billconner
Well-known member
Thought this video was interesting. Shows a nice curb detail, and IMHO the value of compaction and not adding water.
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Not all slabs crack
- Thread starter billconner
- Start date
ConCretin
Well-known member
The headline caught my attention when it posted on YouTube. There is nothing in the design or materials that would add to crack resistance and in fact, I'd have expected more cracking than usual since the slab is restrained from contracting by the curb and poles. If I recall correctly, the concrete was placed at a 5 or 6 inch slump and Bondo specifically mentions that didn't use a water reducer, which means the concrete has way too much water, which would increase shrinkage. Sometimes you just get lucky.....either that or the cracks just haven't opened up and are covered in dirt.
Actually I just recalled one thing that would add to crack resistance; the mesh is all laying in the dirt so no worries about it resisting contraction. That channel is pretty representative of average residential concrete work but I wouldn't use it a guide to a high quality slab.
Actually I just recalled one thing that would add to crack resistance; the mesh is all laying in the dirt so no worries about it resisting contraction. That channel is pretty representative of average residential concrete work but I wouldn't use it a guide to a high quality slab.
txvwnut
Well-known member
When I did my slab for my building my concrete guy said to do cross beams and vibrate it if I don't want any cracks. I did exactly what he said and don't have any cracks. Five inch thick for the flat with twelve by eighteen inch footers and beams.
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OP
billconner
Well-known member
I agree I thought the curb edge would have exacerbated the cracking.
If anyone tells you concrete doesn't crack, run.
A cubic yard of concrete has about 30 gallons of water in it. That water evaporates during curing and something has to give.
All concrete cracks; First rule in concrete Engineering class.
Water should never be added at the job. The concrete mix is designed and the design is tested by the concrete supplier beforehand to establish reliability and predictability. The batch leaves the plant with the mix components and quantities in the truck matching the predetermined design . Adding anything, especially water, messes that up.
If the driver needs to add water then you should turn the truck away because something is wrong.
A cubic yard of concrete has about 30 gallons of water in it. That water evaporates during curing and something has to give.
All concrete cracks; First rule in concrete Engineering class.
Water should never be added at the job. The concrete mix is designed and the design is tested by the concrete supplier beforehand to establish reliability and predictability. The batch leaves the plant with the mix components and quantities in the truck matching the predetermined design . Adding anything, especially water, messes that up.
If the driver needs to add water then you should turn the truck away because something is wrong.
OP
billconner
Well-known member
Certainly concrete does crack but, as seen above and in other posts, it doesn't have to, at least not visibly, if prep and procedures are meticulous.If anyone tells you concrete doesn't crack, run.
A cubic yard of concrete has about 30 gallons of water in it. That water evaporates during curing and something has to give.
All concrete cracks; First rule in concrete Engineering class.
Water should never be added at the job. The concrete mix is designed and the design is tested by the concrete supplier beforehand to establish reliability and predictability. The batch leaves the plant with the mix components and quantities in the truck matching the predetermined design . Adding anything, especially water, messes that up.
If the driver needs to add water then you should turn the truck away because something is wrong.
ConCretin
Well-known member
The vast majority of concrete cracks are caused by shrinkage. I've seen relatively large slabs on flat ground and vapor barrier slide inward a 1/2" or more and not crack while much smaller slabs placed on dirt with perimeter haunches, penetrations. etc. crack in several places.
The freer the slab is to contract without restraint, the less likely it is to crack. Ironically, the presence of reinforcing actually increases the likelihood of shrinkage cracks due to the internal restraint it presents. There are many more variables such as mix water, aggregate size and weather to name a few but as slabs get bigger, shrinkage cracks become inevitable.
Base prep is often mentioned as being critical to preventing cracks. It's obviously important but compaction has no bearing on shrinkage cracks and in geotechnical terms the bar isn't that high to support the loads imposed on an average residential slab. As long as the base provides uniform support, it's pretty hard to overload a slab and cause structural cracks.
The freer the slab is to contract without restraint, the less likely it is to crack. Ironically, the presence of reinforcing actually increases the likelihood of shrinkage cracks due to the internal restraint it presents. There are many more variables such as mix water, aggregate size and weather to name a few but as slabs get bigger, shrinkage cracks become inevitable.
