Question
Asked 22 May 2014
The brittleness of high strength concrete.
Due to its fined microstructure, HSC possesses different mechanical properties compared to normal strength concrete. One common characteristic is its increased brittleness. Many approaches have been used to judge the material brittleness. One method is using the ratio between the tension strength to compressive strength: the lower the ratio, the more brittle. Another is the linear portion in the stress-strain curve observed in uniaxial compressive concrete. The larger the linear portion, the more brittle. In fracture mechanics, some brittleness quantifiers have been defined according to different models In fictitious crack model (FCM), a characteristic length (lch) that is defined by combining the fracture energy (Gf) with elastic modulus E and the tensile strength of material (ft). The smaller the value lch, the more brittle the material. Another common used brittleness indicator is the critical crack extension length (ac). the larger the value ac, the less brittle. When it decreases with an increase in compressive strength, we can say the brittleness increases with strength. Apart from these, what are the other approaches that can be used to define brittleness of HSC?
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what is the relationship between fracture energy and brittleness ?
All Answers (8)
IRANDAMP
Discussing about ductility of plain concrete seems a little weird to me. I think ductile or brittle behavior should be investigated for a component rather than a specimen. So let's consider an RC column instead of cylindrical concrete specimen. A well detailed concrete column might be quite ductile regardless of its concrete type. While crushing strain has some contribution, most concrete elements are ductile thanks to their confinement reinforcements. As a result, and in the case of HSC, more confinements are required which might be hard to achieve, if not impossible. The so called FRP might be a possible confinement alternative.
University of Technology Malaysia
Dear seyed, there are two different topics in here. One is the material ductility itself and the second one is the 'structural ductility' . Well, the one you're talking about is most probably the latter, as it deals with structural behaviour. In the structural ductility we understand that the ductility of a structural member can be enhanced through confinement,etc. but for material wise, research shown that the brittleness of concrete increased with the concrete strength. Say 90MPa concrete is more brittle than 30MPa. This can be observed by concrete's stress-strain curve itself which has published elsewhere (attard, 1990 etc.) The drastic drops at post-peak indicating clearly that HSC is brittle compared to NSC even in plain material. Hence, several approaches have been proposed to determine the brittleness of concrete (types of approaches pls refer to my question's explaination above). My concerns are the suitability of these approaches and is there any other approach which is relevant but not about the 'structural ductility'. Anyway, thanks for your answer. Appreciate it :)
IRANDAMP
Still I cast doubt on validity of material-level evidences. Virtually all concrete elements, either structural or unstructural, have some sort of reinforcement which would govern their ductile/brittle behaviors.
Anyway, I think crushing strain of concrete can be a good indicator of its ductility. The more the crushing strain, the more ductility would be achieved.
A good example is lightweight EPS concrete. Attached pic shows a tested lightweight EPS concrete with density of 800 kg/m3 and compressive strength of only 5 MPa. Crushing strains of 1.2% to 2% were observed which are much more larger than those of normal or high strength concretes (which are 0.1% to 0.3%). No sudden or explosive-like failure was observed during the tests.
University of Technology Malaysia
As far as i concerned the crushing strain is one of the most acceptable brittleness indicator for concrete material. The example that prescribed in the previous comment is exactly the example of 'material' brittleness as the EPS concrete test as shown in the attachment. The sample tested is 'material' without reinforcement. If it is pointless to discuss about ductility of plain material, why the mentioned tests are carried out on EPS concrete? And why the example used is for testing on material-plain-concrete without reinforcement?
Example: HSC is more brittle compared to NSC. But slender reinforced HSC could be a ductile structural member (compared to short conterparts) but it is still considered as brittle structural element if compared to slender reinforced NSC column. A structural member, even with reinforcement will carry the properties from its constituent material.
The material based investigation can give insight to the structural element inherenting some material properties. As in this example, the findings shown HSC is brittle so extra care have to be given to structural member made of HSC, but can we say reinforced HSC column is not brittle because it is a stuctural member but not a material? Of course, reinforcement can increase the ductility of strcutural members up to certain degree but a strctural members which made of brittle material will still behave in the same manner.
Missouri University of Science and Technology
You have mentioned several approaches for evaluating the brittleness of concrete. Others may include the post-peak tail length on a 3-point bending load-displacement curve.
Koneru Lakshmaiah Education Foundation
Respected Ma Chau Khun sir,
The field approached by you is very interesting and also vast, so more research can be done in this regard. It's very interesting topic. The following article link may help you for your kind information.
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