Ready-mixed concrete is an intermediate input material in the entire construction process. Therefore due importance must be given to other steps in the construction process viz, shuttering, formwork, reinforcement placements and placement of concrete, vibration, compaction and curing of ready-mixed concrete on its arrival at job sites. Double stage centring should be strictly avoided in case of Ready-mix concrete.
5.6.1 Ordering process for Ready-mixed concrete: Before ordering for ready-mixed concrete the engineer has to look into the following details and properly order the grade and type of concrete required for the construction as per specifications.
- Type of building
- Exposure conditions
- Cement content
- Water content, water cement tario
- Strength requirements
- Type of aggregates
- Size of Aggregates
- Slump requirements.
- Admixtures type and quantity.
- Time of delivery.
Table 5.6 Typical Sample Report of Fresh Ready Mixed Concrete
5.6.2 Handling Ready mixed concrete at construction sites: The following points can be followed at the site to get the good quality concrete.
(a) Verification of delivery challan
- Grade, quantity and dispatch time of concrete while receiving the transit mixer at site.
(b) Workability of concrete
- Slump- a measure of workability. ( Designed to be 125+/-25mm @ site for pumpable concrete)
- The slump will be higher than 150 mm for columns and pile foundations.
- It shall be noted that increase in the slump (or workability) is due to admixture only and w/c will be kept according to the design parameters.
(c) Duration of consumption
- Concrete, normally, is designed to have the workability to last for two and half hours from the time of dispatching.
- After two and half hours the workability tends to fall. In this situation re-dosing of admixture is recommended to increase the workability. However, it should be done with proper care and planning.
- Site personnel will have to contact QA/QC engineer to know the amount of re-dosing and method of re-dosing required.
- It shall be noted that this practice is deemed as very occasional due to problems like, traffic jam, pipeline choking, pipe line shifting, problem at sites, etc.
- In general, concrete shall be consumed within two and half hours after dispatching and shall be used within three hours even after re-dosing.
(d) No water addition to fresh concrete
- As adding water to fresh concrete will increase the w/c ratio, the strength and durability of concrete will get affected adversely and so water shall not be added, strictly.
(e) Mixing of fresh concrete before pumping
- Before unloading concrete into pump hopper for pumping, the fresh concrete shall be mixed in transit mixer drum at full rpm for 2 minutes.
(f) Placing and compaction of concrete
- Fresh concrete shall be placed in the form work, which is made water tight to prevent any leaking of slurry water from the concrete in such a way that height and distance of fall and spacing of rebars are good enough to prevent any segregation.
- Optimum compaction shall be given and over vibration shall be avoided to prevent segregation and loss of water from fresh concrete.
- Over vibration and escape of water from fresh concrete will result in reduction of volume of the freshly laid concrete.
(g) Early curing and prevention of plastic shrinkage cracks
- Plastic shrinkage may occur when concrete is in Plastic stage during hot weather and windy conditions.
- Tarpaulin or plastic sheets shall be used to cover the freshly laid/finished concrete surface in order to prevent loss of water in the form of evaporation.
- Early curing, as early as 1 to 2 hours after finishing of concrete, shall be done in the form of sprinkling of water to prevent development of plastic shrinkage cracks.
- A full wet curing in the form of ponding of water or gunny sack wrapping shall be made after 12 to 14 hours approximately.
- If plastic shrinkage crack is observed, it shall be removed immediately by working the concrete surface with wooden battens. Otherwise, in combination of delayed curing, these cracks could develop through the thickness of slab.
Will be Continued in Issue 5..