We get it, weather in Michigan can have a mind of its own. This can be a real struggle when dealing the ever-changing conditions. Pop-up rain showers and thunderstorms can prove to be quite a challenge when navigating concrete pours, especially during the summer months. What are some of the issues that can be caused by rain, and what can you do to be prepared in the field and minimize the damage that could be caused by these pop-up threats?
If rain is expected to occur on newly placed concrete pavement that has not hardened, we recommend covering the surface with plastic sheeting. The sheets should be weighted down continuously along all outside edges to prevent them from blowing off in the wind.
You may expect some marring of the concrete slab surface from plastic sheeting used to protect slabs from rain. Except for a blotchy or mottled appearance, there is nothing wrong with surfaces affected by plastic sheeting. A similar appearance can occur when using plastic sheeting to cure concrete.
Climatic conditions during a rain event can be conducive to good concrete curing. During a rain shower, the humidity is at or near 100% and there is little chance for evaporation of mix water. Temperatures are generally moderate during rain, which is also beneficial. In these situations, the rain essentially provides a beneficial “moist” curing environment.
Any additional water on the pavement surface that is mixed into the concrete surface by direct impact or by finishing action will elevate the surface water-cement ratio, potentially reducing durability. A contractor should remove any excess water with a burlap drag or wait until it drains from the surface before refinishing or re-texturing.
If the rain is extremely hard and occurs soon after concrete placement or during placement, such that the surface mortar is significantly distorted, it is likely that the rain has affected the water-cement ratio near the surface (within about 0.25 inches). Corrective action is usually necessary in this circumstance – we recommend diamond grinding the entire surface in the affected area. Equipment and technologies have been developed that provide a “diamond-ground” slab surface that is attractive and durable. However, we do recommend that a small area be treated for owner approval prior to grinding an entire slab.
Proper planning prevents poor performance. Concrete contractors need to be proactive and aware of the risks of rain events. Being proactive and developing a wet weather plan are keys to success. Have enough plastic on site to cover the portion of the pour that is vulnerable should a rain event occur. Note: the timing of a potential rain event after completion of a pour must be considered. In typical summer weather 1 to 2 hours may be sufficient to resist rain, but in cooler weather a rain event that takes place many hours later can cause damage.
If you would like more information on “Concrete Pavements Exposed to Rain” check out this great article in Concrete International.