Concrete degradation

Concrete is mostly damaged by the corrosion of reinforcement bars due to the carbonatation of hardened cement paste or chloride attack under wet conditions.

On the surface of concrete pavements, the ASR can also cause pop-outs, i.e. the expulsion of small cones (up to 3 cm (1 in) in diameter), corresponding to aggregate particle size.

When complete (i.e., when all Fe2+ ions are also oxidized into less soluble Fe3+ ions), pyrite oxidation can be globally written as follows: The sulfuric acid released by pyrite oxidation then reacts with portlandite (Ca(OH2)) present in the hardened cement paste to give gypsum: When concrete is carbonated by atmospheric carbon dioxide (CO2), or if limestone aggregates are used in concrete, H2SO4 reacts with calcite (CaCO3) and water to also form gypsum while releasing CO2 back to the atmosphere: The dihydrated gypsum is relatively soluble in water (~ 1 – 2 g/L) at room temperature and thus mobile.

It can easily be leached by infiltration water and can form efflorescences on the concrete surface while the insoluble Fe2O3·nH2O remain in place around the grains of oxidized pyrite they taint in red-ocre.

It is a surreptitious and dangerous form of localized corrosion because the rebar sections can be decreased to the point that the steel reinforcement are no longer capable to withstand to the tensile efforts they are supposed to resist by design.

The ten-storey Queen Elizabeth hospital in Kota Kinabalu contained a high percentage of chloride causing early failure.

The characteristic feature of delayed ettringite formation (DEF) is a random honeycomb cracking pattern similar to this of the alkali-silica reaction (ASR).

In fact, this typical crack pattern is common to all expansive internal reactions and also to restrained shrinkage where a rigid substrate or a dense rebar network prevents the movements of a superficial concrete layer.

When H2S and HS− anions are further exposed to atmospheric oxygen or to oxygenated stormwater, they are readily oxidized and produce sulfuric acid (in fact acidic hydrogen ions accompanied by sulfate and bisulfate ions) according to the respective oxidation reactions: or, The corrosion often present in the crown (top) of concrete sewers is directly attributable to this process – known as crown rot corrosion.

[8] Thaumasite is a calcium silicate mineral, containing Si atoms in unusual octahedral configuration, with chemical formula Ca3Si(OH)6(CO3)(SO4)·12H2O, also sometimes more simply written as CaSiO3·CaCO3·CaSO4·15H2O.

The detrimental reaction proceeds at the expense of calcium silicate hydrates (C-S-H, with dashes denoting here their non-stoichiometry) present in the hardened cement paste (HCP).

TSA was first identified during the years 1990 in England in the United Kingdom in the foundation piles of bridges of the motorway M5 located in the Kimmeridgian marls.

Simultaneously, carbonic acid (H2O + CO2 ⇌ H2CO3) dissolves calcite to form soluble calcium bicarbonate: So, when all the chemical ingredients necessary to react with C-S-H from the hardened cement paste in concrete are present together the TSA reaction can occur.

Consisting primarily of CaCO3 this secondary deposit derived from concrete is known as "calthemite"[12] and can mimic the shapes and forms of cave "speleothems", such as stalactites, stalagmites, flowstone etc.

Gypsum is easily washed away in wastewater causing a loss of concrete aggregate and exposing fresh material to acid attack.

As a preventive measure sewage may be pretreated to increase pH or oxidize or precipitate the sulfides in order to minimize the activity of sulfide-reducing bacteria.

Concrete slabs, block walls and pipelines are susceptible to cracking during ground settlement, seismic tremors or other sources of vibration, and also from expansion and contraction during adverse temperature changes.

Therefore, the volume of the fresh and very young concrete undergoes a contraction due to the hydration reaction: it is what is called "chemical shrinkage" or "self-desiccation".

It is not a problem as long as the very fresh concrete is still in a liquid, or a sufficiently plastic, state and can easily accommodate volume changes (contraction).

Later in the setting phase, when the fresh concrete becomes more viscous and starts to harden, water loss due to unwanted evaporation can cause "plastic shrinkage".

Cracks form in case of a too short, or too poor, curing when young concrete has not yet developed a sufficient early strength to withstand tensile stress caused by undesirable and premature drying.

It can be done by letting the formworks in place for a longer time, or by applying a hydrophobic thin film of an oily product (curing compound) at the concrete surface (e.g., for large slabs or rafts) to minimize water evaporation.

As a consequence, a network of fissures with the characteristic honeycomb pattern also typical for the cracks resulting from the expansive chemical reactions (ASR, DEF, ESA) forms.

Different approaches and methods have been developed to attempt to quantitatively estimate the influence of cracks in concrete structures on carbonation and chloride penetration.

This creates numerous small air-filled micro-cavities in the hardened concrete serving as empty volume reserve to accommodate the volumetric expansion of ice and delays the moment tensile stress will develop.

Air entrainment makes concrete more workable during placement, and increases its durability when hardened, particularly in climates subject to freeze-thaw cycles.

Concrete in buildings that experienced a fire and were left standing for several years shows extensive degree of carbonatation from carbon dioxide which is reabsorbed.

It may be necessary to repair a concrete structure following damage (e.g. due to age, chemical attack, fire,[24] impact, movement or reinforcement corrosion).

The general principles of repair include arresting and preventing further degradation; treating exposed steel reinforcement; and filling fissures or holes caused by cracking or left after the loss of spalled or damaged concrete.

The filling of cracks, fissures or voids in concrete for structural purposes (restoration of strength and load-bearing capability), or non-structural reasons (flexible repairs where further movement is expected, or alternately to resist water and gas permeation) typically involves the injection of low viscosity resins or grouts based on epoxy, PU or acrylic resins, or micronised cement slurries.

Degraded concrete and rusted, exposed reinforcement bar (rebar) on Welland River bridge of the Queen Elizabeth Way in Niagara Falls, Ontario .
Example of flat piece of concrete having dislodged with corroded rebar underneath, Welland River bridge across Queen Elizabeth Way in Niagara Falls, Ontario .
Typical crack pattern associated to the alkali-silica reaction affecting a concrete step barrier on a US highway (photograph, courtesy of the Federal Highway Administration , US Department of Transportation ).
Carbonation-initiated deterioration of concrete at Hippodrome Wellington , Belgium .
Typical crack pattern of the alkali–silica reaction (ASR). The gel exudations through the concrete cracks have a characteristic yellow color and a high pH . The fatty aspect of the exudations imbibing the concrete porosity along the cracks is also a distinctive feature of ASR.
Example of secondary efflorescence in parking garage exposed to diluted road salt from vehicles entering the garage during winter.
Stalactites growing beneath a concrete structure as a result of calcium hydroxide being leached from concrete and deposited as calcium carbonate; creating calthemite forms beneath the structure.
Calthemite flowstone-stained orange from rust , from steel reinforcing bar, deposited along with calcium carbonate.
Concrete heavily degradated after long-time exposition to seawater in the tidal zone