Workability of concrete & Workability tests on concrete

*  Workability :-

·        Property of concrete which determines the amount of useful internal work necessary to produce full compaction.
·        It is the property with which concrete can be compacted 100% with regards to made of compaction & place of deposition.
·        Slump Test used for In situ determination of workability.

*Factors affecting Workability:

Factors	Effect on Workability
Water Content	o   As water content increases, fluidity of mix increases so workability increases
Aggregate	o   Higher the aggregate size, higher the workability. o   Big size aggregate, surface to be wetted is less.
Shape of aggregate	o   Angular & flaky aggregate have large surface area hence is less workable. o   Rounded cubical shape aggregates have less surface area. So less amount of paste is required for lubrication. Hence they are more workable.
Aggregate /Cement	o   Higher the ratio, leaner ratio workability.
Surface texture	o   Smoother the surface, higher the workability.
Grading of aggregates	o   Well graded aggregates are more workable.

 

*Workability Tests:-

1.      Slump Test
2.      Compacting Factor Test
3.      Vee Bee Consistometer
4.      Flow table Test
5.      Compressive Strength Test

 

1.     Slump Test:-

•         Concrete slump test or slump cone test is to determine the workability or consistency of concrete mix prepared at the laboratories or the construction site during the process of the work.

·        Concrete slump test is carried out from the batch to check the uniform quality of concrete during construction.
·        Dimensions of mould ------
·        Top dia. = 100 mm, Height = 300 mm, Bottom dia. = 200 mm,
·        Four layers, 25 strokes/layer
·        Tamping Rod: dia. = 16 mm, length = 600 mm

·        Sampling of Materials for Slump Test-----

•         A concrete mix (M15 or other) by weight with suitable w/c ratio is prepared in the laboratory and required for casting 6 cubes after conducting.

P     Procedure for concrete slump cone Test:-

·        

·    Clean the internal surface of the mould and apply oil.
·        Place the mould on a smooth horizontal non-porous base plate.
·        Fill the mould with the prepared concrete mix in 4 approximately equal layers.
·        Temp each layers with 25 strokes of the rounded end of the tamping rod in a uniform manner over         the cross-section of mould.
·        For the subsequent layers, the tamping should penetrate into the underlying layer.
·        Remove the excess concrete and level the surface with trowel.
·        Clean away the mortar or water leaked out b/w the mould and the base plate.
·        Raise the mould from the concrete immediately and slowly in vertical direction.
·        Measure the slump as the difference b/w the height of the mould and that of height point of the               specimen being tested.

Result of Slum Test on Concrete:-

·        When the slump test is carried out, following are the shape of the concrete slump that can be observed:

·        

·       True Slump: - True slump is the only slump that can be measured in the test. The measurement is taken b/w the top of the cone and the top of the concrete after the cone has been removed.
·       Zero Slump: - Zero Slump is the indication of very low water-cement ratio, which results in dry mixes. This type of concrete is generally used for road construction.
·       Collapsed Slump: - This is an indication that the w/c ratio is too high, i.e. concrete mix is too wet or it is a high workability mix, for which a slump test is not appropriate.
·       Shear Slump: - The shear slump indicates that the result is incomplete, and concrete to be rested.

No.	Type of Concrete		Slump
1	Concrete For road construction		20 to 40 mm
2	Beams & Slabs		50 to 100 mm
3	Mass Concrete		25 to 50 mm
4	Normal RCC work		80 to 150 mm
5	Concrete to be vibrated	10 to 25 mm
6	Impermeable work	75 to 120 mm

          

    2. Compacting Factor Test:-

·       Degree of workability is measured in terms of internal energy required to compact the concrete thoroughly.
·       Compacting Factor test is useful for concrete mixes of medium and low workability.

·      
C.F =

·        

·       

 

    3. Vee Bee consistometer:-

·       It determines the time required for transforming, by vibration, a concrete specimen in the shape of a conical frustum into a cylinder.

·       It is suitable for concrete having slump less than 50mm.

·       Consistency of the concrete is expressed in VB degree which is equal to recorded time in seconds.

·       It is a good laboratory method, particularly for very dry mixes.

·       

   4. Flow Table Test:-

·       It is the laboratory test which gives an indication of the quality of concrete wrt consistency, cohesiveness & the proneness to segregation.

·         Flow% =

·       Range 0 to 150.

·       Order of stability of test from low workability to high workability.

·       Vee Bee consistometer<Compacting Factor test<Slum test<Flow table test

   5. Compressive Strength Test:-

·       Compressive strength test is the capacity of material or structure to resist or withstand under compression. The compressive strength of a material is determined by the ability of the material to resist failure in the form cracks and fissure.
·       In this test, push force is applied on the both faces of concrete specimen and the maximum compression that concrete bears without failure is noted.

