Hairline Cracks in RC Slab

You have not stated width of slabs or what reo you have used.and if there are any strenthening beams.

as for the concrete I would have used 40 mpa, what you have stated is what we use as driveways in Australia

Alf Arnold Be Mba.

Is it dangerous structurally?

Not at all. You are carrying a 28cm RC slab x8m span.

ITS SELF WEIGHT IS 700kGF/m² already!

Fine Cracks at bottom centre only means that the steel is almost at the kink of the (yield point)--linear Stress-Strain Curve.

You still can go to double the loading upstairs on this slab with live/dead loads.

Ramil

The structural danger is obviously related to the bearing surfaces and styles of support being provided to the floors between the cracks. You may have to calculate for additional columns and support members if the slabs are off-ground. Mesh is not a support consideration, only a strengthening strategy.

Cracking is a universal problem caused by shrinkage during the curing process, and usually avoided by pouring in smaller contained and supported surfaces. Large surfaces are sometimes saved from cracking by the addition of CO₂ during the curing process, I'm told. Also check out the sites of various pozzolan provider companies to see what's new in that area.

When I laid a large concrete surface on ground, I had it deliberately cut into smaller areas, and the cuts sealed with epoxy to provide a smooth surface and expansion joints. The cuts did not go all the way through the floor, since it was expected to crack along the lines of the cuts as the places of least resistance anyway.

Mark

Cracks way be formed when the curing process happens too quickly. It is more likely to happen with a higher ambient temperature. Generally any temperature above 75 deg F or 22 deg C would benefit with cooling to slow the curing process. While building bridges, above these temperatures, we would typically hose down (cover with water) the concrete every hour for the first 48 hours after it has set.

What's the width of the slab? Is the crack running in the main rebars direction or along the secondary bars? Probably the temporary formwork support was removed before the concrete achieved its required strength for self support. Another possibility is the position of the bottom bars may be too high i.e too much concrete cover. All this need to be checked.

Anyway, you have to seal the 10cm deep cracks with epoxy resin injection to prevent corrosion to the rebars.

Whether it is safe or not should be determined by a professional Engineer after evaluating all the design parameters. Of interest would be whether the rebars was designed correctly, and then, was it placed accordingly. Make sure to check position of the rebars as it affects the slab capacity tremendously, especially the bottom bars. Then look into the concrete strength. You mention that the strength is around 28mpa. I am not sure how that value was obtained. Is it insitu strength i.e. from core samples or is it based on the cube test records during the construction stage? If it is from the cube test samples, then you will still need to take core samples for compressive strength testing to determine the insitu strength. After getting all this datas, then only you can evaluate whether the slab is safe or not.

Cheers.

You need to provide reinforcement information. If the reinforcement is properly designed there may not be any concerns at all.

leneiar cracks are indicative of structural defects but there are other reasons which can cause this it can be due to exothermic reaction due to imbibed quick setting chemicals or any other additives in concrete also if calcium content is more the reinforcemts can also contribute to this if they are corroded which may not be the case with you (reinforcement can swell due to corrosion and can cause cracks when concrete is in green or grey stage) last and least thing to think is due to bending movement

remedy can be cut a v groove of about 2 inch and pour only epoxy resin and top it with 100 mesh sewed sand and allow it to be like that ofcourse you can clad it later with flooring materials