Steel rebars & concrete

With the balcony formwork finished several days ago, it was finally time to install the steel reinforcing bars and pour the concrete. I've been looking forward to this for a rather unusual reason - after finishing architecture school back in 1998, I worked for a structural engineering company overseeing and inspecting high-rise concrete construction in New York City. Having spent a year and a half doing that, I was curious to apply my knowledge first-hand, because I'd never had the opportunity to do the actual hands-on work myself (the NYC unions would not have liked that)...

1998 - New York City - overseeing high-rise concrete & rebar in New York City

Anything made of concrete is always contains a combination of steel bars, know as "rebar", and concrete, all held in place by the formwork. Concrete is actually made up of cement, sand, aggregate (or small stones) and water - these can be mixed in various proportions to achieve different strengths.

June 29th - the empty formwork

After the forwork is made, the rebars are installed - they provide critical tensile strength to hardend concrete which is very strong in compression, but not in tension. The rebar adds tensile and torsional strength while preventing cracking, making the concrete extremely strong and durable. It's probably safe to say that my rebar was overkill for our needs, but it was done correctly! Besides providing strength to the concrete, the rebar also needed to help anchor the balcony railing posts in place. With the help of our Building Control officer, Ron, we developed a great detail for the post anchors which had the rebars woven in-between. When our neighbor and local handyman stopped by, he was shocked by the amount of steel and detail in the rebar - I took that as a compliment...

Adding the bottom row of rebar

When it came time to pour the concrete several days later, I have to admit I was not looking forward to mixing it - particularly after my experience making mortar for the ridge tiles. Sure enough, it was a lot of work and quite exhausting. A concrete mixer would have made the whole thing take an hour rather than a day, but for such a small area, it was probably not worth the trouble to rent it, transport it to the site, and then carry it upstairs.

I only encountered one problem, but it turned out to be a major one - I ran out of sand and cement halfway through the pour! Looking back, it was quite funny - a classic DIY mistake of not calculating quantities - I had eyeballed the bags of sand & cement and figured it would be enough. Fortunately I was able to jump into a mini-cab and quickly buy the needed materials just a few minutes before the supply shop closed...otherwise it wouldn't have been so funny...

Pouring the concrete for our little balcony in London may not have had the scale and glamor of high-rise construction in New York, but it somehow felt like coming full circle...

The first railing post installed with the anchoring system in place

Another railing post installed

The top layer of rebar installed

July 1st - Stuffing wet newspaper into the gap between the brick and the formwork - a trick of the trade learned from my neighbor

Final alignment and leveling of the railing posts before pouring concrete

Mixing the first batch of concrete

After "pouring" the first bucket of concrete

Filling up the remaining area with concrete - the dark wood on the left is intended to remain exposed and act as a window ledge and as a stop for the concrete. Because it's higher than the lip of form-work, the resulting slope will allow water to run down and away from the window, while preventing any pooling of water.

Several buckets later after flattening and shaping the concrete to the final slope

Using a wood stud to flatten and shape the wet concrete

Halfway finished, but running out of materials! Decided at this point to run down to the supply house (before they closed) to pick up several bags of sand & concrete.

Resuming work - this is where the future balcony doors will be

Pouring the last part of the balcony

The finished balcony...just don't step on it yet!

Balcony Formwork

With the ridge tiles done, the roof is basically finished - so it's nice to finally start doing something else. The next area to focus on is finishing the dormer - but before working on the carpentry and painting which will be quite involved, I figure it makes to start at the bottom. The bottom of the dormer will be a narrow "Juliet" balcony which will sit in front of a very large series of windows that will open (on one side) like a set of French doors. The idea is that one day, we'll be able to open these doors and step out onto the balcony to enjoy a view if the beautiful neighboring gardens and trees below.

The Dormer -afternoon of June 22nd

The balcony will be made of concrete so that it's strong enough to allow the balcony to extend beyond the face of the brick wall, and also to help hold together all the loose bricks that remain exposed. The first step in working with concrete will be to make the wooden form-work that will hold the wet concrete in place before it dries.

Before starting the form-work, the first task was to remove some obstructions that were in the way - the ends of the vent stack for the plumbing, and the downspout for the old gutters. These were quickly cut away, revealing that some of the brickwork had been removed, leaving a gap. I soon realized that before moving forward, this gap had to be repaired before the form-work could go up - whoever installed these pipes really butchered the wall and the brick. Because it was so bad, the rest of the day was taken up by repairing and filling in this gap. It was only on the following day that I was able to do the actual form-work, and that ended-up taking the whole day...but at least it was finished. Another item to check off the list!

The obstructing pipes before being cut away

After the pipes were cut, the gap and damage to the brick (from when the pipes were originally installed) was revealed

To fully repair som eof the decorative brickwork would have involved removing most of the surrounding bricks, and possibly causing more damage. To avoid this, I decided to span the gap with a plate of unused steel from the structural work. The plate was held in place by mortar.

Brick were then laid over the steel plate and the gap was slowly filled.

The completed repair.

June 23rd - the form-work in place.

This space is where the concrete will go. The left side is the form-work, the right side is the wood "wall plate" that used to hold the original roof rafters.

The completed form-work