The three golden rules for pouring concrete on site

construction details have a heavy impact on the actual quality of the concrete in a structure.

RULE No. 1 

CONCRETE MUST BE CAST INTO ITS FINAL POSITION ACROSS AN ESSENTIALLY VERTICAL PATH.

Horizontal paths must be avoided. This must be taken into consideration in the design drawings for reinforcing bar arrangements.

  Figure (a) depicts the right way to reinforce a beam to ensure that the forms are filled speedily and satisfactorily. If the bars are arranged as depicted in Figure (b), the coarse aggregate will not pass readily between them. As a result, the concrete will have to flow horizontally, inducing segregation and lengthening the time needed to fill the formwork. Moreover, such arrangements leave insufficient space for the vibrator. 

Concrete should not be dumped in a pile for subsequent spreading with vibrators. Rather, it should be poured in each and every spot where it is needed.

RULE No. 2 

THE VIBRATOR MUST BE ABLE TO REACH THE BOTTOM REINFORCEMENT 

Figure (c) shows the right way to reinforce a beam. With 65-mm spacing (somewhat smaller on site due to the height of the ribs), a standard 50-mm vibrator will be able to reach the bottom reinforcement. The solution depicted in Figure (d) is wrong, for it leaves insufficient room for the vibrator.

Figures (e) and (f) show two further cases in which the vibrator is able, or unable, to reach the bottom reinforcement.

RULE No. 3 

CONCRETE CONSISTENCY MUST BE IN KEEPING WITH THE REINFORCEMENT ARRANGEMENT. AS A GENERAL RULE THE CONCRETE SLUMP SHOULD BE NO SMALLER THAN 60 mm. 

Unless the reinforcement is arranged very openly and spaciously or powerful vibration methods are used, overly dry concrete is characterised by the following. 

* The required strength can be reached in test specimens (but not on site) with a lower proportion of cement. The real strength of concrete can be lower. 

* Since the control specimens can be compacted with no difficulty, the laboratory trials will furnish good information. 

Insitu placement and a good surrounding of the reinforcement will be difficult to achieve and the loose consistency will lower actual on-site strength.

reference

Manual for Detailing Reinforced Concrete Structure to EC2