Mechanical Bonding


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Old 10-23-05, 09:57 PM
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Mechanical Bonding

Dick or others -

I have never seen any mention of mechanical bonding. What I mean by this is when a new footing is being poured next to an existing slab and it is highly desirable to avoid uncontrolled cracking or failure at the slab joint (cracking along a control joint would be OK of course). I'm just referring to driveways, patios or walkways of course.

The books I have read just recommend thoroughly wetting the old concrete 12 hours before pouring the new footing or slab and perhaps this is reliable.

Is there any accepted technique of using concrete fasteners or similar which would be installed into the edge of the old slab (thus having a large bolt head protrude from the slab) so the new footing has some purchase on the old slab?

I'm not really concerned with differential settlement but complete failure. One example might be the case of adding brick edging to an existing driveway (over 4"+ footer) or adding a walkway which connects with an existing foundation/patio. In the latter case there would usually be an expansion joint but in some cases where the walk/riser would be covered with flagstone or brick it might not be acceptable to have an expansion joint.

Some have said that caulk can be used lieu of an expansion joint but from what I understand caulk that is supposed to look like mortar rarely fools anyone.
 
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Old 10-24-05, 07:57 AM
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Mechanical Bonding

It would be rare to pour a footing next to a driveway slab or sidewalk and wany them to be structurally connected. A footing is structural and a slab is not. The exception would be a thickened edge floating slab for a lightweight structure.

There is no way to limit cracking at the joint between old and new concrete. It is much easier to get a good bond if the old face is clean, but rough. Painting the face of the old concrete with a bonding agent (there are many - Acryl 60, Laticrete?, etc.) prior to pouring will increase the bond between the old and new concrete.

I don't understand your definition of "complete failure". Certainly, the joint between old and new concrete will not transfer stresses (tension, shear and flexural) as well as continuous concrete does. The exception would be a high strength adhesive, which could be difficult and unpredictable. Your example seems to be for relatively lighly loaded concrete.

The generally accepted method to join old and new concrete slabs is to use a bonding agent in conjunction with steel dowels drilled into the existing and projection into the new concrete. This is to prevent differential settlement. In the case of patching concrete, the steel is normally not needed. The effectiveness of the steel is limited by the slab thickness since steel in the middle ot a 4" slab has about 1 1/2" of cover.

The term expansion joint is very vague when it comes to concrete since concrete does not expand. Concrete shrinks and brick expands. Caulk is used to seal a joint, not to hide it. Isolation joints are meant to keep a joint filled and prevent debis from getting in, not to seal.

Dick
 
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Old 10-24-05, 11:14 AM
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By complete failure I meant that the bond fails and then over time a large gap forms due to settling etc. For finished concrete, an isolation joint is not unsightly but I rarely see them used for brick over concrete at say the junction of a walkway and step riser.

Just to give you you a more concrete example - I demoed a 10' concrete walkway that led from my concrete driveway (original driveway/walkway connection was control joint only) to a raised 8" thick patio slab (isolation joint here) that is part of the house foundation. Just by luck I was able to preserve the rebar that protruded from the driveway. I then poured a new 4" thick slab 3" below original grade so I could mortar in pavers on the walk and end up level with the driveway. I also put split brick on the patio riser.

I considered putting in an isolation joint between the walkway and patio slab but I looked at several houses in my neighborhood that have the same brick treatment (done at time of construction of house) and I could see no isolation joint, just mortar. So for aesthetics I did not put in a isolation joint and just cleaned the patio slab as well as possible and used bonding agent. I then put in a mortar joint between the riser split brick and paver bricks on walkway. So in effect what I have done is connect the driveway and house with a slab with only a control joint at the driveway. The control joint is only about 1" deep on a 7" thick structure (4" slab + 3" brick + mortar) so it will probably have little effect. I'm not too concerned since it is light duty as you said and I used good construction practices (soil compaction, plenty of rebar) but I'm thinking that it may have been better to move the isolation joint next to the concrete driveway. Also, since the patio slab is about 8" thick I could have drilled and placed steel dowels to reinforce this connection.


The other project I did was to put a footing in next to the driveway so I could install brick edging along the driveway (flush with driveway). The footing is 8" wide (brick is placed at right angles to the driveway) and 4" thick with rebar and about 10' long (top of footing is 3" below driveway surface). My concern here is if a car does put a wheel on the brick will it hold up? It's very common to see driveways with brick edging - the problem I see is that a 4" thick footing will only overlap the 4" driveway slab vertically by about 1" so perhaps they also thicken the slab near the footing for brick edging?

Thanks for any comments.
 

Last edited by AlexH; 10-24-05 at 11:41 AM.
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Old 10-24-05, 12:12 PM
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Mechanical Bonding

Alex -

The long and short of it is you cannot try to tie everything together outside and on soil and not expect the individual pieces to move or react differently. As an old friend said "You can't build or tune a piano outside".

