Pdx 42" update

[Macrinehart]

Ready to pour the hearth! Unfortunately I'm a day behind so I may have to wait a week for Jesus, Constantino and Machete to be ready.

I setup my fullsize template and it looks good. The front of the oven opening lines up with the front of the yardstick in the image below.

[Macrinehart]

It's a beautiful day, and I was impatient so I did the hearth by myself. I finished in about 4 hours, hopefully I didn't mess it up. Little concerned to be honest, the hearth set up a lot faster than the pad, so I felt like it was a bit of a race to get all the concrete in while I could still work it.

The top seems a little sandy, this doesn't feel right at the gut level but maybe it's OK? You guys tell me if I just made a big mess.... Here are some pics.

[MarkJerling]

Looks good. Once it's set, keep it wet for a few days by spraying it down, but don't let it freeze. Even if the top is a little sandy that's not to say it won't be strong enough. After about 2 days, spray it down with a hose and quite high pressure. If the water does not dig holes, it will be fine. The water should blow off any loose sandy material.

[Gulf]

It does look a little rough. But, the hearth slab does not have to be a "burnt finish". It just needs to be level.

[Macrinehart]

Even if the top is a little sandy that's not to say it won't be strong enough. After about 2 days, spray it down with a hose and quite high pressure. If the water does not dig holes, it will be fine.

I suppose time will tell. So now that I've cleaned up and got a glass of wine, the narrative version of today. Up at 6:40 AM - thank you cats - I cooked up the elk breakfast sausage, downed four patties with coffee and toast and hit the road to home depot for tool rental and concrete. (Every big workday should start with a hearty breakfast!)

Rented a concrete mixer, float, and the truck, and baught 25 bags of concrete plus flow guard. Asked for help loading the concrete and then proceeded to Complete the full load without help, 1500 lbs, round 1.

At home preping the job site included sifting 25 bag of concrete to the back yard...this time I used a cart, but thats still counts for two sets of 25 reps with 60 lbs bags. Getting a good workout! Then there was some running around for missing supplies and after my guilty pleasure of the 2 cheeseburger meal with a coke at McDs, I was ready to start mixing at Noon.

So I read that safety thread, and for gear I had a pair of rubberized, full arm chemical gloves (they were cheap, so why not?) Face shield and respirator. I was not interested in breathing concrete dust today. But after 10 bags I could barely see through the face shield so that came off.

It took a bit to get my rhythm, but once I got going the sun came out, it got up to about 55F, and it was a good day for back breaking outdoor labor. I got another rep of lifting the 60 lb bags into the concrete mixer, and then there were another set of lifting the 5 gallon bucket of concrete up to stand, those were squats, and considerably heavier than 60 lbs!

So four hours in, I still have a functional back and got a solid workout. About half way through I was not liking the texture of the poured concrete. But what can you do? I just decided to try and keep working it so it didn't set up too much.

Thanks for the tip Mark! I setup a tent over the concrete with some stakes, plumb line and a tarp. Then I cleaned our the space under the stand and setup a space heater there. The whole thing is covered by my canopy and some tarp walls. Hopefully that's enough to keep it above freezing. Supposed to be getting down close the freezing at night for the next couple of days. We'll see.

[Macrinehart]

So we hit a low of 32 last night but with the space heater going and build covered it was very comfortable warm when I check this AM. I fixed up my tenting rush job from last night and gave the hearth slab a drink with the sprinkler set on shower.

It does look a little rough. But, the hearth slab does not have to be a "burnt finish". It just needs to be level.

There is an ever so slight slope to the front. I notice that water pools up on the slab but over a few minutes it gradually drains off the front. This is not unexpected. When I checked my level the last time the bubble was between the lines but touching one side indicating this slope. I decided further tweeking of the stand wasn't necessary.

Yesterday I tried to put a bull nose edge on after taking that picture, but mostly just made a mess that I'm sure will have to touch up when the forms come off.

[NCMan]

Contrary to common myth, spraying a cement product down w/water to keep it wet is not good for it, despite what some may have read or believe. It actually weakens the outer surface of the product and can cause spalling, etc. The best method is to retain the existing moisture, slow down the drying process and not add any additional water to it. It's tough to compete w/all the internet myths out there (there are so many), so people believe it. It's just not true. Having said that, as Gulf said, it'll be ok in the end. Not pretty, but it will probably be structurally sound enough to build on.

[MarkJerling]

I'd be interested to see some research. I've not seen any literature as to disadvantages of wet curing concrete.

See for instance: Microsoft Word - Articles1. English.doc (irbnet.de)

And: Role of Concrete Curing (cement.org)

And: Curing Concrete, Chapter 12 (memphis.edu)

As I work in the construction field, I'd be most interested to learn of studies showing the opposite result.

