self build waterproof basement formwork membrane concrete

Maximising Thermal Mass

Either bare concrete walls, bare brickwork, bare stone, plastered or tiled.

If you are tempted by the recent series "The World's Most Extraordinary Homes"; or the recent publicity following the refurbishment of the brutalist South Bank Centre in London; or my argument you should maximise thermal mass, then you might deliberately have concrete walls inside and your insulation envelope outside.

You then have the choice between plastering and tiling your concrete, covering it in brick or stone or leaving it deliberately bare.
  self build basement

Smooth, rough, timber grained, homely or industrial?

Another example of the growing popularity of bare concrete appeared at the end of 2018 with this article in Dezeen, the design magazine:

What this picture introduces, that isn't in any of the other images that follow, is the idea that a frame beneath the ceiling can be made to house services without covering it over so that the room still benefits from the ceiling's thermal mass.

Is cement as bad for the environment as some claim?

I heard a very interesting "fact" on an environmental report on the BBC News channel just before Christmas 2019.

"cement is responsible for 3 times as much greenhouse gas as flying."

That might lead some in the UK to say we must use less cement. But the whole picture is very different.

flying cement

On the chart above, Chinese and Indian citizens fly the fewest miles per capita. But on the chart below, China and India produce, and use, most of the world's cement.

cement by country

In the UK we already restrict our use of cement to what is fairly beneficial whereas we should probably virtually stop flying completely, if we want to stop global warming.

(This was written before Covid 19)
  unfinished interior walls

The point of thermal mass is that it softens the impact of extremely cold weather and it softens the impact of completely hot weather. Cement used for thermal mass can save many times as much energy as was used to produce it if it reduces our need for heating and cooling year after year after year.

concrete   brick   stone
thermal mass concrete   thermal mass brick   thermal mass stone

Timber and insulation don't let heat penetrate so they store very little.

Concrete blocks are less dense so they store less than the 3 materials above.

Steel allows a lot of heat to penetrate but the heat moves along and is lost elsewhere. Steel doesn't store heat.

How do you stick plaster or tiles to bare, structural concrete?

Good structural concrete and better still waterproof concrete can all be too dense to provide a key for any plaster or paint.

If you know that you will want a good key later, the most thorough way is to remove the cement surface from the concrete and expose aggregate.

Removing the cement surface mechanically could take months

For entirely different reasons, this contractor daubed a thick coat of retarding chemical on his stop-end formwork and jetwashed it clean after the formwork was removed. This is quick, simple and safe.

Clearly, plaster or tiles would bond strongly to a surface like this.

how to plaster bare concrete use retarder and jet wash.

Over the years only a couple of my clients buried their electrics in the concrete. Look carefully at these images for any sign of electrics, as well as making your mind up if you like the idea.

These first images are all of the refurbished South Bank Centre including a few ideas exhibited inside by school children.

I have deliberately tried to pick out blemishes. Concreting is hard work so imperfections are to be expected. However, bare steel reinforcement should be covered over. Here you see holes that housed the threaded bar during the pour. Some filled in, some not. Or perhaps some of the hole repairs were so poor that they came out with jet washing during the refurbishment. Note of caution: don't leave these holes in waterproof concrete walls in case some of your repairs fail to be waterproof, use my FRP threaded rods to avoid the risk and avoid the cost of filling in the holes.

South Bank Centre refurbished brutalism

Here I think you see ends of bare steel that were inside the formwork to control the concrete width.

brutalism bare steel

This is the front that faces the Thames.

brutalism front elizebeth hall

This detail of the front shows walls that are actually beams up in the air. Notice how mucky the formwork must have been inside. The underneath needed patching up.

brutalist southbank centre

Waffles give the engineer depth to make the roof strong whilst reducing the weight in the roof.

Years ago, when I was the site engineer on a multi storey car park, we hired in plastic waffles to create the voids. But years later producing something similar in a basement ceiling over a swimming pool I bought polystyrene void formers.

brutalism waffles

Inside the Centre now.

