Fig.1: Concrete Pour Placement
Precautions during Concrete Pouring
The ACI 304R-00 has provided several precautions for pouring concrete which include:- The time between adding water to dry mixture and concrete pouring should not exceed 30minutes for normal condition where temperature is no greater than 30 degree. However, this time would decrease to 20mintues in hot weather condition. If it is need, admixture can be used to increase this time period.
- Suitable equipment and technique should be considered to make sure that concrete drop vertically to the specified locations without any restrictions.
- Free fall of concrete need to be continuous and possible separation due to free falling of concrete over reinforcement or other embedded objects should be prevented.
- Free distance of concrete is limited to 0.9 to 1.5m, and occasionally it is restricted to 0.6m.
- It is required to ensure that concrete is placed at or close to its final position to prevent segregation.
- It is recommended to discharge concrete into the forms directly and avoid the application of equipments such as chutes or hoppers. This is possible if forms are wide and open.
ACI Code Recommendation for Concrete Pouring of Different Reinforced Concrete Elements
In this section, suitable method of concrete pouring for various reinforced concrete elements will be discussed. The elements include columns and walls, slabs, inclined slabs, and curved members. If these suitable techniques are applied correctly, then the occurrence of segregation and honeycomb can be prevented.Pouring of Concrete in Columns and Walls
If the height of these elements is large, then it is recommended to place concrete in layers. The layer thickness ranges from 300mm to 500mm and the time between successive layers should not exceed 30 minutes for normal weather condition and 20 minutes for hot weather condition. Each layer should be compacted properly using vibrators. Figure 2 explains correct (A) and incorrect (B) method of concrete pouring into deep forms in layers. Figure B is not desired since concrete would strike form face and segregation would occur.
Fig.2: Pouring concrete into deep forms in layers, A) Proper way of pouring concrete, B) incorrect method of pouring
When height of forms in not large and concrete poured from the top of the forms, then the ACI code recommends discharging concrete into light hopper and then place concrete into its final location as explained in Figure 3.
Fig.3: Recommended way of pouring concrete into narrow forms
Lastly, if steady concrete pouring is not achieved and concrete strike against the form as explained in Figure 4, the concrete would suffer from separation and hence honeycomb at the bottom would prevail.
Fig.4: Improper pouring of concrete into narrow forms such as walls and columns
Pouring of Concrete in Slabs
The ACI Code recommends placing concrete into face of concrete in place as shown Figure 5 in rather than away from the face of concrete in place as shown in Figure 6.
Fig.5: Pouring concrete into the face of concrete in place as recommends by ACI 304R-00
Fig.6: Incorrect pouring of concrete which should be prevented during slab construction
Pouring of Concrete for Inclined Slabs
Commonly, pouring concrete of inclined surfaces is more challenging compare to horizontal surfaces and hence it needs more efforts to avoid segregation. The ACI 304R-00 recommends the application of baffle and drop at the end of the chute to avoid concrete separation and make sure that concrete stay on the slope. This process is illustrated in Figure 7.
Fig.7: Pouring concrete on slope slab using baffle to prevent concrete separation
However, if such technique is not used, then it is highly possible that concrete would suffer separation and large size aggregates would settle at the bottom as shown in Figure 8.
Fig.8: Improper pouring of concrete, large size aggregates settled at the bottom
Pouring of Concrete for Curved Members
Curved members could be beams, columns, walls, or slabs. The ACI 304R-00 recommends to pour concrete in layers as explained in Figure 9.
Fig.9: A) Correct method of concrete pouring, B) Incorrect method of concrete pouring
Concrete Pouring Using Pumps
Nowadays, pumps are widely used to pour concrete instead of other conventional equipments. It is advised to extend the hose of pumps as close as possible to the final position of concrete to prevent segregation and other undesired effects. Figure 10 and Figure 11 shows the desired way of placing pouring concrete using pumps for both right and curved members respectively.
Fig.10: Extending pump hose near final location of concrete
Fig.11: Extending pump hose close to the final position of concrete
References:
- ACI Committee 304, 2009. Guide for Measuring, Mixing, Transporting, and Placing Concrete. Michigan: American Concrete Institute.
- ASCC, N.A. Free Fall of Concrete. St. Louis: American Concrete Institute American Society of Concrete Contractor.
- Gates Concrete Forming Systems, 2018. Gates Concrete Forms. [Accessed 10 February 2018].
- Mohamed A. El-Reedy, 2009. Advance Materials and Techniques for Reinforced Concrete Structure. Boca Raton: CRC Press.
- Morgana Matus, 2013. Inhabitat. [Accessed 10 February 2018].