradient in the slab ? do i assume 20 C° difference between the top and the bottom of the slab ?

3-Do i have to prescribe an admixture to accelerate hardening since there are cases of live load traversing the concrete at a very young age ? . Also i keep noticing in literature that rebar reinforcement consists of a single mesh put in the middle of the slab and that they only reduce cracking of the concrete rather than contributing to tensile resistance why is that true ?



thank you

RE: rigid pavement design

I'd refer to the Portland cement association in your country for standards,etc. Missing so far also are the characteristics of the ground below the slab. Most pavement type methods also need to know load repetitions per year, etc.

Of course passing this on to an experienced engineer sometimes is the way to go.

RE: rigid pavement design

(OP)
I wouldn't be here if we actually had experienced engineers , the ones we had were bs i might as well be the only professional around . However load repetitions per year are not mentioned in Westergaard method !

Thanks

RE: rigid pavement design

I'm probably not helping, but. Why only front wheels of a truck? Usual design for truck load apply the rear axle loads. Is your calculation done on a free computer program? If so, that explains it. Get out the dust covered calculator.

There out there nomographs where you can put in low number of repetitions, soil data, etc and it maybe sufficient for your purposes.

RE: rigid pavement design

(OP)
No i am using a spreedsheet on excel , the reference axle load in French standards is 13 t compared to 10 t in most countries so i assumed a 6.5t per wheel load . as for the effect of repetitions i will be using a different software

RE: rigid pavement design

Try PCA method. Link

RE: rigid pavement design

3
If this is a container terminal pavement, your loads and concrete thickness are way too low. You need to consider much higher loading, mostly due to the loading/unloading equipment to be used. As a result, your concrete thickness will be higher.

As pointed out by OG you might need to consider repetitions of load as well as all axles to exert load on the pavement. If the tensile stress at the bottom of the concrete layer exceeds 50 percent of the modulus of rupture of the concrete, you need to consider repetitions. If the stress is lower than 50 percent, the repetitions are irrelevant. For analysis, you can use Westergaard theory, PCA procedures, AASHTO procedures, elastic layer analysis, finite element analysis or other procedures.

You also need to consider the mix design to be used for the concrete. Use the largest coarse aggregate you can get for the available placement capability. This will keep the cement content lower for the same strength of concrete, thus reducing shrinkage which contributes to cracking. Further, the larger aggregate will provide better load transfer across non-doweled joints or cracks.

You asked about wire mesh. The purpose of the wire mesh is to keep cracks that happen to occur from drying shrinkage or thermal movement as tightly closed as practicable. Its purpose is not to provide tensile reinforcement to the slab to resist bending, thus the reason it is typically placed in the middle third of the thickness of the slab.

I would suggest that you study some of the publications of the Portland Cement Association (PCA), and a variety of textbooks on pavement design, such as those by Huang or by Yoder and Witczak.

The US Army Corps of Engineers has some good design information on container pavements as well.

The last container terminal pavement I analyzed was for roller compacted concrete and the thickness of the pavement was in the 460mm range with a poor subgrade condition. If you have a competent subgrade, I would expect your concrete thickness to be in the 250 to 300mm range.

RE: rigid pavement design

I agree the pavement should be more than 6" thick.

RE: rigid pavement design

(OP)
Thank you. Pre-existing pavements consists of a 25 cm concret I think they have 25 mpa compressive strength and underlained by a 30 cm unbound material. These pavement were reinforced with a double mesh of t8 rebars I don't know if you guys have the same symbols basically t8 means a diameter of 8 mm, these slabs suffered excessive breaking at the angles and side due to the absence of dowel bars obviously. So my guess a 25 cm concrete with at least 35 mpa with a single mesh of t8 underlained by lean concrete would suffice, what are your thoughts?

RE: rigid pavement design

Quote:

these slabs suffered excessive breaking at the angles and side due to the absence of dowel bars obviously.