•   The test is carried out using 150mm concrete cubes on a universal testing machine or compressive testing machine.

·       As per IS: 516-1956 Compressive testing machine (2000Kn), 150 mm*150 mm*150 mm steel cube moulds or cylinder having dia 150mm and length 300mm are used.

Preparing of material for cube test:-

·       

·       All the materials must be brought and stored to an appropriate temperature of 27±3°C. Cement must be uniformly mixed with a trowel in order there exist no lumps.

·       1>Calculated Water, 2>50% coarse aggregates, 3>fine aggregates, 4>Cement, 5>50% sand.

·       Hand mixing: - The process must be done on the rectangular pan until a homogeneous mix is obtained.

·       Dry mixing of fine aggregates and cement>addition of coarse aggregate with even distribution>addition of calculated water in batch till consistency is achieved.

Procedure of Compressive Strength of Concrete or Cube Test:-

 

·         Place the prepared concrete mix in the steel cube mould for casting.
·         Once it sets, after 24 hours remove the concrete cube from the mould.
·         Keep the test specimens submerged underwater for stipulated time.
·         As mentioned the specimen be kept in water for 7 or 14 or 28 days and or every 7 days the water is        changed.
·         After the removal from the water, the specimens are ready for test on UTM (Universal Testing       machine).
·         Weight of sample is noted in order to proceed with testing and it must not be less than 8.1kg.
·         Testing specimens are placed in the space b/w bearing surfaces.
·         Care must be taken to prevent the existence of any loose material or girt on the metal plates of machine or specimen block.
·         The concrete cubes are placed on bearing plate and aligned properly with the center of thrust in the testing machine plates.
·         The loading must be applied axially on specimen without any shock and increased at the arte of 140kg/sq/min. till the specimen collapse.
·         Due to the constant application of load, the specimen start cracking at a point & final breakdown of the specimen must be noted.

   * Calculations:-

·         The Compressive strength of specimen can be calculated by dividing maximum load carried by the cross-section area of the specimen cubes.
·         Compressive Strength of Concrete =
·         The Surface area of the specimen is
                                                                = 150*150 = 22500mm² =225cm²
·         Assume, The Max compression load is 450KN.
·         1KN = 1000N ; 450KN= 450*1000=450000N
·         Then the compressive strength of concrete = 450000/22500 = 20N/mm² = 203kg/cm². 

    *Mix Design:-

·         When the proportions of cement, aggregate & water are adopted based on arbitrary standards the concrete produced is termed as Nominal Mix Concrete.
·         Ti may be adopted for concrete of M20 or lower grade.

*Steps in mix-design (IS: 10262 – 1982 concrete mix design):

·         Find target mean Strength (fm) = fck + 1.65 σ
·         Get W/C ratio based on 28-day strength of cement

        Grade of concrete                        Assumed σ

               M10-M15                                    3.5

               M20-M25                                    4.0

               M30-M50                                    5.0

·         Get water content based on workability requirement.

*Grades of concrete:-

 

    *Defects in concrete:-

·        Crazing: - It’s the network of fine random cracks on the surface of concrete of mortar caused by shrinkage of surface layer.
·        Cracks: - Acceptable limits for the surface width of cracks 0.1 to 0.3 mm.
·        Efflorescence: - Fluffy white patches on the surface of concrete members. Efflorescence is caused by salty water generally.

*Segregation:-

·        Segregation of the constituent material of concrete (cement, sand & aggregates) because they have different specific gravity.

·        It is of three types

è  Water separating out from rest of the material (bleeding).

è  Coarse aggregate separating out & setting down.

è  Paste Separating away from aggregate.

è   It is seen in badly proportioned mixes or when concrete is dropped from height (>1.5m) is prone to segregation.

·         As per IS: 456, maximum height to free fall is 1.5 m.

*Bleeding:-

·        In this water from concrete comes out to the surface of concrete because of low specific gravity.
·        Rich mixes are prone to bleeding than lean mixes.
·        It leads to formation of pores inside & hence, cause decrease in strength.
·        It can be reduced by use of uniformly graded aggregate, entraining agent, finer cement.
·        It is seen in highly wet concrete of badly proportional mixes.
·        When water comes up with cement particles to the surface it is called ‘Laitacnce’.

*Creep (plastic or yield):-

·        Continued deformation with time under a constant load.

·        In general creep increases when aggregate content & relative humidity is low or when cement content, Air Entrainment, water cement ratio & temperature are high.