Plain Cement Concrete (PPC)| Column Formwork | Concrete Curing
Plain Cement Concrete (PPC)
Plain cement concrete is concrete without steel reinforcement. Plain Cement Concrete is often used in foundation beds, and this is provided as based concrete for laying the foundations and flooring. The purpose of providing a PCC bed is to have a uniformed leveled surface for foundations and flooring.
Preparing Plain Cement Concrete Bed
The grade Plain cement concrete shall be a minimum of M10 before laying Plain cement concrete. The soil on which it is to be laid shall be well compacted or drummed.
It is generally made of one part of cement, three or four parts of sand, and 6 to 8 parts of 40-millimeter aggregates. It is mixed thoroughly, either in mechanical mixers or by hand, mixing depending upon the volume off work.
The concrete is then placed uniformly to a thickness of 100 millimeters to 150 millimeters and is thoroughly compacted using rummers. It is to be ensured that the aggregates are well embedded in the motor, and there are no voids.
Sometimes, this may require the use of cement mortar on the surface of plain cement concrete to fill the surface voids.
Plain cement concrete bed is the first concrete work in any construction. Plain cement concrete provides a leveled surface for foundation, and flooring typically cement , sand and aggregate in a proportion of one is to three is to six or one is the four is to eight is used.
It is preferable to use 40-millimeter aggregates. The thickness of the plain cement concrete would usually be 100 to 150 millimeters.
Cement absorbs moisture when exposed to the atmosphere, resulting in the formation of Granules or lumps due to chemical reaction and loses its strength. Hence, cement has to be stored properly to maintain its freshness and strength.
Column Formwork or Shuttering
Columns carry the load of the structure, and hence they are critical in the structure. It is important to align them for verticality so that load is transmitted correctly.
The column shuttering should be strong enough to pressure fresh concrete and remain in position during concreting.
Column Formwork Shuttering Process
To hold the concrete, shuttering should be firmly in place with proper alignment, a concrete pad called Starter is cast before fixing the shuttering.
The thickness is about 45 millimeters 2. 60 millimeters, and the dimensions are precisely the same as the dimensions of the proposed column.
The Starter should be cured for a day or two so that it is hard enough to fix the shuttering around it. The column box or shuttering for columns is made of plywood sheets or steel sheets fabricated with adequate stiffness.
A thin film of oil or grease should be applied to the shuttering’s inner surface to enable easy removal of the column after the concrete hardens.
Shuttering should be aligned appropriately to its verticality, and diagonally should be checked to ensure accuracy in dimensions.
It has to be thoroughly supported with props or ties before pouring the concrete to not move horizontally or vertically during concreting.
The gaps near the shutter joints should be sealed to prevent any leakage of slurry.
Appropriate space is to be provided between the shutter’s inner face and reinforcement by fixing cover blocks. It is preferable to remove the shutters after 24 hours of casting, and they need to be removed earlier; it should not be before 16 hours.
The removal of shutters has to be done gently and without jerks or hammering so that the edges of the columns are not damaged.
Care should be taken regarding column shuttering. A starter should be built to hold the shuttering and prevented movement.
The diagonal of the shuttering should be checked to ensure dimensional accuracy. Gaps at shutter joints should be sealed and cover blocks placed. Ensure that shuttering does not move while concreting.
For a structure to be durable, it should be free from cracks. One of the significant issues in concrete is the development of shrinkage cracks.
This can be avoided by keeping the surface moist when it’s hardening; this process is called Concrete curing.
Concrete Curing Process.
Curing is the process of maintaining moisture of freshly placed concrete to ensure proper hardening, attaining desirable strength, and durability. Curing keeps the concrete surface moist and reduces the shrinkage. Cracks curing should be started at the earliest.
It has to be started when the surface is hard enough for a person to walk over it without concrete damage till such time, the surface moisture may be maintained by splashing or spraying water without force.
Ponding is used for flat members like slabs. It is done by impounding the water to a depth of around 50 millimeters. The slab is to be divided by constructing small buns with lean mortar or mud.
Buns are filled with water. Ponds are replenished with water at frequent intervals.
The wet covering is generally used for columns, sloped surfaces, beams, concrete walls. The concrete surfaces are to be covered with wet soaky bags.
Water is sprinkled on these coverings at regular intervals to ensure continuous wetness. These should not be allowed to dry even for a short duration.
Sprinkling or Spraying Curing Method
Sprinkling or spraying method is generally used for masonry and plaster. The frequency of spraying or sprinkling should ensure that the surfaces are continuously moist.
It is advisable to cure the concrete members for a minimum period of 10 days. Intermittent curing is more dangerous than not curing as it leads to cracking.
Concrete curing is essential to achieve the strength and durability of Concrete. Curing should start as soon as the concrete is stiffened and should be continued for a minimum of 10 days for Ordinary Portland Cement and 14 days for blended cement.