Base prep is often mentioned as being critical to preventing cracks. It's obviously important but compaction has no bearing on shrinkage cracks and in geotechnical terms the bar isn't that high to support the loads imposed on an average residential slab. As long as the base provides uniform support, it's pretty hard to overload a slab and cause structural cracks.
LopezBart
Well-known member
Concrete doesn't cure due to water evaporating from the mix; it's a process where the Portland cement in the concrete hydrates and chemically locks up the water. Modern curing agents slow the evaporation of water from the mix to prevent cracking; this used to be done by covering the slab w/ cloth, etc. Reference:A cubic yard of concrete has about 30 gallons of water in it. That water evaporates during curing and something has to give.
Firebrick43
Well-known member
I have a 30’ x 50’ and a 30’x 30’ slab with no cracks.
12” of clean number 8 stone packed 5 passes for each 4” lift, felt like walking on a paved surface.
Topped with stegowrap including up the sides to keep the concrete bonding to the walls.
6 bag with fiber and 4” slump brought to 6” with superplastisizers. The slab shrunk over a half inch leaving 1/4 to 3/8 gaps along the walls. There was some footers tor the main support beam in the basement but the were poured before putting the stone in and topped as well with steggo so the top would slip over as it shrunk.
In the 30x 30 I did put some hairpins in the corners as the only steel reinforcement.
I think having plastic underneath and keeping the slab wet for a week helps as well to keep differential shrinkage com the top to bottom causing the slab to curl until it can support itself and cracks to relief the stress.
12” of clean number 8 stone packed 5 passes for each 4” lift, felt like walking on a paved surface.
Topped with stegowrap including up the sides to keep the concrete bonding to the walls.
6 bag with fiber and 4” slump brought to 6” with superplastisizers. The slab shrunk over a half inch leaving 1/4 to 3/8 gaps along the walls. There was some footers tor the main support beam in the basement but the were poured before putting the stone in and topped as well with steggo so the top would slip over as it shrunk.
In the 30x 30 I did put some hairpins in the corners as the only steel reinforcement.
I think having plastic underneath and keeping the slab wet for a week helps as well to keep differential shrinkage com the top to bottom causing the slab to curl until it can support itself and cracks to relief the stress.
Firebrick43
Well-known member
But evaporation is still going on and if the water has left due to evaporation on top or being sucked out the bottom into due sub grade then there will be a reduction of strength and possible shrinkage differences because the chemical reaction of hydration stalls without the moisture.Concrete doesn't cure due to water evaporating from the mix; it's a process where the Portland cement in the concrete hydrates and chemically locks up the water. Modern curing agents slow the evaporation of water from the mix to prevent cracking; this used to be done by covering the slab w/ cloth, etc. Reference:
Extreme high strength precast components are cure in water tanks or under constant sprinklers.
Firebrick43
Well-known member
I agree in therory but some common materials that can’t be compacted in the traditional sense such as pea stone will displace just from concrete men walking over it.Base prep is often mentioned as being critical to preventing cracks. It's obviously important but compaction has no bearing on shrinkage cracks and in geotechnical terms the bar isn't that high to support the loads imposed on an average residential slab. As long as the base provides uniform support, it's pretty hard to overload a slab and cause structural cracks.
Uneven bottom surface then creates tooth that limits shrinkage slippage.
So your opinion is that we can detect moisture content in cured concrete? By cured I don't mean at 28 days. Concrete continues to change volume for many years as witnessed by observing the condition at joints including where slabs meet foundation walls.Concrete doesn't cure due to water evaporating from the mix; it's a process where the Portland cement in the concrete hydrates and chemically locks up the water. Modern curing agents slow the evaporation of water from the mix to prevent cracking; this used to be done by covering the slab w/ cloth, etc. Reference:
"Not visibly" are the key words. Shrinkage cracks are normal process. Meticulous, I'll have to add that to our specs.Certainly concrete does crack but, as seen above and in other posts, it doesn't have to, at least not visibly, if prep and procedures are meticulous.
