Consolidation of fresh concrete in congested concrete members should be performed using proper concrete pouring and compaction procedures. Otherwise, the concrete element suffers from strength deficiency and durability loss due to problems like honeycomb and poor concrete cover in addition to expensive and time-consuming repair work.
Structural and seismic design requirements, embedded conduits and pipes, and boxouts are factors that cause congestion in concrete members.
Contractors have to practice special care and consideration while selecting concrete pouring and consolidation techniques to ensure adequate compaction in congested areas and achieve structurally sound and aesthetically pleasing concrete. Additionally, it is essential to consider high flowable concrete and place the concrete in its final position.
Finally, adequate concrete consolidation in congested areas needs good detailing on the part of the designer and proper planning and controlling on the part of contractors.
- What are the Causes of Congestion in Concrete Members?
- Consolidation Techniques for Congested Areas
What are the Causes of Congestion in Concrete Members?
1. Congestion of Reinforcement
Seismic and structural requirements lead to steel congestions in reinforced concrete elements. Steel congestion occurs in areas where additional reinforcement is needed.
For instance, around formed openings, especially in thin wall sections, columns intersecting other concrete members, beam/beam junctions, deep beams, mat foundation, post-tensioned members, and bridge deck slabs.
Structural and seismic design requirements lead to the extensive placement of ties at the bottom and top of columns. Closely spaced steel bars would restrain coarse aggregate; hence concrete cover is made of cement and sand grout.
Consequently, concrete cover would be prone to drying shrinkage followed by surface cracking and crazing and, ultimately, loss of durability.
2. Electrical Conduit, Pipe Sleeves, and Other Embedded Objects
Electrical conduits, pipe sleeves, and other complex structural embedment can hinder proper placement and concrete consolidation. The electrical designers generally specify the usage of several 25 mm to 150 mm diameter conduits in localized areas for cable trays.
Formed boxouts in slabs and walls are prone to congestion because the concrete flow under the boxouts and close formed openings is limited. The provision of a construction joint or introducing access openings within the boxouts can facilitate proper placement and concrete placement.
Consolidation Techniques for Congested Areas
Proper concrete compaction can be achieved by giving special attention to construction practices in three specific areas:
1. Concrete Placement and Consolidation Techniques
The concrete placement, as close as possible to its final position, is vital for good compaction of concrete in congested areas. Concreting supervisors or site engineers should use hoppers and trunks for crane and bucket applications.
However, when pumps are used to place concrete, it is recommended to connect a wire-reinforced rubber hose to the boom pipe to pour close to its final position.
When the distance between steel bars is too small, and as a result of which, the congestion is too extensive, the use of a lie-flat hose is advised. This type of pipe can go between closely spaced steel bars and pour concrete into its final position. It is possible to cut the hose to easily remove it when fresh concrete rises in the forms.
In congested wall elements, provide ports on one side of the wall to facilitate proper compaction of concrete. The ports can be placed on 0.6 m x 0.6 m grids. When concrete reaches the first lower line of ports, the ports are closed, and vibrators are moved to the next upper row of ports.
Transparent plastic plate can be utilized as form face in congested areas to provide additional visual access. This would allow the team engaged in the concrete placement to observe the problems during concreting and take proper measures to tackle them.
Another strategy that can be used to obtain adequate compaction in the congested area is the use of internal vibration through obstruction-free vertical runs. The vertical runs with at least a cross-section of 10 cm x 15 cm can be installed through which internal vibrations can be produced into concrete in congested areas.
The maximum spacing between vertical runs is 61 cm. If vertical runs cannot be installed without compromising structural integrity, the engineer should determine suitable construction details and procedure to obtain adequate compaction.
2. Use of Chemical Admixtures
The addition of chemical admixtures, such as high-range water-reducing admixtures, can increase concrete workability and, consequently, improve the concrete compaction in congested areas.
Chemical admixtures can increase concrete flowability without influencing the water-cement ratio. One should be aware that chemical admixtures cannot replace the requirements for good compaction by vibration.
3. Use of Modified Mixtures
Modified mixtures are considered for congested concrete elements when the proportioned mixture’s flow to the form face is questionable.
Modified mixture contains a reduced maximum nominal aggregate size to make sure that concrete would have high flowability.
Table-1 presents properties of internal vibrators, flexible shaft, and motor-in-head vibrators that can be used to consolidate concrete in congested reinforced concrete members.
Table-1: Properties of Internal Vibrators, Flexible Shaft, and Motor-in-head Vibrators
|Group||Diameter of vibrator head, mm||Recommended frequency, vibrations per min (Hz)||Radius of influence of vibrator, mm||Rate of concrete placement, m3/h|
|1||20 to 40||150 to 250||75 to 150||1 to 4|
|2||30 to 65||140 to 210||125 to 250||2 to 8|
It is the process of reducing volume of voids, air pockets, entrapped air in fresh concrete using mechanical energy.
1. Structural and seismic requirements that lead to the placement of high reinforcement ratio in concrete elements such as top and bottom of columns.
2. Embedded pipes and conduits
1. Place concrete in its final position using proper means like connection of fiber-reinforced hose or lie-flat hose to the pump hose. The lie-flat hose can go between steel bars and can be cut while concrete rises in the forms.
2. Add chemical admixtures to concrete to improve its flowability.
3. Use a modified mixture; use a small maximum aggregate size.
1. Structural deficiency
2. Loss of durability
3. Expensive and time-consuming repair work