Concrete Question??

[Randy Meijer]

I have wondered about this for a long time and decided this weekend, while setting some fence posts, that someone here would probably be able to enlighten me.

Why do they put sand and gravel in concrete??

[Steve Clardy]

Concrete [Portland cement] bonds to sand, rock.
Without the sand rock, you have nothing to bond to.
Portland will crumble by itself

[Vaughn McMillan]

Steve pretty much summed it up. Cement powder on its own, when hardened, doesn't have a lot of shear strength (IIRC). It's also expensive for the amount of volume it fills. Add some sand (to make mortar or grout), and it gains strength, but by volume it's still costly. Add some gravel (to make concrete), and it gains more strength, and the cost goes down because there's more volume for the same amount of cement powder. Add too much sand and/or gravel, and the strength eventually drops, because you get to the point where there's insufficient cement paste to hold the rocks and sand together. The balance between all the separate ingredients controls the compressive and flexural strenght of the finished product.

Also, the type of rock used for the gravel plays a big part in the overall strength of the concrete. If you make concrete with a light rock like pumice for the gravel, the resulting structure is light but it has comparatively little compressive strength. (Yet potentially decent flexural strength.) On the other hand, if you use very hard and heavy rock like magnetite (iron oxide), the compressive strength of the concrete can be off the charts. I worked in my dad's material testing lab back in high school, and we once tested a mix design using magnetite aggregate. To test concrete, you make 6" diameter cylinders that are 12" high, then after they cure, you put them in a hydraulic compression machine that squashes the cylinder to the point of breaking. Most regular concrete (3000 to 4000 psi mixes) broke within the first third of the dial gauge on the compression machine. We had some magnetite cylinders that redlined the machine and still didn't break. (The ones that did break were spectacular explosions, too.) It's been too many years to remember the actual numbers, but I believe the average breaking strength was in the 8000 to 10,000 psi range. Normal cylinders weigh about 30 to 35 pounds; magnetite cylinders were twice that heavy.

To answer your original fencepost question, they put the sand and gravel in your fencepost mix because it's less expensive and stronger than just using cement powder alone.

- Vaughn

[Perry Holbrook]

To the point, your question is actually a matter of syntex. "Concrete" is the mixture of cement and aggragate, usually sand and gravel. "Cement" is a product made at high temperature in a rotary kiln, the primary incredient being gypsum. BTW, cement was discovered by accident and chemist can not fully explain why it works. A lot of folks use the terms interchangeably by mistake.
Perry

[Kyle Kraft]

I realize you're from Texas, so it may be different due to environmental conditions, but in my neck of the woods, wooden posts set in concrete have a useful life of about 5 years before they rot off at the ground. We pour a small concrete pad in the bottom of the hole or fill the hole with 4-6" of crushed stone for a base then backfill and tamp around the post with earth.

Kyle in K'zoo

[Ben Grunow]

Ditto what Kyle said. Put a little gravel under the end grain of the post and then pack the dirt back in around it. I use the round end (2 1/2" diameter cap) of my 6' iron digging bar to comact as I fill the hole checking for level a few times as I go. Much better and easier. Good luck.

[Randy Meijer]

Thanks for all of the replies. They were very helpful.

I've pulled plenty of concrete fence footings where the wooden posts have rotted out......not what I would call fun!! I now use steel posts for the extra longevity.

[Frank Hagan]

My theory on fence posts has been that if you encapsulate the post with concrete, when the wood shrinks you have a perfect place for moisture to gather and rot to begin. Like putting the post in a big coffee cup in the ground.

So I put the post in the hole, tamp earth around it, then pour in a mixed but dry sack of post-hole concrete mix, and tamp again. The dry concrete mix will absorb moisture and harden, and you don't have the hassle of mixing the stuff in a wheelbarrow. And if the wood shrinks away from the concrete, water can still migrate into the soil beneath the post, and not be in constant contact with the wood.