I am attracted by the variety of finishes available with concrete. I would like this as one wall in a room with completely different finishes on other walls.

brutalism inside

Little differences in the thickness of the boards remain in the finish.

brutalism small differences in formwork boards

As do bigger differences in the boards.

Lighting can be used to highlight the details.

brutalism bigger differences in formwork boards

I was struck by how similar in colour, and therefore boring, so much concrete was.

LED lighting might be used to add colour to it.

brutalist bare concrete lighting

You can look at some different approaches to using bare concrete on this Concrete Centre web page.

And here at the Nobu Shoreditch they might have used precast panels and fixed them to the walls.

concrete wall panels

Just in case you were wondering exactly what is brutalism?

Ashmole Primary School Brutalism

The year 5 schoolchildren came up with these ideas as well.

Ashmole Primary School ideas 1



Ashmole Primary School ideas 2

The series on the BBC comprised of 4 episodes and they studied about 3 homes in each. Of the 12 or so, about half had bare concrete inside.

None of them, as far as I noticed, had any of the rusty metal popular on Grand Designs in recent years.

Quite a few of the owners were architects.

One of them said 'perfect architecture becomes very boring, accept mistakes'.

I'm not sure I would accept the degree of mistakes I have seen sometimes.

By complete coincidence, a self-builder who used my rods and waterproofer a few years ago was also an architect.

He sent me a few photos showing how he got on, which was very well.

This one is the outside of his basement before he backfilled.

It now seems a very good example of what could be achieved deliberately.
  wood grain in concrete

Here are 30 or so images from the series and at the end a few photos selected from my work - most were never expected to be seen bare, but it shows you what might be normal using timber boards for shuttering. A couple of other photos were experiments.

I like the idea of bare concrete but I would want more variety, more timber grain and better workmanship so that there were fewer voids and less distinct lines where snots of concrete were bashed off.

At the very end I have added a couple of photos showing instances where the formwork wasn't quite right and no one did anything about it before the concrete was poured. My formwork method using timber makes it much easier not to make these mistakes.

bare concrete walls

bare concrete walls

bare concrete walls

At this junction of 4 panels we seem to have everything going on. Perhaps mastic was used between panels and the concrete later patched up.

bare concrete walls

This is a shot of the panel system. Note between the top horizontal panels and the vertical panels beneath the round waler plates on the threaded rod that goes through the wall.

bare concrete walls

bare concrete walls

This looks like they used timber shuttering with the boards in 2 directions.

bare concrete walls

In this photo you can see how they chose to fill in the holes where the threaded rods had gone through.

bare concrete walls

An electric light.

bare concrete walls

And a socket. I don't know why there is a different surface either side of where some panels were joined together. Perhaps the timber face was damaged along the edge and patched up before this wall was formed.

Or the panels might have bent back and they had to chisel concrete off where it was proud then made good, and this might be the repair we see.

bare concrete walls

bare concrete walls

Timber around the window. Perhaps that covers external insulation.

bare concrete walls

I think these vertical lines have to be something to do with spray foam or mastic between the formwork panels.

bare concrete walls

Just a bit of theory about the durability of concrete surfaces.

Concreting gangs on big sites realised long ago that if they stick the vibrating poker down between the steel and the formwork the surface would look better with fewer air holes. This is because the vibrator makes tiny pockets of air join up into bigger bubbles that float to the surface and escape - except that where bubbles come into contact with a solid formwork face the bubbles stick. They don't float. They don't escape.

Unfortunately, if the gang don't always keep the poker moving, they could vibrate the timber surface too much and that vibration can cause bleeding. That means that the vibrating formwork knocks the solid stones away, then the solid sand, then the solid grains of cement and the concrete surface against the formwork is very watery and won't get much strength. It would wear in places, such as around a light switch.

  1. It was found, years ago, that a vegetable oil release agent on the shuttering created more air bubbles in the surface of the concrete than other types, presumably those that smell of diesel.

  2. In Japan they had a building boom throughout the 90s and by the noughties they were having to fix problems.

    The academic conclusion was that no-one had liked using the vibrator so compaction was poor.

