Sidewalk Standard Details and Specs: Base Materials, Reinforcement, and Jointing 1

[brav]

Hello all,

TL;DR: A few questions about contemporary public sidewalk specs
1) If modern practice stresses the importance of free draining bases for most new roadway construction, why do so many design standards still allow/spec dense graded / crusher run for base material? It seems that this practice is a carryover from a time when we thought that the most important function of a base layer was to add to structural capacity rather than to help move water away from our structures.
2) If most agencies abandoned jointed reinforced concrete pavements long ago, then why do we still see these techniques often used in sidewalk construction? Do you still use reinforcement in your sidewalk structures? If so, why in sidewalks but not in jointed plain concrete roadway construction?
3) Do you use transverse expansion joints in your long sidewalk projects? If so, at what intervals? If not, why not?
4) Why do most places allow hand/tool jointing of concrete sidewalks? All my concrete knowledge tells me that joints must be at least 1/3 of the thickness of the slab. Most concrete details spec at least 4" thick concrete, many 6".

I'm updating my municipality's standard details and specifications (which haven't been substantially updated in ~50 years). Although I'm sure other agencies are in similar situations, it seems our specs are particularly dated. For example, our standard concrete roadway detail was still based around JRCP with 5"-7" thick slabs and 40'-60' joint spacing. Perhaps more troubling, none of our pavement details include base drainage structures despite our mountainous locale receiving 45"+ of rainfall per year and sitting near the 40th parallel (i.e. we experience a *lot* of freeze/thaw cycles). I'm now on my sidewalk spec, and after having recently updated our concrete pavement specification I am caught wondering why I shouldn't make similar changes to my sidewalk structures. Basically, if jointed plain concrete pavement is the typical preferred treatment for local roads with low truck volumes, why wouldn't we design sidewalks in the same way?

Thanks!

 

[Ron]

It seems that this practice is a carryover from a time when we thought that the most important function of a base layer was to add to structural capacity rather than to help move water away from our structures.

That hasn't changed. The primary purpose of a base material is structural support...not drainage. Drainage can be handled in a variety of ways and through the base is not one of the preferred methods.

For Item 2, most agencies have not abandoned jointed concrete placement. Continuous reinforcement is nice and is used more often, but it is still not the norm. As for sidewalks, they are often cheaply built by available labor so the more basic the design, the better.

For Item 3, concrete volume is almost never greater than when it is first placed (it shrinks). Expansion joints are only necessary where you have extreme temperature swings relative to the temperature at the time of placement....usually farther north than your location. I would not place an expansion joint any closer than every 200 feet for a sidewalk.

Hand tooling goes back to the simplicity of the design. You obviously want to control cracking and sawcutting is certainly preferred, but it is also something that is not necessarily amenable to sidewalk construction, particularly for short runs. You need a saw and you need someone on the crew who knows a bit about concrete technology....for a lot of municipal sidewalk construction, you will have neither. Further, many sidewalk placements can occur in a few hours so the crew would have to come back later in the day for sawcutting...probably won't happen.

Hand tooling works, particularly if you control the concrete thickness tolerance.

 

[brav]

Thanks for the quick and thorough reply, Ron. Following up on a couple things below:

The primary purpose of a base material is structural support

This 2017 FHWA Tech Brief on Bases and Subbases for Concrete Pavements states quite clearly otherwise (or maybe I'm interpreting it incorrectly?):

"The key characteristic of a good quality rigid pavement foundation is not the strength of the support, but rather the provision of uniform support that is free of any abrupt spatial and material changes. Rigid pavement design relies on the structural carrying capacity of the PCC and on the uniformity of support provided by the base layers. As such, the pavement design engineer should not attempt to use the base/subbase layers simply to increase the overall structural capacity of a rigid pavement system or to reduce the thickness of the PCC layer. In most rigid pavement designs, the PCC design thickness is relatively insensitive to the foundation strength or stiffness and, therefore, slightly increasing the slab thickness is more economical than structurally increasing the thickness of the base layer to achieve the necessary structural capacity. A pavement design engineer should evaluate the potential causes of a non-uniform foundation and design the base or subbase layer to mitigate their effects."

