Concrete Question

[HitchHiker]

Not sure if it matters or if there's anything that can be done about it?

Concrete pour Friday... 9/9 (5000psi, Cemex) and about two hours later we got a good solid rain. Contractor didn't cover it.

Did use some Kurez DR VOX on it.

I'm seeing some spider cracking (9/12).

Anything to be concerned about?

 

[Pickettpuncher]

The spider web stuff isn’t much of a concern. Most of the time when I’ve seen them it’s just surface cracks from being closed up and drying too fast. Think of a crust on a pie and the crust breaking as it dries. What would have been a concern is if you got enough rain, and the concrete to water ratio got to high. If it didn’t beat divets in it, you’re ok.

 

[HitchHiker]

I reckon that’s just the cream that came to the top

 

[Pickettpuncher]

As long as the ground underneath it was compacted properly you shouldn't worry. Concrete is stupid and never makes sense. if you saw cut it in 8' or 10' squares for relief cuts 1" deep, it will decide to crack across the square. it should be fine with the spider veins. Was it hard before the rain came? Does the finish look ok? With 5000PSI you shouldn't ever worry about the small spider veins. in the hot summer, we set up sprinklers to keep the concrete cool after it is finished so that it will help with the curing. Hope that helps.

Anytime I pour for homeowners I tell them to keep it soaked for a few days if it’s 75 or higher. Slows the process down a lot and keeps the cracking down

 

[roadrunner]

The surface evaporates at a faster rate, especially as the heat of hydration brings the cement temp up during its transition time.

It's quite normal and an eye appeal thing more than anything else.

 

[Rainyday]

Broom finish?

 

[manitou1]

Had a smooth finish slab poored in Sep of 2020. It wasn't hot, maybe averaged 60-70 degrees during days and 50s at nights during the cure with a light brief snow on it about two weeks into cure.
It literally has thousands of spiderweb type cracks... hairline. Some of the bigger ones can be felt with the fingernail, but there has been no futher cracking in two years.
Is this a concern?
What caused it? (very dry climate, btw)

 

[Reburn]

Concrete cracks.
That is a guarentee.
If you feel its a problem and want to chase down nothing call the builder or engineer and pay for an inspection.

 

[Travis907]

One thing is definitely for sure, concrete cracks but spiderwebbing is common and you shouldn’t worry especially with a solid compacted base! I know it makes you question quality and stability but if it was done correctly then you have no worries. It’s more of an eye sore than anything.

 

[Drenalin]

Spiderweb cracking is almost never a concern for anything other than aesthetics. Concrete can be a real sob and do some ugly crap on you. If it was set enough for the rain not to beat the finish up, then getting wet was probably the best thing that could have happened to it. Wind is often the culprit for that type of cracking, generally for the first 12-24 hours after placement.

 

[Fjellvei]

Agree with other input here on concrete being wetted as a good thing. If you take exception to the aesthetics, could rent a big boy diamond buffer from home Depot and roast it for a few hours to see if you like what's right beneath the surface more.

 

[NebraskaStickHunter]

Like others have stated just cosmetic most likely. The top dries faster than the middle of the slab 90%+ of the time there is not enough cure on the slab if cure is even used. Best way to prevent future slabs from doing this is to have misters over the slab to increase humidity and make sure the grade is wet before you pour on it. Concrete is fickle and you will only have to deal with the drying shrinkage cracks for 50+ years before the rest breaks up .

 

[WV Mountaineer]

Wetting concrete can weaken psi in the early stages before setting. However, it won’t harm it as it starts to take a decent set. And, on large pours, needed in order to get a smooth finish if it’s hot and in direct sun and or wind. As you are fighting the clock.

As stated, cracking is what concrete does. Floating brings the water to the top. And adding water slows it set time. By adding water to slow it down, it further creates the temperature difference between the surface and interior. The surface starts to harden and cool while the inside is still hot. If it’s in the sun and wind, you must start adding water if it’s setting up too fast. As the surface sets and cools hours after the pour, it contracts. The inside is still much warmer and doesn’t contract with the surface Causing the cracking.

The wind and sun are the deciding factors of how intense the temperature differences get. Because too much of either requires water to be added. That’s why you see this type cracking in finishes that had a lot of water added while it was being worked.

It doesn’t hurt strength. And, it’s nearly impossible to prevent on hot sunny days in the sun when there is a wind blowing accelerating the drying time.

 

[HitchHiker]

Broom finish?

No. but they fiber reinforce it.

 

[KurtR]

Supplying concrete is my job. I have seen about every thing go wrong that can. How much fly ash was in it would be a question. What was the slump would be another. What was used for water reducer and was the air content bellow 3 since it was power troweled. If it did have air and was sealed to fast that will cause problems. How much water did they spray on top to finish it? What aggregate was used? Did they wet the grade before it was poured? While the spider webs are probably mostly aesthetic I would still be pissed paying what it costs now and that’s what I get. It’s a guarantee that it will crack but when cut at the right time to the proper depth you should have little to no cracking other than where you direct it. We have a pretty severe freeze thaw cycle in South Dakota and the guys that do it right you can see the results vs the short cutters. I have poured at 100 degrees and at below 0 and pretty much everything in between. Sometimes there is no explanation it’s concrete doing concrete shit.

 

[EastHumboldt]

There’s two kinds of concrete. The kind that has cracked and the kind that is going to crack. Sleep well everything will turn out the way it was supposed to.

 

[Agross]

KurtR nailed it. I’ve been pouring flatwork for 20 years, and couldn’t have said it better.

 

[darrenk75b]

You are likely fine. Excess water reduces concrete strength and increases shrinkage. Consequently, your upper surface may be a bit weaker and have an increased evidence of cracking. The rain is unlikely to affect the overall strength of the concrete. If a high-wear surface (driveway, etc), you may get some surface spalling down the road, but it is primarily aesthetic.

P.s. I'm a structural engineer specializing in concrete parking structures.