• If the on-site slump test fails, the site engineer may use his/her contractual power and reject the concrete and suspend the concreting process.
• However, it is believed that a slump test is just a measure of concrete consistency, and it may not cause the reduction of concrete strength. So, the site engineer can allow the contractor to continue concreting work, but proper slump adjustments should be made.
• If the concreting process is permitted to be finished, the contractor still needs to prove that the finished work complies with other contractual requirements, such as a compression test.
• Engineers should be aware that the workability is related to both strength and durability of concrete. If the concrete attains the designated strength, the durability may still get affected.
• If the concrete slump value is too high, the site engineer may allow the mixer to revolve for 8 to 10 minutes to reduce the slump. If the test still fails, one may add cement to the concrete mix to minimize slump value.

For more details: https://theconstructor.org/concrete/site-slump-test-fails/176167/