Both the FHWA Mobile Concrete Lab engineers and my local LTAP agree with the brief, as does my old pavement design professor. Although it doesn't state explicitly that drainage is the most important function of base layers, proper drainage helps improve everything that degrades (pumping, frost heave, soil expansion) what it says is the most important: long-lasting uniform support.

Regarding item 2 - the hinge qualifier in my claim is reinforced vs non-reinforced jointed concrete pavements. Again, if the FHWA and the ACPA are to be believed (and the textbook authors, too) only a handful of agencies now use slab-wide reinforcement when they are constructing new jointed concrete pavements, and no one uses 40-50' joint spacing any longer. Whether agencies use dowels at joints (versus aggregate interlock) seems entirely dependent on expected ESALs / percentage of truck traffic.

As for sidewalks, they are often cheaply built by available labor so the more basic the design, the better.

I do think the need to simplify construction techniques to accommodate less ideal laborers and inspection processes is a major factor here - definitely agree with you there. I suppose if I told most any contractor that the most important thing is a uniform base and then specified #57 clean stone, they would look at me as if I had two heads (particularly on the grades we have to deal with around here...). Most contractors perceive a compacted base to be a uniform base, despite significant vertical deviations. I've recently been working with local material suppliers to prepare something in the middle - a 50/50 blend of #57s and #8s or #9s, which is somewhat comptactible but also drains rapidly into my underdrains.

On simple construction - why do so many details still specify steel reinforcement in sidewalks? This seems to me to be the place where construction most deviates from specification. There's one reason I can see to still ideally require mesh - to (hopefully) reduce vertical faulting at joints in an attempt to improve ADA compliance. But improperly placed mesh (like straight on grade or top of base course) doesn't provide this function and even properly placed mesh is attacked by water intrusion and eventual blowouts caused by rusting all joints.

I would not place an expansion joint any closer than every 200 feet for a sidewalk.

I'm glad we're on the same page here. My current detail specifies expansion joints every 40 feet. I have never been able to figure out why. For the same reasoning, I also questioning the utility of expansion material against most cold joints - like on longitudinal joints where both sides of a new 5' wide sidewalk are constrained between, say, a curb and a wall. This is another common aspect of municipal sidewalk details. Seems like the practice just accelerates water intrusion into the structure.

the crew would have to come back later in the day for sawcutting...probably won't happen.

You've got me there - and also the likelihood that they often cut into curbs. I just wish more contractors would use tools with bits more than 7/8" deep. Our sidewalk spec is 6" thick, so a 7/8" deep tool joint seems severely inadequate to me.

Thanks again for your time and thoughts.

 

[Ron]

@brav....I agree with the FHWA tech brief; however, it is limited to rigid pavement subbases, not pavements in general. Your post mentioned "bases" not subbases so I responded in that context alone.

For rigid pavement subjected to high loading, drainage at the subbase level does take precedent over subbase strength since the pavement surface design has hopefully accommodated the strength issue.
I recently had an additional width added to my pool deck apron. It is 4" thick. The joints were tooled to match existing control joints. The depth of tooling is about 1/2 inch. So far, two of the joints have cracked as they should, but others have not yet cracked and probably won't. There are no other cracks in the addition.

One advantage of tooling joints is that it occurs while the concrete is in a plastic state. As the concrete gains strength, that reduction in cross-section that is already in place allows initial microcracking to occur and ultimately full depth cracking to relieve shrinkage stresses. The initial microcracking from the surface down actually increases the effectiveness of the tooling depth, thus allowing proper cracking to relieve full section shrinkage stresses.

 

[brav]

Ah, pardon my generalization. Good point about the microcracking around tooled joints that occurs as the concrete cures.

I recently had an additional width added to my pool deck apron.

Did you elect to use any steel reinforcement in this concrete?

 

[r13]

I think this paper is a must read, as it addresses many, if not all, of your concerns. Also note, that each local has its own pavement design criteria that shouldn't be overlooked. Link.

 

[Ron]

@brav....no reinforcement.

 

[r13]

In my local, I have yet to see a residential walkway/pavement having any form of reinforcement. Tooled or saw cut joint every 4'-5', and full depth control joint every 40'-50', no dowels. I think it is convenient for staggering concrete placement.