Concrete can be a fickle material sometimes, since there's variations between batches of concrete mixed, and external factors can change how concrete cures. Also, concrete can be a challenging medium to work in because of the long cure time, waiting days to see mistakes can make you wonder exactly where you went wrong.
Luckily, the post-mortem of concrete can tell us a lot about where any shortcomings were. Though even the best prepared plans can go awry, here are a few common problems and their solutions.
Concrete Crumbly When Cured
This sometimes happens when you remove the concrete from the mold before it has completely cured, or if your water to cement ratio was too dry or too watery. See the Mixing and Pouring Concrete Lesson for tips on how to mix concrete perfectly every time.
Concrete Has Bubbles After Removing From Mold
Bubbles are caused by not vibrating the concrete sufficiently after pouring. Take the time to really vibrate and shake up your pour in the mold and get all those bubbles. For deep molds try degassing after each short pour to release the bubbles as you go. Learn more about vibrating concrete in Lesson 2.
Ouch, Concrete Burned Me!
Concrete is alkaline, the opposite of acidic on the pH scale, which means it will cause burns when in contact with skin when it is wet. There are some steps you can take to minimize contact, like proper PPE, but there's also some triage treatment in case of emergencies. You can find all that info in introduction in Lesson 1.
Lots Of Flashing on My Concrete
Flashing is caused by over-pouring in molds. While flashing isn't hard to deal with, it can add time to finishing your concrete piece, and larger flashing artifacts can sometimes break off and mar the finish of the cast piece.
How to remove flashing is covered in this Lesson.
My Casting Is Stuck In The Mold
Did you you a release agent? There are specially formulated sprays that are applied to molds that allow castings to easily slip out after curing. However, regular cooking spray also works (and is probably more readily available).
Taking the Next Steps
You've completed the basics, now it's time to take your concrete projects to the next level.
Though the projects in this class were small-scale, there's no limit to how big your next concrete projects can be. Here's a few more projects that use the skills learned in this class and take concrete concepts a little further:
Your concrete creations are sure to be the perfect addition to your house, and will last a lifetime. Get messy with concrete and make something awesome! Then, share your creation with everyone with an Instructable!