I'm a mechanical engineer building 1

[jmf]

I'm a mechanical engineer building a 40' x 50' steel column storage shed on my property that will store a large RV, boat, tractor, and other heavy equipment.

The slab foundation is a 40' x 50' monolitic slab (5" thick in middle, 12" thick on edges). Used 6" x 6" - 6 gauge welded wire (which came in a 5' x 150' role) in middle (at least 6" overlap and wired together) supported by solid fire brick to a 3" height. The edges have 3 No.4 rebar in a ":." shape (looking on end). The column footers have four No.4 rebar crossed (and wired together) with four No.4. The column footer also has a 20' No.5 rebar bent in the middle (in a V shape) around each pair of column J bolts.

With the contractor checked the pad location with a lazer level, we discovered that the end of the 50' mark the slab needed to be raised 28" to be level. (Note: Eyeballing shallow slope hill is difficult.) I got a local dirt contractor (and neighbor) bring 12 loads of dirt to the sight. Unfortunately, it was 12 loads of red (Georgia) clay mixed with gumbo. So, I had the dirt contractor (and still a friend) move the clay away and haul-in 12 loads of sand (Note: I live on the Georgia coastal plain where sand and clay is plentiful, gravel does not exist nearby. I'm 15 miles from a town of 5000 folks and the ready-mix company.) The dirt contract packed the sand as it was spread with his D5 Cat. I then rented a 9000# impact strenght reversible vibratory pad compactor (the only one in the area), and compacted it the next day.

The concrete contractor (who's from the next county and is not my neighbor nor relative) compacted the trenched thickened edge with his 3000# strenght pad compactor, then placed the rebar and wire over a 6 mil black plastic sheet.

Yes, the termite control company sprayed the footers.

The weather in mid-June was predicted to be low 70s in the morning, then mid to upper 80s, partly cloudy, 90%+ RH, and 30% chance of late afternoon thundershowers. Because this weather pattern will likely exist until mid-October, I decide to pray and pour concrete. (I was unable to get the concrete poured in early spring and can't wait until until fall for better and less hot weather conditions.)

On Friday (3 days after the dirt was compacted), the pad was poured with 40 yards of 5000 psi concrete. Used 5000 psi, instead of the standard 3000 psi concrete for durability reasons. (E.g., Salt water from a boat bilge or water softner will mess-up 3000 psi concrete.)

The pour began at 7:00 AM and the concrete contractor completed finishing work by 11:00 AM. However, he had not yet cut the contraction joints (he highly recommended that the joints not be cut for another 4 to 16 hours.) He left the site (for another job) stating that he would be back at the end of the day or early morning to complete the contraction joint cuts. By Noon, I had sprayed a concrete curing compound on the slab. (There is no water supply or well on this 30 acre lot yet.) Temperature at Noon was probably in the mid 80s.

By 1:00 PM, I estimate that temperature had risen to the low to mid 90s.

At about 5:00 PM the slab experience about 1/2" of thunderstorm rain over a one hour period. The steam (or visible water vapor) that was evaporating from the slab was a mystical sight. After the rainfall, I noticed that the curing compound did not wash off the pad. There were no marks on the pad surface and it appeared to be in good shape. We may have had some additional light (less than 1/4" rainfall during the night.

The next morning came, but the contraction joints did not get cut until about 11:00 AM (Saturday). (The contractor did show to cut late the previous day due to thunderstorm activity and did another job first.)

Shortly after the contraction joints were cut, my wife (and then I) noticed hairline cracks in the concrete. The cracks had a mosaic or map-like appearance all over the slab. The crazy looking cracks appeared more numberous about 4 hours after the cuts and are about 1 to 3 feet apart.

Were the contraction joints cut too late?

Was this crazy pattern cracking due to the cold rainfall on the hot concrete surface?

Could the cracking become a structural concern?

Is the cracking only a visual appearance problem.

What should I tell my wife? (I was a genius before this.)

Thanks for your insightful information and suggestions.

