why was my concrete "rocky"?

[SpiderTrev]

In addition to all the other chemicals in there?

as far as i was aware the composition was all okay to be in the soil, once i have leached it to keep the pH down. however your comment made me look into it again. i couldn't find a complete SDS for the cement i am using but i did find others and it turns out there will be some amount of Hexavalent Chromium Cr in there. i'm not sure if it is going to be the kind of amounts i need to worry about but i will do some research before i completely commit to putting food in these planters! i found a paper about the leaching already that i will read, i'm sure there are more too.
is there any other chemical you think i should be aware of?

thank you for the heads up!

 

[SpiderTrev]

You would need to sift out the larger stones, first, before using it.

i have some chicken wire that should be about the right size. i might try that. maybe i wont bother with it, i'm sure i can find somebody that will want it. haha

 

[JohnD]

Garden sieve.

 

[SpiderTrev]

Just to check, did you tip out and shovel over all the ballast?

The stones tend to settle to the top of the bag.

You can buy a plastic spot board with a rim, for mixing cleanly on.

https://www.screwfix.com/search?search=Mixing+board&brand=faithfull

https://www.diy.com/search?sort=product.price&term=Spot+mortar+board+

Heavy Duty Tuffspot Mixing Tray - Black | Wickes.co.uk

Heavy Duty Tuffspot Mixing Tray - Black

www.wickes.co.uk

i tipped out the whole bag and i would have shoveled over it while i was mixing it with the cement i would guess. i mixed the dry ingredients a lot before adding any water, enough that it was all one consistent look and texture.

 

[SpiderTrev]

Garden sieve.

good idea. i have got one i made a while back. thanks

 

[mrrusty]

Never made planters, but when I'm using concrete in fairly thin applications I have added stranded fibreglass on the recommendation of a builder. Have others any thoughts on this?

 

[Dereekoo]

Never made planters, but when I'm using concrete in fairly thin applications I have added stranded fibreglass on the recommendation of a builder. Have others any thoughts on this?

Stranded fibre glass is there to prevent thermal cracking during drying, on small planters I would imagine the dimensions involved do not particularly lead to any drying stresses being set up so fibre not necessary

 

[stuart45]

Look.into the way paving slabs are produced and the materials used, as it's similar in that it's thin concrete in a mould. Paving expert website will probably have some info on the subject.

 

[Harry Bloomfield]

Have others any thoughts on this?

A long time since I last laid any large amounts of concrete, but I heard the stands act, similarly to steel reinforcing, though not as strong, though an awful lot easier to add to the mix.

 

[Dereekoo]

A long time since I last laid any large amounts of concrete, but I heard the stands act, similarly to steel reinforcing, though not as strong, though an awful lot easier to add to the mix.

Watched a video on a concrete slab reinforced with glass fibre verses rebar and the rebar resisted 4x the load, also at failure the fibre slab was in 2 halves after loading whilst the rebar still held the concrete together in a fashion. I don't think we are ever going to see rebar replaced by fibre glass in structural reinforced concrete slabs/beams

 

[stuart45]

Fibres are used more to reduce minor cracking in the slabs. Rebar is there to hold the cracked pieces together.

 

[Harry Bloomfield]

Watched a video on a concrete slab reinforced with glass fibre verses rebar and the rebar resisted 4x the load, also at failure the fibre slab was in 2 halves after loading whilst the rebar still held the concrete together in a fashion. I don't think we are ever going to see rebar replaced by fibre glass in structural reinforced concrete slabs/beams

No, but did they test fibre reinforced, versus no reinforcement?

 

[Dereekoo]

No, but did they test fibre reinforced, versus no reinforcement?

No, but at a guess I don't think it would take much load before failure due to the tension properties in concrete being about 10% of its compressive strength so for a 30N/mm2 concrete the tensile strength would be 3N/mm2 which is 150 times less the HY reinforcement

 

[Dereekoo]

Look.into the way paving slabs are produced and the materials used, as it's similar in that it's thin concrete in a mould. Paving expert website will probably have some info on the subject.

I thought paving slabs were compressed during manufacture to give a denser and more durable product, the planters will be compacted by gravity and prodding

 

[SpiderTrev]

I thought paving slabs were compressed during manufacture to give a denser and more durable product, the planters will be compacted by gravity and prodding

.. also kicking, i did kick them a lot, don't forget that