Underpinning piers to support a steel

[clairelouisecook]

We are working on a 1920's house with no foundations. The house is having an extension with the front of the house being knocked through and steels sitting on piers supporting the existing house. Building control have suggested dry packing the piers rather than wet filling the pier void. What is the view on dry packing rather than wet fillling?. We have found a dry pack product called ' Flow cable' but it does say not to be used below 5 degrees C. Anyone any ideas or experience of this product or an alternative which is ok with the cold wheather. Advice would be appreciated.

 

[roughcaster]

Your post will get a better response in the "building" forum.

Roughcaster.

 

[^woody^]

Is the question to do with underpinning or filling a pier void?

If its piers, just bung some concrete down, or use brick when building them up.

Don't over complicate things with a "special product"

 

[alittlerespect]

Hi

A 'dry mix' is not dry in the real sense - a dry mix is a normal mortar mixture with just enough water to make the ingredients interact. You should add just enough water that the mix is friable and when a small amount of the mortar is cupped in the hand it retains its shape. You then use the dry mix to fill any gaps, in this case between the steel rsj and the foundation pad to the brick piers. Depending on the size of the gap/s you can use normal building sand for say gaps up to 25mm, and a sharp sand mix for gaps up to 50mm - width can be anything you like as the width doesn't impact to greatly just curing time will be a bit slower. You will need to leave the beam supported until the mortar has reached its design strength - setting time is usually 24 hours, reaching design strength allow say 7 days - there's nothing to stop you building on top of the rsj during this period. Suggest that you use a mix of 1 part portland cement to 3 parts sand (good and strong!)

One comment - you will need to put some concrete pads down on the top of the piers to support the rsj and transfer the load over a wider area than just have the base of the rsj resting on the brickwork - this should have been specified by your Architect/Designer.

 

[andywc]

hi

while the question was posted a few weeks ago and i'm sure you've progressed on site now, it's an interesting question and one which i've just dealt with on-site with an underpinning contractor too... so it's sorts of topical...

i guess the reason why the building control officer suggested dry packing was because he wanted to make sure that the pier was fully supported. i've had situations in the past where a contractor wet filled a void/pad and then found that the wall/pier above had moved. while there can be various reasons for this, a possible one is the slight shrinkage of the fresh concrete as it cures. builders will know that when casting a cube for testing it shrinks away from the timber/metal former after a few days and the same is true of concrete cast in the ground.

if a 'dry pack' is adopted then it should be well rammed into the space between the top of the wet filled concrete no earlier that 24 hours after it was cast [subject to the size of the pour and prevailing temperatures of course]. in this way the shrinkage will already have occurred in the curing concrete and the dry pack [which doesn't shrink nearly as much, if at all] can be compacted into the void. typically i suggest that a 75 void is left for dry packing.

of course, you may find that by the time you've temporarily supported the piers over the hole you're about to fill with concrete, then it may be more economic to take down the pier and rebuild it...

as ever the prevailing site conditions will govern and the above are only generalisations....

hope this helps

andrew

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Andrew Cartlidge BEng(Hons) MIStructE CEng
Chartered Structural Engineers
http://www.beamcalcs.co.uk