My concrete contractor did a stamped patio for me but his crew botched the job. There were huge imperfections in the job and I made these known the day after they were done. The contract I signed states that i pay 50% at the start of the job and the other 50% after the job is completed. I have paid the first half but not the second. The contract also stated that all defects related to workmanship would be corrected in 5 days. It was a week before they did the first repair attempt and over another week after the second repair attempt. They seem to make the concrete look worse each time they try a fix. Would this be considered a material breach of contract? Also I do not want to pay the other half at the moment since they have made the patio look even worse and in my eyes this job is still not complete, should i hold out the second payment till they fix it?