7/30/2019 6-composite-beams-en.pdf

1/5

EARTH BASED TECHNOLOGIES

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 12 mm for theAuram blocks 290 & 295 and 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 incompression. The concrete cast in the U shape is normally 1 cement: 2 sand: 4 gravel . The vertical mortar in between the U blocks is cement sand mortar CSM 1: 3 of 1 cm thickness. Three types of beams havebeen 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 12 mm. 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 direction s)- 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 12 mm on the top and 2 steel bars 12 mm on the bottom.

- 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

7/30/2019 6-composite-beams-en.pdf

2/5

Triple height beams (1 U blocks dow nwards, 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 12 mm on the top and 2 steel bars 12 mm on the bottom.

- 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 thesection vertically positioned (see drawings above).

Casting a single height b eam

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

7/30/2019 6-composite-beams-en.pdf

3/5

Casting a double height beam

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

7/30/2019 6-composite-beams-en.pdf

4/5

Testing a trip le height b eam 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 wit h 5.54 mm deflection Section of the double height beam 298, tested on 2m span

7/30/2019 6-composite-beams-en.pdf

5/5

Casting a triple height cantilevered beam

Laying triple height composite beams: 2 m beam and 1.6 mcantilevered 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