rigid pavement design 4

[killswitchengage]

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

 

[oldestguy]

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.

 

[killswitchengage]

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

 

[oldestguy]

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.

 

[killswitchengage]

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

 

[r13]

Try PCA method. Link

 

[Ron]

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.

 

[r13]

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

 

[killswitchengage]

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?

 

[r13]

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.

 

[killswitchengage]

yep near the joints otherwise the interior is perfectly fine

 

[r13]

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.

 

[killswitchengage]

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

 

[r13]

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.

 

[BWCSA]

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.

 

[killswitchengage]

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

 

[r13]

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.

 

[killswitchengage]

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 .

 

[r13]

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.

 

[Settingsun]

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?