Concrete cracks 3

[chollan2]

Hi,

While out walking, I looked at cracks. Cracks in driveways, cracks in sidewalks, cracks in pavement, etc.

Things I have noticed.
1. No expansion joints on driveways of 8 or more feet.
2. No expansion joints of different materials side by side.
3. No expansion joint on concrete on an angle.
4. No expansion joints above piping.
5. Irregular forms. Not square. Again. No expansion joints.
6. Discontinuity. An expansion joint stops, not continuing on to the adjoining structure.

I should have pictures to show exactly what I mean. I see driveways 20 feet across, with NO expansion joints.
You can clearly see driveway where the crack forms almost in the center. (I don't have a digital camera)

Something that is funny, I guess they plan it that way, is that every 3rd sidewalk block, they cut part of the curb.
So, when the sidewalk expands, the cut expands causing a crack in the street cement.

If you have a driveway and the slab is wider than say 8 feet, check out the cracks.

I don't know why the industry or contractors don't install expansion joints. Every driveway I see will eventually need replacement, due to a simple expansion joint.

Walking around noticing things,

 

[Ron]

OK...first a few things on terminology. The cracks you see have NOTHING to do with expansion. They are drying shrinkage cracks. They are caused by improper CONTROL joint design and improper timing of the sawcuts. Another common joint type is an ISOLATION joint. This occurs where features of one type need to be isolated from adjacent concrete. An example is a column blockout.

Next is a CONSTRUCTION joint. This is used to divide construction done at different times (starting and stopping at convenient points).

Last is the EXPANSION joint. It is rarely needed as concrete continues to get smaller through its life, not larger unless exposed to large temperature differential from its time of placement. Expansion joints are useful in pavements and sidewalks every 100 or so feet to allow for thermal expansion.

Most cracks in residential construction are caused by ignorance and poor quality control.

 

[chollan2]

>>Another common joint type is an ISOLATION joint. This >>occurs where features of one type need to be isolated >>from adjacent concrete. An example is a column blockout.
I see this occurence when they place a square of some material in the center of a slab of cement. I would have to think the cracks are caused by thermal expansion, just because of the way it looks.

Ok, when building a sidewalk, they put grooves into the cement every 3 feet or so. Why do they bother doing that to sidewalks and not to driveways,etc? What type of joint is that and what is it's purpose?

>>unless exposed to large temperature differential from its time of placement.
That is pretty much everywhere on earth. I don't know of a single bridge that does not incorporate expansion joints.
I will agree with you that concrete shrinks, but I think a vast majority of the cracks have to do with expansion. Not only of the concrete, but anything that adjoins the concrete.

A lot of the cracks have to do with plain old water errosion. The water softens up the dirt and concrete, eventually causing it to crack.

Regards,

 

[GeoPaveTraffic]

chollan2, Ron is correct, concrete only gets smaller with time NOT larger. Remeber that concrete produces heat as it cures and then cracks form as it sets and cools. The reason there are joints (groves) in sidewalks every 3 feet or so is to encourage the shrinkage cracks to form at those locations. The rule is to make groves or sawcuts to form rough squares in the concrete. Therefore, if the sidewalk is 4 feet wide the groves are cut every 4 feet.

As for driveways, most of the joints are cut prependicular to the driveway. Therefore if you have a 10 foot driveway, you will have a joint about every 10 feet. Now if you have a driveway over 15 feet wide or so, then you need to have a joint formed down the center of the driveway. This is often not done.

All of these joints are to control cracking caused by the concrete as it cures. The normal thermal expansion of concrete does NOT create cracks.

As for your comment about square items placed in the middle of concrete; it does not matter if they are steel, concrete, or wood. If you place a squre hole in concrete or place concrete around a square something, cracks will tend to form at the corners. You can sometimes control the cracks by placing groves (joints) at the corners, but not always.

Lastly, I have read two of your posts where you say that water softens concrete. That is simply not true. Water does NOT soften concrete, in fact concrete should be cured at 100 percent humidity.

 

[chollan2]

Hi,

I suppose shrinkage is responsible, but not exclusively. I believe it is a combination of shrinkage and expansion.

>>All of these joints are to control cracking caused by the >>concrete as it cures. The normal thermal expansion of >>concrete does NOT create cracks.
Then what is the purpose of putting expansion joints on bridges? Why bother if nothing is going to crack. Please explain this.

>>Lastly, I have read two of your posts where you say that >>water softens concrete. That is simply not true. Water >>does NOT soften concrete, in fact concrete should be >>cured at 100 percent humidity.
It does. That is the #1 cause of errosion. Concrete does not last forever. Puddles form, Seepage occurs, Concrete is porous. Seepage occurs in dams all the time. I wish I had a digital camera to show pictures of down spouts and water damage. The water damage errodes the surface and soil. The cement cracks and the water further errodes the cement.
I have no idea why you say that water is harmless to concrete?

