Compressive Strength of Concrete Hollow Blocks (Formula)?

[andreaaa]

Hello! My group mates and I are working on a project concerning potential reinforced concrete hollow blocks using junk shop materials. We're pretty new to the entire process, and we're not sure of how we will arrive to our results (if the final results will be declared via machine or derived through a certain formula). Either way, we've decided to include an equation for further explanation. Our aim is to compare the compressive strength—the topic we are researching on. Knowing which equation to use is somehow troubling since upon searching, results show the usage of the Hilsdorf Equation; although, according to a webpage I've encountered (https://theconstructor.org/practical-guide/compressive-strength-concrete-blocks-masonry-units/13966/), "the load at which masonry unit fails and the maximum load divided by gross sectional area of unit will give the compressive strength of block". 

We'd really appreciate a formula for compressive strength (and, if possible, an explation as well). We're newbies and we wish to have a smoother experience throughout this project. 

Thank you in advance!

Edited November 15, 2017 by andreaaa

[studiot]

Hello andrea and welcome to ScienceForums.

Unusual project, but still. You should be able learn a lot about (composite) materials from it.

Please be more specific about what you mean by 'reinforced'?

Steel rods? Steel Fibres? Other fibres? Something else entirely?
Are you making the concrete blocks yourselves?

Also the configuration of the hollow blocks is important.
There are two possibilities depending on how you apply the load.

 

Finally a word about this line

2 hours ago, andreaaa said:

"the load at which masonry unit fails and the maximum load divided by gross sectional area of unit will give the compressive strength of block". 

 

This is the ultimate failure strength of the block, not the usually quoted compressive strength.
The compressive strength that is used for design purposes and specified in catalogs or standards will only be a fraction of this.

 

 

[andreaaa]

@studiot

 

Hello and thank you!

Yes, we will be making the concrete hollow blocks ourselves; there would be three types, all containing a specific junk shop material (shredded rubber, cans, and metal for each respective type). 

Also, if it's alright to ask for a little more help, I've been researching more on the coverage of our project for the past few hours, and I found a video demonstrating compressive strength testing. I somehow consider it to be a reliable source as it isn't certified by a corporation that I am quite familiar with, and I think that I'm slowly figuring it out (although, of course, that could change). Here are the details:

 

Sample: Ordinary Concrete Hollow Blocks 

Size: 340 L x 90 W x 20 face thickness x 25 web thickness

Weight: 8.21 kgs 

Maximum load: 207, 573 N

Compressive Strength: 1060 psi 

 

I tried making sense of it and tried to arrive at a certain formula, but I was wondering how they arrived at that result, and I somehow considered that there may be a missing factor that wasn't exactly given/stated. Although, I watched another video of theirs, but it was concerning concrete bricks. I figured it out with the basic knowledge I have of a formula I derived myself and converting N/mm2 or MPa to psi, although that is another thing entirely and I'm not certain of my methods. 

Could there be something else missing? Is this accurate and compatible with this specific project that we're working on? I apologize for asking too much, we really need to figure it out, and we have no idea if we're getting there or not.

Thank you so much!

(also, the video I stumbled upon if more details are needed: 

 

[studiot]

We're always very pleased to help someone who is doing much of the work themselves and in your case it is very good work.

 

 

I hope if you break any blocks your testing equiment has a shield in front of the block under test. They can literally explode on failure, throwing debris about all over the place.

Don't worry about the certification of the testing people in your video, it was a good video.

You may be aware that ASTM stands for American Standard Test Methods. The was not a British or EU standard.

Because of the shape of the hollow blocks they will respond differently to compression compared to a solid block (the brick you mentioned).

So before we talk about a formula (very difficult for hollow blocks) let us decide what you are actually testing for i.e. the aims of your project.

You mention that you are going to incorporate different ingredients into your concrete mix.
Presumababy you want to compare the stengths of blocks made with each.
Have you given any thought to predictions as to what might happen?

I suggest you investigate the following in relation to this.

Does concrete loose or gain volume as it sets. i.e. does it shrink or expand?

Do you have any idea how and why concrete fails?

You have reported a stength in terms of a stress (PSI) What do you know about stress and what is the difference between stress and load?

 

[andreaaa]

@studiot

Hello again (this is quite an extended delay for a reply, I apologize)! Thank you for taking time to view the video and your approval. And, also, side note: 

23 hours ago, andreaaa said:

I somehow consider it to be a reliable source as it isn't certified by a corporation that I am quite familiar with

There was a slight typo in there. It IS certified by a corporation I'm familiar with. Yikes.

