how much weight is needed to break concrete block

[vickerm]

followed instructions for project on Does the ratio of sand to cement affect the strength of the concrete. We have the cured blocks and have added 75lbs to the bucket and the block isn't even cracking. How much weight is required???

[deleted-71447]

Hi,
Welcome to the Ask an Expert forum.
The weight required will depend on a variety of factors, including the dimensions of the concrete bricks and the experimental setup. To minimize the required weight, make thin bricks (too late for that, I realize), support them by the extreme outer edges during the experiment, and support the load in the center of the brick on as narrow a brace as possible.
What dimension are your bricks? What sort of equipment are you using to test their strength?
Chris

[bradleyshanrock-solberg]

Concrete is a really interesting substance. It's a mix of hard rocks and a sort of "glue" made of softer stone that behaves as if it is one big stone most of the time but has very different rules for breaking. Mostly it's easy to mold and pour compared to just sticking stones or bricks together and using mortar, but there are some very important properties you need to know about if you are trying to break it.

1. Concrete is extremely strong in compression mode. If a block of concrete is just sitting on the ground and you try to crush it by putting weights on top, it can withstand tremendous forces. This is why we build skyscrapers out of concrete, or partly why.

2. Concrete is much weaker in "shear" mode. If you take a long, thin slab of concrete, support it on the left and right side but not in the center, and then put a weight on the center, it will break pretty easily.

An irrelevant but interesting fact is the reason skyscrapers built of concrete don't fall down (they get big shear forces from wind) is that they're reinforced with metal. Some clever person discovered that if you embed tough steel rods in the concrete, sideways (shear) forces get turned into compression forces. A building made of reinforced concrete can actually have a bomb hit it, the concrete cracked into multiple blocks and the reinforcement bars sticking out and will still be structurally sound because even in pieces, the reinforcements turn most forces into compression forces on the blocks of concrete that remain. Such a building will also have big steel girders running up and down to take most of the forces, but concrete is always the foundation and generally part of the structural support after the frame is built.

Getting back on topic - a typical brick is going to be extremely tough to break by just putting supports on the end and sticking a weight on it.
The geometry is wrong. The only way I can think of to do it easily would be to clamp it on one side and hang weights off the other side and even that isn't very likely to work. You need the kind of force a sledgehammer can bring to bear to break up a brick of concrete and that's tough to control or measure. If you could cure a long and thin "brick" more along the geometry of a board than a brick, the measurements would be much easier.

[deleted-71447]

I should have added this link, which includes the basic experimental procedure:
https://www.sciencebuddies.org/science- ... p010.shtml
The recommended dimensions of the "bricks" in this experiment are 4x4x20 cm. Maybe they should be thinner.

[vickerm]

thanks for the help. We realized we made them too thick, about 2 1/2 inches thick. Tried the bench on each side and hung a bucket from the middle, used 75 lbs and still didn't crack. We repoured them, making them 1/2 inch thick but most broke taking them out of the mold. Have enough though and will try again after they have cured. thanks again for replying.