Concrete placing process carried out in four stages, casting area preparation, conveying of concrete, placing of concrete and finally curing of concrete.
To ensure and produce structures with high quality each stage of concrete placing should be properly implemented.
Casting Area Preparation
- Formwork: formwork is a temporary mold used to form and hold concrete in place until it hardens, formwork can be made of steel or wood, prior to pouring concrete, formwork should be cleaned properly and it should be painted with proper oil to allow quick removal of formwork, prior to casting formwork support should be checked to ensure the stiffness and ability of formwork to retain concrete without bulging or opening.
- Reinforcing steel: prior to placing concrete, reinforcing steel should be checked properly and confirmed that it is as per the design and drawing, steel should be cleaned from any rust, cleaning rust can be done by steel brush and if it is insufficient sandblasting can be done, also also steel should be cleaned from any residual or sticking material such as old concrete, oil, grease. this very important to ensure good bonding with concrete. enough chairs should be provided to prevent the movement and distortion of reinforcing steel during the placing of concrete
- Embedded items: sometimes slab or any structural member will enclose items such as pipes, anchor bolts, and instruments, prior to placing concrete drawing should be checked properly for any items.
Conveying concreteprior to placing concrete order, casting area should be checked and cleaned again, concrete will be delivered to the field by a concrete mixer, the time from batching concrete to placing concrete should not exceed 90 minutes or as per the local specification. concrete can be placed by direct discharge from a mixer into the forms, crane and buckets, pumps, conveyors, buggies, wheelbarrows, pneumatic placers, small railcars, or a combination of two or more of these methods.
The placing method will depend on the quantity of placed concrete and available space and equipment, if the concrete placed directly from the mixer, a small chute shall be used to avoid segregation of concrete. if the pump used to place concrete, pump hose should be placed near the bottom of formwork, if the hose is short it should be extended. usually opening are made in steel to allow the phose to reach the bottom of the member then it will be closed again when the concrete reaches this opening.
Each concrete mixer should be tested for slump test to ensure the concrete possesses the required workability, also cubes or cylinders samples should be taken to perform compressive strength test and other durability tests.
Compacting of Concrete
Compacting of concrete is vital and very important, proper compacting of concrete will produce a dense concrete with high strength and low permeability, this will result in durable concrete. in field there is two common vibrators, internal and external vibrator, internal vibrator called poker, this poker immersed in concrete and vibrate in a harmonic way to expel the entrapped air and produce dense concrete, the diameter of poker ranges from 20 mm to 150 mm. Pokers can generate vibration with a frequency ranging from 70 to 200 Hz, and acceleration from 0.7 to 4m/s2 at speeds of 5500 to 15,000 vpm (vibrations per minute).
External vibrators are used in congested reinforcing areas, where it is difficult to use poker vibrators, external vibrators are fixed at the outside of formwork at a different point, therefore formwork should rigid and supported sufficiently.
Curing of Concrete
Curing is very important to provide concrete with high strength and durability, the purpose of curing is to avoid concrete from losing water, extensive evaporation of concrete water will produce a porous concrete with high permeability and low strength, concrete should be kept moist for a minimum of two weeks, this can be attained by covering the concrete with Hessian sheets, hessian sheets should be kept wet all the time.