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

 

[killswitchengage]

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

 

[Settingsun]

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.

 

[BWCSA]

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.

 

[BWCSA]

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.

 

[BWCSA]

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.

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.

 

[BWCSA]

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.

 

[Settingsun]

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.

 

[killswitchengage]

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 .

 

[Ron]

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.