Concrete degradation mechanisms

Concrete is a base material in civil engineering. Concrete may be used on some pipework, especially when dealing with water systems. Conditions and stressors that may result in concrete degradation of typical underground piping are:

— Physical attack:

● Salt crystallization;

● Freezing and thawing attack;

● Abrasion, erosion and cavitation;

● Fatigue/vibration;

● Settlement.

— Chemical attack:

● Efflorescence and leaching;

● Sulphate attack;

● Acids and bases;

● Alkali-aggregate reactions (AAR);

● Aggressive marine water;

● Biological attack (including MIC).

The specific case of concrete piping with a steel cylinder core will be described in Section 4.3.11.4.

Detailed information on concrete ageing management and degradation mechanisms in nuclear power plants is contained in IAEA Nuclear Energy Series No. NP-T-3.5 [111].

4.3.11.1. Physical attack

(a) Salt crystallization

The movement of salt solution causes physical salt attack by capillary action through concrete and its subsequent crystallization through drying. The process is repeated through wetting and drying cycles. Figure 57

FIG. 57. Concrete slab experiencing deterioration due to salt crystallization after one year of exposure; arrows indicate concrete surface deterioration and lines the level of solution in wetting cycle (copyright 2005 CSIRO Australia; reproduced with permission) [112].

presents a concrete slab after a one-year exposure to cyclic wetting and drying in sulphate solutions [112].

Crystallization and recrystallization of certain salts (e.g. NaCl, CaSO₄ and NaSO₄) can generate expansive forces resulting in physical concrete breakdown. The mechanism is similar to water freezing and thawing. Structures in contact with fluctuating water levels or in contact with groundwater containing large quantities of dissolved salts are susceptible to this deterioration. Above ground moisture is drawn to the concrete surface where it evaporates, leaving salt crystals growing near surface pores. The result is a deteriorated area just above ground level. Salt crystallization problems are minimized with low permeability concretes and where sealers or barriers have been applied to prevent water ingress or subsequent evaporation.

(b) Freeze–thaw and thermal cycling

When water freezes, it expands by about 9%. As water in moist or cracked concrete freezes, it produces pressure in concrete pores. If the pressure exceeds concrete’s tensile strength, the cavity will dilate and break. The cumulative effects of freeze–thaw (FT) cycles and disruption of paste and aggregate may lead to expansion and cracking, scaling and even concrete crumbling.

Deterioration of concrete by FT actions may be difficult to diagnose as other types of deterioration mechanisms such as alkali-silica reactions (see Section 4.3.11.2(d)) often go hand in hand with FT. Typical signs of FT are:

— Spalling and scaling of surfaces;

— Large chunks (centimetre-sized) coming off;

— Exposing of aggregate;

— Usually exposed aggregate is uncracked;

— Surface parallel cracking.

Thermal cycling at higher temperatures can be an issue at hot locations within nuclear power plants and can cause similar concrete spalling to FT cycles and concrete strength loss.

(c) Abrasion and cavitation

Abrasion occurs when solids transported in water flowing over the concrete surface abrade the concrete, causing surface pitting and aggregate exposure. Water flowing around certain concrete surface profiles can cause cavitation (negative pressure) at concrete surfaces resulting in pitting [98].

(d) Fatigue

Fatigue in concrete occurs similarly to that in metals as described in Section 4.3.5.

(e) Settlement

Settlement can impact on concrete similarly to metals as described in Section 4.3.7.

Concrete structures and components susceptible to settlement are those built with minimum foundations, those subject to large fluctuations in water table elevation and those subject to soil erosion and improper drainage.

4.3.11.2. Chemical attack

(a) Efflorescence and leaching

Efflorescence is a crystalline deposit of salts, usually white, occurring on or near concrete surfaces following percolation of a fluid (e.g. water) through the material, either intermittently or continuously, or when an exposed surface is alternately wetted and dried. Occasionally efflorescence may be a symptom of chemical reactions such as sulphate attack, or it may indicate leaks in water retaining structures or undesired leakage of moisture through a structure. Typically efflorescence is primarily an aesthetic problem rather than one affecting mechanical properties

or durability. In rare cases, excessive efflorescence deposits can occur within concrete surface pores causing expansion that may disrupt the surface [113]. Figure 58 shows efflorescence in a water retaining structure.

