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# Pavement and hollow blocks

## Pavement and hollow blocks

(OP)
Hi,
I have few questions in mind about the aggregate choice for making pavers and hollow blocks.

How would the natural sand compare to crushed (washed and screened of course ) gravel, to limestone aggregate?
What would be the best option in terms of quality of products?

I have heared that first fraction( 0-4 mm) of limestone aggregate cannot be used in concrete mixes and it has to be replaced with sand. Is that true? If so does it also apply for making pavement or only for more serious constructions as tunnels , etc ?

Im talking about hydraulic vibropressed pavers and blocks.

Thanks a lot for your time and knowledge!

Best regards,
Stefan

### RE: Pavement and hollow blocks

What brand of machine are you using? What kind/size of blocks are you making?

Different types of machine made concrete products have different "ideal" aggregates. I assume you can control the vibration in terms of cycle time and amplitude.

The fines are very critical and the top size for aggregate and may need to be different. If you blend a coarse and fine aggregate to get a final gradation and you have more latitude. The best blend will be slightly different for the pavers than the block. - The particle shape also has an effect because the extreme amount of vibration to get the maximum density. Rounded particles can be compacted better compared to angular.

Keep in mind that you are using zero slump or low slump concrete mixes.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

### RE: Pavement and hollow blocks

(OP)
Its something im thinking about doing in the future.
The machine will be able to produce blocks, curbs , pavers of different shapes.

Ok, so you are saying that natural sand , which is rounded, is better ?
I actually thought it would be the opposite as angular shapes should interlock with each other and produce higher strenght products. Considering theres no pumping and workability isnt a problem i thought crushed gravel would be the way to go.
Do you mind to explain why is rounded better? Im really curious.
Does it mean that natural sand blows away the limestone products? As limestone needs to be crushed and by crushing theres no way to produce round particle shape?

In case i start it i would be making it of natural and/or crushed gravel. Some deposits of gravel and sand, that are nearby and would be economical to use, do not contain many fine sand so it would be used for crushing mainly. However some competitors are using the limestone aggregate so thats why im interested in how it compares with natural and crushed gravel and sand.

Thanks a lot for replying. I really appreciate.

### RE: Pavement and hollow blocks

Both angular and rounded aggregates can be used.

Zero slump concrete for the type machines you would need does not have the same parameters as normal concrete.

Rounded aggregates are better than angular (normally) because they flow better to fill the mold more evenly and uniformly. Since you have a a fully confined mold volume, they move easier under vibration as the top plate goes down to the required elevation to maintain vertical dimensions.

The difference between angular and natural is usually only different in term of rate of production. I worked for company that had 2 machines operating 24 hours a day, 7 days a week for 51 weeks a year. With that volume, the amounts to about 500,000 units a year. Our block tested about 40% over the minimum requirements.

For concrete block, the cheapest way to increase strength is to add water to the mix and not cement. - That is an example of some of the differences from normal poured concrete.

When you get a machine, the manufacturers have service departments that can help to set up some standard mixes for you aggregates.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

### RE: Pavement and hollow blocks

(OP)
Thanks for clarifying. Mind if i ask what type of aggregate the company used while you worked there ? What would be the standard mix for this type of concrete ? You dont have to be 100% accurate, im just interested to get the general idea of the proportions.

### RE: Pavement and hollow blocks

All of the aggregates we used were siliceous glacial deposits that were rounded (coarse and fine). The only exception was when lightweight block were made we used manufactured lightweight aggregate, some crushed limestone 1/4" (washed) and a small amount of siliceous sand (rounded). Only 15% of our CMU were lightweight (95 pcf).

We had a totally automated plant had the ability to store huge amounts of different aggregates.

Different machines have different advantages depending on the local the local aggregates. Before you buy a machine send aggregate samples to the equipment manufacturers and a list of what you choose to produce.

The European large production pallet size machines do well for pavers, but do not produce quality concrete block well. We used a high production American small pallet machine that was not designed for pavers, but it made pavers (8000 psi specification) that tested at 12,000 psi in test runs.

All the engineers in the world cannot give you specific recommendations without aggregate samples and a list of products to be made and the required specifications.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

### RE: Pavement and hollow blocks

(OP)
Well it would be too much of an investment for new production lines so im looking for second hand machines. I am looking for full automatic lines however those machines would be either 10-20 years old or smaller scale which i would prefer but im unable to find quality small automatic plants.Any recommendations ?

Dick, you seem to have really good knowledge and loads of experience so i have one more question i must ask you which is unrelated to the topic.

Is there any use for sand fines that are washed away at screening plant? Those particles are probably less than 100 microns.

### RE: Pavement and hollow blocks

To answer your post about the fines, it makes no sense to put back some materials that were removed. Try to get the processing of the aggregates straightened out.

You will not find any used automated plants available that would be economical to operate. Just a single new mold could be $5,000 to$10,000.

What country are you in?

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

### RE: Pavement and hollow blocks

(OP)
You didnt understand me, i dont want to put the fines back im wondering if there is any industry that could use it, or product being made of those fines ? Im from Serbia. Well many second hand machines come with several molds although i dont know in what condition are they in.

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