You obvously have some soil settlement in your area, but who doesn't? Materials of different composition, shape, moisture content and temperatures move differently and you cannot stop the process. You need to allow building elements move.

I would never set brick in mortar unless it was for absolutely necessary architectural appearances. You can have durability problems, especially with clay pavers. Your 7" slab structure is really only 4" (the slab thickness), the control joint may do some good if it is in the right place (the concrete).

When good joints are contructed using steel across the joint, one side of the stell is not anchored, but is permitted to move horizontally - typical highway joint.

You seem to have thought about everything. I hope your project is as tight as you want it to be.

Dick
 
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Old 10-24-05, 12:33 PM
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Originally Posted by Concretemasonry
Alex -


I would never set brick in mortar unless it was for absolutely necessary architectural appearances. You can have durability problems, especially with clay pavers.

Dick
I appreciate your comments but I don't really understand this. In my area, in the front yard at least, clay pavers are always mortared in. Also, several of the downtown sidewalks in my area have several blocks of mortared pavers. When we say pavers are we really talking about the same thing? For example, the bullnose "bricks" that the various brick companies make can only be mortared and they are the same material and finish as the pavers. These pavers are 2 1/4" thick, with rough bottoms and sides and have a smooth almost glazed top surface. They are fired at higher temps than common brick and have no lugs. In fact I'd be less inclined to use these in sand - these pavers have square edges and I prefer the pavers with rounded edges for use in sand.
 
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Old 10-24-05, 02:55 PM
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Mechanical Bonding

You are apparently looking at a local application trend limited to clay pavers. The market for clay brick pavers is limited because they should be set on a concrete slab.

The use of clay pavers, such as you describe, set on a concrete base is a market reserved for lighter, small scale applications. The clay pavers are not structural unless supported by a concrete slab. Interlocking concrete pavers are a structural element in a flexible pavement system when place on a granular base.

The practice of setting pavers on a granular base has been the standard for years throughout the world. This is especially true when heavy loads such as heavy gantry cranes, airplanes and heavy road traffic or even for lighter loads such as driveways and streets. The ability to remove and replace interlocking pavers is one of the reasons they are used over areas where access to utilities (urban streets and sidewalks) is needed.

Interlocking concrete pavers are usually available in 60mm (2 3/8"), 80 mm (3 1/8") and 100mm (4") thicknesses. The pavement strength is gained from the shape, method of installation and type and thickness of the base. Lightly loaded areas such as sidewalks and patios would use 6 or 80 mm. Driveways and streets would use 80 mm. Civil applications such as airports and ship unloading gantry areas would use 100 mm. The minimum compressive strength of an interlocking concrete paver is required to be in excess of 8000 psi, with 10,000 psi available and 12,000 psi possible, but not needed.


Concrete under pavers can have a negative effect of durability. Mortar joints will crack, especially with hard brick. This permits moisture to be trapped, leading to freeze-thaw problems in most of the U.S. The clay also has different shrinkage and creep properties

To see some of the uses of concrete pavers set on a granular base take a look at the Interlocking Concrete Paving Institute (ICPI) web site.

Dick
 
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Old 10-24-05, 03:40 PM
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Actually clay pavers are required to exceed 8000 psi compressive strength in most states and have a water absorption rate of 8% or less.

Most people choose clay pavers for aesthetic reasons. The downside is that they are expensive. The ones I used require 4.5 pcs per sq ft (with 1/2" grout lines) and are between $1 and $2 ea in small qtys depending on color. Factor in the concrete base and mortar (a 2:1 sand - portland mix is recommended) and the cost over concrete pavers is several fold and thus best used in small installations.
 
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Old 10-24-05, 06:58 PM
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Pavers

Alex -

Your cost figures are a little higher than the ones I have been using. The cost and structural limitations of a slab/paver installation now make me understand why the use is as rare as it is.

Regarding, the specifications for most construction materials, they are minimums not actual. When we set product standards specifications, the wide regional variation of construction materials, and increasing improvements have left the standards much lower than the level of the products actually produced. In the concrete products industry, few people pay attention to the standards because they are too low and the specifications have not caught up with manufacturing technology and marketing needs.

As an example, the minimum compressive strength of a concrete paver ( ASTM 936) is 8000 psi. Most pavers just happen to be close to 9000 or 10,000 because that is what the market needs (not specified). Some producers may make even higher strengths just for marketing purposes. The strength actually has little to do with the load carrying capacity of the pavement. The absorption maximum for concrete pavers is 5%. Many paver producers are in the 2 - 3% range without doing anything special.

I just the specs are just for low cost sellers.

The key to the successful use of a product is the application.

Dick
 
 

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