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I mixed 1:1.5:3 by hand and using a bucket. I mixed the concrete on a plastic sheet but on hard soil not a concrete surface. I am not sure what I have done wrong. The portland cement bag suggested 1:4:3.5. It seems like that people here use more sand than coarse aggregates. I created this pad for a small sliding gate motor. Is it going to have problems? What went wrong? Too little sand?enter image description here

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GorillaApe
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    Any chance of too little compaction? Have you had a go at it with a tamper, or just dumped it in there and called it a day? The more coarse aggregate you have, the harder the pounding needs to be. – TooTea May 14 '21 at 09:39
  • I just dumped it and moved it a bit with the shovel. Is compaction required for such small projects? I am a beginner. – GorillaApe May 14 '21 at 09:55
  • That's exactly what house footings look like. They're poured fairly dry for speed and vibration isn't usually applied. Foundation walls need vibration to close voids, but some typically remain. – isherwood May 14 '21 at 12:53
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    Compaction, no. Vibration, yes. You failed to vibrate the air pockets out of it. Most commonly I see people just stab the concrete with a trowel for a few minutes before smoothing out the top of it. – MonkeyZeus May 14 '21 at 15:33
  • Practical Engineering has some interesting videos on concrete, doesn't directly answer your question but I'd recommend giving them a watch. – Lio Elbammalf May 14 '21 at 19:03

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If you want no holes, you have to compact it, easiest is with vibration but that needs a special tool. Or you can poke a stick in and out of it for a while. I've had great success by fastening a bit of wood plank on the business end of a vibrating sander and using that as a vibrating float.

Don't worry though, you're not building a suspension bridge, it's not going to come apart. If it was a thin load-bearing shape like a slab, then it would be worrying to have it "pre-fractured" like that, but this is a big chunky block whose only job is to be heavy and take a few screw anchors.

If the holes are an aesthetic problem, you can always cover it with a mortar made with a mix of fine sand and cement, apply like plaster.

For next time:

Since this is not a load bearing item like a slab, you can put in a bit too much water so it flows better. It will be a bit weaker, and have a bit more cracks, none of it matter for what is basically a concrete boat anchor. Or you could mix in some concrete plasticizer, which makes it a lot easier to work with without compromise in solidity, but that's a bit expensive and way overkill for this job.

bobflux
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  • The simplest poor man's tamper (apart from just jumping in and stomping the hell out of it) might be a length of 4x4 lumber (long enough so that you can do the job comfortably standing up). Thicker blocks of concrete need to be compacted in several passes (dump a layer of concrete in, give it a decent thumping, repeat). – TooTea May 14 '21 at 11:45
  • Would a different mix like the one that local contractors use with more sand than coarse aggregates would require less compaction? I am saying that because I have seen pros pouring concrete and they do no bother at all with compaction. I have some concrete repair ready mix so I may cover the sides. However this will be backfilled. I am afraid if drilling this would crack my concrete – GorillaApe May 14 '21 at 11:49
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    "I have seen pros pouring concrete and they do no bother at all with compaction", that is almost certainly not the case. You either missed it, or didn't recognize their actions as compaction. – longneck May 14 '21 at 11:54
  • Your mix looks a little dry. Did you follow exact directions when mixing? The wetter the mix, the lesser the amount of compacting.... also, compact in stages.. – JACK May 14 '21 at 12:18
  • @JACK Since my plastic sheet got holes I might missed some water and cement. Next time I will use a bigger sheet on a concrete surface not dirt.It seemed like too much aggregates. But I also skipped compaction. I was afraid to use more water since it weakens the final product – GorillaApe May 14 '21 at 12:23
  • @GorillaApe In the future, mix it in a plastic mixing tub readily available in your home store. FYI, adding a bit more water for a gate motor wouldn't weaken the mix. – JACK May 14 '21 at 12:52
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    If you didn't see the pros compact a slab, it probably had some plasticizer in it so they just run a vibrating screed and it sets in just like magic. – bobflux May 14 '21 at 15:30
  • Builders here in the UK used to use a squirt of washing up liquid as a plasticizer! – Bonzo May 14 '21 at 20:38
  • a little dirt can help too. – Jasen May 15 '21 at 00:18
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    @TooTea another way to vibrate the mix is to tap the outside of the boxing with a hammer, not hard but repeatedly all over. This is one time when it is practical to dual-wield hammers for double the effect. – Criggie May 15 '21 at 01:04