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

Just in case you think the roof is finished...

...it's not.

One might imagine that now with the slates done, the roof is finished...

...but unfortunately that's hardly the case.

There's still a long list of things that needs to be done before the roof work is officially finished, the scaffolding can come down, and I can go back to New York:

• The Ridge tiles need to be installed along the sloping edge of each roof plane. These tiles are triangular shaped and cover the gaps where the roof planes meet (at one of the dreaded corners) with a bed or mortar. The tiles are placed on the mortar which holds everything together and seals all the openings. (You can see one of these ridge tiles sitting on the edge in the picture above.)
• The eaves, which are plywood, need to be be protected from the rain so they don't rot away. They can either painted (which would mean they would have to be re-painted every few years), or - preferably - covered by a waterproof sheet (so they won't fade or need repainting).
• The gutters need to replaced because we took the old ones out in order to remove the roof joists back in January as part of the structural work. Besides being in the way, the old gutters were metal and rusting away - it was only a matter of time before they started falling off...
• There's a small area of re-pointing to be completed in the brick party wall over the flat roof.
• The flashing needs to be completed along the brick wall - primarily where the flat roof is.
• Once the flashing is completely finished, I need to apply a special waterproof caulk in the open joints where the lead is inserted into the brick. The caulk is formulated to bond between the brick and lead, but allows each to expand and contract properly - this avoids the problems associated with water getting into cracked mortar, which is the traditional way to seal the joint.
• Our neighbors' TV antennas, which are cluttered along the face of the dormer, will need to be relocated.
• And now on to the dormer - the first element of the dormer that needs to be done is all of the final exterior trim-work and facing. The plan is to do this with wood and then paint it (beacuse it will be easily accessible for repainting)
• Once the dormer is trimmed out, the wood needs to be treated for exposure to the rain & damp weather with a protective coating for the wood. Once that's dry (2 days later), all joists and screw holes need to be patched with an exterior filler. After drying and sanding, the wood is ready to be primed, and then finally painted with 3 coats of black exterior paint.
• Once the dormer is painted and finished, the final flashing/cap can be installed over the top edge. this will prevent water from coming in along the edge of the dormer where the slates stop. The trick with this is that the detail needs to span both types of roofing: traditional slates and modern membranes, yet look the same for visual consistency.
• The vents for the boiler, kitchen exhaust and bathroom exhaust need to be cut through the brick wall in the rear and the vents need to be installed. This has to be done before the scaffolding comes down because otherwise there is no way to install the vents and patch the holes. The plumbers will do the boiler vents and I'll take care of the others.
• The hole from the old boiler vent needs to be patched (once the plumbers take it down) - otherwise we'll have a 4" hole on the side of the building!
• The windows need to scraped and painted (not replaced) because in a few years we would like to replace the windows with efficient double-glazed windows. 
• Paint the exterior stone sills and other decorative stone elements that frame the windows. These are probably the most distinctive features of our building's facade.
• The floor of the Juliette balcony needs to be poured in concrete. To do this, the wood form-work must be made, followed by the placement of steel reinforcing bars, followed by the brackets that hold the railing posts. Once all of that is done...then you can pour the concrete...
• Once the balcony is poured and dry, the balcony railing can be installed.
• Finally...the scaffolding needs to be cleared of all the debris that's piled up since November, and then it can be taken down by the scaffolding company!

A Concrete Ring

With the main steel beam in place, we can start reinforcing the top of the brick wall with the concrete ring beam that our structural engineer is calling for.

The main reason for this concrete beam is to help stiffen the top of the brick wall from the lateral roof forces that want to push those walls out (typical in all pitched roofs). Because I want to open up the space, we've removed all the ceiling joists which used to brace the walls against the push of the roof. The other reason for doing this ring beam is help consolidate the top courses of brick together. Because the roof has been in such a state of disrepair, the top courses of brick are barely holding together due to water damage, so i would like to tie them all together.

A concrete beam is made up of concrete (sand, cement and water) and steel reinforcing bars - know as rebar. Several days ago I ordered the rebar along with rebar straps (know as links here) that had to be custom bent. The straps tie together the long steel bars and help provide additional strength to the final beam.

Before any steel can be put in and concrete poured, the guys had to make space for the beam - they did this by removing and cutting out bricks that were in the way. Once that was done, they installed the rebar and then put up the plywood formwork which helps keep the liquid concrete in place before it dries. Once it dries, the plywood is removed and you have a concrete beam!

It may sound easy, but this process took much longer than expected and our Polish crew was rather unhappy that it took them over a week to finish the ring beam.

Delivery of the steel rebar

The bricks removed and cut at the top of the brick wall.

The steel rebar links

The assembled rebar "cage"

The finished concrete beam

The concrete ring beam with the steel beam in the foreground