Concrete Degradation, Causes and Remedies

Counteracting degradation in concrete

Per prevent degradation of concrete it is necessary to intervene on the factors that prevent thewater inlet in the cement matrix and the consequent deterioration of the reinforcements.

These factors are: the reduction of the permeability of the cement paste due to the presence of pores, the presence of a layer of iron cover adequate, the composition of concrete with the right proportion of the constituent elements.

Il degradation of concrete in structures depends on the interaction of these with theenvironment that surrounds them.
One of the conditions that most determines the progressive deterioration of concrete is the presence of humidity.

Concrete damaged by rising damp – Getty images

In order for the material to be subjected to aggression it must have a certain porositywhich allows the entry of thewater in the cement matrixand with it a series of aggressive agents (even oxygen itself is for armor).

The more difficult it is for water to penetrate the pores, the more the concrete will be durable.

The durability of concrete

Durability is the ability of materials to keep their physical and mechanical characteristics.
In order for the safety levels of a structure to remain the same throughout the useful life of the work, it is necessary to guarantee the durability of the material.

For what concern concreteit must last over time, resisting the aggressive actions of the environment in which it is found.

In the absence of external aggressions, the concrete should even improve its capabilities due to the continuous reaction process between water and cement.

How is concrete made?

Il reinforced concretethat is, the one used for the construction of structural elements, is made up of three components main:

• l’steelcomponent present in the armor
• the cement pastemade up of clay or limestone rocks;
• l’aggregate o inertusually gravel or crushed stone.

Il cement in powder form, mixed with thewateroffers good resistance to compression, hardening over time by a process of hydration. Once hardened it maintains its stability even under water.

What concrete looks like when poured – Getty Images

The function of theinert is instead that of being the resistant skeleton and to make the concrete mass as compact as possible. This is achieved by choosing assorted aggregates of different sizes, so that they can fill the spaces to the maximum empty.

The fundamental parameter that determines the resistance of concrete is the water/cement ratioThe water in the mixture performs the function of hydrating the cement, that is, the taking phenomenon.

Its function is also to make the dough workable. It is therefore necessary to find the right compromise: a lower a/c ratio implies an increase in the durability to the detriment of the workability. On the other hand, an air/c ratio that is too high compromises the resistence final concrete.

Causes and effects of concrete deterioration

The factors that influence the characteristics of concrete, leading to its progressive degradationThey are different.
They can be external factors o internal factors.

The external ones do not depend directly on the material, and are:

• Environmental factorssuch as temperature, humidity, the presence of chlorides and sulphates.

The internal ones are strictly linked to the composition of the material itself, and are:

• How to performthat is, the laying of the concrete and its subsequent compaction and maturation.
• Design solutionssuch as the thickness of the concrete cover.
• Concrete technologythat is, the type and quality of the cement, the water/cement ratio, the quality and size of the aggregates.

All these factors can generate one or more of the following in the concrete effects:

• Corrosion of metal reinforcementdue to external atmospheric agents, due to the low thickness of the concrete cover.
• Sulphate attack of the cement matrix.
• Ice formation between the cement matrix and the aggregates.
• Surface washout of concrete by acid water.
• Micro cracking induced by hygrothermal variations combined with static stresses during operation.

Example of low concrete cover thickness resulting in corrosion – Getty Images

But how do they do it? aggressive agents to enter a concrete element? Through thewaterwhich transports them through the interconnected pores present in the cement matrix.

Corrosion of metal reinforcement

The corrosion from the metal armor It involves the transformation of the Iron (Fe) contained in the steel into rustor oxidized iron.

Rebar corrosion phenomenon – Getty Images

This mechanism is activated by the process of carbonation of concretethat is, the reaction of thecarbon dioxide contained in the air with the lime contained in the concrete. This reaction generates the carbonate and drastically reduces the highly basic condition present in the pores.

