Problems / Pathologies
Common problems found in concrete monuments
Reinforced concrete was the material of choice for many architects of the modern era, and they exploited the material in a multitude of creative and innovative ways. Many of the modern era's most extraordinary structures demonstrate the potential of reinforced concrete and illustrate the material's rapid evolution over the twentieth century. Like many modern materials, reinforced concrete has raised new and distinct conservation challenges. These issues relate to the lack of appropriate techniques and materials to meet conservation needs, the current lack of knowledge on the efficacy and durability of existing repair solutions, the shortage of training opportunities, and the lack of technical and guidance resources available to professionals.
It is the attrition of a surface produced by friction or rubbing caused by phenomena such as the passage of vehicles or pedestrian traffic on pavements, or the impact or sliding of materials from landslides. This wear can also be produced by the action of heavy particles in water that circulates at high speed, such as in the case of bridge piers, reservoirs, dikes, and other hydraulic works, etc.
The action of acids on hardened concrete consists of the transformation of calcium compounds (calcium hydroxide, hydrated calcium silicate and hydrated calcium aluminate) into calcium salts that break down the concrete. Due to the marked alkaline character of the cement paste, caused by calcium hydroxide from the hydration of the silicates, the different types of acids attack, generating the aforementioned calcium salts.
Action Freeze - Defrost
It is a typical injury in outdoor concrete in climates where temperatures drop below zero degrees, and is based on the freezing and subsequent defrost of water absorbed by the concrete inside its open pores. The mechanism of deterioration is as follows: when the water freezes, its volume increases by 9%. If it is partially occupied, the pores of the concrete and the free space occupied by the air is equal to or greater than this percentage, when the frost occurs there will be sufficient free space to absorb the expansion. But if the degree of saturation is greater than the critical 91%, the free space will be reduced and the expansion of the water will be prevented from expanding, causing tensions that generate surface disintegration.
Reinforcement Cage Corrosion
Any metal can suffer two types of corrosion: electrochemical or galvanic corrosion and corrosion by oxidation. Of the two, concrete suffers the first, since in the second there are no oxidation-reduction reactions that cause electron transport.
The pathological process of this erosion presents a physical character, from the eroding agent to the erosion mechanism, resulting also in a loss of surface material produced in a more or less slow and continuous way. The most common agents are normal atmospheric phenomena (rainwater, temperature changes) acting together, so this erosion is also known as "weathering". It affects the exterior surfaces of buildings, with more intensity the higher the level of exposure and the weaker the porous structure.
It consists of the reaction between the air CO₂, which penetrates through the pores of the concrete, and the calcium hydroxide, coming from the cement and subsequent reactions during the hardening of the concrete. The final product of the reaction is the formation of calcium carbonate and water: Ca (OH)₂+ CO₂→ CaCO₃+ H₂O . This damage is also considered as a natural deterioration process.
Appears when a section is subjected to an axial compressive stress acting on the part's centre of gravity. This type of load, typical of pillars, does not appear independently in reality, as it would require a very high precision in the load centering mechanisms.
Cuando en una sección de un prisma de hormigón armado actúan únicamente fuerzas tangentes se dice que está sometido a esfuerzo cortante puro; pero esto en realidad no sucede, ya que siempre que hay cortante aparece un momento flector.
Within the concept of bending we will distinguish between simple and compound bending. It is said that a section is subject to simple bending when it supports a bending moment but not an axial force, so for example the beams are calculated assuming this type of bending as the axial is usually negligible. The capacity of reinforced concrete to withstand simple bending will depend fundamentally on the mechanical amount of the tensile reinforcement.
It consists of the reduction of volume that suffers the hardened concrete when it is exposed to an atmosphere that causes the drying; that will be greater when the atmosphere is drier and when the presence of wind exists. If the decrease in volume is prevented, tensile stresses are created, which, like thermal contractions, if they exceed the resistance of the concrete mass, cause the element to crack.
