concrete cracked.?

[banjodeano]

guys, i just laid 5mtr by 2mtr of concrete, 150mm thick, but now its drying there is a crack developing right across the middle, what have i done wrong,? was the mixture too wet, can i just fill the crack in later.?
Thanks in advance

 

[noseall]

It is drying out too quickly and needs protecting from the sun.

Wet sacking, spraying with water or covering with plastic sheeting usually works.

 

[banjodeano]

It is drying out too quickly and needs protecting from the sun.

Wet sacking, spraying with water or covering with plastic sheeting usually works.

Ah thanks for that..so ideally it should have been done on a cloudy day as well...

 

[noseall]

You can lay concrete on a warm day and have no problems, as long as it is say kept in the shade all day.

suction from beneath can be a factor also.

Common (building) sense says that on a warm day and in direct sunlight, curing concrete requires protection.

The shrinkage cracks may not be all that detrimental and are possibly only in the surface. I have seen 500mm thick foundation concrete with minor surface cracking, whereby the cracks only go down a few mill.

5 or 6mm of cracking ain't gonna bother a heap of concrete none.

 

[pinenot]

5 x 2, so where is the crack, can you sketch it...pinenot

 

[^woody^]

can you sketch it...pinenot

They tend to look like jagged lines

Untitled

• ^woody^
• 5 May 2013
• 1

 

[banjodeano]

5 x 2, so where is the crack, can you sketch it...pinenot

there was actually two or three that spread from one side of the 5meter length to the other....i sort of blended the surface over the cracks with a trowel when it was nearly dry, doesnt look too bad now,,,i just wondered what i did wrong

 

[dann09]

1. "what i did wrong" was not coming on here or somewhere similar before starting.

2. Preparation: assuming no hard core, did you pre-water the earth below the concrete?
Was the earth beaten down?
Was a membrane used? Not always necessary, but can be useful.

3. Was the pour vibrated - by hand or mechanically?


4. Did you tamp down the surface from equal benchmarks to leave a level surface before the water flashed?

5. Did you use simple form/shuttering boards at the edges? They will retain the pour.

 

[JohnD]

In southern France, which tends to have hot summer sun, it is the practice to do prep in the morning, and pour in the afternoon, avoiding peak sunshine. It will then form a set overnight.

Covering and keeping it wet for at. least a week, preferably two, while it cures, will greatly improve hardness and strength