Even with the best commercial-grade equipment, operator error can ruin an expensive pour. Diagnosing concrete surface defects often leads right back to how the vibratory screed was handled. The most common error I see is over-vibration.
If an operator leaves the machine running while standing still, or moves at a snail's pace on a wet mix, he will drive out all the coarse aggregate and bring an excessive amount of water and cement dust to the top. This creates a thick "paste layer." While it looks beautifully smooth when wet, it is structurally disastrous. When this watery layer cures, it shrinks aggressively, leading to a network of fine surface cracks known as "crazing."
Another serious issue caused by bad screeding is blistering. If the operator uses the screed to completely seal the top surface before the internal "bleed water" has had a chance to rise and evaporate, that water gets trapped just below the dense surface. As the water and trapped air try to escape, they form blisters—hollow bumps on the surface of the slab that will pop and break under the first sign of foot or vehicle traffic. Operating this machine requires reading the concrete, understanding its moisture content, and knowing exactly when to pull the trigger and when to back off.