Initial Thermal Contraction
They are the movements that take place in the mass of the concrete, during the process of set and hardening, due to the difference of temperature that exists within her. Due to the heat of hydration of the cement, the interior of the mass is at a higher temperature than the surface area of the piece in contact with the environment; as this heat cannot reach the exterior, especially in very thick pieces or in massifs, a thermal gradient is produced from the interior of the piece to the surface (the higher the temperature of the concrete and the lower the exterior)
Leaching is a non-hazardous injury to concrete that occurs when water dissolves components in concrete. Hydrated Portland cement contains up to 25 to 30% calcium hydroxide, Ca(OH)2, which is soluble in water. This component will be leached from the concrete. If water passes through cracks or joints, the leaching can also erode the internal concrete. In porous concrete with a high water-cement ratio, leaching can remove sufficient calcium hydroxide to reduce the strength of the concrete. However, this is usually only a cosmetic problem.
Oxidation of Reinforcements
Oxidation is the transformation of a metallic surface in contact with oxygen into oxide. It is a chemical process by which a metal surface reacts with the oxygen contained in the air around it, producing a superficial layer of oxide of the metal in question. Basically it is a process of recovery of the natural state of this material. In this process there is no loss of material.
It is a segregation caused by the settlement by gravity of the heavier solid components (aggregates and cement) towards the bottom of the formwork; and the ascent of water from the concrete to the surface. This process, known as exudation, is produced from the moment of pouring and compacting; and consists of the following: the water from the mix flows to the upper face of the concrete until the beginning of setting, at which time it decreases sharply.
Plastic shrinkage cracks appear when surface water evaporation is greater than the amount of water exuded from the interior of the concrete. This situation, and the delay in curing and protecting the concrete, causes an increase in capillary tension in the water-filled pores that cause the cracks.
Presence of chlorides
The role that chlorides take in this mechanism is to penetrate through the network of pores in the concrete whether totally or partially filled with water (unlike CO₂ which is only harmful in air pores), and initiate the corrosion reactions by depassivation of the oxide film that protects the steel. This allows the steel to enter the solution, favouring the transport of electrons between the anode and the cathode.
Reaction of the Alcalis
The attack of alkalis is more similar to that of sulphates than to that of acids, since such an attack is on a single substance not like the acids that attack all. The difference between the attack of the sulphates and the attack of the alkalis is that in the first case the reactive substance is the cement, and in the second case it is the aggregates. The reaction of alkalis is manifested in two ways: alkali-silica, which is the most common, and alkali-carbonate.
The attack by sulphates is the most harmful and destructive of the different attacks that can develop the anions that make up the salts. The sulphates centre their attack on the cement, creating strongly expansive components that cause the destruction of the concrete.
The torsional stress or torsional moment is produced by the forces parallel to the section and which do not cut the axis perpendicular to it (torsional pairs, which are equal and opposite) and those located moments which have the direction of the axis perpendicular to the section.
It is the phenomenon that suffers a section that is subjected to an axial traction effort that acts in the center of gravity of the piece. As with compression, traction is an effort that rarely occurs independently, due to the impossibility of matching the centering mechanisms of the loads.
Urban Contamintation by Deposit
Also called "urban dirt" we define this injury as the accumulation and permanence of contaminating perticules on the facades of the building of any material, including concrete, either on its external surface, or inside the surface pores. For this injury, the possible chemical reactions of these particles with the mineralogical components of the façade material are not considered. The contamination or dirt by deposit constitutes the first step in the process of contamination or urban pollution and consists of the positioning of contaminating particles on the surface of the facade or inside its surface pores. It is a primary or simple contamination.
Urban Pollution By Differential Washing
The planes of the facades present different reliefs, highlights and changes of plane that cause distortions in the path of the water film, with changes in speed and concentrations of dripping, the end result is a marked heterogeneity in the effect of this interaction water - facade, causing more or less intense washing in some areas and internal deposits of relative intensity in others. This heterogeneity of dirt-washing, which makes the dirt stand out more in the whole, is what we can call Urban Contamination by differential washing.