Concrete Basics FAQ’s – Why did it crack?

I know that this post may seem out of place as I have already been making DIY projects out of concrete and providing tutorials for many years. However, much of what I have learned along the way has come from actual experience – Isn’t that really the best way to learn?!

What is concrete?

The terms concrete and cement often get used interchangeably but actually are not equal. Concrete is the combination of cement (often Portland cement but not always as some use a rapid setting hydraulic cement) and other coarse aggregates or additions. The aggregates can be sand, gravel or crushed stone. Once cured the cement and it’s additions become one very hard rock-like mass/compound.

How is Cement Made?

To make portland cement natural mined raw materials such as limestone, shale, iron ore, and clay, crushed and screened the rock, are heated by cement manufacturers to extremely high temperatures in a cement kiln. These materials form a small ball called “clinker” that is very finely ground to produce portland cement.

How/Why Does Concrete Harden?

Concrete becomes hard through a chemical process called hydration. The Cement particles react with water to form new stable very hard compounds that coat and bind the aggregate particles in the mix. The hydration process happens quickly at the beginning and then slowly to gain strength over time. Curing plays an important role in strength development and durability of the concrete. To ensure a good cure continuously wetting the exposed surface thereby preventing the loss of moisture from it will help.

If water evaporates too quickly, it will weaken the finished product with stresses and cracking.

Why did my concrete crack or break?

There are a few problems that could be the reason:

Water-cement Ratio:

Adding too much water to the concrete mix (water-cement ratio) will change the density and therefore compromise the strength of the cured concrete. The high amount of water can create shrinkage in the concrete during curing and therefore crack. In concrete crafting we sometimes need a fairly fluid concrete to get it into the small mold cavities. Instead of adding extra water a special additive made for the mix would be better to add such as the Flow Control for Rapidset Cementall.

Improper Curing:

For concrete to gain compressive strength (anywhere from 2500 psi to 5000 psi, pounds per square inch) it needs the chemical reaction to have proper curing. This curing is dependant on the reaction of the water and the cement (portland or other) in the mix. The temperature and weather conditions is also important as if it is very warm/windy the water will evaporate very quickly and thus not doing it’s job. To remedy this problem you can cover the curing concrete with plastic to prohibit the loss of moisture or also mist with water during the initial curing time. The ideal curing temperature is between 50ºF -90ºF. Heat will also accelerate the curing with some mixes (as Rapidset Cementall) and make for not enough working time. Using very cold water can slow the setting, and also using warmer water may be an accelerator. Each manufacturer has agents specifically made to slow or increase the setting/curing of their mix. A solution of citric acid can slow the setting of Rapidset Cementall.

Since cold weather leads to colder concrete, the set time can be delayed. Accelerators added to the concrete can help. Addition of 2% (by weight of cement) of calcium chloride is the traditional way to accelerate the hydration reaction—it is very effective and reasonably cheap

Lack of Reinforcement:

In the sidewalk concrete slabs you see rebar inserted for strength. However for smaller crafts the addition of Fiberglass fibres act in a similar way. Fiberglass reinforcement helps prevent cracking due to stress or age. Also when I dip fabric in the Portland cement mixture (concrete draping) the fabric fibres act like a reinforcement.

If the concrete piece is subject to some breakage from trying to force it from a mold perhaps a reinforcement in the mix will give it the necessary strength.

Expired Concrete Mix:

The bag of Concrete mix may also be past its prime date. As much as we would like to think; concrete mixes that we buy at the home improvement store do not last forever. The bags of dry mixes will lose their ability to be strong concrete. The storage of these mixes is important as moisture and fluctuations of temperature will affect the concrete mixture. The cement bags absorb moisture from the air, the hydration process will start. Concrete mix will start to lose its strength gradually as some little amount of moisture content is always present in the air. With this process, over a period of time, the strength of cement reduces. If I am in doubt I like to mix a small amount and do a test. After a year of improper storage a bag of concrete mix may have lost 50% of its strength.

When I started concrete crafting I did not realize that the concrete would get ‘old’ and I would wonder why the finished piece just did not seem as strong as the usual ones I had made. It would set but the concrete’s durability would be highly compromised and if I applied pressure it would break.

Proper Storage:

To get the most longevity from your bag of concrete store it in stable dry conditions in airtight containers. Often the paper bag is lined with plastic to prohibit moisture from entering but it will still lose its ability to bind in only a few months. To get as long as possible life I often transfer the mix to a sealed bucket and bring indoors from the shed.

Improper Winter Storage:

Since I live in Canada I know well about freezing! If you make a vessel (concrete pots, planter, bowl etc) and leave it outside in the winter it may collect water. As it gets cold the water will freeze and expand; that expansion will force the concrete to crack and break. Depending on the size and thickness it may be more or less destruction.

‘If’ I leave these concrete planters out through the winter I may tip them on their side or cover to avoid the collection of moisture. A concrete bird bath may also need to be covered.

Use of Wrong type of concrete:

Just like when cooking or baking it is key to use the right ingredients. I often get this question since many people think ‘concrete is concrete’ – all the same! But that is not the case at all! ‘Just like sugar can not substituted for flour. I have many Concrete Crafting projects that all use specific methods and therefore suggested specific Concrete mixes or recipes.

I have written this post with suggestions of which mix to use for certain uses. Often the prices of the mixes also varies a lot since some the fast-set mixes are quite different from the cheaper ‘everday’ regular concrete bags. Generally they can not be substituted… Rapidset Cementall is a favourite mix to use for its extra strength (uses calcium sulfoaluminate instead of portland cement). It does however have a very fast set time. This may be a problem for your specific concrete craft, especially if the weather is also not accommodating (see above). The additives (aggregates) in some mixes will not work well for a small mold.

The final ‘look’ of the concrete will also be different depending on the mix you use. Some show more of the large grain sand and some show less if the mix is finer. This will show in the surface of the concrete as well. We see this in the well worn sidewalks.

These issues should not deter you from trying some concrete crafts. As with anything new start with the simpler projects. I have a Tip & Tricks for concrete crafting just to help with all the unique concrete projects. Happy making!