Are the damages near joints? Can you elaborate a little further. A photo will tell a lot.

RE: rigid pavement design

(OP)
yep near the joints otherwise the interior is perfectly fine

RE: rigid pavement design

Expansion joint, or construction/control joint? Reinforcing detail across the joint is very important, as under cyclic wheel load, even slight differential settlement can cause a lot of headaches.

RE: rigid pavement design

(OP)
Yes i am well aware of that and i think you didn't notice i was referring to a preexisting pavement and using it as a reference for design

RE: rigid pavement design

I know. So you need to pay attention to the existing joint design to see how it has failed. I would place my rebars on top face with a spacing that satisfies temperature and shrinkage requirement.

RE: rigid pavement design

With regard to design procedures. Since you're talking about French axle load standards, I assume that the terminal is located in France? PCA procedures, AASHTO procedures and the like, while all fine, won't result in a calculation note that is compliant with applicable codes in the terminal's jurisdiction. Use a guide or calculation procedure that references/uses European (EN, NF-EN) or where relevant French (NF) codes.

I personally like "TR34: Concrete Industrial Ground Floors" (4th ed. 2016) for everything that has to do with slabs-on-ground. It's a UK publication that follows the Eurocodes, so you could use the procedures in it for your project, just make sure that all parameters that can vary per country are aligned with the French national annexes instead of the UK's. It uses Westergaard's model for the static loading checks (with vehicle loading = static wheel loads * dynamic coefficient). It also has an Annex that outlines a separate minimum thickness check under loading repetitions / fatigue. This Annex uses a procedure common in road design standards/codes: calculate the number of "equivalent standard axles", then put that number into an empirical formula to determine the thickness requirement.

Like others have pointed out, axle loads will be much higher than the 130 kN you have in mind. A reachstacker can have up to 1100 kN on its front axle. Contact pressure for container handling equipment on air tyres will be around 1.0 N/mm² versus 0.7 N/mm² for a truck. If general cargo / heavy piece goods will be handled, tyre contact pressures will become even higher, up to 10 N/mm² , since these are moved on trailers with full rubber or steel tyres. Talk to your client about expected vehicle loading.

With regard to slab thickness. Your first suggestion of 20 cm of concrete doesn't seem enough. Just to compare: road construction standards vary from country to country (and even state to state within countries), but as a rule of thumb concrete roads that will be driven on by trucks will normally have at least 19 cm of concrete paving with a stiff (bound) layer underneath it (25-35 cm depending on the material). So for a container terminal which will be much more heavily loaded, 20 cm will never be adequate. Your second post with the 25 cm seems more in the right direction, depending on the rebar, the base layers underneath and the equipment that will be used.

I wouldn't use saw cuts for crack control in this kind of application and I would also maximize the spacing of expansion joints as much as possible. Instead use lots of rebar top and bottom for both crack control/distribution and handling the heavy equipment loads.

RE: rigid pavement design

(OP)
Thank you all

BWCSA i like your reply its very thorough , however i wanna point out a few things :

-You said reachstacker can have up to 1100 kn on its front axle , but you do know that it has a large area of contact so i don't believe it will develop considerable pressure under its wheels .
- You mentioned rebars , i am thinking of only using welded wish mesh on the top side of the slabs and nothing on its lower side because according to my preliminary calculations the concrete will resist both single and repetitive wheel loadings with its tensile strength alone so why should i put rebars cages ?
-I have additional questions regarding dowels and lean concrete foundation layer , must i design dowels in shear , bending etc or should i only prescribe dowel diameter , FeE and its length and spacing as per regulations ? . About the foundation in lean concrete do i need to prescribe welded wire mesh or extensive curing is enough ?

thanks again

RE: rigid pavement design

1. Where is the dowel to be placed, across construction joint, or expansion joint?
2. The lean concrete sub-base should be cured to develop minimum strength to allow imposed construction loads. And, prior to place the foundation, it needs to be roughened and thoroughly cleaned (pressure wash will do). Why wire mesh, wire mesh is only required on the top face of the pavement.

