I am poring concrete into forms that are are fairly thin. I now that they are thinner than would be acceptable to most but it is my project. I am using a wire fencing to reinforce. The mix ratios for concrete makes sense ( Google Portland cement mixes). My problem is that I don't know the ratio of Portland cement to gravel and sand in an 80 lb bag of regular high strength concrete. So I don't know how much Portland cement I can add and increase strength. I don't want to buy gravel sand and cement and then mix. My forms hold 80 lbs of concrete. Its convenient. Look at the pictures to see what I'm doing. These are pretty strong but I would like to maximize the strength of each panel. Any help or advice would wonderful.
https://drive.google.com/file/d/0B5J...w?usp=drivesdk

 

If you are already using 5500psi concrete mix, why would you need to add more cement? Or if using the regular high strength (4000psi), why not switch to the 5500? It's only a couple of bucks more than the regular stuff. It's for counter tops and slabs as thin as 1.5". With your wire, you might be able to go a bit less even. Since you won't really have that much stress once in place, you might not need that even.

After reading a few similar replies here and other places, basically if you want to throw a 16 oz cottage cheese container of cement into the mix, it probably won't hurt anything and may make it marginally stronger.

As you said, there's no real way of knowing what's in the bag in the first place. The concrete engineers (or whatever they are) specify exact size and amount of aggregates, exact size, amount and moisture level of sand and exact amount of cement to arrive at what meets the specification and usage.

 

Well the only reason I'm trying to increase the strength is because I'm trying reduce the number of times I break the slab. As it is right now I break 1 in 10 slabs. I have just recently tried 4 pours adding about 2 lbs of cement. I was surprised at the creamer consistency of the pour but I will have to see if this addition will reduce the breakage. Thanks for your thoughts.

 

Too much cement can actually weaken the slab, though it's appearance and texture are as you said "creamier". That's actually not usually a good thing when placing concrete. It's like peanut butter. The really creamy stuff usually has more oil and less ground peanuts.

There was a very good (but pretty technical writeup I found while looking and it explained all the various ingredients and what they did for the mix, but weren't specific in quantities for the reasons I listed. Fine silica and fly-ash were mentioned as increasing the strength somewhat, but you have to reduce the cement, which you obviously can't do with the bagged mixes. Doubt you could even find the stuff as a bagged good.

 

Yeah your right I also read too much cement will weakin the slab. I'll just have to see. I thought that maybe some one out there might have done some testing and some rule of thumb to work off

 

The rule of thumb is one part sand, one part aggregate and one part cement. Pretty much what you're going to find in the bags.

 

When and how do the slabs break? Is it when you un mold them? Or when you tip them up? Or when you fasten them together?

 

Have you considered adding glass fibers to the mix ?
That will make your panels much more break resistant.

I think the panels are too thin for the fence to be of much help.

 

I thought it was 2 or 3 parts sand I know with mortar it's 1 part mortar and 3 parts sand, I thought concrete was similar except for the type of cement and addition of aggregate.

 

Again I want you to understand that I understand how to mix good concrete from sand ,agragate , and cement. I just wanted to know if any one new of a rule of thumb about increasing strength of standard 80# bags of concrete by adding additional cement to the mix. Adding fiberglass fibers in a great idea. But is much more expensive than adding a little cement. The panel usually breaks coming out of the mold. Once vertical I have not broken a panel.

 

http://sealwall.com/uploads/Acrylic_...t_TechData.pdf I would try a part jug of some of this(there's different brands) in the mix depending on how much you need.Around 15 bucks at the big boxes I think.

 

Are you using a form release agent before every pour? Can you spare the time to let the slab cure an extra day or two before you take it from the form? Could the slab be drying out too quickly?
A little Portland would surely help harden and strengthen the slabs. I don't know how much by ratio would be beneficial. I would not add much. you could actually estimate pretty well how much cement is in the mix. Measure out a gallon, weigh it, sift all the fines and coarser aggregate out leaving only the cement. Weigh each component. Adjust for the fact that some of the cement will still coat the fines and gravel. Then you know the ratio of cement to everything else in the 80# bag. Probably in the neighborhood of 10% max more cement would be safe. Maybe start with less and work up until you have a suitable formual. About the time you get it figured out you will be done. And here is another thought. Where do the slabs break? Can you still use them just fasten them together the way you are attaching the slabs now?