I designed a concrete mix of a mean strength of 60 MPa but on testing I got a maximum strength of 55 MPa after 56 days and this was after adding about 20% fly ash and a superplasicizer. The aggregates that I used were coral limestone. Looking at the failure pattern all of them failed (i.e the aggregates and the mortar)
In your case, it is the aggregate strength that limited the concrete strength. It is well known that in low strength concrete, cracks propagate through paste/mortar and ITZs, and in high-strength concrete, cracks can break aggregate particles. Paste/mortar have to be broken to lead to the concrete failure.
I mixed mortar which was cured in autoclaved approx. 6 hr. I got ca. 120 MPa. For the same mix added C. aggregate in the same curing method, I got +-120 MPa. Failure founded on aggregate for both broke down and ITZ.
Mr Alex sir,
In normal strength concrete size of the aggregates is 20mm app. Crack grows in between the aggregates.
In high strength concrete size of aggregate is 10mm app. Crack grows by breaking aggregates. Packing of aggregates plays a vital role in the compressive strength of concrete
Hence size of aggregate, strength of mortar and fineness of the admixture are the primary parameter.
Strength of aggregates is secondary parameter.
In your case, it is the aggregate strength that limited the concrete strength. It is well known that in low strength concrete, cracks propagate through paste/mortar and ITZs, and in high-strength concrete, cracks can break aggregate particles. Paste/mortar have to be broken to lead to the concrete failure.
WITH A GOOD SELECTION OF MATERIALS + CAREFUL FORMULATION YOU CAN OBTAIN A HIGH STRENGTH CONCRETE;
BUT ATTENTION, TO OBTAIN HIGH PERFORMANCE CONCRETE, DO NOT FORGET THE DURABILITY.
I am agree with the answer of Mr. Hongyan. Since, you are dealing with high strength concrete, you have to use a kind of aggregate that has a higher strength compare to your mortar. Otherwise, the crack will propagate from mortar and ITZ, and aggregate will be less effective.
I am agree with Leon Raj that strength of concrete depends upon size of aggregate, strength of mortar and fineness of the admixture . but strength of aggregate is also crucial in high strength concrete as strength of mortar and ITZ can be more than the strength of aggregate and specimen fails due to break down of aggregate.
I think that's depend on the percentage of the superplasicizer you have used.
If you have got fluid concrete, I mean workability, in this case maybe the aggregate go to the bottom of specimens and in other side you fin just mortar. It means that this concrete isn't homogeneous (distribution of aggregate inside concrete), so you got this strength
The strength of aggregate is equally important in case of high strength concrete. Granite aggregate will provide better strength than lime stone aggregate. I suggest you to check your mix design once again, because by strengthening the mortar phase also help you to achieve 60-70 MPa.
First revise your mix design 20 % super plasticizer is too much. Second the point in high strength concrete really mainly on using smaller nominal maximum size aggregate which increases the bond between the paste or mortar and aggregate. Thus, the crack propagates through the aggregate itself. So revise the mechanical properties of aggregate you got through well-known tests (crushing value, impact value, and abrasion value). These tests will indicate to you whether the compressive strength of your mix design will increase or not as the relation is direct proportional. By the way the compressive strength of aggregate may reach 170 MPa but this does not mean you will have concrete with compressive strength of 170 MPa, however, as I said the higher the value of compressive strength of aggregate the higher compressive concrete you get. Also for normal (Max. size around 20 mm); for high (max. size around 14 mm) and you can reach to 60 and 70 MPa without chemical additives. Just adjust your concrete mix.
the failure is mainly occurred within aggregate skeleton since the mortar already have high strength..regards
Anyone can help me how to evaluate the strength of aggregate itself. e.g
If i am going to design a concrete mix of 100 MPa strength then what should be the strength of aggregate and how we can measure the strength of aggregate itself. Is there any standard test method for measurement of aggregate stregth. Thanks
To answer Mr. Kaffayatullah Khan's question above:
Aggregates have irregular shapes and cannot be tested in compression like you would do for a cube or a block. We use indirect tests to determine their strengths and that is The ACV(Aggregate crushing value) and AIV(Aggregate Impact Value). The ACV is done for aggregates used for normal concreting and the AIV for road surfacing aggregates. There is also the Abrasion Resistance test for aggregates to be used for wearing surfaces.
Standards provide methods for doing the tests.
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