Browse Forums Owner Builder Forum 1 Jan 20, 2021 12:48 pm Hi, first time building anything, first time posting here, so please forgive me. I'm trying to calculate what I need to order for the concrete for my footing post holes. If it's relevant, it's going to be a single story, slightly raised, 8m x 4m house. I think I've done the calculations right, but I'd really like the confirmation (or re-direction) of someone who actually knows what they are doing! Here's what I've come up with... 12x post holes/footings holes Hole dimensions: Diameter = 300mm Depth/height = 600mm So, approx 0.5m³ of concrete is needed. Will be buying local 19mm concrete aggregate mix. $80/m³ Assume aggregate density = ~1500kg/m³ As recommended by manufacturer... Cement:sand:aggregate = 1:3:3 and, 1m³ concrete = 13x 20kg bags cement (260kg cement) So, 0.5m³ concrete = 130kg cement. 1:3:3 130:390:390 (kg) 390kg = 26% of 1500kg This is where I confuse myself... So, should I order: 0.3m³ of aggregate mix 7bags of cement Or do I need to change the mixing ratio of the 1:3:3 to something like 1:6 because I don't need to add the sand? In which case... 1:6 130:780 780kg = 52% of 1500kg So, I should order: 0.5m³ aggregate mix 7bags cement Please help! TIA! Re: Calculate concrete orders for footings. 2Jan 20, 2021 4:06 pm Hey Kloe Welcome to the forum If you are hand batching order bulk Concrete has a density of 2400kg/m3 Try batching by Volume here General purpose concrete mix Easy Guide to cement Cheers Chris Designer,Engineer (Civil,Const & Envir),Builder,Concrete & Masonry Contract.Struct Repairs Re: Calculate concrete orders for footings. 3Jan 21, 2021 1:43 pm Why dont you need to add the sand, im confused? If possible, and if youre in Sydney (dont know prices elsewhere), it would probably work out cheaper for you to order .5m3 through minicrete, and just barrow it into your holes. Would also be significantly easier, and you would get a much more reliable, and consistent mix. I would also increase the diameter of the post holes to 400mm. Re: Calculate concrete orders for footings. 4Jan 24, 2021 9:01 am I don't need to add sand cause the 19mm agg mix I'm buying already has sand blended in. All you need to do is add the cement powder and water. Unfortunately not in Sydney. Live in a tiny rural town in the middle of nowhere, so this is definitely the cheapest way for me to get it, especially seeing as I don't need much. Was going to be over double the price to get the pre-mixed stuff. Thank you for taking the time to reply to my post though! Re: Calculate concrete orders for footings. 5Jan 24, 2021 9:06 am StructuralBIMGuy Hey Kloe Welcome to the forum If you are hand batching order bulk Concrete has a density of 2400kg/m3 Try batching by Volume here General purpose concrete mix Easy Guide to cement Cheers Chris Thank you for the links. I'm still unsure what to do about the cement:sand:agg 1:3:3 ratio though, seeing as my agg is a blended mix and contains the sand already. I guess I would just add the 3:3 and make it cement:blendedmix 1:6...? Do you happen to know if this is right? Re: Calculate concrete orders for footings. 6Jan 24, 2021 9:42 am too much sand....by bulk I mean order all components separate then batch it by volume/shovels (read the cement companies instructions), mix in a cement mixer, make sure the sand, aggregate is dry, add water & check the slump, etc,etc Designer,Engineer (Civil,Const & Envir),Builder,Concrete & Masonry Contract.Struct Repairs Re: Calculate concrete orders for footings. 7Jan 24, 2021 11:40 am Kloe StructuralBIMGuy Hey Kloe Welcome to the forum If you are hand batching order bulk Concrete has a density of 2400kg/m3 Try batching by Volume here General purpose concrete mix Easy Guide to cement Cheers Chris Thank you for the links. I'm still unsure what to do about the cement:sand:agg 1:3:3 ratio though, seeing as my agg is a blended mix and contains the sand already. I guess I would just add the 3:3 and make it cement:blendedmix 1:6...? Do you happen to know if this is right? You are definitely correct with the math, your ratio will essentially be 1:6. Thats all. Goodluck. To my understanding early saw cuts are to control shrinkage cracks, so doing them now would be pointless. Control joints may reduce ugly cracking during periods of soil… 3 9826 Hi all I need a guidance on how far I need to space expansion joints in the concrete driveway and its type (keyway/foam.) Contraction joints are at 3m max for a 125 slab. Thanks 0 10921 |