Composite Beams
This technique is extensively used in Auroville since 1993. U-shaped CSEB are reinforced with reinforced cement concrete. Reinforcements vary with the span, but the rod diameter cannot exceed ø 12 mm for the Auram blocks 290 & 295 and ø 16 mm for the Auram blocks 240 & 245.
The U blocks are used as lost shuttering, but they also help the compressive strength of the beam. Hence it becomes a composite technique as the reinforced concrete work in tension and the U block works in compression. The concrete cast in the U shape is normally 1 cement: 2 sand: 4 gravel ½’. The vertical mortar in between the U blocks is cement sand mortar CSM 1: 3 of 1 cm thickness. Three types of beams have been developed:
The U blocks are used as lost shuttering, but they also help the compressive strength of the beam. Hence it becomes a composite technique as the reinforced concrete work in tension and the U block works in compression. The concrete cast in the U shape is normally 1 cement: 2 sand: 4 gravel ½’. The vertical mortar in between the U blocks is cement sand mortar CSM 1: 3 of 1 cm thickness. Three types of beams have been developed:
Single height beams (only 1 U block)
- The U block is laid on a bed of sand of 1 cm and the concrete in cast in the U shape.
- After 1 month curing the beam is returned and laid on the wall.
- The maximum span for a single height beam will be limited to 1.5 m with 2 steel bars ø 12 mm. ø 6 mm stirrups are laid at 25 cm c/c.
- After 1 month curing the beam is returned and laid on the wall.
- The maximum span for a single height beam will be limited to 1.5 m with 2 steel bars ø 12 mm. ø 6 mm stirrups are laid at 25 cm c/c.
 Casting a single height beam on the ground |  Single height beam reversed on the wall |  Single height beam section |
Double height beams (2 U blocks in opposite directions)
- The bottom part of the beam is cast first in a reversed position and after 1 month it is returned:
Either on the ground and the top part is precast in other U blocks or the incomplete beam is lifted with care and the concrete is cast in situ into other U blocks.
- The maximum span for a double height beam will be limited to 2.25 m with 2 steel bars ø 12 mm on the top and 2 steel bars ø 12 mm on the bottom.
- ø 6 mm stirrups are laid in the thickness of the vertical mortar to link the tensile and compressive bars of the beam.
- The horizontal mortar joint is with cement sand mortar CSM 1: 3
Either on the ground and the top part is precast in other U blocks or the incomplete beam is lifted with care and the concrete is cast in situ into other U blocks.
- The maximum span for a double height beam will be limited to 2.25 m with 2 steel bars ø 12 mm on the top and 2 steel bars ø 12 mm on the bottom.
- ø 6 mm stirrups are laid in the thickness of the vertical mortar to link the tensile and compressive bars of the beam.
- The horizontal mortar joint is with cement sand mortar CSM 1: 3
 Casting a double height beam on the ground |  Lifting a double height beam |  Double height beam reversed and cast on the wall |  Double height beam section |
Triple height beams (1 U blocks downwards, 1 plain block in the middle, 1 U block upwards)
- The bottom part of the beam is cast first in a reversed position.
- After 1 month it is returned and the incomplete beam is lifted with a lot of care and the rest of the beam (plain block in the middle and U block on top) is done in situ in.
- The maximum span for a triple height beam will be limited to 3 m with 2 steel bars ø 12 mm on the top and 2 steel bars ø 12 mm on the bottom.
- ø 6 mm stirrups are laid in the thickness of the vertical mortar to link the tensile and compressive bars of the beam. Note that this triple height beam is rarely precast fully on site as it is too heavy to lift.
- The horizontal mortar joint is with cement sand mortar CSM 1: 3
- After 1 month it is returned and the incomplete beam is lifted with a lot of care and the rest of the beam (plain block in the middle and U block on top) is done in situ in.
- The maximum span for a triple height beam will be limited to 3 m with 2 steel bars ø 12 mm on the top and 2 steel bars ø 12 mm on the bottom.
- ø 6 mm stirrups are laid in the thickness of the vertical mortar to link the tensile and compressive bars of the beam. Note that this triple height beam is rarely precast fully on site as it is too heavy to lift.
- The horizontal mortar joint is with cement sand mortar CSM 1: 3
 Casting a triple height beam on the ground |  Lifting a triple height beam |  Triple height beam reversed and cast on the wall |  Triple height beam section |
The main limitation of this technique is the handling of long beams:
- If it is fully precast on the ground, it becomes too heavy to lift by hand. A maximum of 2 m span can be precast and lifted.
- Triple height beams of more than 2 m cannot be precast and they should be lifted incomplete. But the beam is fragile as the section of concrete is only 5.5 cm x 17 cm. Therefore it should be lifted with the section vertically positioned (see drawings above).
- If it is fully precast on the ground, it becomes too heavy to lift by hand. A maximum of 2 m span can be precast and lifted.
- Triple height beams of more than 2 m cannot be precast and they should be lifted incomplete. But the beam is fragile as the section of concrete is only 5.5 cm x 17 cm. Therefore it should be lifted with the section vertically positioned (see drawings above).
Casting a single height beam
 Adjusting the U blocks on a bed of sand |  Casting cement sand mortar CSM 1: 3 in the vertical joints |  Casting concrete 1: 2: 4 in the U blocks |
Casting a double height beam
Casting a triple height beam
Testing a triple height beam 240 of 2.5 span
Testing a triple height beam 298 of 2 span
 Laying the U blocks on a bed of sand, above the plain blocks |  Adjusting U blocks |  Inserting steel bars in the vertical joints of the double height beam |
 Casting the vertical joints of a double height beam |  Casting concrete 1: 2: 4 in the U blocks |  Smoothening the top of the beam with mortar CSM 1: 3 |
Casting a triple height beam
 Inserting steel bars in the vertical joints of the triple height beam |  Casting a triple height composite beam |  Finishing the triple height composite beam |
Testing a triple height beam 240 of 2.5 span
 Testing a 2.50 m span triple height beam 240: 1280 kg/m: 4 mm deflection without cracks |  Section of the triple height beam 240, tested on 2.5 m span |
Testing a triple height beam 298 of 2 span
 Testing a 2 m span double height beam 298: 1750 kg/m: cracks with 5.54 mm deflection |  Section of the double height beam 298, tested on 2m span |
 Laying triple height composite beams: 2 m beam and 1.6 m cantilevered beams |  Laying plain blocks in the middle of the beam |  Laying the top U blocks |
 Bending the bars of the column |  Casting concrete 1: 2: 4 |  Cantilevered beam near completion |