    The solution they proposed was that in future a self-compacting concrete (SCC) should be used.

    The issues with SCC would be:

    • Additional cost and complexity.

    • Grout loss through any voids in the formwork - which could prevent waterproof concrete being waterproof, as well as compromise strength if grout loss was severe.

    Self-compacting concrete, to be successful, needs

    1. A concrete producer whose staff at the batching plant understand what they are doing and using. They need specific training and supervision.

    2. A supervisor on site ensuring voids in formwork are minimal and very thin indeed.

    3. A supervisor on site to ensure that the concrete is pumped properly. Self-compacting concrete is not self-levelling. A supervisor is needed to make sure the workforce don't just let the concrete flow around the walls on its own, because the grout will have flowed and the large stones will have stayed put.

    4. A supervisor is needed to make sure that the concrete is delivered to the bottom of the wall properly. Segregated self-compacting concrete is still segregated.

    5. A supervisor is needed to make sure that there is still some vibrating to help all the air escape without causing segregation or bleeding.

maximising thermal mass

In my opinion, timber boards instead of panels are far less industrial looking and the grain far more interesting.

maximising thermal mass

Deliberate use of boards of slightly different thickness.

maximising thermal mass

maximising thermal mass

You can get variety as well from using a mix of planed and sawn timber.

maximising thermal mass

They didn't fill the gaps between the boards behind the sink with mastic or foam, neither did they knock all the snots off, creating variety with the large wall.

Whereas, if you still have the Concrete Society page still open in another tab, I think they have ground and filled some of the joints between formwork to get such a good finish.

bare concrete walls

bare concrete walls

bare concrete walls

This ear stuck on a door made me realise they missed a trick.

Any timber board in the formwork could have anything carved into it. Where this wooden chopping board is carved out, the concrete would be proud. Names, pictures, puzzles, games ..... ears.

So many things would be possible.

bare concrete walls
  bare concrete walls

Is the wall at the back concrete or timber? It could be concrete.

bare concrete walls

Timber and concrete side by side.

bare concrete walls

Another way of dealing with the holes left by steel threaded bars.

bare concrete walls

bare concrete walls

Looks like the vibrator did not compact the concrete in the corner. Interesting they did not bag it up, which means filling it in by rubbing in with a semi dry cement and silver sand mix.

bare concrete walls

bare concrete walls

bare concrete walls

bare concrete walls

Timber around the window opening again.

bare concrete walls

bare concrete walls

bare concrete walls

This blemish is where they emptied one lorry load of concrete and compacted the concrete. But the last of that load remains in the concrete pump tubes until a new load arrives, is discharged and pushes that older concrete out.

In this case, the next load was delayed so the concrete in the pipe began to set. When it came out it wouldn't compact properly.

The gang should have expected this and after the new concrete was coming through they should have come back with the pump, put some runny concrete on top of the old and worked it thoroughly with the poker to mix the two together and avoid this voiding.

Sometimes the voiding is a cone shape. That's when the pump pipe was above the concrete already in and dribbling slowly while waiting for the next load. That dribbling wasn't compacted.

bare concrete walls

They avoided continuous vertical joints in this wall.

bare concrete walls

April 2018. Examples of concrete are popping up everywhere. This next photo is a link to a page with many more photos of this very thin concrete house overlooking the Pacific Ocean.


The following are my photos.

These boards were scaffold board timber without steel ends. A variety of snots, good grain and knots.

Notice the fibreglass threaded rods that I cast in to avoid leaving holes in otherwise waterproof concrete. Near top left you can see one rod has been cut off flush and you can see the thin nut that kept the forms apart.

bare concrete walls

These walls were formed with regularised timber which was all saved to use inside the new house being built. You can see where the line of fibreglass rods was cut off. Regularised timber has rounded edges, so the wall has proud lines of concrete too substantial to knock off completely.

At the top of the page you saw the effect retarder had on the concrete surface and that it was easy to remove the surface with a jetwasher. This had made me wonder since whether reaching in and painting retarder in the gap between regularised boards might make these snots easier to remove?