>>As for driveways, most of the joints are cut >>prependicular to the driveway. Therefore if you have a >>10 foot driveway, you will have a joint about every 10 >>feet. Now if you have a driveway over 15 feet wide or >>so, then you need to have a joint formed down the center >>of the driveway. This is often not done.
I wonder why they don't? So they can just replace the driveway in a few years?
I also notice that the angle of the slab creates cracks as well. It is funny seeing the cracks form on new cement.

I'm not a structural or cement engineer. I just noticed the similarities in cement cracks and wondered why they continually design cement to crack.

Walking around noticing things,

 

[Ron]

chollan2...it is obvious you are neither a structural engineer nor a "cement" engineer (the proper term is concrete for our discussion...cement is the "glue" that holds it together). GeoPaveTraffic has clearly and correctly pointed out some reasons for joints.

Water can erode concrete, but it does not soften it. Water can erode most anything if continuous impingement occurs. Static water does not soften concrete...it generally enhances its properties including strength. Dynamic or flowing water causes erosion, but it is a mechanical function, not a chemical one.

As for why there are "expansion" joints in all bridges...part of the reason for such joints (they are actually isolation joints) is that each bridge segment is usually resting on a separate foundation, thus works somewhat as an independent system....and certainly independent from the adjacent abutment and pavement, from which it clearly needs isolation, but not necessarily because of "expansion".

 

[chollan2]

Hey,

Well, thanks all for enlightening me. I've been surfing around seeing how concrete is made, How it is laid, how it is cured, etc.

It peaked my interest when I walked around and look at things. I noticed the driveways and how cracks form if the length is over say 8 or 9 feet. Also, when concrete is on a slant. Plus a few other obvious overlooks.

Some of the concrete I see looks as though it is caused by expansion, but I suppose it could be caused by contraction.
I am not totally sure about it yet.

I am trying to identify cracks and determine why they occur.
Some cracks I can't identify why they occur. They must be either curing or settling cracks.

I like the (What I call) Arch Crack and sometimes flower crack. It happens when a crack forms an arch around a corner. If you have 4 corners and 4 arches, it forms a circle. More arches form a flower.
I believe that is from water erroding the soil underneath causing the cement to lose support and crack at the corner.

The crack that is odd is the crack that will form on two or more different pieces of concrete and run as though it is one crack. Even though the pieces are different ages. That seems odd to me as though it was an expansion thing, rather than a contraction thing. That, plus the piece in the middle along with the L-type pieces that look as though they are caused by expansion rather than contraction. But, in either case, a control joint would have prevented the crack. Why they don't put control joints in is beyond me and I guess nobody really knows?

Till next post...Cheers!

Walking around staring at cracks,

 

[blake989]

chollan2, some very astute observations you have made. I'll try to help you understand those cracks you see that appear to be caused by expansion. In a way, yes you are correct. Joints are cut or finished into concrete to allow for contraction. Either way, in home construction these joints are just cut and left alone. To be done correctly, they should be cut then sealed (usually with caulk). When they are not sealed($$$), and say the seasons change and temperatures fall, the concrete will contract and the joint will open. Meanwhile you drive your car across it or whatever and sediment collects in the opened joint. Summer rolls around again and the concrete WILL close this joint, the sediment in the joint tries to prevent this return to normal condition (expansion), in most instances concrete will spall.

 

[unclesyd]

I check out new driveways the same as chollan2

My neighbor, who just happens to be a building inspector, just had new driveway poured with some of the driest concrete I've ever seen. It was almost as dry as machine grout. It would just flow from the truck and then only by using a shovel. The concrete was steel fiber reinforced no other admixes.
No water added on site.

They were just able to screed it then was hand troweled by 6 finishers. Control joints were made at 10 ft intervals on a 11 ft wide driveway. The joints were tooled in to about 1/2 thickness of the 4" slab. There are two curved sections with about a 6' radius with both ends truncated (squared off) to about 4" wide. There was control joint separating the arches from the slab.
As stated above the control joints (9) broke through the slab on the second day due to shrinkage and all the joints have opened to about 1/8" in two weeks.
The most unusual crack was in the arches were at one week old both cracked as if on schedule. They started on the curved part and progressed perpendicular to the side against the road, asphalt on clay base. The crack has about 3-4 branches but seem to rejoin the main branch. One other point, as of today the control joints at the driveway end of the arches haven't cracked. I had the job of pointing out the cracking, needless to say he wasn’t a happy camper.
They have had enough people look at this I should have had a concession stand.

I had mentioned to him that he needs to seal the cracks to prevent water intrusion. This discussion reinforces my SWAG, thanks.


PS:
True Story
My old concrete buddy finally cracked, really, and is now in a state hospital. He always said that concrete will drive you nuts. He wrote supposedly two excellent books on concrete and published them himself and then wouldn't sell them. I asked to buy one and his comment was he had decided not to sell them as working with the book it would be too easy to get his job.

 

[TPNY]

DaveFlax,
I think I've noticed about 4 other posts from you recommending type K! You're not a $$manufacturer or a distributer$$, are you???