In conditions of basicity lower (already with pH < 11) the ferro loses its protective film of iron hydroxide and thus suffers the corrosiondue to contact with oxygen.

To reduce the phenomenon of corrosion of the armors necessary reduce porosity of the concrete. It is possible to act on the water/cement ratio at the time of its establishment, or increase the degree of compaction at the time of casting.

Another way to counteract the phenomenon is to act directly on the ferrothrough processes of galvanizing o cathodic protection. You can also act directly on the exposed iron by spreading a special two-component mortar that acts as a protective layer against corrosion (an example is the Mapefer from the Mapei).

However, they are always essential: correct maturation and the correct sizing of the iron cover; the latter can be restored following damage by means of new jets of malta specific (an example is the malta tixotropica from the Weber).

Sulphate attack

L’sulphate attack consists in the reaction between the sulfate ion SO₄²‾typically present in brackish waters and in some soils, and the products present in the cement matrix.

The attack by the sulphate ion can be externalif it comes from the soil or water in contact with the structures, or internalwhen already present inside the concrete.

For the transport of the ion through the porosity a means such as the is indispensablewater.
It, once in contact with the limereacts by forming the gesso (calcium sulphate dihydrate). Gypsum in turn reacts with hydrated aluminates and forms ettringitewhich causes an increase in volume and consequently to phenomena of swelling e lamination.

L’sulphate attack may be responsible for a drop in mechanical resistance due to the descaling.

Ice formation

Freeze and thaw cycles within the cement matrix they cause a variation in the stress state with consequent cracking e disintegration of the material.

It is therefore important to pay attention to the degree of saturation of concrete: if it is lower than the critical saturation, the material will not suffer damage.
To keep saturation at bay, it is necessary to set a limit to the water/cement ratioin order to reduce the capillary microporosity of concrete. It is also possible to promote the presence of pore large in the form of air bubbles, perhaps by introducing a sanding agent (an example is the Mapper from the Mapei).

In case the presence of a porosity excessive, you can intervene with water repellent treatments o obstructions with silico-fluorides.

Concrete surface washout

It consists of theexport of material of the cement matrix by running waterThe phenomenon is aggravated if the water is acidic.

Preventing Concrete Degradation at the Design Stage

To ensure the durability of concrete in structures it is necessary to carefully study the concrete mix design (or its composition) at the project stage; in particular the water/cement ratio significantly affects the resistenceon the workability and on the presence of porosity.

Almost all the phenomena seen so far have as a consequence the opening of cracks in concrete, which significantly alters the durability of the material, exposing the armature all’aria.

Cracks in concrete – Getty Images

To prevent degradation at the design stage, the addition of additives to be inserted into the mix, which They make the dough more fluid without the need to add water outside of the calculation during the casting phase.

Fluid concrete being poured – Getty Images

It is also necessary to study the environmental conditions to which the structure will work, so as to prescribe a steel more or less sensitive to the corrosion.

Another very important issue in the project stage is the sizing of the concrete coveras a protective layer of concrete for reinforcement.

In the executive phase it must be carefully verified that the concrete used actually corresponds to the one in the project; it is also necessary to ensure the correct compaction in the casting phase (through the vibration of the still fresh concrete).

Concrete Vibration – Getty Images

The next one too maturation must be followed, checking its speed evaporation of the water.

The legislation requires some specific checks to the SLE (Serviceability Limit States) to take into account all aspects relating to design.

The resistant reinforcement must be protected by an adequate concrete covering. The structural elements must be verified at the cracking limit state according to § 4.1.2.2.4. In order to protect the reinforcement from corrosion, the concrete covering layer (reinforcement cover) must be sized according to the aggressiveness of the environment and the sensitivity of the reinforcement to corrosion, also taking into account the tolerances of the installation of the reinforcement; for this purpose, it is useful to refer to UNI EN 1992-1-1. Chapter 4.1.6.1.3. NTC 2018