Concrete is widely used construction material all over the world. It is composed of aggregate, cement and water. Composition of concrete varies to suit for different applications desired. Even size of the aggregate can influence mechanical properties of concrete to a great extent.
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Peculiarities of Concrete[edit]Response to tensile and compressive loading[edit]
Concrete is strong in compression but weak in tension. When tensile loads are applied, concrete undergoes fracture easily. The reason behind this phenomenon can be explained as follows. The aggregates in concrete are capable of taking compressive stresses so that concrete withstands compressive loading. But during tensile loading cracks are formed which separates the cement particles which hold the aggregates together. This separation of cement particles causes the entire structure to fail as crack propagates. This problem in concrete is resolved by the introduction of reinforcing components such as metallic bars, ceramic fibres etc. These components act as a skeleton of the entire structure and are capable of holding aggregates under tensile loading. This is known as Reinforcement of Concrete.
![]() Material Properties[edit]
If a few random cracks like that and you know that it was reinforced, I would have him (in lieu of tearing out and replacing now) give you an extended warranty for say 6-12 months against further cracking (and keep photos of current cracking), and he fills the current cracks - if all that small probably with concrete caulk, which can be colored. Micro Concrete: 5 Things You Need to Know By Daniel Wanat 19 September, 2018 August 28th, 2019 No Comments MicroConcrete, otherwise known as micro cement (although there are some small differences between the two), is an increasingly popular design product.
Concrete may be referred to as a brittle material. This is because concrete's behaviour under loading is completely different from that of ductile materials like steel. But actually concrete differs from ideal brittle materials in many aspects. In modern fracture mechanics concrete is considered as a quasi-brittle material.[1] Quasi-brittle materials possess considerable hardness which is similar to ceramic hardness, so often it is called ceramic hardness. The reason for ceramic hardness can be explained on the basis of subâcritical cracking that happens during loading of concrete. Subâcritical cracking in concrete which precedes ultimate failure, results in nonâlinear StressâStrain response and Râcurve behaviour. So concrete obtains hardness from subcritical failure.[2]Also concrete has a heterogeneous structure due to uneven composition of ingredients in it. This also complicates the analysis of concrete by producing misleading results.
LEFM and Concrete[edit]
Linear Elastic Fracture Mechanics yields reliable results in the field of ductile materials like steel. Most of the experiments and theories in fracture mechanics are formulated taking ductile materials as object of interest. But if we compare the salient features in LEFM with results derived from the testing of concrete, we may find it irrelevant and sometimes trivial. For example, LEFM permits infinite stress at crack tip. This makes no sense in real analysis of concrete where the stress at crack tip is fixed. And LEFM fails to calculate stress at crack tip precisely. So we need some other ways to find out what is stress at crack tip and distribution stress near crack tip.
LEFM cannot answer many phenomenon exhibited by concrete. Some examples are
Cracks In New ConcreteFracture Process Zone (FPZ) in concrete[edit]
In LEFMPA, during cracking, no specific region is mentioned in between the area which is cracked and that which is not. But it is evident that in concrete, there is some intermediate space between cracked and uncracked portion. This region is defined as the Fracture Process Zone (FPZ). FPZ consists of micro cracks which are minute individual cracks situated nearer to crack tip. As the crack propagates these micro cracks merge and becomes a single structure to give continuity to the already existing crack. So indeed, FPZ acts as a bridging zone between cracked region and uncracked region. Analysis of this zone deserves special notice because it is very helpful to predict the propagation of crack and ultimate failure in concrete.In steel (ductile) FPZ is very small and therefore strain hardening dominates over strain softening. Also due to small FPZ, crack tip can easily be distinguished from uncracked metal. And in ductile materials FPZ is a yielding zone.
When we consider FPZ in concrete, we find that FPZ is sufficiently large and contains micro cracks. And cohesive pressure still remains in the region. So strain softening is prevalent in this region. Due to the presence of comparatively large FPZ, locating a precise crack tip is not possible in concrete.
Pre-peak and post-peak response of steel and concrete[edit]
If we plot stress (Pascal) vs. strain (percentage deformation) characteristics of a material, the maximum stress up to which the material can be loaded is known as peak value (ft{displaystyle f_{t}}). The behaviour of concrete and steel can be compared to understand the difference in their fracture characteristics.For this a strain controlled loading of un-notched specimen of each materials can be done. From the observations we can draw these conclusions:[3]
Pre-peak
Fixing Micro Cracks In Concrete
Post-peak
Fracture mechanics of concrete[edit]Concept of fracture energy[edit]
Fracture energy is defined as the energy required to open unit area of crack surface. It is a material property and does not depend on size of structure. This can be well understood from the definition that it is defined for a unit area and thus influence of size is removed.
Fracture energy can be expressed as the sum of surface creation energy and surface separation energy. Fracture energy found to be increasing as we approach crack tip.
Fracture energy is a function of displacement and not strain. Fracture energy deserves prime role in determining ultimate stress at crack tip.
Mesh Size Dependence[edit]
In Finite Element Method analysis of concrete, if mesh size is varied, then entire result varies according to it. This is called mesh size dependence. If mesh size is higher, then the structure can withstand more stress. But such results obtained from FEM analysis contradict real case.
Size effect[edit]
In classical Fracture Mechanics, critical stress value is considered as a material property. So it is same for a particular material of any shape and size. But in practice, it is observed that, in some materials like plain concrete size has a strong influence on critical stress value.[4] So fracture mechanics of concrete consider critical stress value a material property as well as a size dependent parameter.
Bažant's size effect relation[edit]
where
How To Fill Micro Cracks In Concrete
This clearly proves that material size and even the component size like aggregate size can influence cracking of concrete.
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Computational models for fracture analysis[edit]
Because of the heterogeneous nature of concrete, it responds to already existing crack testing models 'anomaly'. And it is evident that alteration of existing models was required to answer the unique fracture mechanics characteristics of concrete.
Earlier models[edit]
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