Wednesday 25 November 2015

Bending steel

After some spraying of blue marker paint on the ground and lots of measuring, we've specified the steel reinforcing bar for the concrete beam to support the gate posts of the field gate. In an earlier post on here we mentioned the reasoning for constructing a concrete beam. Now we've taken a view on the likely forces that will be imposed on the concrete. We've also considered how the posts will be replaced in future if significant repairs are required.

Some research on YouTube has made us instant experts on how to set out rebar (Steel reinforcing bar) and to create shuttering to hold the concrete used to form the beam. We've had to understand the British Standards for common rebar shapes used in the construction industry. This allowed us to produce specifications for the amount and shape of rebar required for the concrete beam to support the orchard field gate. It is a cantilevered design to prevent the gates from sagging in use. We requested a quotation from a local steel supply firm Twiggs for rebar bent to our custom requirements. The price was so reasonable I actually rechecked they'd included the cost of bending within their price. We've placed the order and await delivery.

The beam will be approximately 200 cm long by 90 cm wide with a depth of approximately 40 cm. It will be capable of withstanding the weight of heavy agricultural machinery which might pass through the gateway. It will be buried from view once the posts are installed.

The reinforced beam will provide reinforced sockets for the gate post (200 mm diameter) and a fence post (100mm). When we're casting the beam we'll mount the gate and fence post in position in the beam. We'll pre-wrap the end of the posts in plastic bubble wrap so we can lift them out once the concrete has hardened. The sockets are also designed to prevent the accumulation of water under the base of the posts. After removing the bubble wrap we'll re-install the posts in the beam using sand to pack the gap between the beam sockets and the posts. This installation method allows the post to be securely mounted, free from rocking/tilt, but allowing for easy removal for future maintenance. If you just mount the posts directly in the concrete when it is poured subsequent maintenance of post removal is difficult.

We are also practising the art of wire tying to fix the rebar in place before pouring concrete. Youtube helps again.  All that remains now is to trick the volunteer team into agreeing to, dig the trench and to mix the concrete when the management team are ready to construct the beam. We'll be incorporating reinforcement fibre into the concrete mix to provide extended life.

Little did we realise when we started the orchard project that we'd become involved in civil engineering and reinforced concrete design.

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