I live in a new home ( 4 months old) My sidewalks are cracked and so is my driveway. Also right ware my garage door meets the floor the concrete is crumbling. What causes that, and should it be covered under a warrenty ?
Unfortunately, cracks in concrete are quite common and it's fair to say virtually unavoidable. However, a good concrete contractor will strive to assure that all steps are taken to try to prevent cracking. Most cracking results from the natural shrinkage of the concrete. Shrinkage can be minimized by proper control of all of the aspects that affect shrinkage.
Excessive cracking can be caused by many factors or a combination of factors including improper concrete mix, overly wet mix, settlement of poorly compacted substrate, ground temperature, air temperature, humidity and wind conditions, lack of construction joints or improper placement of such joints, bonding of concrete to the foundation block or concrete, not allow for contraction and expansion, improper floating, troweling and finishing, improper time between each step of the process and a whole bunch more which only a concrete expert could explain.
Not to point a finger in any direction, but often a general contractor is overly concerned about his scheduling and might push the envelop when ambient conditions are not favorable for a quality job. This is when a concrete subcontractor who is a real professional would balk at the contractor's pressure because his reputation may end up suffering as a result. Also, this is why price should not be the only criteria for selecting contractors/subcontractors. The lowest price subcontractor might do all possible to speed up completion of the job so he can squeeze another job in the same day while disregarding proper timing and length of time for each step of the process.
The crumbling could be the result of improper mix, improper floating, timing or finishing. Or, if you have a garage door opener it may not be adjusted correctly and the door is hammering the edge of the slab excessively. The same could be possible with a manual door where the spring is not set properly and the door is slamming hard on the slab.
The National Association of Home Builders guide to Quality Standards states..."Cracks exceeding 3/16" in width or 1/8" in vertical displacement shall be repaired." The corrective measure in the standard states..."The contractor will repair cracks exceeding the performance standard by surface patching or other methods as necessary."
Your contract with the builder should spell out what warranty was included. If it didn't or if you bought a spec home the purchase agreement should spell out the warranty. If neither did, most states have laws covering residential construction that normally state the warranty for construction work is one year from acceptance of the property or certificate of occupancy or notice of completion. And, lastly any honest and fair contractor would step up and fix the problems at least to meet the quality standards promulgated by NAHB. Good luck.