Hi all, last year i put down a very good, level & hard slab, left it 8 weeks to cure, then put d/brick constructuion on the slab, all was in good order, & i went away for 2 weeks at Xmas, came back to find annoying shrinkage cracks in the larger open areas exposed between walls such as garage, lounge & family rooms.
i've put down numerous shed floors of far greater area & never seen a crack - so was very disappointed & annoyed by this development.
i sort advice from experienced commercial builders, grano workers & engineers - their advice was mostly all the same more a cosmetic annoyance not structural. in some parts of WA we now have to put large sub-slab beams, when a slab is directly heated by the sun, the difference in slab thickness between normal 100mm and deeper sub-beam areas is effected by different heating & cooling and cracks result.
the reason nothing happens with the shed floors i'm assured is becos firstly these r a common depth, secondly the floor is poured undercover and concrete dries evenly without direct sun on it.
the basic rules seem to be that "shinkage" cracks take a few weeks/months to appear & only "run" while exposed to the sun & don't get very wide - ie i couldn't get a point of a pen knife into the crack even though it could be 3 to 4 m long. structural cracks usually open up very quickly (ie within 3 weeks) - r much wider (fit a 5 or 20 cent coin into & may get wider).
i was told once the roof was on the cracks would not "run" any further - to date this has been the case. theres probably no reason to worry about treating these cracks as floor covering will hide them. however i dont like having them in the garage so looked for a "repair" solution.
now the roof is on i applied a product called Polyfilla, powder filler, Exterior use for bricks, concrete etc, non shrink & very hard once dry, but can be "papered" back to floor level. the product i used was in powder form, & i found it good to mix into a fine paste.
i painted the solution into the cracks covering about 35 mm wide for the full length, then " worked" the grout like solution into the crack with a spatula. i've found it works best if the filler is slightly "domed" over the crack. i left it to dry for 48hrs, papered back & the result to date - 5 days has been very good, hard to see any evidence of the cracks. with a good concrete paint on the garage floor it will look like it was when it was first poured. i'm now encouraged to do all cracks regardless of whether they will have floor covering over or not - just to satisfy my annoyance!
i've posted this to try & calm those who have similar experience - it upset me to look at the little devils, but the fixing has to date worked very well - i'll keep those interested posted should cracking re-appear or start to run again - however i'm sure the roof covering stopped the cracking process, this product certainly does the filling.
we r now into the plumbing & electrical work getting ready for render & gyprocking.
hope this helps others - cheers tony