· In simple word segregation means separation of one from another.
· The term Segregation of Concrete in Civil Engineering means the separation of constituents of concrete from each other.
· Segregation of concrete is also defined as the separation of some size groups of aggregates from cement mortar in isolated location resulting in to deficiencies of those materials in other locations because of differences in size, density shape and other properties of ingredients in which they are composed.
Causes of Segregation or What are the causes of segregation in Concrete?
The causes and conditions due to which the segregation in concrete occurs are as follows –
a) The main reason of segregation in concrete is the differences in size of particles and sometimes the specific gravity of the mix constituents differs from each other.
b) Use of poorly graded aggregate, for example flakiness.
c) Bad proportioned of concrete mix.
d) Using high water cement ratio (happens when concrete mixed by unskilled worker at site)
e) Transportation of concrete for long distance.
f) Poor compaction of concrete after placing.
g) Over compaction of concrete.
h) Specific gravity of aggregate is in between 2.6-2.7 g/cc and for cement it is in between 3.1-3.6 g/cc, due to these differences, the aggregate separates from the matrix and loose its homogeneous nature.
i) Dropping of concrete more than 1 m height in case of underground foundations like and rafts etc.
j) If concrete conveyed through conveyor belts, wheel barrow, long distance haul by dumper promoting segregation of concrete.
Types of Segregation or Forms of Segregation:
There are two types or forms of segregation,
· In case of first one, (Segregation due to the separation of aggregate) the coarser and the heavier particle tens to separate out or settling down from the rest of the surrounding substances since they travel further along a slope or settle more than the finer particle. This type of segregation occurs when the concrete mix is too dry i.e. the mixing of water in the dry mix is lower than its design aspects.
· In case of second one, (Segregation due to the separation of cement slurry), generally occurs in wet mixes, is manifested by the separation of cement slurry or grout (cement + water) form the mix. This type of segregation occurs when the concrete mix is too wet i.e. addition of water to the concrete mix is higher than its specified mix design.
Effect of segregation in Concrete:
Following’s are some effect of segregation in concrete –
a) The segregation in concrete will affect the strength of the concrete. When segregation occurs in concrete the strength of concrete decreased as compared to it’s designed / target strength.
b) It is very difficult to compact a segregated concrete.
c) When segregation occurs in concrete sometime the excess mortar comes to the top of the surface resulting in plastic shrinkage cracks.
d) A homogeneous mass throughout the structure cannot obtain when the concrete becomes segregate.
e) As a result of segregation after pouring of concrete, the lower portion of the concrete pour of any significant depth is stronger than the upper portion.
f) Segregation results the porous layers, rock pocket, sand streaks in hardened concrete.
g) Honey combing, high permeability of concrete, poor finishing of surface are also some harmful effect of segregation in concrete.
How to prevent or minimize or control Segregation in Concrete:
To control Segregation –
a) The concrete mix should be properly designed with most favorable quantity of water i.e. not too wet or not too dry.
b) If the concrete does not have far to travel and transferred directly from skip or the wheelbarrow to the final position of in the formwork, the danger of segregation is small.
c) During placing of concrete more than a height of 1.50 m it is possible to avoid the dropping of concrete by passing the concrete through a inclined chute, keeping the inclination between 1:3 to 1:2 for the smooth travel of concrete from top to bottom, and particularly with changes of direction or by discharging concrete through an obstacles reduce the possibility of segregation.
d) By using a correct method of handling, transporting and placing of concrete the chances of segregation can be minimized.
e) The segregation in concrete can be controlled by using air entrain agents, admixtures and pozzolanic material in the concrete mix.
f) The formwork in which concrete should be poured should be air tight so that leakage of slurry prevented which leads to the control of segregation.
g) If any segregation is observed in concrete, remixing should be done to make it homogeneous again.
h) Proper compaction of concrete prevents the segregation in concrete.
i) Some field quality tests should conducted to control the segregation of concrete.