It will likely depend upon if there was negligence on the trucking company or their driver. For instance if it was a truck that was loaded with rocks or gravel and they were not tied down properly then you may be able to prove negligence. By proving negligence the trucking company should be liable for rocks that fell off of the truck cracked your windshield.
However if instead the truck kicked up a rock from the roadway that then hit your vehicle and broke your windshield then it would be doubtful that the truck driver or his trucking company could be held responsible since that was an unintentional road hazard.
Every individual operating a motor vehicle within Texas has a duty to exercise reasonable care in the operation of that motor vehicle. The burden of reasonable care makes drivers accountable for acts which they knew would likely result in injury to person or property damage and for acts which they should have known would likely result in injury to person or property.
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Texas state law identifies a driver's failure to use reasonable care as negligence, so normally you would have to prove that that reasonable care was not taken by the truck driver or trucking company in order for their insurance company to accept your claim.
For your incident you may need to show that the driver had knowledge that the rock that came from his truck and could have somehow prevented it from coming loose and striking your car. To find out for certain what your rights are in this situation discuss it with your insurance agent and/or the Texas Department of Insurance.
If you cannot make a claim with the 18 wheeler's insurance company then to get your windshield replaced you will need to place a claim with your own comprehensive coverage. | 
