BSF News

What’s been going on

This week we have placed concrete to the suspended slabs to Section 1 of the new building. This is the three storey section which Music, MFL, Maths and English classrooms.

The Steel erectors started work erecting the steel frame to the last part of the building, Section 3 which includes the larger classrooms to Food Tech, Engineering, Science and Textiles as well as humanities. The first part of the steel work to be erected is for the large central atrium/ dinning area, this will be bounded by circulation spaces, ICT suites, the Senior Management Team offices and the Enterprise Centre it will form the heart of the new College.

In order to provide weather protection to the slabs below we concreted the 2nd floor slab first. The concrete is pumped from the ground up to the metal deck. A vibrating poker is used to ensure that the stone aggregate within the cement is evenly distributed and ensure that the concrete fills all the space around the steel reinforcement and the profile of the metal deck. Once placed the concrete is smothed off to the finished level using hand trowels. The finished level is set with a rotating lazer beam. The concrete finishers can then measure down from the plane created by the spinning lazer to ensure that the slab has been laid to the correct level at any point on the slab.

The concrete slab will form the finished floor to your classrooms with carpet and vinyl being laid direct to the concrete. In order to ensure that the finished concrete is free from surface irregularities which would not be acceptible on a finished floor it is necessary to continually smoth and polish the concrete as it sets. To achieve this finish mechanical trowels called power floats are used (see above). A float is the name given to the type of trowel used to finish concrete. These devices can only be used once the concrete has hardened sufficiently to take the weight of the machine approximately 3 to 4 hours after the concrete has been placed. The surface of the concrete is then polished using rotating steel blades and later in the process using a rotating circular steel pan. Because the blades are angled slightly the reaction to the polishing effect will cause the float to move in one direction, by mounting two sets of blades spinning in opposite directions this force can be directed by altering the angle of the blades to each set. This allows Բide onԠpower floats like the one above to be directed around the slab. Sometimes these devices are nicknamed ԨelicoptersԮ

This photograph shows the concrete being placed to the 1st floor slab, you can clearly see all the reinforcing steel mesh which is incorporated in the slab as the concrete is placed. In earlier bulletins it was highlighted that whilst concrete is strong in resisting compressive (pushing) forces, it is comparatively weak at resisting the tensile (stretching) forces which are induced in the top of the concrete when the slab bends. By introducing steel mesh into the concrete we produce a composite material that can resist both the downward pressure of walls, people and furniture and the bending forces created in the slab spanning between the beams below. If you look closely you can see the boom and pipe of the concrete pump. This is clearly shown in the photograph below.

The photograph below shows concrete being dispensed from the delivery wagon into the mobile concrete pump. The concrete mix has to be carefully designed to ensure it has the required strength (i.e. enough cement powder in the mix) but still is sufficiently fluid to flow through the pump up to the slab. If you think of the weight of concrete (2 tonnes per cubic metre, twice that of water) it gives you an idea of the power of the pump; capable of lifting the concrete over 30m up if necessary (only about 9m on this project). Concrete pumps can reach internal pressures of 70 bar.

If concrete were to harden inside the pump this very expensive piece of plant would be ruined, so it is vital that the supply of concrete is continuous to the pump once the pour is started. To ensure that the concrete is the right consistency to pass through the pump each load is tested prior to placing. If a load is too stiff or too wet it is rejected and sent back to the supplier. To ensure that the concrete provided achieves the correct strength, cube moulds are filled with concrete for each 50m3 of concrete placed, i.e. every 6 deliveries of concrete. The concrete cubes created are taken to a laboratory and crushed in a testing machine after 7 and 28 days. The force needed to crush the cube determines the strength of the concrete. In addition to these site test cubes the supplier is continually taking test samples at their mixing plant to control the quality of the concrete dispatched from the plant.

Over the half term we have excavated the area behind the existing sports hall to construct the new external staircase that will provide fire escape and access for roof maintenance to the new building. In order to allow completion of the stairs without interfering with the operation of the College we have also moved your access to the sports pitches round the back of the staircase slab to the doors to the mini-sports hall. This provides you with a shallower incline to walk up and down. This new path will be concrete over its entire length to give mud free access to the pitches. This will mean improved access for staff and students for Sports day in July.

Elsewhere we have also completed the installation of our biometric recognition site entry system. Only those who have been inducted and have registered their hand on our system can gain entry to the site. As each person enters they place their hand on a sensor and the turnstile will only spin if the hand is recognised.

Coming soon

• Erection of Structural Steelwork

  All steel will be erected by the end of June.

• Upper floor slabs

  The decking to Section 3 – the Science, Humanities and Design & Technology Classrooms, will start at the end of June after the completion of examinations. The Section 3 slabs will then be prepared and cast during July.

• Ground floor slabs

  From June into the Summer Holidays we will be preparing ground floor slabs. This is not a particularly noisy activity with only small machines able to be used beneath the upper floors. There will be the sound of steel reinforcement mesh being laid out, but as this is not being placed on a metal deck this will not be as loud as that from the upper floors.

• Envelope

  This is the word we use to generically describe what goes on the outside of the building to protect the internal finishes from the elements. The first activities, walls to Section 1 and roofs to Section 1 and 2 are due to start Monday 21st June.

Regards,
John England – Carillion Project Manager