Leaching of cementitious materials involves transportation of ions from a material’s interior through its pore system outwards into its surroundings. In the leaching process solid concrete compounds are dissolved by water and are transported away by diffusion based on concentration gradients, or convection through water flow. Induced leaching of calcium hydroxide from concrete may cause an increase in porosity and permeability and a decrease in mechanical strength. Leaching also lowers concrete’s pH, which threatens the rebar’s protective oxide layer and may lead to carbonation.

(b) Sulphate attack

All sulphates are potentially harmful to concrete. Sulphate attack of concrete is caused by exposure to excessive amounts of sulphate from internal or external sources. Internal sulphate attack occurs when a soluble source of sulphates is incorporated into concrete at the time of mixing, due to the presence of natural gypsum or pyrite in the aggregate and admixtures. External sulphate attack is most common and occurs when water containing dissolved sulphates penetrates the concrete. The sulphates react with calcium hydroxide and if enough water is present, will cause expansion and irregular concrete cracking and thus progressive loss of strength and mass. The degree of sulphate attack depends on water penetration, the sulphate salt, its concentration and type, the means by which salt develops and concrete binder chemistry. The results of sulphate attack can be excessive expansion, delamination, cracking and loss of strength. Figure 59 illustrates the mechanism of sulphate (sodium) attack and presents an example of the resultant cracking. It has been reported that at a concentration of about 0.2% sulphate content in groundwater concrete may suffer sulphate attack. Magnesium sulphate can be more aggressive than sodium sulphate and there are three key chemical reactions between sulphate ions and hardened cement pastes:

(1) recrystallization of ettringite, (2) formation of calcium sulphoaluminate (ettringite) and (3) decalcification of the main cementitious phase (calcium silicate hydrate) [116].

Concrete pipes and other underground structures may be exposed to attack by sulphates in soil and groundwater.

The severest attack occurs on elements where one side is exposed to sulphate solutions and evaporation can take place at the other [117]. Structures subjected to sea water are more resistant to sulphate attack because chlorides present form chloro-aluminates to moderate the reaction. Concretes using cements low in tricalcium aluminate and those that are dense and of low permeability are most resistant to sulphate attack.

FIG. 58. Efflorescence in water retaining structure [114].

A rare form of sulphate attack is through the formation of thaumasite as a result of reactions between calcium silicates in the cement, calcium carbonate from limestone aggregates or fillers and sulphates, usually from external sources [118]. Thaumasite sulphate attack forms slowly and can result in a soft, white, pulpy mass that causes total disintegration of the concrete and exposing rebar. Serious damage to concrete or masonry due to thaumasite formation is, however, uncommon. Figure 60 presents a subsurface concrete pier affected by thaumasite sulphate attack in the United Kingdom.

(c) Aggressive chemical attack

Concrete usually does not have good resistance to acids which may attack concrete by dissolving both hydrated and unhydrated cement compounds as well as calcareous aggregate. In most cases, the chemical reaction

FIG. 59. Concrete cracking due to sulphate attack: (a) mechanism, (b) cracking due to sulphate attack (adapted from Ref. [115] by permission of the International Federation for Structural Concrete/Fédération internationale du béton (fib), (www.fib-international.org.).

FIG. 60. (Left) Thaumasite sulphate attack on subsurface concrete pier and (right) scanning electron microscope image showing thaumasite formation [119, 120].

forms water-soluble calcium compounds, which are leached away. Degradation may be increased in the presence of cracks or near poor joints.

(d) Alkali-aggregate reactions

Aggregates containing certain constituents can react with alkali hydroxides in concrete. Reactivity is potentially harmful only when it produces significant expansion. This alkali-aggregate reactivity (AAR) has two forms [121]: alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR).

ASR is of more concern than ACR since aggregates containing reactive silica minerals are more common.

(i) Alkali-silica reaction

ASR forms a gel that swells as it draws water from surrounding cement paste. Reaction products from ASR have a great affinity for moisture. In absorbing water, these gels can induce pressure, expansion and cracking of aggregate and surrounding paste. The reaction can be visualized as a two-step process:

(1) Alkali hydroxide + reactive silica gel → reaction product (alkali-silica gel);

(2) Gel reaction product + moisture → expansion.

The amount of gel formed in concrete depends on the amounts and types of silica and the alkali hydroxide concentration. Gel presence does not always coincide with distress and thus does not necessarily indicate destructive ASR.

For ASR to occur there must be (1) reactive forms of silica in aggregate, (2) a high-alkali pH pore solution and (3) sufficient moisture. If one of these conditions is absent, ASR cannot occur.

Typical visual indicators of ASR may be a crack network (Fig. 61), closed or spalled joints, relative displacements of different parts of a structure, or fragments breaking off of concrete surfaces (pop-outs) (Fig. 62).

Because ASR deterioration is slow, risk of catastrophic failure is low. However, ASR can cause serviceability problems and can exacerbate other deterioration mechanisms such as those that occur in frost, de-icer or sulphate exposures.

FIG. 61. Map cracking (i.e. pattern cracking or alligator cracking) in a concrete wall due to an alkali-silica reaction [122].

The best way to avoid ASR is to take appropriate precautions before concrete is placed. Standard concrete specifications may require modification to address ASR. Careful analysis of cementitious materials and aggregates should be done and a control strategy chosen to optimize effectiveness. If aggregates are not reactive by historical identification or testing, no special requirements are needed.

(ii) Alkali-carbonate reaction

ACR is relatively rare because aggregates susceptible to this reaction are usually unsuitable for use in concrete for other reasons, such as strength potential.

(e) Aggressive marine water

Concrete in service may be exposed to aggressive waters, the most common deleterious ion being sulphate [123] and others being acids and chemical by-products from industrial processes. Some locations have sea water or brackish water in contact with concrete. Most sea water has a pH of 7.5 to 8.4 and contains about 3.5% soluble salts by weight [124]. Concrete exposed to marine environments may deteriorate as a result of the combined effects of chemical action of seawater constituents on cement hydration products, AAR (if reactive aggregates are present), crystallization pressure of salts within concrete (if a structure face is subject to wetting and others to drying), frost action in cold climates, corrosion of embedded rebar and physical erosion due to wave action or floating objects.

(f) Biological attack

Growth on concrete structures, lichen, moss, algae, roots of plants and trees penetrating into concrete at cracks and weak spots may lead to mechanical deterioration from bursting forces causing increased cracking. Such growth can retain water on concrete surfaces, leading to high moisture content and increased risk of deterioration due to freezing. Microgrowth may cause chemical attack by developing humic acid that dissolves cement paste [125].

Formation of capillaries within the concrete during the hydration process and the capillary action of water provide a means for microorganism penetration.

The metabolism of microorganisms results in the excretion of acids that contribute to cementitious material degradation. In environments where reduced sulphur compounds are present, such as sewers, production of sulphuric acid by sulphur oxidizing bacteria (thiobacilli) produces a corroding layer on the concrete surface that penetrates the concrete. This is the same mechanism as MIC in metallic pipe. Microbes have extremely diverse

FIG. 62. Pop-outs caused by an alkali-silica reaction of sand-sized particles [121].

modes of metabolism, are natural inhabitants of soil and can survive extreme environments such as the inner wall of geothermal cooling towers [126]. Concrete can also be corroded by acids produced by fermentative bacteria that are natural soil inhabitants [127].

4.3.11.3. Irradiation

Irradiation of concrete can lead to deterioration via a process of aggregate expansion and hydrolysis [111]. At levels encountered by buried concrete piping at nuclear power plants, however, it is unlikely to be a concern.

