cement floor and foundation

sipherlucian84

hi i poored foundations and floor last sunday for some coal bunkers im building.
im pretty worried now, the mix was 5 sand - 1 cement and 1 stone, they seem weak, i can hit them with a lump hammer and make a dent like they are soft.
the foundation is 8" deep and 12" wide, the floor is around 2" but theres quite a strong floor under that about 2-3" thick, theres also a foundation under the back where the fence is in the pics, id say thats 4" deep 12" wide so ive just added to it. i still have to add another inch or 2 to the top so i was going to do it with 3 -1 ration and with bonding compound.
im building with cement blocks at the back 7 high, and red brick at the front and sides, im pretty worried now can someone suggest something, i want these bunkers to last, hope to hear from you, thanks!

wayoutwest

sipherlucian84 - I'm afraid that you have'nt used the right mix for the concrete slab -it is far too weak -more like a mortar mix [apart from the one of stone]. Mortar is for sticking courses of blocks together and is'nt meant to be applied in thicknesses over about 20mm.The slab should be made of 1 cement+5 concrete aggregrate[which is mainly gravel with a bit of sand]. I woud be inclined to get a pickaxe and smash the whole lot out before it fully hardens and start again .....sorry.

sipherlucian84

****e, i was afraid this would be the answer, maybe if i remove the top few inches and put in a stronger mix? i was thinking it might be ok as it is only cause the foundation and floor are so deep

wayoutwest

With all your blocks for the bunker+the coal itself -there's going to be a lot of wieght bearing down on the slab. Its a bit hard to see what your foundation is comprised of -is it old pathways etc?. Usually you would dig down,removiing soil,until you get to natuarally compacted clay or stoney stuff,then put a layer of hardcore[broken bricks and stones-not old mortar]and tamp down really hard with top of sledgehammer.If I came to areas of existing concrete I would test how solid its by giving it a hefty ole whack with sledgehammer,and if it cracked/sounded slightly hollow/vibrated a bit-then out it would come.I would then pour in maybe 8 " of concrete.Why do you need to add another 1 or 2 inches of screed?- did you get your level wrong?

sipherlucian84

wire came loose on the cement mixer by the time i got it sorted it was dark and theres no light around my area to see what you are doing, so i got it as level as i could see. the cement that was there would not budge, it was part of the driveway, had to get a con saw to cut out where the foundation was going and break that up with a sledge i regret not getting a kango for that cause it was really solid! i had hardcore and wacker etc to do what you are saying but because the cement was so hard i ended up leaving it i thought it was pretty dam strong.
i did dig down to solid soil too, the far side with the fence used to have a wall there that i removed, i left its foundation which was 4" x 12" and simply added to the top of that

wayoutwest

4" of the old wall foundation sounds like it would do instead of hardcore. So what is the thickness of your poured slab a]above this and b]above you areas of compacted hardcore?

sipherlucian84

no hardcore, left the slab of cement you see in img 1311 there as it was so strong and ignored the hardcore, so the slab left there was up to 3" thick, what i poored on top starts at 1" thick and goes up to 6" thick, because the driveway was on a slope you cant tell in the pics, what ive put on top is flatten out, so foundations start at 8" deep at the top and are 6" thicker towards the end, ya get me ? by the way thanks for your help so far!

wayoutwest

I can see now -so the corner of the original slab [nearest us],is os only covered with an inch or two of VERY weak concrete :eek:. Even if the correct mix was used,there would be a weak point here. If you are laying a slab,rather than strip foundations,it wants to be thick enough to have the strength be self supporting .IMHO the combination of weak mix,varying thickness and varying hardcore structure[especially at points where bunker walls are going to be placed upon] could easily lead to the slab failing,leading to the walls cracking......sorry.

sipherlucian84

ok, have to start again so ! lesson learned, thanks for taking the time to help me i appreciate it! ya wanna build it too !?

wayoutwest

I suppose you'll have a second chance at getting it level:).good luck