 

[jmf]

Correction: The contractor did NOT cut the contraction joints later in the day (of the concrete pad pour) due to thunderstorm activity. It was cut around 11:00 AM (the next day).

 

[nhyankee2]

More than likely the crazy cracks or alligator cracking are a result of shrinkage at the surface which could be due to any number of causes.

bottom line: they NOT an indicator of structural failure!

More than likely they are a result of the finishers 'blessing' the slab with excess water.

 

[jmf]

Sorry for re-editing this. I hope I got it right this time.

I'm a mechanical engineer building a 40' x 50' steel column storage shed on my property that will store a large RV, boat, tractor, and other heavy equipment.

The slab foundation is a 40' x 50' monolitic slab (5" thick in middle, 12" thick on edges). Used 6" x 6" - 6 gauge welded wire (which came in a 5' x 150' role) in middle (at least 6" overlap and wired together) supported by solid fire brick to a 3" height. The edges have 3 No.4 rebar in a ":." shape (looking on end). The column footers have four No.4 rebar crossed (and wired together) with four No.4. The column footer also has a 20' No.5 rebar bent in the middle (in a V shape) around each pair of column J bolts.

With the contractor checked the pad location with a lazer level, we discovered that the end of the 50' mark the slab needed to be raised 28" to be level. I got a local dirt contractor to bring 12 loads of dirt to the sight. Unfortunately, it was 12 loads of red clay mixed with gumbo. So, I had the dirt contractor move the clay away and haul-in 12 loads of sand (Note: I live on the Georgia coastal plain where sand and clay is plentiful, gravel does not exist nearby. I'm 15 miles from a small town and one ready-mix company.) The dirt contract packed the sand with his D5 Cat.

The concrete contractor completed site leveling and compacted the trenched thickened edge with his 3000# impacty strenght vibation pad compactor. I rented a 9000# impact strenght reversible vibration pad compactor and compacted it again the next day. The day before the pour the concrete contractor placed 6 mil black plastic sheet, rebar, and welded wire.

The weather in mid-June was predicted to be low 70s in the morning, then mid to upper 80s, partly cloudy, 90%+ RH, and 30% chance of late afternoon thundershowers. Because this weather pattern will likely exist until mid-October, I decide to pray and pour concrete. (I was unable to get the concrete poured in early spring and can't wait until until fall for better and less hot weather conditions.)

On Friday (3 days after the dirt was compacted), the pad was poured with 40 yards of 5000 psi concrete. Used 5000 psi, instead of the standard 3000 psi concrete for durability reasons. (E.g., Salt water from a boat bilge or water softner will mess-up 3000 psi concrete.)

NOTE: The concrete contractor had previously warned me that 5000 psi concrete was more expensive and has a greater chance for crack development because it contains more water. I responded that (compared to 3000 psi concrete) 5000 psi concrete is only an additional 5 to 6 dollars per yard, contains a lower water ratio, and is more durible.

I do realize that concrete work is a special process. A crack-free pad is highly dependent on skilled people and other things (e.g. weather) that are difficult to control and independently verify during the pour and afterward during the cure.

The concrete contractor has been in the concrete business for over 25 years and has poured concrete pads for many commercial and residential buildings, large warehouses, and parking lots, and was highly recommended by my storage shed's steel fabrication company. However (like all concrete contractors in this area), he is not familar with the ACI standards. Too bad the ready-mix company and other ACI members don't make a simplified standard available (i.e. free) for the typical concrete contractor or hold local seminars.

The ready-mix company is an ACI (in-state chapter) member and proportions their mixes per a computer program.

The big challenge with concrete is to have a good compacted base, well placed reinforcement, getting the concrete poured in the early morning (before it gets hot), resist adding extra water to it, proper curing, and the weather.

The pour began at 7:00 AM and the concrete contractor completed finishing work by 11:00 AM. However, he had not yet cut the contraction joints (he highly recommended that the joints not be cut for another 4 to 16 hours.) He left the site (for another job) stating that he would be back at the end of the day or early morning to complete the contraction joint cuts. By Noon, I had sprayed a concrete curing compound on the slab. (There is no water supply or well on this 30 acre lot yet.) Temperature at Noon was probably in the mid 80s.

