Types of aggregate to use to resist cracking

[yahoo123]

Hi, I want to get your oppinions on what kind of aggregates are ideal for resisting cracking (particularly due to drying-shrinkage and curling in boiling sun). I need to build an exterior slab on grade which will not have any construction or control joints because it will serve as a containment pad incase of chemical spills during truck loading. It will be reinforced.

I have read that large aggregates are ideal becase it requires less water and also have a higher fracture energy. However, I also read that smaller aggregates are better against cracking because it increases the tension strength of the concrete. I'm thinking about setting the max aggregate size to 3/4" and using a very low water-cement ratio (achieved by water reducers).

Also is rounded or granular coarse aggregates better for resisting cracking? Again I read conflicting recommendations.

 

[JAE]

I've never heard the small aggregate claim. Always understood that larger aggregates were better...and a well graded mix of aggregates is even better yet.

 

[flamby]

To resist cracking, rebar (or some alternative reinforcement) is useful. Aggregate claim, as JAE noted, is unheard of.

Ciao.

 

[apsix]

I've 'heard' that dirty sand, ie. too much very fine sand, will promote shrinkage due to the requirement for more water.

 

[shin25]

The problem with using small aggregates is, it increases the surface area in the mix that needs to be covered with paste. Therefore, more water is needed to maintain a given slump. Which, inturn increases the shrinkage of the mix.

In genaral, aggregates with high modulus of elasticity and low absorption will produce concrete with low ultimate shrinkage. Quartz, limestone, dolomite, feldsper all are high modulus aggregates.

 

[miecz]

As noted above, larger well graded aggregates help to reduce shrinkage. I think your 3/4" maximum size (ASTM C33 grading No. 67) is appropriate, unless your slab is more than 12 inches thick. Theoretically, rounded aggregate needs less water and cement to produce workable concrete, with no loss in strength. I've never seen this concept put into practice, though.

 

[yahoo123]

All of the following quotes are from various sections of the ACI 224R-01:

"There are two measures that can minimize cracking. The
first is to modify the materials and mixture proportions to
produce concrete with the best cracking resistance or the
greatest tensile-strain capacity. This can require careful
aggregate selection, using minimum cement content for interior concrete, restricting the maximum aggregate size.
The attempt made to produce a concrete with a large tensile strain capacity can limit the maximum aggregate size to a value somewhat below that which might be the most economical."

"The aggregate that imparts concrete with the highest tensile-strain capacity can increase the water requirement and the cement requirement, offsetting the benefits of high strain capacity."

"7.3.4.2 Crack resistance—The tensile strain that concrete can withstand varies greatly with the composition of
the concrete and the strain rate. When strain is applied slowly, the strain capacity is far greater than when the strain is applied. Therefore, concrete in the interior of a large mass that must cool slowly can undergo a large strain before cracking. If concrete contains rough-textured aggregate with small maximum size, the strain capacity will be high. There is an optimum, however, with respect to aggregate size. Concrete with smaller aggregates requires more cement for a given strength. This results in more heat, a higher maximum temperature, and greater subsequent strain due to cooling. Therefore, these effects can offset the strain increase developed from the use of smaller aggregates."

"The extensibility represents how much the concrete can be strained without exceeding its tensile strength and is the sum of creep plus elastic-strain capacity. The latter is largely related to the composition of the aggregate and can vary widely. Typically, some concretes of highly quartzitic gravels have a low strain capacity and a high modulus of elasticity. Concretes with a low strain capacity are much more sensitive to shrinkage due to drying (and to a decrease in temperature) and will be subjected to a greater amount of cracking."

 

[connect2]

Not sure of the size or thickness of your slab with no joints. You may want to look at ACI 350. Your minimum steel requirements may get to about 0.06Ag depending upon unjointed size of single pour. The slab curling you mentioned has nothing to do with coarse aggregate type and everything to do with sub-base conditions and ambient air conditions and how you handle them.
A uniform set of curing conditions through the slab is what you are looking for regardless. A slow uniform cure through the slab is what you want, re plastic shrinkage. Dry shrinkage you need sufficient reinforcing. How thick is your slab? Recently poured a 30,000 sq.ft, 3 ft. thick mat foundation slab in 13 hours, no joints. Sealed it and then tarped it with insulated tarps. That requirement to use insulated tarps in the middle of August threw the Contractor for a loop. We had 6 monitoring points with 3 sets of thermo couples at each point to monitor the Delta T through the slab over time. The temp peaked at day 5, dry shrinkage will be going on for the next year.
Some aggregate types and sizes I agree do help, but curing conditions and reinforcing are the answer at the end of the day to plastic shrikage and dry shrinkage. You really need to consider the thickness of your slab and its unjointed size to resolve this issue.

 

[miecz]

Well said connect2.

 

[yahoo123]

Actually aggregate size does affect slab curling.

ACI 360R-06
"Significant curling of slabs-on-ground has become more
prevalent in the past 30 years. This is partly due to the
emergence of more finely ground cements, smaller maximum size
coarse aggregates, and gap-graded aggregates, all of which
increase the water demand in concrete."