Large Scale Central

Adventures in Concrete

Track and Trestles
#1

Dennis has a “Split Jaw Crossing” that you’re supposed to be able to drive vehicles and such across, but it would bend a lot on the rather squishy sandy ground, so we decided to make a concrete pad to support it. Personally, I would have set a 3/4 inch board into the top of the form to make a recess for regular track to hide from vehicle wheels.

Stan told us that an 80 pound bag of concrete mix makes a cubic foot. Being a little suspicious of Stan’s information, we asked the guy at the local ACE hardware, and he agreed.

Emboldened with this information, we built our form. I measured the inside dimensions of 71 by 17 by 3.5 inches. The handy calculator told me that’s 4224.5 cubic inches. Dividing that by 12 cubed gives me 2.44 cubic feet. 2 1/2 bags.

After a hurried trip back to the hardware store for more mix, we got our pad finished.

Any of you who have worked with concrete will know why it took 4 1/2 bags, not 2 1/2: An 80 pound bag of mix makes 0.6 cubic feet. Any of you who haven’t worked with concrete, but might consider it in the future now know: an 80 pound bag of mix makes 0.6 cubic feet.

Shadow helps Dennis level the concrete while I wonder how the calculation could be so far off.

Tom resists temptation.

Finished! I rounded the corners with a sidewalk tool. Not quite right in one corner, and I don’t know that it won’t hold water.

I’ve also been casting many stones of mortar mix for making mountains. They go on the other end of the yard.

Except that the bags are half my weight and quite a task to move around, I rather like working with this stuff. I guess if I keep it up, I’ll be more able to move them.

#2

I’ve done a lot of concrete work n my RR as you probably recall. I gave up on 80# bags a few years ago; they are just too tough for me to handle. I pay more and buy the 40# bags. I also invested in a 1.5 Cubic foot mixer a few years ago. Sure beats hand mixing if you are doing a lot.

#3

So Tom, let me get this straight. You said (and I quote): “An 80 pound bag of mix makes 0.6 cubic feet. Any of you who haven’t worked with concrete, but might consider it in the future now know: an 80 pound bag of mix makes 0.6 cubic feet.” In other words, Tom, if I mix an 80-pound bag of cement with sand and stuff, I will wind up with 0.6 cubic feet of working see-ment, correct?

Sorry, I just had to mess with you because of the repetition, intended or not, was just too easy to screw with.

Seriously (why now?), you’ve done a fine job. Myself, I woulda looked at all those 80-pound bags at Home Depot and headed right for the door. 'Cause like Jon, 80 pounds, heck, 40 pounds, is way more than I wanta handle. OK, maybe if it’s donuts…:slight_smile:

#4

No, I used 80 pound bags of concrete mix, not an 80 pound bag of cement. Concrete is rock, sand and cement mixed together, ready to add water. Cement is the powdery glue that you mix with sand and rocks to make concrete.

We’ve also used “type S mortar,” which has some plastic in it and no rocks. It’s stickier and finer, since there are no rocks. That’s what I use for casting stones. I understand this stuff is sticky and won’t come out of a cement mixer well. You will barely pour out of a bucket, you have to scrape it out.

We’ve also used something, I can’t remember what it’s called, that’s sand and cement, without the rocks. It’s finer than concrete, and works for making things like building bases. Dennis just builds a form out of 3/4 inch square sticks and pours a building base wherever he wants one. I don’t know how much an 80 pound bag of this stuff makes. If I have to fiddle with his trestle again, we’ll make a base for it out of this stuff.

Dennis has been using all this stuff for years, but this summer is the first I’ve fooled with it except once when Susan and I set up an antenna tower. Then, we just dug a hole, set the tower in, and poured the stuff into the hole. I think it was a much bigger bag as it took both of us to move it and it bent the wheelbarrow.

I use a bucket and trowel for casting stones, wheelbarrow and hoe for mixing bigger things.

#5

The bagged stuff that is only sand and cement is known as Sand Mix concrete. It’s usually used for thinner pours or when even pea gravel is too large for the purpose of what the concrete is being used for.

A bit of a quiz here. Does fresh concrete, as it comes from the mixer, heavier or lighter than cured concrete ?

The answer will be posted later.

#6

Heavier - Most of the water evaporates during the curing process which can take weeks to fully complete.

EDIT to put back my original answer, Steve confused me and I changed it - I had read it correctly the first time.

#7

Gotta be lighter, with the water gone.

Q. What weighs more… One hundred pounds of bricks, or one hundred pounds of feathers?

Edit yeah, RTFQ. I read the question wrong, too. That was was to do, with that question. :slight_smile:

#8

Yes - I’m dyslexic !

#9

So what is the difference between sand mix concrete and mortar? I used mortar to pour the foundation for my green barn.

#10

Dan Padova said:

A bit of a quiz here. Does fresh concrete, as it comes from the mixer, heavier or lighter than cured concrete ?

The answer will be posted later.

The question is “as it comes from the mixer”, so it is heavier since it still contains all the water. RTFQ.

#11

According to a structural engineer friend, it takes 29 days for concrete to reach FULL cure = full strength. 14 days is sufficient to begin working with (adding anchors, etc) without weakening the slab.

FWIW.

Bob C.

#12

The name for Cement plus Sand plus Water mix is “Screed” .

Commonly used as a surface finisher , it is harder wearing than concrete .

Mike , (one time builder’s labourer during school holidays)

#13

You can also get denser stuff like “hydraulic cement” mix, often used for patching.

Greg

#14

There are actually a lot of different mixes for different purposes. I don’t know why we use “Type S Mortar” for casting stones except that Dennis Rayon says that’s the one to use.

#15

Concrete weighs the same weather it’s freshly poured or fully cured. The water doesn’t evaporate, it chemically reacts with the cement to form the paste that bonds the aggregate and sand together. The next time you are near a concrete pour of a wall or beam or perhaps a column, go back the next day and put your hand on the formwork. Feel the heat? That’s a chemical reaction.

Concrete takes 28 days, not 29 to cure to about 98% of it’s full strength. It takes another 185 years to completely cure.

#16

When estimating sack concrete I always assume that whatever the footprint of a sack laying flat is exactly as much concrete you’ll get out of each sack.

#17

Vic , that’s a bit dodgy , because there is a good chance that you will have just enough to do the job in hand----minus about a whisky bottle full .

So buy an extra bag . Then store it carefully when you find you did have enough after all .

Thus sayeth the Law of Sod , for yea , verily , it is writ large in the Annals of Building Things , writ by the builder of the Pyramid of Buggerit , so named when he climbed to the top to put in the final pointy bit only to find he had not mixed enough . This prompted the practise of building from the top downwards .

Mike

ps That’s why Stonehenge was never finished , they ran out of wood for the roof .

#18

No mention of the use of reinforcing rod, at all…on a pour of that type, if any real traffic is expected, it should be considered. While curing, it usually is wise to keep the surface wet, for several days, by using old felt bags, or something to maintain the moisture.

There is also the maybe, old fashioned task, of “Floating” the surface, after the pour, in order to maintain a finished surface, and drive the stone content away from the top inch.

But I guess the “Experts” on this thread know best, and can explain why these ideas were not mentioned.

#19

Fred mentions old felt bags .

There must be any number of old bags who have been felt available for the job .

Mike

#20

Mike,

:slight_smile: