Diagnosing delayed ettringite formation in concrete structures

There has been a number of cases involving deteriorated concrete structures in North America where there has been considerable controversy surrounding the respective contributions of alkali–silica reaction (ASR) and delayed ettringite formation (DEF) to the observed damage. The problem arises because the macroscopic symptoms of distress are not unequivocal and microscopical examinations of field samples often reveal evidence of both processes making it difficult to separate the individual contributions. This paper presents the results of an investigation of a number of concrete columns carrying a raised expressway in North America; prior studies had implicated both DEF and ASR as possible causes of deterioration. Although the columns were not deliberately heat-cured, it is estimated that the peak internal temperature would have exceeded 70 °C and perhaps even 80 °C, in some cases. The forensic investigation included scanning electron microscopy with energy-dispersive X-ray analysis and expansion testing of cores extracted from the structure. Small-diameter cores stored in limewater expanded significantly (0.3 to 1.3%) and on the basis of supplementary tests on laboratory-produced concrete specimens it was concluded that expansion under such conditions is caused by DEF as the conditions of the test will not sustain ASR. In at least one column, DEF was diagnosed as the sole contributory cause of damage with no evidence of any contribution from ASR or any other deterioration process. In other cases, both ASR and DEF were observed to have contributed to the apparent damage. Of the columns examined, only concrete containing fly ash appeared to be undamaged. The results of this study confirm that, under certain conditions, the process of DEF (acting in isolation of other processes) can result in significant deterioration of cast-in-place reinforced concrete structures.     

The formation and role of ettringite in Iowa highway concrete deterioration

Some Iowa highway concrete constructed of coarse carbonate aggregate exhibits premature deterioration, which is, in part, caused by the growth of secondary minerals, including ettringite. Petrographic scanning electron microscope (SEM) and energy-dispersive analytical X-ray (EDAX) studies were conducted to determine the abundance, spatial location, and morphology of ettringite and the spatial relationship of ettringite to the occurrence of oxidized pyrite and coarse/fine carbonate aggregate.In poorly performing concrete (<16-year service life), ettringite completely fills many small voids, occurs as rims lining the margin of larger air entrainment voids and as microscopic disseminations in the paste. Pyrite (FeS₂) is commonly present in coarse aggregate, and goethite [FeO(OH)], one of its oxidation products, is observed in many concrete samples. Sulfate ions derived from pyrite oxidation apparently contribute to ettringite formation. The direct precipitation of ettringite from solution was responsible for most of the observed ettringite in voids and cracks. Microscopic ettringite, which commonly occurred in the paste, most likely was formed by the replacement of calcium aluminate. Severe cracking of cement paste is often spatially associated with ettringite, which strongly suggests that ettringite contributed to cracking and resultant deterioration.     

The relevance of laboratory studies on delayed ettringite formation to DEF in field concretes

Many laboratory studies of delayed ettringite formation (DEF) have been conducted on thin mortar bar specimens, heat treated, and then immersed in water. Under these conditions, rapid diffusion of alkali hydroxide into the surrounding water occurs and necessarily reduces the alkali hydroxide concentration of the mortar pore solution. Results reported recently by Famy indicate that the DEF process is triggered as a consequence of such leaching. When it is prevented by immersion into alkali hydroxide solution instead of water DEF expansion is delayed or prevented entirely. Results reported by Zhang indicate that 51-mm mortar cubes behave differently than more leaching-susceptible mortar bars when exposed to the same wet environment. Mortars that show severe DEF as mortar bars remain almost free of DEF symptoms if they are stored as cubes, even after 900 days. Attention is called to the fact that DEF in concrete is found commonly in thick concrete members where the possibility of leaching is remote. For such concrete, the reduction in internal alkali hydroxide concentration that occurs with ASR can substitute for the effect of leaching. It is postulated that without effective reduction of alkali hydroxide concentration by one or the other process, DEF remains latent.     

Studies on delayed ettringite formation in heat-cured mortars: II. Characteristics of cement that may be susceptible to DEF

Expansions of mortar bars, stored over (but not in) water after simulated steam curing to 85 °C, were related to certain cement compositional parameters. The relationship is expressed in the form of a “delayed ettringite formation (DEF) index.” The DEF index is computed as the joint product of the SO₃/Al₂O₃ molar ratio of the cement, the sum of its SO₃ and Bogue C₃A percentages divided by 10 and the square root of the alkali content expressed as equivalent % Na₂O. The mortars studied were made with 18 different cements, prepared from a set of six representative clinkers by incorporating Terra Alba gypsum to total SO₃ contents that were 1% below optimum, at optimum and 1% above optimum (as defined in ASTM C 563). Measurements of expansion were recorded at intervals for up to 1400 days. Severe cracking and prominent DEF-induced expansions were observed in mortar bars derived from four of the six ‘oversulfated’ cements and lesser expansions from three of the six cements prepared at optimum SO₃ contents. No expansion was found for cements of DEF index below a threshold value; above this value expansions were approximately proportional to the difference between DEF index and its threshold value. The relationship confirms the significance of all three compositional parameters making up the index, e.g., the SO₃/Al₂O₃ molar ratio, the joint contents of SO₃ and C₃A, and the alkali content, in influencing the extent of DEF-induced expansion. In these measurements, the apparent pessimum effect for SO₃ content previously reported by others was not found, although SO₃ contents examined spanned the supposed pessimum value of 4%. Rather, expansion increased with increasing SO₃ content for mortars made with all clinkers exhibiting expansion.     

A concrete performance test for delayed ettringite formation: Part I optimisation

Delayed ettringite formation (DEF) is a rare internal swelling reaction of concrete that, in a wet environment, may considerably reduce the durability of a structure or a member that has been prefabricated in a factory. As a result of a large number of studies, the main causes of this problem have been established and this paper proposes a test method to predict the susceptibility of a concrete to DEF and provide long-term protection against this hazard. After an examination of the feasibility of a test based on wetting and drying cycles, the results of an optimisation study based on a factorial experimental plan are presented. The test is in four stages: concrete manufacture, a heat treatment that simulates steam curing in a factory or the heating that occurs in mass cast in-situ concrete, wetting and drying cycles and the monitoring of the longitudinal expansion of specimens that are immersed in water. Optimisation essentially concerns the last two stages and resulted in the decision to apply two drying cycles (38 °C at a relative humidity of approximately 30%) and wetting cycles (in water at 20 °C) followed by permanent immersion in water (at 20 °C). This test is applied to concrete that has undergone early age heating. It provides a means of evaluating the potential reactivity of “mix design–heating cycle” pairs as realistically as possible within a reasonable period of time.     

A concrete performance test for delayed ettringite formation: Part II validation

Delayed ettringite formation (DEF) is a rare problem of concrete, whose reaction mechanisms have been investigated by a large number of studies. In order to develop a performance test, the authors have conducted a feasibility study and an optimization study followed by this validation study. A performance test was previously developed to evaluate the risk of expansion as a result of DEF for a given “concrete/heating” combination to be evaluated. This paper presents the results of the validation study and explains the necessary conditions that must be applied for this test to be used as part of a rigorous preventive procedure. Data has been collected to reproduce the heat development in concrete under actual conditions on site or in a prefabrication factory. Concrete/heating combinations were studied for which 10 or 20 years' experience of the use of the concrete in wet environments existed. After nearly 300 days of testing, all the laboratory results reproduced the behaviour observed in-situ. On the basis of these macroscopic measurements and microscopic observations, DEF susceptible concretes can be distinguished from concretes that have never caused problems, despite being heated to 80 °C in early age. This test program therefore, confirms the reliability of the proposed performance test.     

Studies on delayed ettringite formation in early-age, heat-cured mortars: I. Expansion measurements, changes in dynamic modulus of elasticity, and weight gains

Mortars were prepared from laboratory cements blended from a set of six representative ground clinkers and Terra Alba gypsum. The addition of gypsum was such that cements containing 1% SO₃ less than the optimum SO₃ content, the optimum SO₃ content, and 1% greater than the optimum SO₃ content were produced. Mortar bars and mortar cubes containing each of these cements were exposed to continuous room temperature (23 °C) curing, or to early-age curing cycles involving maximum temperatures of 55 and 85 °C, followed by long-term exposure at 100% RH over water, but not immersed in water. Measurements of expansion, dynamic elastic modulus, and weight gain were recorded at intervals of up to 900 days. Severe cracking and prominent delayed ettringite formation (DEF)-induced expansions were observed in 85 °C cured mortar bars derived from four of the six “oversulfated” cements. Much smaller expansions were observed in mortar bars from two cements with optimum SO₃ content cements also cured at 85 °C. No expansion or other visible indication of distress was observed for any of the 55 °C or continuously room-temperature-cured mortars. The dynamic elastic modulus increased progressively on prolonged exposure for the unaffected mortar bars, but it decreased precipitously after the onset of expansion in affected mortar bars. Significant weight increases also accompanied the processes of expansion. Mortars that showed severe cracking and deterioration when exposed as mortar bars suffered almost no visible damage when exposed as cubes.     

Some factors affecting delayed ettringite formation in heat-cured mortars

Although more than 10 years of studies on delayed ettringite formation (DEF) have led to consensus in numerous areas of past disagreements, some questions remain experimental work is needed to complete the knowledge of this pathology. Following this objective, this paper studies the influence of pre-existing microcracking, wetting/drying cycles and the type of sulfated addition on DEF in steam cured mortars. The mortar specimens were prepared using an Ordinary Portland Cement and two types of sulfate were added to the mixtures: calcium sulfate (CaSO₄) or sodium sulfate (Na₂SO₄). The results confirm the well-known effect of temperature: no expansion was observed in any of the mixtures cured at room temperature. Moreover, no expansion was observed after 800 days for the reference mortar or for the mortar containing calcium sulfate but all the specimens of heat-cured mortars containing sodium sulfate expanded markedly after about 50 days whatever the supplementary treatments applied (thermal shrinkage or wetting/drying cycles). These results show the significant role played by alkalis in the occurrence of delayed ettringite. The supplementary treatments intended to cause prelimiray microcracking of the specimens did not promote expansion but contributed to a slight acceleration of the reaction. The ultimate values of expansion were similar to those obtained with sound mortars.     

Delayed ettringite formation symptoms on mortars induced by high temperature due to cement heat of hydration or late thermal cycle

Cases of delayed ettringite formation (DEF) have mainly been detected on mortars or precast concretes steam-cured according to a predefined temperature cycle during hydration. The present study shows that other situations in which the material is submitted to a temperature cycle can induce DEF expansions. Mortar bars were made with three different cements (types 10, 20M, and 30). As a first heat treatment, the mortar bars were steam-cured to reproduce the temperature cycle they would undergo if they were at the center of a large mortar member. The dimensional variations of these specimens were studied for 1 year. After 1 year, half of the specimens were steam-cured for 1 month at 85 °C. The expansions were followed for two more years. The early-age steam-cure-induced expansions for mortar types 10 and 30. Late steam-curing induced expansions for the three cements tested. In one case (cement type 20M), the early-age steam cure has suppressed or delayed the expansion induced by the late steam cure. A scanning electron microscopy (SEM) study showed that typical DEF symptoms are associated with the expansions.     

Impact of unrestrained Delayed Ettringite Formation-induced expansion on concrete mechanical properties

The consequences of Delayed Ettringite Formation (DEF) on the mechanical properties of concrete still remain imperfectly known. It is generally recognised that this pathology can decrease the strength of the material and its Young modulus.The comparative study between the expansion of concrete and the evolution of its dynamic modulus carried out in this study on a great number of samples, demonstrates that two types of swelling behaviours can be observed: a linear and a sigmoidal.A relationship between modulus and expansion rate is highlighted but it only remains in the case of linear swelling, which does not generate any significant damage.In the case of sigmoidal swelling, the damage process starts for expansions greater than 0.1% that can consequently reduce by 60% the dynamic modulus and by 65% the compressive strength. The relationship between modulus and expansion rate established for linear swelling cases is temporary verified for sigmoid swelling ones at the inflection point. At this point, the supersaturation is assumed to be mostly consumed and cannot further damage the matrix. Thereafter, the phase of stabilization of the expansion begins, and can be concomitant with a rehealing of the matrix induced by the continuation of cement hydration.     

The effects of chemical environment on the nucleation, growth, and stability of ettringite [Ca3Al(OH)6]2(SO4)3·26H2O

Ettringite is responsible for both the initial set of Portland cement and for premature concrete deterioration. A new method of ettringite crystal growth by combining calcium hydroxide and aluminum sulfate solutions was devised to reliably produce crystals that could be seen with a light microscope (45×-320×). The nucleation, growth, morphology, and stability of ettringite in the presence of over 300 chemicals and admixtures, many of which are present in the concrete environment, was then investigated. The plasticizers sorbitol, citrate, and tartrate were found to inhibit ettringite nucleation and growth, as did certain lignosulfonate air-entraining admixtures. The Type B set retarder borax inhibited ettringite formation at <44 ppm. The consequences and implications of this are discussed.     

Thermal stability of ettringite in alkaline solutions at 80 °C

The thermal stability of synthetic ettringite was examined in NaOH solutions up to 1 M after 12 h of heat treatment at 80 °C, with or without the coexistence of C₃S in the system. Ettringite was found to convert to the U phase, a sodium-substituted AFm phase, over the heat treatment in the absence of C₃S. The presence of C₃S, leading to C-S-H formation, prevents the U phase formation and results in the conversion of ettringite to monosulfate. Sulfate ions generated from ettringite decomposition mostly remain in the solution, but some is incorporated into C-S-H. During subsequent storage at room temperature, the majority of monosulfate slowly converts back to secondary ettringite under moist conditions, using the supply of sulfate ions from the solution and C-S-H. The observations support the current mechanism of delayed ettringite formation (DEF).     

The behavior of silicocarbonatite aggregates from the Montreal area

In a previous paper, it was concluded that silicocarbonatite aggregates from the Francon quarry, Montreal contributed to durability problems in Portland cement concrete. Results show that, at 2 days after casting, concrete made with silicocarbonatite aggregates contained over 1.5% more Na₂O than similar bars made with Exshaw limestone aggregates. A reaction involving the rare mineral dawsonite in the silicocarbonatite is thought responsible for the higher Na₂O content. In turn, this caused increased expansion of concrete bars made with alkali expansive aggregates. Also, concrete made with alkali-carbonate reactive Pittsburg aggregate showed more expansion when cured at 80 °C than bars cured at 23 °C. Concrete bars made with Exshaw limestone aggregates cured for 4 h at 85 °C showed late-stage expansion, which is attributed to delayed ettringite formation (DEF). However, no expansion was shown by heat-cured concrete prisms or mortar bars made with silicocarbonatite aggregates. Release of alkalis, aluminates and carbonates by the dawsonite reaction may have inhibited DEF. Concrete bars made with nonreactive Nelson dolostone and 10% silicocarbonatite cured at 80 °C for 4 h showed up to 0.15% expansion after several years at 23 °C and 100% relative humidity (R.H.), indicating that a deleterious reaction did occur.     

Dual effectiveness of lithium salt in controlling both delayed ettringite formation and ASR in concretes

The influence of lithium nitrate on expansions due to delayed ettringite formation (DEF) and alkali-silica reaction (ASR) has been investigated. Effects of the lithium salt were examined in heat-cured mortars and concretes containing one or both damage mechanisms. The mortars and concretes made using reactive and/or non-reactive aggregates were subjected to heat treatment consisting of a hydration delay period of 4 h at 23 °C followed by steam-curing at 95 °C and then stored in limewater. Results showed that the lithium salt admixture was able to reduce the occurrence of deleterious expansion due to delayed ettringite formation in addition to controlling alkali-silica reaction in cementitious systems containing one or both mechanisms. In concretes made using non-reactive limestone aggregates, incorporation of lithium nitrate in a proportion of 0.74 M ratio of Li to (Na + K) was found to control delayed ettringite formation during the one-year period of this study.By analyzing the leaching properties of lithium and other alkalis from mortars during storage, it was found that a substantial amount of lithium was retained in the cementitious system in a slightly soluble form, and is expected to be responsible for reducing DEF.     

