Concrete

CONCRETE 

 

                           Concrete is an artificial stone, which is made with a mixture of cement, fine Aggregates, coarse aggregates and water. This concrete is hardened into a strong stone due to chemical reaction between cement and water and we can get good strength concrete by using ½ of water cement ratio. We can make concrete in different strength by changing mix proportions, which are used for different situation.

 CONCRETE MIX DESIGN

 

                  Concrete mix design may be defines as the art of selecting suitable ingredients of concrete and determining their relative proportions in the object of producing concrete of minimum strength and durability as economically as possible. 

                  The main purpose of concrete mix design is to ensure the most optimum proportions of the constituent materials to fulfil the requirement of the structure being built.

 

 CEMENT 

 

Cement is used in construction work in the form of 50-kg bags. Ordinary Portland cement is the main type of cement that we use in concreting. The initial setting time shall not be less than 45minutes and the final setting time shall not more than 10 hours for OPC. 

There are various types of cement which are,

o      Ordinary Portland Cement (OPC) o    White Portland Cement (WPC) o       Portland Pozzolana Cement(PPC) o   Low Heat Portland cement

o      Rapid hardening cement        Figure 7.1 – Cement o         Coloured Portland Cement  o    Sulphate resisting Portland Cement

CEMENT STORAGE

 

•      The storage sheds shall be of weather proof construction and floors shall be free from all possibilities of flooding

•      The cement shall be bags stacked at least 100mm to 200mm clear above floor level over wooden planks and joints

•      A spacing of 600mm shall be kept between exterior walls and the stacks

•      Cement bags shall be placed close together in stack to reduce circulation of air to the minimum

•      Cement bags shall not be stacked more than 10 bags high to avoid lumping under pressure  

 FINE AGGREGATE 

Sand is natural sediment made up mainly of silicon compounds in granular form, which is the product of rock weathering. It is gritty with no veal plasticity. The particles normally range between 0.06 mm to 2.00 mm in diameter sand should be clean and free from     excessive         silt       and chemical impurities well graded and not too but

             Figure 7.2 – Checking quality of sand by palm

depending on the nature of work on. Sea sand should never be using for any construction, very reason due to salt in the sea sand. 

 

 QUALITY OF SAND

                               The best fine aggregate shall pass 4.75 sieve and retain on 150 microns sieve. The good quality of sand shall have less percentage of clay in it. Presence of excessive clay can be determined in two ways.

1. Hold some dry sand and drop it. If the sand adheres to palm, it has excess clay.  

Figure 7.3 – Checking quality of sand by measuring cylinder Take a glass full of water and add some sand to it. Then shake it and allow the sand to settle. After 2 hours observe it.

 h₁ – height of sand layer

                                     h₁                             h₂ – height of mud layer

 

 

         If,    h₂      *  100  <  8%  the sand is good for use.

                 h₁

 

 COARSE AGGREGATE 

Aggregate should be clean and free from organic

impurities. The presence of organic material in aggregate results to poor concrete. Aggregate should be free from oil, coating of dust and clay as these will affect their proper bounding of material. 

Excessive amount of fine dust or stone may prevent the particles of stone from being properly coaled with cement and this reduce the strait of concrete for structural coarse aggregate generally use 20mm stones (some other sizes are 37.5, 14, 

Figure 7.4 – Coarse aggregate

5 mm)

 

 

 WATER 

Only good water to be used generally which is fit for drinking, directly or other contaminated water should not be used, as they will have an adverse effect on the concrete. Therefore much care is taken for the ingredients. Tests are carried out especially for sand and aggregates beforehand. 

Particle size distribution is determined by sieve analyzing test for both. When concrete is going on slump test and cube tests are carried out at the site. 

 

REQUIREMENTS OF CONCRETE MIX DESIGN

 

                     The requirements which form the basis of selection and proportioning of mix ingredients are,

•      The minimum compressive strength required from structural consideration

•      The adequate workability necessary for full compaction with the compacting equipment available.

•      Maximum water cement ratio and maximum cement content to give adequate durability for the particular site conditions

•      Maximum cement content to avoid shrinkage cracking due to temperature cycle in mass concrete

 

 ADMIXTURES

                     

                                  Various types of admixtures are used in concrete to enhance the performance of concrete. Concrete admixtures are defined as the material other than the aggregate, water and cement added to the concrete. 

      Types of admixtures:  

•      Retarder –  Delays concrete setting time. Accrete chemical brand was used for site concrete   

•      Super-plasticizer  – Increases concrete workability and viscosity. Supercrete chemical brand was used for site concrete

We used Optima 183 and MasterEase 3401 as the concrete admixtures at the site. They are super plasticizer for low viscosity concrete, improve the rheological properties for high performance concrete. And also they save the time and cost in the project.

 

CONCRETE BONING AGENT 

 

                                          In our site EMACO 157D PRIMER (Barra Emulsion 157D) was used as concrete bonding agent.  The bonding agent is a white aqueous emulsion of styrene-butadiene copolymer latex, specially formulated for use in cementitious mixtures to improve wear resistance, durability, waterproofing and bonding properties. This provides excellent adhesion to concrete thereby fully bonding the old and new concrete layers together.  

 

QUALITY CONTROL OF CONCRETE WORKS 

 

National development of a country depends on a large extent of essential infrastructure such as housing, roads, and bridges etc. These infrastructures have to be reliable and trouble free. In order to have reliable and trouble free usage it is necessary to exercise quality control of their construction works. In civil construction, a special effort has to be taken for quality controlling since production is usually done at sites. 

Ø  Everyone in the project team plays a vital role in the QC; hence everyone has to be aware of the QO system and inspection procedure.

Ø  Quality       must    be        always given    preference      over speed      and      economy.        (a poor quality work delays the job and cause additional financial on the project) 

Ø  Using poor quality materials cannot        carry    out       good

quality construction but it is often

Figure 7.11– Concreting slab

possible that poor construction is done using good quality materials. 

Ø  Mistakes should not be allowed to repeat under any circumstances. 

Ø  Ignorance, negligence, carelessness and greed lead to construction failures. 

Ø  Essential material testing and concrete testing must be carried out at site. 

The overall responsibility for assuring QC as per required standards, specifications and drawings rests on the Contractor. Anyhow the workers, foremen, supervisors, engineers, the consultant and the contractor form a link of the quality chain.  

 

 

 

 

 

 

 

 

 

 

 

 COMPACTING OF CONCRETE

 

                                         The removal of entrapped air during production of concrete and the uniform, dense arrangement of the constituents of concrete are effected during the compacting of corners. The density consequently, the strength and durability of concrete depend upon this operation. Concrete should be thoroughly compacted during the operation of placing and thoroughly worked around the reinforcement, around embedded fixtures and into corners of the form work. Concrete is compacted by vibration, during which the vibrator communicates rapid vibrations of low amplitude to the particles, as a result of which the concrete becomes fluid, its mobility is increased and the particles in movement under the force of gravity occupy a more stable position with which volume of concrete is least.

 

                    In our site, we used porker vibrator to compact the concrete for mass concrete (such as concrete for slab, strip foundation and etc), and for small concrete (such as Stud columns, Linlals and etc) compaction was done manually by using a steel rod.