RE: rigid pavement design

(OP)
1-obviously across construction and expansion joint i thought that was mandatory , but this doesn't seem to have be in relation to my stacker question !
2- This is a very informative rule ( cleaning the the sub base ) thanks but when using granular foundation they are by default dirty so why the hell ?
Thanks a lot .

RE: rigid pavement design

1. I think you are well aware that these are two different joint, thus the detail for dowelling differs too. You can find many good examples online, or from within the past threads in the forum.
2. It is up to you to clean the sub-base or not, as it's non-structural, but can provide additional protection through a little house keeping/cleaning.

RE: rigid pavement design

British slab-on-ground tables from the 1980s gave ~250mm as the unreinforced slab depth required for 13 tonne forklift axles, assuming dowels or edge thickening. This tallies with you observations that the interior is fine but the edges have been damaged. The thickness varied a bit depending on subgrade stiffness and concrete strength.

To put the question to bed, is there any chance of container forklifts, reach stackers etc on this pavement? There are certainly cases where those machines aren't normally used like RTG/RMGC terminals. Truck breakdown could call for them though, or they might just tow the trailer with another vehicle.

Container pavements do tend to go wrong though...

BWCSA, TR34 requires structural sagging reinforcement, doesn't it?

RE: rigid pavement design

(OP)
Steven 49 yes French Heuristic recommendations for dowel size spacing and length are present in a specific document but they only mention its minimum FeE .
And yes stacker reach do exist , and let me clarify that i am talking about a quay an exterior zone where ships unload their containers and which are then obviously loaded onto trucks or left aside . i don't know what BWCSA adn TR34 are

RE: rigid pavement design

32mm dowels x 450mm long at 300mm centres are about right. Grade 250 steel (250MPa yield stress). Does FeE mean yield stress?

I don't think reinforcement in the lean concrete sub-base is done, but maybe it is in places I haven't worked.

BWCSA is one of the people who replied above. I was asking a question about the British TR34 design method which I'm only a little familiar with. I was under the impression that it doesn't apply to unreinforced slabs and slabs with only crack-control reinforcement at the top face; it only applies to slabs with bottom structural reinforcement. I think that it gives thinner slabs than the design methods for unreinforced slabs because it assumes the slab has structural reinforcement, and was hoping BWCSA could elaborate on this.

RE: rigid pavement design

Quote (killswitchengage)

You said reachstacker can have up to 1100 kn on its front axle , but you do know that it has a large area of contact so i don't believe it will develop considerable pressure under its wheels .

Sure, but per Westergaard formula's (center condition) the max stress is both proportional to the total and to ln(Le/r), with r the (equivalent) radius of the loaded area and Le the radius of relative stiffness. So the increased contact area (larger r) will only partially mitigate the effect of an axle load that's almost 10 times higher than that of your truck.

The reach stacker was just one example of what you might encounter on a container terminal, not sure it's relevant for you case. Like steveh49 hinted at: there are different terminal types with different equipment types. Ask your client about equipment and other loading that needs to be accounted for in your specific case.

RE: rigid pavement design

Quote (killswitchengage)

You mentioned rebars , i am thinking of only using welded wish mesh on the top side of the slabs and nothing on its lower side because according to my preliminary calculations the concrete will resist both single and repetitive wheel loadings with its tensile strength alone so why should i put rebars cages ?

When I mentioned rebar, I meant to say reinforcing steel in a general sense (not an native English speaker). Wire mesh/steel fabric/steel netting/whatever you want to call it is obviously the most time-efficient and economical way to reinforce this type of element.