LopezBart
Well-known member
We've poured two slabs w/o cracks in the barn on my folk's place; each was 24x20 done on subsequent days due to limited access - used long chutes to get the far slab first. Well compacted pit run + plastic with lots of rebar, six-sack mix with fiber. No slitting.
LopezBart
Well-known member
So your opinion is that we can detect moisture content in cured concrete? By cured I don't mean at 28 days. Concrete continues to change volume for many years as witnessed by observing the condition at joints including where slabs meet foundation walls.
Here's some info on shrinkage (the concrete kind): https://insights.basf.com/files/pdf/Shrinkage_Of_Concrete_CTIF.pdf
It's a complex topic...
Thanks for posting this billconner. Maybe this thread has run its course, but I would like to say that I was very excited to hear all about how this crack-free casting was constructed. Things I was looking to hear all about:
1. Sub base and base grading, material, compaction.
2. Slab structural design and details.
3. Mix design, aggregate prep and type, batching, slump selection, admixtures used
4. Concreting, vibration, finishing
5. Moisture retention/fogging prior to initial set, curing initial set to final set, curing after final set, use of curing membranes
But this is what I heard much to my disappointment:
1. The owner did it (competently apparently)
2. Chicken wire stomped into the mud.
3. "I don't like chemicals in my concrete." No understanding of concrete. We haven't used water to get slump in over 30 years. Hopefully the owner called the batch plant and ordered a good mix with 0.45 w/cm ratio max and 20% type F fly ash or 50% grade 120 slag.
4. Did they ever use that ********? No final finishing shown.
5. No curing details shown. Owner must have done it.
I would say that this project's early success is due to the owner. We cast crack-free flatwork and other castings all the time. Light construction needs to step it up and give us good concrete castings like the rest of civil construction. No excuses.
1. Sub base and base grading, material, compaction.
2. Slab structural design and details.
3. Mix design, aggregate prep and type, batching, slump selection, admixtures used
4. Concreting, vibration, finishing
5. Moisture retention/fogging prior to initial set, curing initial set to final set, curing after final set, use of curing membranes
But this is what I heard much to my disappointment:
1. The owner did it (competently apparently)
2. Chicken wire stomped into the mud.
3. "I don't like chemicals in my concrete." No understanding of concrete. We haven't used water to get slump in over 30 years. Hopefully the owner called the batch plant and ordered a good mix with 0.45 w/cm ratio max and 20% type F fly ash or 50% grade 120 slag.
4. Did they ever use that ********? No final finishing shown.
5. No curing details shown. Owner must have done it.
I would say that this project's early success is due to the owner. We cast crack-free flatwork and other castings all the time. Light construction needs to step it up and give us good concrete castings like the rest of civil construction. No excuses.
Wiz02
Well-known member
The guys that built my garage in 2013 did a great job in general and the 30x24 slab has no visible cracks over 10 years later.
However, not only did they do a textbook preparation job, but I live near a quarry and there was only a few inches of soil after excavation before hitting solid rock.
The inspector said that could have pinned the slab instead of excavating footers. They needed a skid steer with a fairly large jackhammer to excavate the footers)
I believe that having the slab poured over solid rock with a miminal amount of soil, is why there are no visible cracks. Of course this is an unusual situation.
However, not only did they do a textbook preparation job, but I live near a quarry and there was only a few inches of soil after excavation before hitting solid rock.
The inspector said that could have pinned the slab instead of excavating footers. They needed a skid steer with a fairly large jackhammer to excavate the footers)
I believe that having the slab poured over solid rock with a miminal amount of soil, is why there are no visible cracks. Of course this is an unusual situation.
LopezBart
Well-known member
Our barn slabs w/o cracks:
- Sub-base was pit run that had been settling with winter wetting for 20+ years. Surface was removed w/ very small Kubota tractor w/ loader.
- Several inches of sand were added & leveled, then vapor barrier, lots of rebar on roughly 18"-24" spacing. Slabs tied together w/ rebar.
- Concrete was six sack mix w/ fiber, used chutes to place it. Relatively water-poor and hard to move (don't remember mix details).
- Screeded, bull floats and then extensively power-troweled.
- Concrete was in barn, so shaded; weather was 70F or so.