WHAT IS CONCRETE ?
TYPES OF CONCRETE COMMONLY USED ?
FOLLOWING ARE THE COMMONLY USED TYPES OF CONCRETE IN CONSTRUCTION INDUSTRY.
a) PLAIN CEMENT CONCRETE (PCC)
b) REINFORCED CEMENT CONCRETE (RCC)
c) PRE – STRESSED CONCRETE
d) PRECAST CONCRETE
e) SELF COMPACTED CONCRETE
HOW TO DECIDE A CONCRETE TYPE?
a) MATERIAL AVAILABILITY
The raw material (aggregate , sand , cement etc) availability decides the type of concrete to be used.
b) STRENGTH REQUIRED
The concrete requires different strengths for different structures. The strength required for the particular structure decides the type of concrete to be used.
c) CONSTRUCTION METHODOLOGY TO BE FOLLOWED
The construction technique to be adopted for a structure decides the type of concrete. Example Pre stressed concrete etc.
d) TYPE OF STRUCTURE
Most of times the type of the structure decides the type of concrete to be used.
SELF COMPACTED CONCRETE (SCC) is preferred in structures where normal pouring is restricted due to rebar congestion or access restricted pouring area. SCC, when pumped from a single point, can fill every part of the structure.
e) AREA OF APPLICATION
The type of concrete shall be decided by the area where it has to be used. In some structures, the reinforcement is so dense that concrete may not pass through it. In those cases, specially designed concrete with small size aggregates or Self compacted concrete (SCC) may be used.
f) CLIMATE AND POURING CONDITIONS
The areas where there is extreme weather conditions like heavy rain , extreme cold, extreme hot specially designed quick setting concrete will be used.
PLAIN CEMENT CONCRETE
A concrete without reinforcement steel is called Plain Cement Concrete (PCC). Design mixes commonly used for PCC are 1:4:8 , 1:3:5 , 1:2:4 , M7.5, M10 etc. Plain cement concrete is high in compression and very low in tension.
WHERE IS PCC USED ?
Plain cement concrete is commonly used over the ground to protect the reinforcement of footings from directly contacting the soil. PCC can also be used for grade slabs (floor) and concrete roads where only compressive loads are there.
PROPERTIES OF PLAIN CEMENT CONCRETE
The compressive strength of PCC ranges from 200 kg/cm2 to 500 kg/cm2. The tensile strength of PCC varies from 50 kg/cm2 to 100 kg/cm2 and density varies from 2200 kg to 2500 kg as per the grade of concrete and aggregates used.
INGREDIENTS OF PLAIN CEMENT CONCRETE
Cement, coarse aggregate, and fine aggregates are used for the production of PCC. Cement (portland cement ) is used as the binding material The coarse aggregate shall be broken or crushed stone or brickbats. The fine aggregate shall be coarse sand. These ingredients are mixed with potable water in the required proportions to form PCC.
PRODUCTION OF PLAIN CEMENT CONCRETE
PCC can be produced in batching plants or mixer machines and some times manually. The thickness of PCC ranges between 50 mm to 300 mm or more depending upon the design parameters.
PLACING AND COMPACTION OF PCC
a) LEVEL MARKING & DRESSING
After completing the excavation, the bottom level of the PCC shall be marked on the ground using a level machine. The centre line from the survey pillars shall be transferred to the ground where PCC has to be done. The surface shall be dressed manually to remove the loose soil the surface level to receive PCC.
b) SURFACE PREPARATION AND SHUTTERING.
The surface shall be neatly dressed and supports has to be placed around using wooden battens. The battens used has to be the same size as PCC preferably. The battens shall be properly supported using proper supports (scrap steel can be used).The dressed surface shall be sprinkled with water to avoid absorption of concrete water by the soil.
c) PLACING AND FINISHING OF PCC
The concrete shall be poured from one end to another. Level pillars at an interval of 2 meters shall be provided for levelling purposes. The concrete shall be properly levelled and rammed in line with the level pillars and end supports. The slump to be followed for PCC shall be around 75 mm. The concrete shall be placed within 30-45 minutes.
PRECAUTIONS TO BE TAKEN WHILE DOING PCC
a) During excavation, care should be taken to excavate only up to the required levels. Do not over excavate. In case if the depth of excavation exceeds beyond the required depth back filling with loose earth is not recommended. In that case, we can place the PCC at that level of doing a plum concrete to that required level. Back filling shall be done with soil and compaction using plate compactors/walk-behind rollers/or Vibro rollers depending upon the situation can also be done.
b) The level of PCC shall be transferred to different locations before starting excavation. Centerline and PCC dimensions have to be marked on the ground before fine dressing using lime powder so that any chance of reworks can be avoided.
d) The surface on which PCC to be laid shall be sprinkled with water.
e) Anti Termite chemical or LDPE sheets may some times be used before doing PCC. A confirmation has to be taken before doing the PCC from the clients/customers.
e) The free falling height of concrete shall be restricted to 1.5 mtrs due to segregation issues.