By 1:00 PM, I estimate that temperature had risen to the low to mid 90s.

At about 5:00 PM the slab experience about 1/2" of thunderstorm rain over a one hour period. The steam (or visible water vapor) that was evaporating from the slab was a mystical sight. After the rainfall, I noticed that the curing compound did not wash off the pad. There were no marks on the pad surface and it appeared to be in good shape. We may have had some additional light (less than 1/4" rainfall during the night.

The next morning came, but the contraction joints did not get cut until about 11:00 AM (Saturday). (The contractor did not show to cut late the previous day due to thunderstorm activity and did another job first.)

Shortly after the contraction joints were cut, my wife (and then I) noticed hairline cracks in the concrete. The cracks had a mosaic or map-like appearance all over the slab. The crazy looking cracks appeared more numberous about 4 hours after the cuts and are about 1 to 3 feet apart.

Later that day, My wife mentioned to me (and I recall that she's right) that the concrete exiting the trucks seemed very water-rich. However, no extra water was added to the concrete according to the ready-mix company's invoice.

Were the contraction joints cut too late?

Was this crazy pattern cracking due to the cold rainfall on the hot concrete surface?

Should I have the concrete tested after 28 days? Which test is best (and cost effective). How can I (now) know whether the concrete contained too much water. Could a 6" (diameter) piece of (blown-under the form) concrete which has a flat side be used as a test sample? The nearest concrete testing company according to a yellow page search is about 120 miles (i.e., ~2.5 hour driving time) away.

I also plan to find a concrete/civil engineer look at it (to be certain it's OK or not).

Could the cracking become a structural concern?

Is the cracking only a visual appearance problem.

What should I tell my wife? (I was a genius before this.)

NOTE: I did not use fiber-mixed concrete, because I have seen too many with cracks and most civil/concrete engineers that I met (who are involved with power plant construction projects) have told me that poly-fiber is great stuff, but it's a waste of money and doesn't really work. Steel fiber is not available in my area (unless I mail order it). A similar non-fiber concrete pad that I had poured for my personal garage (22' x 35') two years ago still has no cracks.

Thanks for your insightful information and suggestions.

 

[Tryan]

Your cracks are most certainly shrinkage cracks caused by all the circumstances you've described!! Hot day , on plastic, and what "slump" was the concrete when it was poured? My bet is water was added to the mix before it was poured.This is a fairly common practice on a hot day to make it easier for the contractor. The ideal slump is about 4". You tell the readymix co what slump you want and you should not add any water on site as this will lead to a weaker strength concrete and also the problem you are experiencing. What happens is this, concrete is poured and screeded then floated.Water bleeds off and evaporates, because it is on plastic the water has to come out the top.On a hot day the top dries out fairly quickly but the bottom is still wet! This creates the situation where the top is shrinking at a faster rate than the bottom causing the slab to "curl" or try to lift up on the edges.Of course with footings around the edges this can't happen and the result is random cracking in both directions. The good news is this is not a structural problem, purely asthetic. I am from a different part of the world to you {OZ} and find some of the practices you describe quite interesting! The mesh you describe sounds like silo mesh and not reinforcing mesh! However the problem you have relates directly to the concrete contractor and his practices.The slab needs protection from the time it is poured as a large amount of the shrinkage has taken place by the time you do your last operation. Two options are 1. After each finishing op'spray a fine water mist over the slab, being careful not to work the extra water into the surface but to let it evaporate just before the next finishing operation.2.After screeding the concrete initially, spray it with a product called ALIPHALATIC ALCOHOL this will help to contain the moisture until the last operation and then the curing oil is applied
This of course is all retrospective and nothing will fix your slab. As a serious concreting contractor you would learn this'if not in your 1st year certainly by your 2nd year! 25 years experience you say!! hmmmmm