To get the required strength from the concrete the good workmanship practice is really having equal importance to the mix design calculations. Especially, for the water retaining structures, water tightness is very important consideration to be achieved through design incorporating good workmanship at the site. Preventing structure from cracks and porosity are main features to be considered for durable structure when doing mix design as well. Site practice during concrete placement is having much importance to achieve the designers’ target as well as durable concrete. The site practice can be focused into three categories as;
- Before Concrete Placement
- During Concrete Placement
- After Concrete Placement
The detailed procedures that are carried out during all three stages are discussed below.
Before Concrete Placement:
Rebars should be cleaned well to remove the dusts with the wire brush or cup brush and check all the bars and drops and any holes are provided accordingly according to the drawings. And the lapping of bars, cover blocks and stools should be ensured before placement of concrete.
Then the formworks should be checked that for any leakages of grout, adequacy of provided supporting elements, rigidity of provided form ties, and accuracy of dimensions and lines. Finally, the formwork surface should be cleaned well for dust free surface.
During Concrete Placement:
The prior checks of the concretes should be carried out just before placing the concrete. Temperature check and slump test are two such tests to be performed. Also, test cubes should be casted for each truck of concrete arrived at site for the purpose of compressive strength test at 7 days and 28 days to ensure the built structure according to the design standards.
For big scale projects, pump car will be used to pour the concrete. While pouring concrete, the concrete should be compacted and levelled simultaneously before reaching its initial setting time. Adequate compaction using poker vibrator should be ensured for non-porous structure. At this stage the formation of cold joints should be strictly avoiding.
After Concrete Placement:
24 hours after placing concrete curing should be carried out for continuously 7 days. Curing can be done in several methods depending on the structure.
- Water sprinkling/spraying – Plastering surfaces
- Wet covering – Covering the vertical surfaces with wet gunny bags (walls, columns)
- Water ponding – All edges of horizontal surface should be placed with quarry dust and pouring water into. Or spreading the gunny bags on the surface and spraying water over the bags. (floor, slab)
Curing is important process to be followed at the site for at least 7 days after placing of concrete. Keeping the moisture and temperature condition is important for hardening process of concrete. This will help to avoid arising of thermal shrinkage cracks in its initial setting out process.