Our project basically aims to use regular hollow blocks as basis for comparison with the modified hollow blocks. If the quality (determined by compressive strength) is greater than the regular hollow blocks, there would also be a comparison between the modified blocks (which of the three types would serve as a—possibly— greater alternative for the regular type). To be specific, this project is our year-long investigatory project (IP) and is considered to be both an experimental and engineering IP, which, I guess, explains why we're aiming for both comparison and (somehow) creation/innovation. 

I've mentioned that we aren't the most knowledgeable in this topic, so we hypothesized that our modified hollow block will be stronger than that of the regular hollow block; as for the results between the modified blocks, we predicted—with our basic knowledge—that the blocks with incorporated metal would be stronger (thinking that it would make sense). 

As for the suggested concepts, I've managed to research and understand what I could:

- Concrete actually shrinks once it has set, but it could expand under hot temperature and a change in the content of moisture.

- The results I found looking it up was mostly filled with structure-based causes and factors, but a really rough summary of the research would be that it depends on the composition of the concrete (if it's the right type of concrete, exposure, rust—again, it was more of concrete in terms of already-established buildings and structures. Sorry about that  ).

- Load = force; Stress = force/area; basically, in this situation, the load would be the pressure applied by the testing machine while stress is what indicates if it breaks (I apologize for the very rough usage of terms here). 

That's what I could research on so far. Again—and I will never get tired of saying this—thank you! It truly means a lot to have your help and guidance so far, we really are struggling on this project, and thoroughly understanding this topic would make all our efforts worth it

(If I misinterpreted anything, I apologize! Please do feel free to tell me if elaboration is needed. Thank you so much!)

Edited November 16, 2017 by andreaaa

[studiot]

10 hours ago, andreaaa said:

- Load = force; Stress = force/area; basically, in this situation, the load would be the pressure applied by the testing machine while stress is what indicates if it breaks (I apologize for the very rough usage of terms here). 

OK so let's start here.

The point about loads is that they are external to the object experienceing the load.
Loads may be forces, which are applied at a point or distributed (spread out) eg pressure.

Mechanics tells us that there are also forces of reaction exerted by the load on whatever is applying the load.
But here you do not need to condsider these.

Stresses are what happens inside the body i.e. they are internal.
And yes they are force divided by the area over which they act because they are always distributed throughout the body.

So let us look at the (deformation) response of bodies to loads and the stresses that these loads generate within the bodies.

 

If these are models of hollow concrete blocks, which do you think is stronger and why?

The point is that the deformation response depends not only on the material strength but also on the shape of the body.

 

Fig3 shows a fat, squat, stout body and the exagerated response as it bulges or bursts under compressive load.

Fig 4 shows a tall slender body buckling under compressive load

Fig 5 shows a 'diagonal shear failure as one part of the body slides over another along a diagonal when under compressive load.
this is intermediate between 3 and 4 where the body is too fat to buckle but not fat enough to burst.

Now if we apply this knowledge to the composite shape in Figs 1 and 2 can you see why there are sub dividing sub walls?

10 hours ago, andreaaa said:

- Concrete actually shrinks once it has set, but it could expand under hot temperature and a change in the content of moisture.

 

Well done for finding this out. This feature of concrete is vitally important in the use of the material because it allows the concrete to be reinforced.

The stress has to be transferred somehow from the concrete to reinforcing material.

 

 

This is where Fig 6 comes in.

If there is a reinforcing nail or bar in the concrete as it sets, the concrete will shrink around the bar and indoing so will grip the bar very very tightly as shown in the right hand part of Fig6.

So the bar is very strongly embedded in the concrete. This embedding is called the bond strength.

So if the concrete tries to pull apart or slide apart the stronger reinforcing material resists as in Fig7.

You mentioned trying diffeent reinforcing materials, what  effect do you think a slippery material like rubber will have?

 

I'm glad you have plenty of time to conduct your project as concrete take time to gain strength. (Industrially steam curing is sometimes used to shorten this).

You should also firm up on your project aims but be prepared to amend them in the light of what you discover and aldo to discuss ideas with your project supervisors.

 

I hope I have given you  some things to think about, for isntance to investigate the effect of shape as well as material properties.

 

It would be a good idea to look at standard test methods to test just the material properties alone so here is a link to that

http://www.icomat.in/assets/articles/failure_pattern.pdf