Your best option is to keep them clean. Wet them before each use, and clean them with water and a sponge, brush, etc. after every test. The scoop is especially sensitive, because it is easy for build-up to accumulate in the back of the bowl, near the handle. Also watch for areas where concrete can easily accumulate (inside the slump cone handles).
You will find yourself in situations where there does not seem to be sufficient time to clean the equipment between tests, do your very best. Fast setting concrete is another challenge, where the concrete seems to harden before you can complete your tests and start cleaning the equipment.
When I finish my slump test, I immediately clean the slump cone and base plate. When the air test is completed, the top section of the air pot is immediately cleaned. Then I dump the base, and submerge it in water and clean it after test cylinders or beams are molded. Unless you are working with fast-setting concrete, the scoop, strike off bar or trowel, and tamping rod can be left in water until you have placed your samples in initial curing.
Dilute hydrochloric acid can be used to partially dissolve the dried concrete, but this can cause corrosion. There are products available on the market that supposedly clean the equipment without corroding it. I've never trusted them enough to try any of their products. Usually, the best approach is patiently chipping the hardened concrete away.
