Following the growth spurt in the week before last we finally received the much needed HB20 blocks last week. The HB20s are 200mm wide blocks comprising a concrete core of 140mm with 2 x 30mm woodcrete webs and do not contain any insulation as they are used for internally.
These blocks have been used to form the structural walls inside the building which form the core of the structure; they run from the basement hall area all the way to the roof and form a key part of the structure. Incidentally, this mass of concrete (about one cubic metre of concrete per 10m2 of wall) provides a significant thermal mass inside the building. Concrete warms up and cools down very slowly; it therefore retains heat for a long time and helps to keep the temperature within the building at a constant level.
The beginning of last week was spent building the HB20 walls along with checking and bracing all of the walls to ensure that they were all ready for the concrete. The lift height is 10 courses above the DPC which equates to 2.5m, taking us almost to the Finished Floor Level (FFL) for the top floor. Now that we can see the level of this floor we can appreciate the outstanding view that the open plan top floor will give us over the Thames Valley.
Everything was booked for Wednesday 27 October at 1pm: a concrete pump followed by 2 ready mix trucks. Midday came along with the first call from the pump company to say there would be a “short delay“. A further call at about 1pm said it would now be 2.30pm. There was no sign of any pump and, on chasing, it was now forecast to arrive at 4.30pm. However, it takes 2-3 hours to pour the walls so there would have been insufficient time to pour in daylight. The pump company will remain nameless but suffice to say that they will not be used again on this project. However, trying to get both a pump and concrete booked in on the same day is challenging in itself at the moment, with projects such as HS2 being cited for soaking up all available concrete locally.
Monday 1 November dawned bright and breezy after a very wet and windy weekend. At 0745hrs, a Camfaud concrete pump arrived on site and was positioned by Dave from Insul Hub. What a difference: the pump was booked for 0900hrs and arrived over an hour early! The first ready mix truck (7.5m3) arrived from Hanson at 1015 and was carefully unloaded into the walls by 1115. This is a trickier than it looks as the concrete has to be worked into all the corners and under the window frames so that there are no voids in the mix. For this reason, a relatively fluid mix is used. The second load of 6.5m3 arrived at 1145hrs and was slightly smaller to ensure that all the walls finished below the level of the blocks; this helps to tie in the next pour above and remove any potential weak spots in the structure.
Thank you for Dave, Matti and Martin of Insul Hub who worked tirelessly to carry out the pour. Next up, the steels are needed to place on the newly poured walls once they have cured sufficiently.