10 lessons learnt from raw concrete construction
Part 2 in our series on owner building the HEY House
This article was written after owner building the HEY House. We've posted it on our website because the experience was invaluable to our practice and might be useful to others someday.

HEY House was really the first time we dealt with raw construction as the finished material surface.

Going into the project, we had limited practical experience about building exposed concrete slabs and block walls, just an academic take on it.

This article is about some of the interesting things we learnt. 



Is raw concrete grey or is it green?
Experience on previous projects found finished and especially polished concrete to be green, and not grey.

Weirdly, the more it was polished, the more green it became.

When we poured the first footing on the HEY House site for a small retaining wall, once the concrete had cured it looked like concrete should - namely light grey. 

Not to be fooled with slurry (which confusingly leaves a milky grey residue on the concrete when dried) we roughly honed it using an angle grinder.

Sure enough, the concrete was that appealing light grey color we associate with and long for in modern concrete buildings.

I asked my product rep at Holcim (the supplier) why their concrete was so grey, when the concrete I had always experienced was slightly green.

The response was the quality of the portland cement, and the limited use of fly ash in the standard mix of Holcim (and probably the reason why they were more expensive). 

Some suppliers add fly ash to their concrete mixes to improve workability of plastic concrete and strength of hardened concrete. As a by-product of coal combustion power stations, the fly ash added to concrete is a supplementary cementitious material that makes it environmentally friendly, and also affects the purity of the green color.

Lesson No. 1: If you want pure grey concrete, it's a good idea to inform your concrete rep so the mix lives up to expectations.

   
(iM)PRECISE SLAB SETOUTS
We document every slab corner and rebate to the millimeter, dimensioning off constant fixed gridlines with normally two points of reference.

However, the reality onsite is concrete is imprecise, and cast in rebates are notorious for wandering 'post pour' due the sheer mass of the liquid or 'plastic' concrete.

In critical face concrete situations (like a window rebate on a polished concrete slab) the solution is to accept the fact and rely on concrete cutting. Deliberately undersize the rebate with enough tolerance to saw cut and pry off a precise edge free from chips. 

Lesson No. 2: If you want precision in concrete, saw cut it.


SOAKER HOSES ON HOT WINDY DAYS DONT MIX
Soaker hoses are fun, especially on large open areas when the day is hot. 

Unfortunately, soaker hoses can be problematic when used to help cure concrete by keeping it moist during the initial hydration cycle (and controlling the rate of moisture loss).

Whilst pouring the HEY House concrete slab, we had a network of soaker hoses setup with great coverage in addition to spray on curing compound. But alas, the wind picked up and blew the water creating dry hot spots on the slab surface. These then supposedly induced hairline cracking due to thermal differentiation or 'shock'.

No matter, the concrete polisher informed that he 'had a way' of filling any cracks using the slurry captured from the grinding process. In truth these cracks ended up looking more like veins, which actually added character to the finished slab!

Lesson No. 3: Flood concrete or cover concrete, especially on windy days.


BEWARE OF WET FORMWORK BEARERS
If someone could have said this to me prior to embarking on curing the HEY House suspended floor slab I would have considered them a saint.

Seriously, from the 'bad experience' that I had not being able to control the soaker hose on the ground slab, I over-compensated and chose to 'flood' the second floor slab for a period of 3 days and pond cure it.

The old fashioned way was appealing, and worked great as the slab had no surface cracking at all. Even when stripped, it was perfect (which was  in reality due to our awesome structural engineers Bligh Tanner).  But controlling the rate of moisture loss through flooding definitely helped things.

What I did not realise is that flooding a second storey slab means water drips everywhere, and formwork bearers have tannins. 

Streams of it saturated the formwork bearers and caused a red tanin to bleed over the sealed face blockwork walls which lead me down a long dark road to recovery (so to speak). 

Lesson No. 4: Don't flood suspended slabs when there are face concrete blocks below.


