Cracks in concrete is a very vast subject that needs a detailed discussion. In this blog, I will discuss quickly some of the main reason for crack formation in building structures. However all points will have to be covered in a series of blogs as one blog will be too long to cover every thing.
RCC or Reinforced Cement Concrete is a material weak in tension.Due to this inherent weakness,concrete is bound to crack,what ever we do. However some cracks can be structurally significant if it is due to underdesign or overloading or faulty construction and so on. Some cracks can be harmless strength point of view,though all crack can result in durability issue.
Is there any crack that is harmless?
Strictly speaking No! All cracks are harmful durability point of view.Today it may be harmless strength point of view,However,water and corroding agents entering the cracks,can create a strength issue tomorrow. Also cracks produce bad effects on aesthetics and induces psychological fear in occupants.So it is mandatory to attend to any crack as early as possible for the health of a building structure.
Causes of cracks
There are various ways we can classify cracks in to.One way is to classify it in to cracks occuring in prehardening stage and the ones occuring after hardening
Prehardening stage :
a) Construction movement can result in cracks.Normal construction movement occurs in formworks and subgrades.Both are results of bad materials or bad workmanship. For example swelling of wood can result in forwork movement and settling of soil due to improper compaction can result in gradeslab settling.
b) Settlement shrinkage : Can be a result of rebar or formwork obstruction
Dense mixes with low water content and adequate compaction can be a solution
c) Setting shrinkage : Plastic or drying shrinkage due to chemical reactions and rapid drying respectively.
Solution : Rapid drying can be reduced by proper protection from wind etc.
Post Hardening stage
a) Drying shrinkage due to loss of water can result in cracks on slabs.
Dense mixes with low water and cement content is a solution.Also adequate curing
b) Chemical action
This can result in expansion of aggregates and rebars
Solution : Non reactive aggregates,low alkali cement,Adequate cover to rebar.
c) Temperature
Climatic changes can create external cracks in buildings
Solution : Adequate expansion joint,designing structurally for temperature stresses
d) Structural cracks
Structural cracks occurs when the stresses are exccessive than the capacity
This can happen due to various reasons as listed below
Over loading
Construction mistakes
Design mistakes/underdesigns
Vibration
Now that we have discussed some of the reasons for crack formation, let us discuss a method each to identify if the cracks are structural and to see if the crack is progressing.
a) Inspect both the faces of the structural member.
If you see cracks on either sides of the member,the chances of it being a structural crack is more.
b) As I have already explained in another blog,redistribution is a virtue and many times alternate load path available due to this redistribution ability comes as a help should a structural crack occur. In such cases,where bridging has occured,the crack may not progress after redistribution.
Stick a thin glass piece across the crack immediately after the crack formation and observe.If the cracks are progressing,the glass will break as it is brittle when the crack widens.
We will try to discuss all the structural reasons for crack formation and the pattern of cracking and the remedies in the subsequent blogs. So wait for the blogs. If you want to subscribe to our blogs,please fill the form here
You can read our other technical blogs here.
Summary
There are various reasons of cracking and some of these can be structural. It is very important to understand the reasons for cracking to mitigate any risks associated with this and also to take rectificaton measures.