Concrete Strength

[nonsinusoidal]

Can someone advice if the strength of concrete depends on the size of the aggregate utilized to composed the concrete? What properties of the materials that make up concrete governed its strength? Thank you

 

[asixth]

There have been many good threads posted about the what contributes to the strength of concrete so I suggest performing a search at the top of the page to get you started.

Basically I am going to summarize the MacGregor textbook for factors contributing to strength of concrete in compression:

1. Water/Cement ratio. Lower w/c ratios give greater compressive strength.
2. Type of Cement. Normal/High Early Strength/Low Heat
3. Supplementary cementitious materials. eg. Fly ash (which delays the strength gain rate)
4. Aggregate. Includes the strength of aggregate, surface texture, grading and maximum size (maximum size is not a significant contributor).
5. Conditions during curing.

 

[Ron]

asixth is correct; although, coarse aggregate size can affect the strength in one way...

Given the same cement content, the larger the coarse aggregate, the lower the water cement ratio. This increases strength relative to a smaller coarse aggregate which will require more mix water for workability and paste coverage.

 

[ishvaaag]

The mechanical aspects of this should be plottable for the less reacting at the surface coarse aggregates. At some point the size of the coarse aggregate particles would become so big that strength would start to diminish. Simply to much rock and scarce surface adhesion by then. But this rarely will occur in plant design targeted for strength.

 

[BigH]

larger agg generally, too, means lower heat of hydration.

 

[nonsinusoidal]

Is there a way to determine the mixture ratios to achieve a certain concrete strength? The current requirements that we have is to provide a 3000 psi concrete strength within 28 days for the concrete used to encased the duct systems. We currently required to use 3/4 of an inch aggregate but it is imperative to change it to 5/8 of an inch. So the mixture rations for the components used to achieve the concrete strength given above has to change, right? I know I can just request a concrete mixture report from the concrete provided and it should provide all of the information I need. However, OUR CURRENT STANDARD gives all of mixture ratios to assist the concrete provided to make the concrete so the strength of the concrete mentioned above is achieved. As a result, I need to modified the current standard to reflect the change of aggregate size to achieve the 3000 psi strength for the concrete. Please advise as appropriate.

 

[Ron]

Concrete mix designs are "off the shelf" for ready mix suppliers. Changing from a nominal 3/4" aggregate to a nominal 5/8" aggegate is not much of a change and is probably not a standard mix design. Usually the range will go from #57 stone (3/4-1" top size) to #89 (3/8-1/2").

To answer your specific question, yes the mix proportions change. If you reduce the coarse aggregate size, you have to increase the cement content and the water to get more paste for aggregate surface coverage. That increases shrinkage and cracking problems.

Contact a local ready mix supplier and get a standard mix design close to the aggregate size you want.

 

[Zambo]

Concrete mix design used to be very easy and I would often make a mix design at site if we had an onsite batching plant. During the last 20 years more use has been made of plasticisers and water reducers which complicates the procedure and makes it less likely that an engineer on site will be making the mix design.

So I agree the best way is to go to a ready mix supplier and tell them what you want. If they have a mix design that they have already used it will save time on testing.

The simple approach is that by reducing the aggregate size the surface area is increased so to keep the slump the same you need more water. Then to keep the strength the same you have to increase the cement content so that the water-cement ratio is unchanged. By doing this there should not be an effect on either shrinkage or cracking. But test cubes (or cylinders) are required to confirm the results before use. You could make compression tests at 3, 7, 14 and 28 days and compare against your current mix design.

You can download a simple software for mix design

http://wareseeker.com/download/concrete-mix-designer-1.rar/3b4ba19713

but I recommend this is just for your interest and certainly should not be used without checking the strength by compression tests.

Where are you working? Are you planning to use admixtures?

 

[conceng]

Zambo,

I respectfully disagree with your saying:

"Then to keep the strength the same you have to increase the cement content so that the water-cement ratio is unchanged. By doing this there should not be an effect on either shrinkage or cracking."

Regardless of the water-cement ratio, increasing the water and paste volume will certainly increase shrinkage and the risk of unintended cracks.

Greg

 

[Zambo]

conceng,

Ron has the same opinion as you, so I concede you may be right as I don't have a reference book to hand.

However, my point is that shrinkage is caused by the release of water from the concrete as it hardens (i.e a reduction in volume of the concrete). This can be controlled to a large extent by correct curing.

But at a given water cement ratio the water combining with the cement during hydration should be the same and therefore more cement means more water combining with it and not causing a loss of volume.

 

[conceng]

Zambo,

You are correct that when keeping the w/c ratio constant, the levels of hydration of the cement would remain the same given identical curing conditions.

However, for example if you used a 0.52 w/c mix with 31 gallons/yd^3(497 lbs cementitious), and another mix of 35 gallons/yd^3(561 lbs cementitious), and we assume complete hydration with proper curing consumes or ties up water equivalent to a .38 w/c ratio. The 31 gallon/yd^3 mix will lose approximately 8.34 gallons/yd^3 with complete drying. The 35 gallon/yd^3 mix will lose about 9.42 gallons/yd^3 with complete drying. Higher loss of volume = greater shrinkage.

Bottom line, the higher the paste volume, the higher the shrinkage. The first thing we do when shrinkage is most critical is try to reduce the paste volume through well graded aggregates, aggregate selection, SCM's (fly ash) and admixture packages.

Greg

 

[Zambo]

conceng,

yes I get your point but the percentage differences you are talking about are quite small. For water loss from the concrete volume you calculate about 1 additional gallon out of 30 gallon, but for shrinkage you have to look at the overall volume of the mix not just the water. The additional volume loss will be small, and 10mm aggregate has been used succesfully for many years.

In this case the threadstarter has another issue, which seems to be compaction concern for his duct encasement. I believe 10-15mm aggregate can be used succesfully, but if admixtures are available in his location to reduce the water content then this will be even better.

 

[Ron]

Zambo...I agree with Greg. I see this routinely in practice. When the paste volume is increased, shrinkage increases. It doesn't take much of an increase to cause cracking to occur EARLIER than would be otherwise anticipate. It is not only a volumetric consideration, but one of increased heat of hydration with the increased paste volume, as well as the rate of hydration will increase correspondingly. Therefore, one has to be much more attentive to finishing and the timing of control joints to help mitigate cracking.

In the microscopic (petrographic) examination of polished sections of concrete, I see more micro-cracking when the paste volume increases and I see more finishing problems.

Ron

 

[Zambo]

Ron,

I believe that you have to consider shrinkage due to a reduction in volume and cracking due to heat of hydration seperately.

For shrinkage excess free water due to aggregate selection water content, water cement ratio and curing are important. I don't think any changes due to coarse aggregate size are the most critical (if the water cement ratio is unchanged), especially considering that for compaction issues the overall quality of the concrete may be a important factor which can be assisted by using smaller diameter aggregate.

Now for cracking due to heat of hydration you have to look at the concrete section. If it is mass concrete it is quite probable that there is no need to consider a reduced size of the coarse aggregate, so no issue. For thin sections with heavy reinforcement or other obstructions to compaction reducing the aggregate size may be an advantage, but the heat of hydration produced in the section should not be excessive.

 

[Ron]

Zambo...you missed my point. I'm not talking about heat of hydration as a cracking issue...I'm talking about the increased heat of hydration as a cause for faster set time, thus changing the timing of when you need to finish and cut control joints. There's not enough paste volume change to cause a heat of hydration cracking issue in the classic mass concrete concept. There is; however, enough of an increase in paste volume to influence the chemistry of hydration with an increase in temperature creating an increase in reaction times.