Sign of a Good Concrete

Sign of a Good Concrete

  A good concrete is one in which all the ingredients are properly distributed to make a homogeneous mixture and it should not shown any sign of

(a)      Segregation

(b)      Bleeding

(a)       Segregation

Segregation can be defined as the separation of coarse aggregate from the main mass of concrete in the plastic stage and it occurs in case of dry mix of insufficient and non - uniform mixing.

Basic reasons for segregation can be stated as follows, if

· There are considerable difference in the sizes and specific gravities of mix constituents used.

· The aggregates used are not well graded.

· In the concrete, sufficient matrix is not available to bind the aggregates.

· The mix is too dry or too wet.

· The mix handling is very improper like dropping concrete from a considerable height, transporting and travelling concrete over a longer length, passing concrete along a chute, particularly with changes of direction and discharging against a obstacle.

· The vibrator is used to spread a heap of concrete over a larger area.

· The vibration is allowed to continue too long.

The remedial measures to eliminate segregation can be summarised as follows;

· Reducing the height of drop of concrete.

· Not using vibration as a means of spreading a heap of concrete over a large area.

· Reducing the continued vibration over a longer time.

· By correctly proportioning the mix.

· By proper handling, transporting, placing, compacting and finishing.

· By taking into consideration of various parameters such as grading, size, shape and surface texture of aggregate with optimum quantity of water to make a cohesive mix.

(b)   Bleeding

Bleeding, known as WATER GAIN, is a form of segregation in which some of water in the Mix tends to rise the surface of freshly placed concrete. This is because of the inability of the solid contitutents of the mix to hold all the mixing water in the place when they settle downwards. water having the lowest specific gravity of all mix constituents.

· Bleeding is predominant in a highly wet mix, badly proportioned and insufficiently mixed concrete.

· Bleeding causes the formation of a porous, weak and non -durable concrete layer at the top of placed concrete.

· In case of lean mixes, bleeding may create capillary channels increasing the permeability of concrete.

· The formation of cement paste at the surface is known as LAITAN C E. The laitance formed in pavement concrete does not have good wearing quality, produces dust in summer and mud in rainy season. It also develops higher shrinkage crack.

Remedial measures to minimise bleeding

· It can be reduced by proper proportioning and uniform and complete mixing.

· Reduction in bleeding is obtained by addition of pozzolanas viz flyash or silica fume.

· Air entrainment effectively reduces bleeding so that finishing can follow casting without delay.

· The presence of an adequate proportion of very fine aggregate particles (finer than 150 micron) significantly reduces bleeding.

· The use of crushed fine aggregate than rounded natural sand can reduce bleeding effectively.

· Bleeding is decreased by increasing the fineness of cement possibly because finer particles hydrate earlier and also because their rate of sedimentation is lower.

· The cement containing high alkali content and a high C3A content produces very le&, bleeding.