Hi soares, I wouldn't be writing the obit for your tree just yet. I don't use cambistat so can't comment on its effectiveness, but here's what I would do;
Back fill the excavated trench with a mixture of old compost and sand, remove the pavers to create a planting circle 4m radius (if you can't create this radius then as large as you can) spread the same compost at about 50mm depth over the top and lightly rake into the paver base (thinking you used sand for that?) ontop of the compost spread a layer of aged forest woodchip again around 50mm thick giving you a total height of 100mm(4") None of the mulch should be allowed to touch the stem of the tree. Water this in with a dilute solution of seaweed extract and fish emulsion. Three days later water again with a solution of raw molasses 2lts in 100lts of water. Repeat the treatments in three weeks time, ensuring the tree is adequately watered inbetween.
If you already have the cambistat and you really want to use it, then go ahead. Just remember if it is effective in reducing foliage growth that will have an impact on the energy levels in the tree, at a time when you really want the tree to be producing an excess of energy for defense maintenance and new growth. New root growth will happen anyway without chemical stimulation, the compost and diluted treatments will provide excellent soil conditions for new root growth and root elongation.
Pavers and tree roots tend not to be the best of friends, indeed the ideal conditions for any construction are the exact opposite of the conditions required for normal healthy growth in any tree, its very important to give and protect as much growing space for the trees we value when we decide to alter the soil environment around our trees.
__________________ Sean  Trees are poems that earth writes upon the sky,
We fell them down and turn them into paper,
That we may record our emptiness.
- Kahlil Gibran
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