What You Need To Know About Chert

Chert is a porous and easily weathered rock that is often found in some concrete mixes. It's a natural component of many soils, but it has effects on concrete that are unwanted in most settings. Chert content is one factor in determining whether a concrete mix is considered high- or low-quality, with low-chert mixes being considered high-quality. The visual effects of chert may be desirable in limited circumstances, but overall, if you want smooth, clean concrete surfaces, you'll want a low-chert mix, including if you're using ready-mix concrete.

It's Naturally Occurring in Varying Amounts

Chert is a common substance in soil; it isn't a pollutant. Some quarries are in regions where chert is very common, and any aggregate mined from there will have a lot of chert mixed in. If you're using ready-mix concrete for a large residential project, you'll want a mix that is either from a quarry that is low in chert, or a mix that has been sorted to remove chert. (These will be marked as low-chert, so you won't have to guess.) Note that sifting can take a long time, and that might influence the price of the mix.

It Leads to Parts of the Concrete Mix Actually Popping Out

What does chert actually do? Because it's so porous, it absorbs moisture easily. That leads to two problems. One is that the moisture can make the chert particles swell, leading them to actually pop out of the concrete's surface while also potentially cracking the surrounding mix. The other problem is that the moisture absorbed by the chert can freeze on cold days, and as that ice expands, it can crack the concrete. The result is a surface that is flat but filled with cracks and odd hollow spaces where the chert popped out. Now, if you want that look, say, for a section of a concrete wall where you want the surface to look weathered, chert could be helpful; you'd pour the regular wall with a low-chert mix and add an additional layer after that, that was made with a high-chert mix and allowed to crack. But for smooth surfaces, a low-chert ready-mix is the way to go.

Larger concrete jobs often rely on ready-mix, but the manufacturer should be able to tell you if the mix is low- or high-chert. When you use that lower-chert, higher-quality mix, you get concrete that continues to look smooth for a long time.

Contact a local concrete service, such as Puente Ready Mix, to learn more.