Classification of concrete & Manufacturing of concrete

*Concrete:-

·         Concrete is a mixture of binding material such as lime or cement, well graded coarse & fine aggregate, water and some admixture.

Physical properties of concrete:-

*Elastic modulus of concrete:

·         It varies from 14*10³ N/mm² to 40*10³ N/mm² & increases with density and to some extent with age.

·         Ec = 5700 √fck N/mm², Min. grade =M15 (IS 456:1978)

·         Ec = 5000 √fck N/mm², Min. grade =M20 (IS 456:2000)

*Poisson’s Ratio:

·         µconcrete = 0.1 to 0.3

·         For dynamic tests its value is slightly high (0.24).

·         The weight of the normal weight concrete is 2300 kg/m³ to 2600 kg/m³.

·         The weight of plain cement concrete is 2400 kg/m³ & for RCC are 2500 kg/m³.

·         As per the IS: 456-1978, the concrete mixes are designed into 7 grades.

·         As per the IS: 456-2000, the concrete mixes are designed into 8 grades.

*Stages of manufacturing of concrete:-

                                I.            Batching

                              II.            Mixing

                            III.            Transporting

                            IV.            Placing

                              V.            Compacting

                            VI.            Finishing

                          VII.            Curing

      1. Batching:-

·         Aggregate cement & water measured with accuracy 0f ±3% of batch quantity.

 

 

     2. Mixing:-

·         Approximate mixing time as per IS-456:1978 is 2 minutes.

·         Generally 20 revolution of concrete in mixture provides sufficient mixing.

 

 

·         Concrete mixers are specified by volume of mixed concrete discharged after mixing of each batch expressed in m³.

·         Sometimes mixes are specified by two quantities, the total volume of ingredients added & the volume of concrete produced for eg.370/310.

·         10% extra cement is added in hand mixing.

·         Tilting mixers are used for large construction works.

·         Non-tilting mixers are used for small works.

·         Non-tilting mixers cannot be used when aggregate size is more than 75 mm.

       3. Transportation:-

·         A belt conveyer is used when the concrete is to be transported continuously & to an inaccessible area.

·         Concrete pumps are commonly used on tunnel works.

·         Best method for transportation of concrete is pumping.

·         For pumping method, the compaction factor of concrete should be 0.90 to 0.92.

·         Diameter of pipe for pumping method is 100 to 200 mm.

·         By pumping method, concrete can be transported 400 m in horizontal & 80 m in vertical direction.

      4. Placing:-

·         Delayed placing of concrete results in a gain in ultimate compressive strength provided the concrete is adequately compacted.

·         As per IS: 456, Maximum permissible free fall concrete may be taken as 1.5 m.

·         Placing of concrete underwater is done by tremie pipe. (Slump 150-200 mm)

      5. Compaction:-

·         Process of removal of entrapped air & of uniform placement of concrete to form a homogenous dense mass is called compaction.

·         Presence of even 5% & 10% voids in Hardened Concrete left due to incomplete compaction may result in a decrease in compressive strength by about 30% & 60% respectively.

·         Surface vibrators/Screed board Vibrators are used for Road surface, plain concrete floors pavement etc. it is effective only if depth is up to 15-20 cm.

·         External Vibrators used only when the section is thin & heavily reinforced where penetration of needle vibrator is not possible.

·         Internal Vibrators are very effective for mass concreting.

       6. Finishing:-

·         Leveling or smoothing of top surface of freshly placed concrete. It is done by screeding, Floating & Trowelling.

·         Screeding: The process of removing of extra particles from the surface of concrete is known as screeding.

·         Floating: It consists of removing the irregularities on the surface of concrete & it is generally done by wooden float.

·         Trowelling: It is final operation of finishing & it gives a very smooth finish.

      7. Curing:-

·         As per IS: 456, curing is the process of preventing the loss of moisture from the concrete while maintaining a satisfactory temperature regime.

·         The test sample should be stored in a place free from vibration, in moist air of at least 90% relative humidity & at a Temp. Of 27°-29°C for 24 hours ± (1/2) hour from time of addition of water to dry ingredients.

·         Curing period for rapid hardening cement is 3 to 7 days & seven days for OPC.

·          Combined water: It is chemically combined with hydration products and it is non-evaporable.

·         Capillary water: It partially occupied the capillary pores that constitute the space in the cement paste remaining offer accounting for the volumes of cement gel & unhydrated cement. It is easily evaporated.

·         Gel water: It held physically or absorbed on the surface area of cement.

 

Method of Curing	Suitability
Shading of concrete work/ Stagnating water	For large concrete surface such as road slab.
Gunny bags	For Structural concrete
Sprinkling of water	For vertical surfaces.
Membranes curing	At place of scarcity of water
Steam (accelerated) curing	For Precast concrete work

 

 

      *Special Points:

·         Steam curing is not suitable for high alumina cement.

·         Membrane curing is used for hilly areas.

·         Steam curing increased ultimate strength & reduced the shear strength of concrete.

·         Concrete heated by steam at 93°C either at low or high pressure.

·         More rapid gain of strength can be obtained by with the help of infrared radiation than steam curing.