When pouring concrete contractors know you can be sure about two things: it will get hard and it will crack.
First, let’s see what concrete is all about: it mainly consists of four components – Portland cement, gravel, water and sand. Water triggers a chemical reaction when mixed with cement, called hydration - this allows the mix of gravel, sand, water and cement to gradually harden over time.
In order to better understand why and how concrete cracks we have to discuss compression and tension, two forces that will always affect our future home. Concrete is very strong in compression but rather weak in tension (tension strength of concrete is about 10% of its compressive strength). This is the main reason why concrete is almost always used together with steel reinforcement. Concrete and rebar are the perfect love story:
1. Concrete is strong in compression but weak in tension while reinforcement steel is strong in tension but weak in compression: the perfect match.
2. Their coefficient of thermal expansion is similar – the two expand and contract at the same rate. If that wouldn’t be the case they would literally tear themselves apart over time.
3. When properly used, cement paste creates a surface-film on the steel preventing it from corroding inside concrete
4. The bond between steel and concrete is very strong - this is why concrete can pass stress to the steel and vice versa.
Concrete cracks before hardening are OK. After, not so much...
Cracks occurring BEFORE hardening are usually the result of settlement within the concrete and/or shrinkage of the surface as water evaporates while the concrete is still fresh.
The water-cement ratio is crucial when we discuss concrete: the minimum amount of water needed to start the hydration process (hardening) is 25% of the weight of cement. In practice however, we’re getting close to 50% if we want to have a smooth, nicely flowing mix. And we want that because concrete has to flow and squeeze through all that reinforcement steel. Why is water content so important? Because every drop of water that exceeds the 25% will NOT be used in the chemical hydration process, instead will get stuck in concrete and evaporate later on, leaving a fine network of capillary voids weakening it and allowing cracks to appear – concrete shrinks about 1.6 mm (1/16 inch) for each 3 meters (10 feet) when hardening.
Plastic-shrinkage cracks are very common in slabs- they are short cracks that appear when surface moisture evaporates before it can be replaced by bleed water coming from below. That's why the surface shrinks faster than the interior causing cracks that can reach the middle of the slab. Like when you are pouring a patio slab on a hot, dry and sunny day:
Cracks occurring AFTER hardening are the ones that should be analyzed carefully. They are the result of applied loads and sub grade settlement.
The two main forces affecting any construction are static loads and dynamic loads: static loads remain unchanged for a long period of time (e.g. a structure’s own weight) while a dynamic load is unstable and moving (e.g. hurricane winds). When designing a house the sum of both loads is taken into account in order to ensure its stability over time. If the final result (our house) does not meet the standards for those calculated loads it will suffer from settlement depending on the action of those forces. E.g. cracks may appear after high winds because of the load they apply on the exterior of the walls, or on load bearing beams and columns if they fail to support their own weight. This is very rare though, and it usually means something must have gone terribly wrong in the construction phase.
The shape and direction of the cracks are important hints as to what is going on. There are three main types of wall cracks according to their direction:
1. HORIZONTAL cracks are almost always caused by an applied load. The following picture is a good example of an excessive load placed on top, as the owner decided to place a 5,000 liters (1300 gallons) water tank on the roof:
2. VERTICAL cracks that are wider at the top or bottom are an indication of settlement:
3. DIAGONAL cracks are usually found in the corner of a window, door, beam pocket or other openings. They are called re-entrant cracks and are of no structural concern unless, of course, they allow you to see your neighbors waiving their hands at you.
As a rule of thumb, when checking a house, you should pay attention to cracks that exceed 6 mm (¼ inch), cracks that leak water, or long horizontal cracks.