Article originally appeared on Bluey Technologies
Why Cracks Form in Concrete Structures
Concrete provides structures with strength, rigidity, and resilience from deformation. These characteristics, however, result in concrete structures lacking the flexibility to move in response to environmental or volume changes. Cracking is usually the first sign of distress in concrete. It is, however, possible for deterioration to exist before cracks appear. Cracking can occur in both hardened and fresh, or plastic, concrete as a result of volume changes and repeated loading.
This involves tensile stresses being loaded onto the concrete, the cracks occurring when the force exceeds its maximum tensile strength. It is important to understand the reasons why cracking occurs, the type of crack formed, and cracks’ effects on structural stability. Once you understand these points you can take the appropriate action. This may mean leaving the crack alone, injecting the crack with an appropriate material, or applying other suitable repair methods.
Evaluating Cracks’ Causes and Status
It is important to identify the primary concern in regard to any cracking. The main concerns are whether the cracks are affecting structural integrity as a result of reduced durability.
Important Points to Evaluate
The type of cracking provides useful information to help understand a crack’s effects on structural stability. Figure 1 presents a summary of the different types of concrete cracks and their possible causes. A crack’s status is critically important. Active cracks may require more complex repair procedures that may include eliminating the actual cause of the cracking in order to ensure a successful long-term repair. Failure to address the underlying cause may result in the crack’s repair being short-term, making it necessary to go through the same process again. Dormant cracks are those not threatening a structure’s stability.
A crack’s environmental conditions influence the extent to which it affects its structure’s integrity. Greater exposure to aggressive conditions increases the possibility of structural instability. Cracks’ sizes range from micro-cracks that expose the concrete to efflorescence, to larger cracks caused by external loading conditions. Noting cracks’ sizes, shapes, and locations can aid in determining their initial causes.
Figure 2 illustrates the types of cracks and their primary causes in relation to their location.
Cracking in Plastic Concrete
Cracks that form in plastic concrete can be categorized as either plastic shrinkage cracking or plastic settlement cracking. These types result from the bleeding and segregation process that occurs when fresh concrete is placed. Such cracks usually appear one to six hours after concrete placement.
Plastic Shrinkage Cracking
As the concrete’s heavier particles settle due to gravity, they push the water and lighter particles toward the surface. This is called bleeding. If you fail to monitor the temperature, wind, and humidity conditions properly the evaporation rate of the surface water may exceed the bleed rate, drying out the concrete’s superficial layer and therefore shrinking it due to dehydration. The concrete beneath the surface layer is still well hydrated, however, and maintains its volume. This applies to oppose tensile forces to the lower part of the drying concrete on the surface, causing a cracked concrete profile.
These plastic shrinkage cracks are usually shallow and usuaLLY 1 to 2 mm in width, which means you cannot repair them with the injection method. They may, however, self-heal through continual cement hydration or by the precipitation of calcium carbonate from the concrete.
If the cracks are wider than 2 mm and do not self-heal, it is important that you repair them with a suitable coating or flood-grouting product to stop them from penetrating the full depth of the concrete slab. If they do become active their reaction to stresses may result in further cracking that weakens the structure either directly or by exposing its reinforcement steel to contaminants that will in time corrode it.
Plastic Settlement cracking
The settlement process is a major factor in concrete strength development. Figure 2 illustrates how plastic settlement cracks form. As the concrete bleeds, the water works its way to the surface. Sedimentation then occurs as the aggregate and cement move downwards under the force of gravity. This separation forms a weaker layer of the concrete near the surface. If steel reinforcing bars are close to the surface and insufficiently covered with concrete the concrete bends back around the restraint and cracks at the apex. Deeper sections of concrete lead to greater separation between the sediment and the water, so it is important to ensure that you cover all superficial restraints adequately to reduce the amount of cracking.
Plastic settlement cracks may also occur in forms involving a sudden change in the concrete’s depth, as it settles more in the deep sections than the shallow ones, forcing cracking at the point of change. A good example of this is waffle troughs, in which the depth changes constantly across the length of the form.
Cracking In Hardened Concrete
Cracking in hardened concrete can result from any one of many causes. These causes include
(a) drying shrinkage,
(b) thermal stresses,
(c) chemical reactions,
(d) weathering, which involves heating and cooling and is linked to thermal stresses,
(e) the corrosion of steel reinforcing,
(f) poor construction practices,
(g) construction and structural overloads,
(h) errors in design and detailing,
(i) externally applied loads, and
(j) poor loading and storage practices.
It is important to understand the factors that influence the above causes of cracking in order to eliminate the cause and select the correct repair method. The following sections explore the causes of cracking in hardened concrete in more depth.
This is the main cause of cracking in hardened concrete. This cracking takes place near the restraints due to volume changes in the concrete. When concrete is exposed to moisture it swells and when it is exposed to air with relatively low humidity it shrinks. If the shrinkage could occur without the use of rebar no cracking would result, but in most cases, the requirements of structural support make this impossible.