Pessimum effect of externally applied chlorides on expansion due to delayed ettringite formation: Proposed mechanism

Mortars and concretes were subjected to a heat treatment cycle consisting of a pre-set period of 4 h at 23 °C followed by accelerated curing at 95 °C prior to storage at room temperature in water or limewater, 0.5 M, 2.8 M sodium chloride solutions. It was found that the specimens stored in 0.5 M sodium chloride solution gave a much greater expansion than those stored in limewater or 2.8 M sodium chloride solution. This pessimum influence of chlorides on expansion due to delayed ettringite formation deviates from the commonly held view that chlorides mitigate sulphate attack in concretes. The mechanism of the pessimum effect of chlorides on expansion due to delayed ettringite formation, and the final products of the associated phase transformations have been proposed. X-ray diffraction and differential thermal analysis techniques were used to follow phase transformations.     

Revealing the temperature history in concrete after fire exposure by microscopic analysis

Concrete structures behave in most cases very well during a fire, after which it is often possible to repair or strengthen the structure to a certain level. This could result in important economic benefits, as costs for demolition and rebuilding can be avoided and the building can be reused faster. In this paper three methods for determining the maximum temperature to which a concrete structure was submitted during a fire are studied. Knowledge of the temperature distribution is necessary to assess the overall damage of a concrete structure. First, the physico-chemical transformations of heated concrete are investigated with scanning electron microscopy (SEM). Secondly, the features visible under the polarising and fluorescent microscope (PFM) are discussed. And third, the influence of heat on the colour of the aggregates is analysed.     

Application of petrography for determining the quality of concrete cured in tropical environment

This paper presents the results of a study on the application of petrographic and chemical techniques for determining the quality of concrete cured in tropical environment. Concretes with water-to-cement ratios (w/c) of 0.30 to 0.70 with an increment of 0.10 were made and cured at 35 and 20 °C, and exposed to different durations of moist curing. The results suggest that most of the petrographic and chemical methods can be used for estimation of w/c and cement content for hardened concrete cured in tropical environment with acceptable accuracy.     

Internal deterioration of concrete by the oxidation of pyrrhotitic aggregates

This paper presents research results on the causes of a severe concrete deterioration, which occurred in many building foundations approximately 2 years after construction. Concrete samples were investigated with X-ray diffraction (XRD) analysis, a scanning electron microscope (SEM) and a petrographic examination performed with a stereomicroscope. It was found that the early cracking of concrete stemmed from the oxidation of the pyrrhotite found in the anorthosite aggregates used to produce the concrete. The oxidation process led to the precipitation of iron hydroxides having a higher volume than the original pyrrhotite does. The presence of micas (biotite) close to the pyrrhotite seemed to promote and accelerate the oxidation process.     

Effects of stress during heating on strength and stiffness of concrete at elevated temperature

Concrete in structures exposed to high temperatures is practically always heated under stress. Yet, there are few experimental studies in which the concrete was heated under stress and then loaded to the peak, and most of these were performed under uniaxial compression. This paper reports on an experimental study of the effects of different heat–load regimes on the stress–strain behaviour of partially sealed concrete under multiaxial compression, at elevated temperature. The specimens were first heated (stressed/unstressed), then loaded to the peak in multiaxial compression. In contrast with previous experimental research, the results show that concrete heated under relatively low compressive stress has lower strength and stiffness than concrete heated without load. The results suggest that the presence of stress during first heating produces a specific damage, which could be the cause for a major component of the load induced thermal strain (LITS) in concrete.     

Post-fire residual mechanical properties of concrete made with recycled concrete coarse aggregates

This paper presents results of an experimental study on the residual mechanical performance of concrete produced with recycled coarse aggregates, after being subjected to high temperatures. Four different concrete compositions were prepared: a reference concrete made with natural coarse aggregates and three concrete mixes with replacement rates of 20%, 50% and 100% of natural coarse aggregates by recycled concrete coarse aggregates. Specimens were exposed for a period of 1 h to temperatures of 400 °C, 600 °C and 800 °C, after being heated in accordance with ISO 834 time–temperature curve. After cooling down to ambient temperature, the following basic mechanical properties were then evaluated and compared with reference values obtained prior to thermal exposure: (i) compressive strength; (ii) tensile splitting strength; and (iii) elasticity modulus. Results obtained show that there are no significant differences in the thermal response and post-fire mechanical behaviour of concrete made with recycled coarse aggregates, when compared to conventional concrete.