Concrete Cracking around Wooden Posts

[Kidbee]

Have done a fair bit of concreting over the last few years. When I put stirrups in there is no cracking of the concrete. Sometimes when I concrete in wooden posts I get a crack from the post running through the concrete to the outside of the original hole.

My thoughts are that the concrete should have been watered while it was curing ie., it dried out, or the post may have swollen slightly.

Would like it if anyone can solve the puzzle for me.

[rwbuild]

The post is dry when you pour the concrete, the post absorbs the water from the concrete, expands while the concrete is going off and in the process creates a stress fracture line in the concrete. Once the concrete has cured, normal thermal expansion / contraction together with any moister / water works the stress fracture and develops as a crack which with time, dirt / dust and small amounts of distressed concrete breaking down inside the crack helps to open the crack up. Also if water gets in the gap around the post, the post is also constantly expanding / contracting and the dirt / dust / distressed concrete exerts additional forces on the concrete.

[Kidbee]

Thanks RWBuild. So the solution is to wet up the post prior to pouring the concrete. Is that right?

[rwbuild]

Thanks RWBuild. So the solution is to wet up the post prior to pouring the concrete. Is that right?

You can, better to seal the post with a thick waterproof sealer bitumen based but dont forget, water can be soaked DOWN from the top also.

What type of posts and location are you having trouble with?

[Kidbee]

Treated sleepers and 100 x 100 treated hardwood posts. Putting them in on my rural block in Buderim. The soil is loamy clay combined with sandstone layers.

[rod1949]

You're combining something that moves with something that dosen't so its only natural for gaps and or cracking to occur. And it would be the same if they were steel posts.

[rwbuild]

Your own brew of concrete or ready mix from local company or ready mix in bags from hardware and how much water you using

[Kidbee]

My own brew in a cement mixer. Bucket of GP Cement, to say between 3 to 4 concrete mix and 1/2 bucket of 20mm crushed gravel and 2/3rds bucket of water. Mix appears to be a good consistency how it behaves in the mixer, though it is drier than wetter if you know what I mean.

[rwbuild]

Drier than wetter has a lot to do with your problem. The chemical reaction of cement+water has to be right otherwise you dont get a complete reaction and therefore no bonding of all the components plus the ground (if on the dry side) and the timber is absorbing the water before the concrete has a chance to set & cure.
I know its easier to get posts & sleepers to stay where you want them with a drier mix but you may as well ram clay in there for all the good you are doing.
Wet the mix up to normal ready mix consistancey, and you will minimize a lot of your problems.