Power washing concrete surfaces can cause real problems. Relatively inexpensive high-pressure power washing units are commonly available. Some of those units can deliver water at pressures well in excess of 6,000 pounds per second (psi)! Moreover, it is not just high-pressure water that’s the problem. The water exits the nozzle at both a high pressure and a high velocity. The resulting momentum is great enough to dislodge not only dirt and debris but also to create flakes, popouts, and even concrete spalls. Good quality concrete will also experience accelerated wear from high-pressure power washing.
However, there exists another problem that is not so obvious and requires a little bit more thought. Concrete is both porous and permeable. It is porous because it contains millions of microscopic and submicroscopic voids. Entrained air voids are just one example of the voids within concrete. As concrete hydrates, the water-filled pore space between cement particles will fill up with hydration products thereby decreasing the permeability of the concrete. For good quality concrete, that process takes a matter of days or weeks. For poor quality concrete, that process may never occur.
Therefore, what happens at the microscopic level and what we see at the macroscopic level are directly related: the permeability has a huge impact on what happens to concrete that’s power washed. Especially because the pores close to the surface (or lack thereof) are the ones that have the greatest exposure to everything that harms the concrete and provide the greatest protection.
High quality concrete with a low water to cement ratio (less than 0.45) typically has few capillary pores and is essentially impermeable. Moderate quality concrete with higher water to cement ratios (greater than 0.45) contains a much larger fraction of capillary pores. Close to the surface, this type of pore structure will actually soak up some surface water; similar to a sponge.
Power washing will only accelerate this absorption. Poor quality concrete contains many relatively large connected pores that typically allow water to slowly travel through the surface layer and into the concrete, like a sieve.
Therefore, power washing creates two potential problems. The first is the aggressive nature of the high pressure and high velocity. We are artificially eroding and abrading the concrete surface. Quite literally we are adding ‘wear and tear’ that is just not necessary. The second problem is that we may actually fill up pores that are meant to be empty. Since those pores are right at the surface, where we need the best durability, we can be creating a real problem. Not only can saturated surface pores expand as the water freezes, but also they can aid in getting chlorides and other harmful chemicals closer to reinforcing steel. In fact, annual power washing can start a devastating vicious cycle.
What’s the answer? Most people know that an automatic car wash (or power washer) will put added wear and tear on their car’s exterior finish. A hand wash will remove the same amount of dirt without damaging the finish. Concrete surfaces follow the same logic. Gentle brushing, mild cleaners, and low pressure can combine to keep a concrete surface looking and performing as it should.