If you are going to clear the surface of cement in order to plaster or tile, it would be quite easy to paint retarder on one side of every board before you fixed it in place. You might partially fill the joints with retarder to stop as much concrete filling the space as well as making what sticks out afterwards easy to wash off.

bare concrete wall regularised timber

This is a mock up I made to explore different thicknesses of timber used together - in part, to make a benefit of regularised timber with rounded edges.

maximising thermal mass different thicknesses of timber wall formwork

But before I filled it the client chose rough sawn timber instead.

Rough sawn turned out to be quite hard to find in a small quantity. These timbers were stocked as barge board for fencing and the timber was quite thin. I doubled up the rods and strongbacks. But if there is a next time I could do better.

maximising thermal mass rough sawn carcass timber wall formwork.

Here you see the thin nuts doing their job.

This timber is regularised and treated 6x2. Regularised timber is often the cheapest, but the rounded edges leave a proud line of concrete.

bare concrete walls

If you use planed timber it costs a lot more money and if you are fussy about the finish you might not want to use planed timber twice.

18mm plywood bends between the 4x2s backing it up and where the ply is nailed to the 4x2s you will see the nail heads in the concrete.

The cheapest planed timber might be floor boards. But floor boards need to be supported underneath every 400mm. If you have 4x2s vertically behind floor boards then you need rods every 400mm as well.

If it were for me and I wanted a timber grain, I think I would use scaffold board timber. 38mm thick only needs the rods and strongback timbers every metre.

You could buy scaffold boards then get them planed through a thicknesser.

Another advantage of a thicker board, compared to 18mm ply, is that you can fix through the brace into the board from the back. This makes striking the formwork much easier. It also means no marks in the concrete from screw heads.

Very few signs of electrics in all these bare concrete walls. I have another page about electrics.

I always use Turbo Coach Screws from Screwfix. M6 dia. 8mm hex head. They drive in and out easily with the right cordless tools and with a 70mm long screw it goes through the 47mm upright and 23mm into the 38mm thick scaffold board without leaving a mark the other side.

bare concrete walls

The joint between the scaffold board needs to be patched over with plywood.

But if all your joints were haphazard then the board above and below a joint would be strong enough to stop the wall length spreading without a patch. But the board ends might bend in under the weight of concrete. That might be a reason for using 6x2s instead.

You should experiment with a sample wall of sufficient height to have the same concrete pressure your actual walls will endure.

I took this photo years ago. The carpenters that formed this wall with plywood did not clean the bottom first so water leaked through sand under the concrete and the rebates were supposed to line up and be continuous, but they aren't continuous and one section of wall is stepped back from the other.

plywood formwork not lined up

This is a pub beside the Thames in Fulham. I doubt it was originally intended that this soffit would be on view. The carpenters would have noticed one sheet of ply caught on another, but they couldn't be bothered to put it right.

plywood formwork not lined up

This slightly odd ceiling effect was because they used polythene sheet over laid-flat scaffold boards instead of oiling the timber. It was an experiment where the concrete would be covered over. Are the creases in the polythene a problem or decorative? They told me the polythene got trapped along some edges and needed cutting off leaving a thin, black line in corners.

If the formwork was covered in overlapping but smaller pieces of polythene, maybe the creases would be prevented and maybe the feint outline of the overlapping shapes would add intrigue?

polythene ceiling waterproof concrete basement

This next peculiar finish was the inside of a garage, so the concrete will remain visible.

The client chose to line the inside of his wall formwork with hardboard to create a very smooth finish.

Perhaps the white material is his release oil and it will clean off. Some of the hardboard got loose when the concrete went in and ended up buried in the wall concrete.

It seems to me a very difficult way to make walls different - but hardboard could work very well on a ceiling instead of polythene. You might get an almost reflective surface.

hardboard walls waterproof concrete basement

I am enthusiastic about maximising thermal mass by having continuous masonry inside all your accommodation with a continuous envelope of insulation outside it. This page tries to give you a lot of ideas how to live with internal masonry.

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