Understand Moisture Content in Aggregates and How To Compensate For It
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Aggregate Moisture Content
The aggregate moisture content and water-cement ratio is critical to the strength of the cured concrete. Too much water and the strength falls. The cement will not hydrate correctly it there is not enough water and again strength will fall.
The only way to ensure the water-cement ratio is correct in the mixed concrete is to compensate for water within the aggregate and sand, then calculated the correct dose of water.
The example below will show you the process and how to calculate the amount of water required to achieve the design mix and design workability of the concrete (the slump) The slump types are below.
s1 – 10-40mm
s2 – 50-90mm
s3 – 100-150mm
s4 – 160-210mm
Added Water Calculation
When working out the moisture/water addition you have to take into account the water absorption value of the materials you are using & increase batch weights to compensate – example for a 25 N/mm² mix with a s3 slump.
Mix design; (Dry weights)
240 cement content
1080 Stone
800 sand
W/C 0.7 @ s3 slump = 168L
You need to know the moisture content of the sand and stone. For this example we will take them as:
Moisture of stone 2.5%
Moisture of Sand 6%
Firstly you will adjust the batch weights;
240 Cement – No change
1080 stone – + 2.5% = 1107
800 Sand – + 6% = 848
Next you need the water absorption values of the aggregates
For this calculation we will take them as:
Stone 1.5%
Sand 1.1 %
Now you can work out how much water is present on the new batch weights;
Stone 1107 x (moisture value – water absorption) = 1% = 11.1L
Sand 848 x(moisture value – water absorption) = 4.9% = 41.6L
You now know there is 52.7L of water present in you aggregates, take this from your total mix water.
The amount of water required is 115.3L to produce the design mix of a 25 N/mm² concrete with a s3 slump.
Aggregate Moisture Content Technical Information
If you would like more information, have a question on aggregate moisture content or you are looking for a concrete technologist please contact us using the contact page.
The calculation above was provided by our UK concrete technologist Phill Moore. You can get hold of him from this link.
For a more technical look at this topic go to the National Precast Concrete Association article on the subject. Click on this link to the article.
Recommendation
Concrete design is a specialist subject. We would recommend that you speak to a concrete technologist to design your mixes or to adjust your current mixes if you think they can be improved.
You can speak to one of our recommended concrete technologists by request. Either complete the contact us form on our contact us page or text Bob Evans. His details are below.
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