Need concrete advice.

[chingmai331]

I'm making a cover for a sewer hole, using concrete and re-bar. Question is concerning the proportions of cement, sand and aggregate (stone). If i use no stones, will the finished concrete be stronger? What about less sand? What is strongest possible to make at home?

[NeverSure]

It won't be strong unless you use aggregate. Concrete is a mixture of cement, aggregate and water. Cement is only the binder (glue) that sets up and holds it together. In itself it isn't strong at all.

If I was mixing a small batch I'd do it by eye and feel but I know what it looks like. Be sure it is well mixed. When it has the right amount of water it will be wet but still hold a peak if you get a shovel full and lay that on the rest of it. If it has too much water it won't be strong. Remember it doesn't harden by drying but rather by a chemical reaction.

I haven't made concrete in LOS, but I always buy what's called ready mix which has the sand, gravel, and cement already mixed to proportion and just add and thoroughly mix in the water.

You don't want cement if you can buy ready mix concrete which is what they should be selling in home improvement stores for the DIY crowd.

Edit. Concrete won't be strong unless you work it down in the form. A trowel is designed to not only smooth the concrete, but to compress it pushing out air pockets and forcing the the components to compact.

Edited March 19, 2015 by NeverSure

[onthedarkside]

The stones are an essential part of concrete strength ..

The proportions of materials for a C20P (P = Portland cement) mix are 1 x cement, 2 x fine aggregate (sand) and 4 x coarse aggregate. If you are using premixed ballast, then 6 of these are mixed with 1 part cement. In an ideal world, where everything is delivered dry, then a water to cement ratio of approximately 0.55 should be used.... For example, if you require 25kg of cement in your mix, then 25 x 0.55 = 13.75kg (liters) of water. This is the maximum amount of water that should be used.

[NeverSure]

Was posting while you were...

[eyecatcher]

OTD is quite correct for the mix of 1:2:4 for general concrete. Thats a strong mix and 1:3:6 is perfectly adequate.

Just a point. Without the aggregate you have made mortar for bricklaying which i am sure you wouldnt ever dream of doing for your manhole cover..

The bagged readymix is generally for concreting posts in the ground. Its a strong mix but very weak in tension. Its not under any stress subsoil so ok but use it as a path and you will have crazy paving before it even dries.

If you feel it needs rebar then the easiest is to get it from global in mesh form. Just get half a metre cut off the roll and cut again to your size they will do it. Simple A142mesh 200mm squares will suffice. Make sure the steel is set about 30 40mm from the bottom.

Wont bore you any longer!

[Seligne2]

I'm amused that you pose this question to TV rather than to Google. There are lots of sites like this: http://www.wikihow.com/Make-Concrete

The aggregate is just filler so you don't waste cement. It is clean sharp sand (emphasis on the word "sharp"; beach sand is round and crap for concrete) and cement that give you strength.

[northernjohn]

Well you pretty well got the ingredients. One other thing I might add is the time it takes to completely dry out is important. Throw some thing over it to hold the moisture in as long as possible. I can't say for Tghailand but we as a ruld in Washington figured 28 days to completely dry out. It was strong enough after 3 or 4 days to drive on yet not at it's full strength. I believe it is at 75% of it's full strength then. Been many years not completely clear on the times other than the 28 days.

[eyecatcher]

I'm amused that you pose this question to TV rather than to Google. There are lots of sites like this: http://www.wikihow.com/Make-Concrete

The aggregate is just filler so you don't waste cement. It is clean sharp sand (emphasis on the word "sharp"; beach sand is round and crap for concrete) and cement that give you strength.

I suggest you look at those sites before coming out with a crass naive statement that aggregate is just a filler so you dont waste cement.

Keep off the diy forum we dont need your advice.

[cooked]

I never wanted concrete to dry out, I was more interested in having the water reacting with the cement to the extent that all the water present was involved in this chemical process. Engineers calculate using the 28 day cube strength which doesn't mean that some load can't be applied before then. The constituents of concrete are marked on the cement bags. 4 of stone,2 of sand, 1 of cement. Mix dry until you have a uniform colour. Add one of water ( use bucketfuls if in doubt) and mix again.

[TPI]

Good advice from everyone, but! Work out the size of the lid you need and make a strong timber or steel form (shape of the lid), when you cut your steel reo cut it to fit your form, if you want a very strong very quick drying concrete throw in a half handful of iron fiiings although you'll have to be quick when you trowel off! Good luck!

[Neeranam]

I did a similar thing recently but used a premixed cement." Something 200 in green writing" - sorry can't remember the name but bought from Do Home. Very strong.

[cooked]

all premixed cement is going to be good.

[sfokevin]

Brush some cooking oil on the wood form before pouring in the concrete to act as a release agent

[northernjohn]

Good advice from everyone, but! Work out the size of the lid you need and make a strong timber or steel form (shape of the lid), when you cut your steel reo cut it to fit your form, if you want a very strong very quick drying concrete throw in a half handful of iron fiiings although you'll have to be quick when you trowel off! Good luck!

For added strength you can throw in fiber glass.