[NCMan]

I have to stress again the importance of keeping the existing moisture in concrete/cement, as opposed to continually spraying it. It's the wetting/drying process that causes weakening of the surface. Ideally, a slab should be covered w/a plastic membrane or a cure/seal product to retain existing moisture, not adding water later, off and on. I also have worked in the construction field for over 45 years (now retired). I've worked on many large projects where the engineers were literally standing over you, telling you exactly what is allowed (and part of the job Specs) and what is not. Never once in all that time, would even one of them allow wetting of concrete. I get what people are trying to do, by retaining the moisture, though. When I poured my slabs for my oven, firstly, I added as little water as possible, but enough to be able to work it. Secondly, when done placing it and it had set enough, I covered it w/plastic. Next, I was sure to keep it from drying too fast, so I placed cardboard over the plastic to keep the sun out. I also left my forms on for as long as was possible. I left it that way for over a week. The slabs stayed "green" for a very long time, as they were allowed to cure as they should. Ideally, I'd say the absolute best way to cure a slab is to flood it w/water (once it is able to be) and keep it that way, fully immersed, but that is not feasible in most cases. On a somewhat related note, this is why when people call it "curing" an oven when starting a fire, it is the absolute exact opposite of actual curing. No one ever cures any cement product by starting a fire to dry it out. Calling it a "drying fire" is more accurate, but it is in no way "curing". Not by any stretch of the imagination. It's what I call another "internet myth". There is no curing going on w/a fire. None.

https://www.cement.org/learn/concret...n-construction

[david s]

I heartily agree with you NC. For years I have preferred to refer to them as drying fires rather than curing fires. Calling them curing fires is confusing. As there is no chemical action taking place when driving off the free water in concrete at the temperatures to which we fire. Regarding damp curing, we do precast concrete and after demoulding cover the casting in wet hessian and seal it up in a plastic bag for at least a week. As you say, it is the constant retaining of moisture that’s important rather than a wet/dry environment.

[NCMan]

I heartily agree with you NC. For years I have preferred to refer to them as drying fires rather than curing fires. Calling them curing fires is confusing. As there is no chemical action taking place when driving off the free water in concrete at the temperatures to which we fire. Regarding damp curing, we do precast concrete and after demoulding cover the casting in wet hessian and seal it up in a plastic bag for at least a week. As you say, it is the constant retaining of moisture that’s important rather than a wet/dry environment.

Yes, David. I am glad to find someone w/similar real world experience to help explain this. There is just so much misinformation out there, that sometimes it takes someone to speak up.

[david s]

So we hit a low of 32 last night but with the space heater going and build covered it was very comfortable warm when I check this AM. I fixed up my tenting rush job from last night and gave the hearth slab a drink with the sprinkler set on shower.



There is an ever so slight slope to the front. I notice that water pools up on the slab but over a few minutes it gradually drains off the front. This is not unexpected. When I checked my level the last time the bubble was between the lines but touching one side indicating this slope. I decided further tweeking of the stand wasn't necessary.

Yesterday I tried to put a bull nose edge on after taking that picture, but mostly just made a mess that I'm sure will have to touch up when the forms come off.

We are currently having a heat wave here and reading your report of an overnight minimum of 32 made me laugh. Our overnight minimum was 30 two nights ago. But that was Celcius not Fahrenheit. 30 C is 85F and when accompanied with extreme humidity is quite uncomfortable. Needless to say we have our aircons running 24/7, Mixing heavy materials in a barrow in the middle (or ar any time) of the day in these conditions is not much fun.

[Macrinehart]

I am glad to see the discussion on best methods for cutting concrete. I stopped wetting my slab after NC's comments, but also concerned about freezing per Mark's comments so kept the space heater going under the hearth since we are experiencing freezing Temps 3 nights this week. Also setup the canopy again and used large tarps to build a tent structure all around for protection from rain, wind (thankfully not much to report) and temp variation. I just checked today and my temp in the tent with space heater going is 60F (15.5C). Outside it's 47F and raining.

I do not have plastic directly over the slab, but I could add that. The tend structure seems to be retaining humidity. My inspection today I can see there are areas of the slab that have firmed up on the surface, but others other like they have a thin sandy layer on top of solid surface. I plan on leaving in the forms for 2 weeks. Possibly longer if I'm not ready to move on by then.

I did get a measure water to concrete ratio as well, and found that I couldn't work with the ratio of 2 liters/60 lb bag. After some experimentation I landed on the ratio of 3.5 liters/60 lb bag. My method was add 4 liters to the mixer, then a 60 lb bag. Then mix about 5 minutes before adding 3 liters and another 60 lb bag. My observation was that if I did <4 liters on the first bag the concrete would cake to the bottom of the mixer and I'd end up with wet concrete spinning around on top of 2 inches for cake-like material. A few times I scraped that material off with a shovel, but found that process was too slow and also just left me needing to add more water to get a workable texture.



Forgive the mess - I finished this tent at dark on Sunday and rushed inside. Actually covering a muddy area with those concrete bags so I don't track a mess all through the house.

[Giovanni Rossi]

I’ve been reading the concrete drying discussion with interest. We poured the footings for the block walls for the basement of our family home when I was in middle school. Once set, they were covered with old towels and blankets. It was summer vacation, and it was my job to keep those coverings wet all the time. I can’t recall the duration, but it was over a week. That was 50 yrs ago and my parents still live there. Not a crack to be found in those walls. Of course, I was just doing as I was told because kids didn’t ask questions back then. Now I know the why.

[NCMan]

I’ve been reading the concrete drying discussion with interest. We poured the footings for the block walls for the basement of our family home when I was in middle school. Once set, they were covered with old towels and blankets. It was summer vacation, and it was my job to keep those coverings wet all the time. I can’t recall the duration, but it was over a week. That was 50 yrs ago and my parents still live there. Not a crack to be found in those walls. Of course, I was just doing as I was told because kids didn’t ask questions back then. Now I know the why.

Bottom line: keep the existing moisture in as long as you can.