This cracking is the result of a combination of factors that influence the magnitude of the tensile stresses that cause it. These factors include the amount and rate of shrinkage, the degree of restraint, the modulus of elasticity, and the amount of creep. Additional factors to be aware of include the type of aggregate, water content, binder type, and the concrete’s mix proportions and mechanical properties.
The amount and type of aggregate and the cement paste are the main influences on the amount of drying shrinkage. To minimize the amount of shrinkage it is best to use a stiff aggregate in high volumes relative to the cement paste. The rate of shrinkage increases with the volume of cement paste. Similarly, increases in the ratio of water to cement in the cement paste increase the level of shrinkage by increasing the potential for volume loss through water evaporation.
The optimum condition for preventing drying shrinkage is a relative humidity of 100%. This is rarely possible, so sealing the concrete surface to prevent moisture loss can control the amount of shrinkage, and the use of suitably spaced contraction joints and proper steel detailing allows shrinkage to occur in a controlled manner.
Volume differentials are likely to develop in the concrete when different temperatures occur across a concrete section. The concrete then cracks when the tensile stresses imposed by a change in volume differential exceed that of its tensile strength. Thermal stresses usually cause cracking in mass concrete structures, the main cause of the temperature differentials being the influence of the heat of hydration on volume change. The heat of hydration is the amount of heat released during the cement’s hydration, causing a temperature differential to occur between the concrete structure’s centre and exterior as a result of either greater exterior cooling or greater heat hydration in the centre (see Figure 4). Either situation puts increased pressure on the exterior as the heat tries to escape from the core.
Chemical reactions in concrete are both due to the materials used in the mix and those it may have come in contact with. The cause of cracking is the expansive reactions between the aggregate and the alkalis in the cement paste. The chemical reaction occurs between active silica and alkalis, producing an alkali-silica gel as a by-product. The alkali-silica gel forms around the surface of the aggregate, increasing its volume and putting pressure on the surrounding concrete. This increase in pressure can cause the tensile stresses to increase beyond the concrete’s tensile strength. When this occurs the concrete cracks to relieve the pressure.
Corrosion of Steel Reinforcement
Three conditions must be present for metals to corrode. These are oxygen, moisture, and an electron flow within the metal. Eliminating or limiting any of these conditions eliminates or reduces corrosion of concrete’s steel reinforcement, thereby reducing the risk of cracking.
Concrete usually provides passive protection to the steel as it forms a protective oxide coating around it in an alkaline environment. However, corrosion may occur if carbonation alters the concrete’s levels of alkalinity.
Corroding reinforcement steel produces iron oxides and hydroxides as by-products. As these form on the steelworks surface its volume increases. This increase in volume increases the pressure on the concrete and causes radial cracking as the concrete fails under the tensile stresses. It is important to address these cracks because as they become larger oxygen and moisture have a greater chance of penetrating the concrete and accelerating corrosion of the reinforcements.
Poor Construction Practices
Numerous poor construction practices can initiate cracking in concrete structures. The following table presents these possible errors.
It is important to pay close attention to the way you load, transport, and unload pre-cast concrete, and how you secure it in place. At any one of these stages, the pre-cast concrete modules can become subject to stresses that overload the structure. If these stresses occur in the concrete’s early ages they may result in permanent cracks. You need to employ lifting procedures that disperse the load across the structure in order to reduce the risk of overload stresses.
Pre-tensioned beams may present cracking problems at the time of stress relief, especially in beams that are less than one day old.
You need to pay particular attention to the storage of materials and operational equipment during the construction phase, as these may generate loads that exceed those that the structure was designed to withstand.
Errors in Design And Detailing
Numerous problems can occur due to incorrect design and detailing, including increased concentrations of stress from poorly designed re-entrant corners, cracking due to inadequate reinforcement, and excessive differential movement from improper foundation design. It is therefore important to ensure that the design and detailing are specific to the particular structure and the loads to which it will be exposed. Overlooking these points may result in cracking, causing a major serviceability problem.
Externally Applied Loads
Most concrete structures are susceptible to external loads that induce tensile stresses through their concrete members. It is important to deal with these loads in the most effective way, so try to disperse the load evenly across the individual members to reduce the risk of uncontrolled cracking. Factors that can reduce cracks’ widths are an increased amount of steel reinforcement and larger concrete sections to disperse the loads more evenly.
The Desired Outcome of Crack Repairs
Once you understand the cause and significance of the cracking you need to apply the appropriate repair method or methods. You should select the repair method based on an evaluation of the crack and the repair’s objective or objectives. Such objectives include;
(a) restoring or increasing strength,
(b) restoring or increasing stiffness,
(c) improving functional performance,
(d) providing water tightness,
(e) improving the concrete surface’s appearance,
(f) improving durability, and
(g) preventing the development of a corrosive environment for the reinforcement.
Article originally appeared on: www.bluey.com.au
*” Editor’s Note: This post was originally published in November 2014 and has been updated for accuracy and comprehensiveness.”