Conc. Shrinkage vs Aggregate Size 1

[STR04]

What effect on a concrete member will it have if a smaller aggregate size (pea rock) is used vs a larger size (#57)? Correct me if I'm wrong but if we keep the same w/c ratio for both mixes (compare apples to apples) we now have to add additional water and cement to our mix for the smaller aggregate because of additional surface area associated with a smaller aggregate size. This means you now have more water to evaporate thus producing greater shrinkage cracks. What effect does the additional cement have to the performance and how does this effect creep? The point is that the contractor will always want to use the smaller aggregate because he can use a smaller less expensive pump. So what kind of limitations do we set for these people when writing our specs and checking shop drawings? Do we say use whatever for foundations, columns, slab on grade but not for elevated slabs & beams or is there a pour limit requirement for pea rock mixes? Where can I find more information on this issue?

 

[DrADK]

STR04

You are touching a very sensitive subject in concrete technology. Concrete shrinkage and the associated mechanisms have caused a lot of discussion in the scientific community not forgetting to mention that is a subject of great controversy not only between the scientists but also between the designers/contractors/site engineers etc. I will try to give some answers to your questions.


What effect on a concrete member will it have if a smaller aggregate size (pea rock) is used vs a larger size (#57)? Correct me if I'm wrong but if we keep the same w/c ratio for both mixes (compare apples to apples) we now have to add additional water and cement to our mix for the smaller aggregate because of additional surface area associated with a smaller aggregate size.

Correct, if you want to keep the same w/c ratio you have to alter the cement and water quantities in order to deal with the surface area change. Let’s go now on the aggregate size and let me write two lines about shrinkage first in order to be clearer what I’m going to explain later.

Shrinkage in cement-based materials can only cause stresses and cracking if the shrinkage is restrained. Next to the degree of shrinkage and the mechanical properties of the material, it is the degree of restraint that determines the magnitude of shrinkage stresses and whether shrinkage cracking will occur. One of the restraint types of concrete shrinkage is the internal restraint caused by aggregates.

Aggregates in the vast majority of concrete mixes have a modulus of elasticity (or bulk modulus) that is several times higher than the modulus of elasticity (or bulk modulus) of the cement paste matrix. Therefore, when concrete dries and starts shrinking, the stiffer aggregates will restraint the matrix shrinkage. Such restraint may cause cracks. The research done on the field of aggregate effect on concrete shrinkage has revealed that the aggregate size (alongside with grading and angularity) has a minor effect on the concrete drying shrinkage (for mixes with same w/c ratio), an effect which can be easily neglected. Vital importance for the drying shrinkage of concrete have the quantity of aggregate used (the volume percentage or volume fraction – larger volumes cause larger shrinkage), the modulus of elasticity of the aggregates used (less stiff aggregate cause larger shrinkage strains – that is not correct for lightweight concrete) and the actual aggregate shrinkage (large aggregate shrinkage will result in larger concrete shrinkage).

All the above it’s a very brief overview. The shrinkage theory, the associated mechanisms are very complex and not well known (and defined) yet. However, the aggregated size does not seem to influence the concrete drying shrinkage for same w/c ratio concrete mixes.

( I assume that you’re talking for mixes with w/c above 0.35, since below that value you might start considering autogenous shrinkage instead of drying)

What effect does the additional cement have to the performance and how does this effect creep?

Don’t forget that alongside with cement you have increased the water quantity keeping the w/c ratio constant. You don’t expect large alteration in the concrete performance but again it depends on the cement you are using, the w/c ratio and the properties of the smaller aggregate you’re going to use.


The point is that the contractor will always want to use the smaller aggregate because he can use a smaller less expensive pump. So what kind of limitations do we set for these people when writing our specs and checking shop drawings? Do we say use whatever for foundations, columns, slab on grade but not for elevated slabs & beams or is there a pour limit requirement for pea rock mixes?

To my opinion you should set the limitations according to:

A) What the standards you’re using, indicate. The standards will guide you on what you have to use for each case.

B) What you want to do. You’re the designer, you’re the engineer, you know what has to be done and how it has to be done. The contractors will always have problems with certain things. But it is us (engineers) who have to keep a higher standard of quality and performance in the structures we’re designing. At the end of the day you will be responsible in the first place if something goes wrong…..not the contractor or the builders.


Where can I find more information on this issue?

I will suggest a couple of papers just to get an idea. I will also give you some web sites where you can register and download relevant papers.

Papers:

1) Carlson, R.W., Drying shrinkage of concrete as affected by many factors, In Proc. Of the American society for testing materials. Part 2, 38 (1939), pp. 419-437

2) Hobbs, D.W., Influence of aggregate restraint on the shrinkage of concrete. ACI Journal 71 (1974), pp. 445-450

3) Hansen, T.C. and Nielsen, K.E.C., Influence of aggregate properties on concrete shrinkage. Journal of ACI 62 (7) (1965), pp. 783-794

4) Saito, M., Kawamura, M., and Arakawa, S., Role of aggregate in the shrinkage of ordinary Portland and expansive cement concrete. Cement and Concrete Composites (13) (1991), pp. 115-121


Sites:

http://www.aci-int.net/general/home.asp

http://www.sciencedirect.com/

I hope I covered your question, if not we can discuss it!

It has become appallingly obvious that our technology has exceeded our humanity
- Albert Einstein

 

[Zambo]

I believe the contractor would always prefer 20mm aggregate to 10mm aggregate due to the lower cost. The smaller the aggregate the higher the cost of the aggregate and also either a higher cement content or additional water reducing plasticiser will be needed to get the same workability. The reason a contractor will request to use concrete with a 10mm coarse aggregate is because he genuinely believes he is going to have placement problems with a 20mm aggregate mix.

Zambo