4.3.11.4. Concrete pipe degradation/failure modes

Failure of PCCP or RCCP pipe is generally due to corrosion of the steel cylinder, prestressing wires or other embedded metal. Corrosion may be initiated from inside or outside the pipe. When steel corrodes, the resulting rust occupies a greater volume than the steel. This expansion creates tensile stresses in the concrete, which can eventually cause cracking, material delamination and spalling. Other impacts can include steel cylinder corrosion leading to pipe leakage, corrosion of rebar which reduces pipe mechanical strength until failure occurs, or breakage of high strength prestressing wires (which is a dominant failure mode of PCCP).

Concrete pipe distress signs include coating cracks and delaminations, corrosion of prestressing wires, broken prestressing wires, longitudinal cracks in the inner core, a hollow sounding inner core, corrosion of steel cylinder or leaks (through joints or steel cylinder). An example of mortar lining degradation is shown in Fig. 63.

Although steel’s natural tendency is to undergo corrosion reactions, the alkaline environment of concrete (pH of 12 to 13) provides steel with corrosion protection. At high pH a thin oxide layer forms on the steel and prevents metal atoms from dissolving. This passive film does not actually stop corrosion, but reduces corrosion rates to insignificant levels. For steel in concrete the passive corrosion rate is typically 0.1 μm per year. Without a passive film, steel would corrode at rates at least 1000 times higher [128]. Proper quality concrete and mortar cover are key to maintaining this protection.

Because of concrete’s inherent protection, reinforcing steel does not corrode in most concrete elements and structures. However, corrosion can occur when the protective layer is destroyed. This destruction occurs when concrete alkalinity is reduced or when concrete chloride concentration is increased to a certain level. An example of RCCP degradation from inside by chlorides is shown in Fig. 64.

FIG. 63. Steel pipe mortar lining damage (courtesy of Simpson Gumpertz & Heger).

The steps of corrosion from the inside by chlorides described in Fig. 64 are:

(1) Diffusion of the chloride through the inner layer of concrete. The thickness of this layer is designed to prevent chlorides from reaching the steel cylinder during the design life; however, cracking of the inner concrete layer may reduce its effectiveness.

(2) The chloride reaches the steel cylinder and corrosion may be initiated. Once initiated and if not stopped the corrosion will further develop.

(3) Over time the corrosion may reach the other side of the steel cylinder (called ‘drilling’ of the cylinder).

(4) Once drilled, the degradation of the pipe may continue with corrosion of rebar, spalling or even breaking of the pipe.

As described in Section 4.3.11.2(f), wastewater or sewer applications are susceptible to biogenic sulphide corrosion, which is a bacterially mediated process of forming hydrogen sulphide gas and its subsequent conversion to sulphuric acid which attacks concrete and steel. Corrosion can be minimized by ensuring proper (steeper) gradient sewers to reduce time for hydrogen sulphide generation, providing good ventilation (which can reduce concentrations of hydrogen sulphide gas and may dry exposed sewer crowns) and using acid resistant materials like PVC or vitrified clay pipe as a substitute for concrete or steel sewers.

Corrosion can be increased by the process of carbonation. Carbonation of concrete is a process by which carbon dioxide in ambient air penetrates the concrete and reacts with hydroxides, such as calcium hydroxide, to form carbonates. Carbonation significantly lowers concrete’s alkalinity (pH), which is required to protect embedded steel from corrosion. The amount of carbonation is significantly increased in concrete that has a high water to cement ratio, low cement content, short curing period, low strength and highly permeable (porous) paste [121].

Since carbonation is a relatively slow process, sufficient concrete cover over the rebar will prevent the carbonation from reaching it.

Concrete pipe can also degrade or fail for mechanical reasons. Poor construction, improper pipe bedding, settlement, or excessive external loadings can place forces on the pipe that can cause pipe joints or other components to fail. Construction damage to applied coatings can also facilitate premature degradation.