Your concrete section will be subjected to the tensile stresses arising from both:
1. The usage loads (wheel loading and possibly others)
2. The restraining of imposed deformations (axial deformation due to shrinkage, internal temperature gradients due to the hydration process during concrete hardening, externally imposed temperature gradients, … restrained by the friction with the foundation )

Stresses from (1) can be kept below the concrete tensile strength by simply thickening the slab, stresses from (2) much less so. That's why IMO breaching of the concrete tensile strength should just be accepted as a given and strategies for controlling the crack formation should be devised, like:
a) spreading total crack width into lots of smaller acceptable cracks by use of reinforcement
b) controlling where the cracking starts and further widening/opening up can happen by inducing them (under a saw cut) or by placing other type of joints (which you could consider as an "artificial crack" through the whole slab)

Most designs will incorporate both strategies to some extent. I'm very partial to strategy (a), that's why I suggested lots of reinforcement (both sides) and as little joint work as possible, but this is just a subjective design choice. Given the right joint work your solution might work just as fine.

RE: rigid pavement design

Quote (killswitchengage)

I have additional questions regarding dowels and lean concrete foundation layer , must i design dowels in shear , bending etc or should i only prescribe dowel diameter , FeE and its length and spacing as per regulations ?

These regulations/heuristics have to come from somewhere, either from experience or they were calculated for a range of assumptions & input parameters that are representative for the application for which the regulations/heuristics are meant. Either way, applying them should lead to the similar results as doing the calculation/design yourself. When in doubt (re)calculating something yourself is never a bad idea tough.

Quote (killswitchengage)

About the foundation in lean concrete do i need to prescribe welded wire mesh or extensive curing is enough ?

I've personally never prescribed steel reinforcement in the foundation layer/base layer (or any other sublayer), nor have i seen it done by others, but that's just anecdotal of course.

RE: rigid pavement design

Quote (steveh49)

TR34 requires structural sagging reinforcement, doesn't it?

Yes. It doesn't literally require it since T34 is more of a general design guide than a "hard" technical specification with prescriptions/requirements, but it does assume it in certain design formulas.

RE: rigid pavement design

Quote (BWCSA)

I'm very partial to strategy (a), that's why I suggested lots of reinforcement (both sides) and as little joint work as possible, but this is just a subjective design choice. Given the right joint work your solution might work just as fine.

I like this too. The only problem is the upfront cost, or at least perception of cost. I'm not sure whether Killswtch is replacing the damaged pavement or extending it but, if replacing, it's only being done because of joint damage. Heavy reinforcement without joints would have meant the original pavement would still be OK. Low cost in the long-term.

I'm also sceptical of the low reinforcement quantities in jointed pavements. The reinforcing strength is typically a small fraction of the concrete's own strength, say 30% as an order of magnitude. Whatever force broke the concrete will yield the steel without much control of the crack width occurring. If you've got close contraction joint spacing, you'd probably get the same performance from completely unreinforced concrete. But, if there is a crack, there will be inevitable questions about the lack of reinforcement.

RE: rigid pavement design

(OP)
Hi
My analysis concluded that for a loaded Rechstacker which seems to be the only heavy load in my case it made me prescribe a jointed 9.84 inch thick concrete pavement with 4 MPa tensile strength for a 25 years life service, the slabs obviously will have welded wire mesh at the top face and doweled joints in its 4 lateral faces ( because Reachstacker does not always travel in a straight line) .
I am using a 14 cm lean concrete which is 5.5 inch width with 1.7 MPa tensile strength , my question is : it seems that French recommendations suggest the extent of the lean concrete layer to be about 19.5 inch farther away compared to the limits of the adjacent slabs and if i am not wrong it also recommend creating tied joints in the same layer between new and older lean concrete masses with deformed bars same way you would do for tied joints for slabs .

What do you guys think ? IMO i think it makes sense in order to avoid lean concrete joints coinciding with slabs joints and ensuring the continuity of the layer .

RE: rigid pavement design

Extending the lean concrete beyond the surface pavement is a good idea. It will reduce the edge stresses on your pavement and mitigate some cracking....it mitigates the radius of relative stiffness issues at the edge.

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