HOW TO CLEAN ORGANIC TANINS OFF CONCRETE BLOCKS
When we finished building and cleaning our first wall, I returned to site the next day to find red bat poop smeared diagonally on it.

No matter how much I scrubbed, or pressure cleaned, it would not come off.

After consulting with the supplier and perusing the shelves of Bunnings, I testing our some chemical approaches but to no avail.

The secret savior?

Nappy-San!

I found if you create a paste with nappy-san and leave it on the block wall (provided concrete block is whitish) the enzyme mixture eats away at the organic tannins and comes off with a basic pressure clean.

Lesson No. 5: There is no right and wrong when it comes to cleaning building materials, so  long as you maintain and do not damage the material finish.

  
'WET SET' STARTER BARS
Starter bars are bits of vertical reinforcement specified by the structural engineer that tie a load bearing concrete block wall into a slab.

The best way (and I thought ONLY way) is to actually set-out the bars before pouring and tie them off at intervals that align with the block core.

Our concreter preferred to place the bars when the concrete was wet, or had just started to loose its plasticity.

This approach was risky, particularly in a face concrete block scenario where there are polished concrete floors. Get it wrong and it wanders outside the line of the wall, and you have to angle grind the steel with no way of covering it.

Lesson No. 6: Beware of novel construction methods that risk material integrity. 


PROBLEMATIC CLEAN OUT BLOCKS 
Clean-out blocks are blocks that have the face cut off them to allow the debris (namely hardened globs of mortar) to be cleaned out of the wall core prior to core filling.

When you are planning to build walls over 3m, you can't just do it in a single pour for stability reasons. The wall needs to be core filled and stabilized before adding more height. This means you need multiple courses of cleanout blocks, one at the base and one mid-way.

Not a good look, especially when walls are face concrete block! The solution is to either convince your structural engineer that the debris in the core is being minimized and have him or her agree to omit the cleanout completely, or orient the cut side towards the cavity so it is not seen.

Lesson No. 7: Don't be afraid of pleading with your structural engineer if design related issues are super important. 


BLOCK WALL SETOUTS & TOLERANCES
Using fluoro paint markers on formwork is like drawing all over face concrete with a fluoro paint marker! It leaves an impression that is impossible to remove. 

Critical building dimensions like grids and corner points should be given by a surveyor with permanent 'mickey pins' on footings and slabs.

Gridlines along the edge of a block wall face are not really useful, particularly if needed after the wall is built. It's best to offset the grid from walls and give references to two gridlines for cross checking.

Block mortar heights and widths are never really 10mm. Our block layer started from day 1 with a series timber spacers that varied in thickness from 8mm to 13mm. These spacers could then be used to manipulate the wall length.


Lesson No. 8: Accurate setouts are fundamental to good architectural outcomes (but millimeter perfect is virtually impossible to achieve in reality).  


HONED VS SMOOTH CONCRETE BLOCKS
Honed blocks surfaces are created through a mechanical process that shaves off a few millimeters to reveal the aggregate in the concrete block. Sounds great and looks great, but this process changes the width of the block by up to 5mm each side (or 10mm overall).

The astute and accomplished block layer (which we fortunately had) will centralize the honed block on a two sided wall so that the inconsistency in width gets blended.

Lesson No. 9: A honed 20.01 standard block is never really 190 wide x 190 high x 190 thick. 


TEST OF CHARACTER
Detailed construction is not easy. Detailed architecture more so.

When working with and around face concrete slabs or blocks, everyone working onsite needs to be on the same page. Especially the labourer! 

If a tradesperson says that it's impossible to achieve', they are either lazy or afraid to take the risks to question the usual way of doing things. Their character is wrong for the project, and for the site.

Good architecture takes a long time to plan. It's generally expensive to create. And always requires skilled craftsmen and their teams to make the ideas come to life.

If you have a bad experience with a tradesperson, it's because they lack the understanding that they are part of something greater.   

Lesson No. 10: Walk away from conflict onsite if tradespeople are undesirable or unwilling to cooperate - life's too short to argue, find someone else even it costs the project in time and dollars.
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