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CRACKING AND SULPHATE ATTACK IN FIELD CONCRETE IN SPAIN Esperanza Menéndez Institute of Construction Science Eduardo Torroja, Spain

AbstractMany concrete structures in Spain are at risk of being subjected to external sulphate attack due to high level of sulphates present in soils and in ground water. On the other hand, some of the existing structures show signs of expansion caused by alkali-silica reaction (ASR). In certain cases these structures also contain ettringite, but usually it is not possible to authoritatively state which of the two processes (ASR of ettringite formation) initiated the deterioration. To the author’s knowledge, to date only one case of internal sulphate attack has been encountered in a field concrete in Spain.

1. External sulphate attack

In order to minimize the risk of external sulphate attack, the Spanish standards and specifications of cement and concrete contain specific limits that are presented below.

With respect to composition of cements, the “Specification for Receiving” contains the following limits for cements to be used in structures resistant to sulphates and sea water.

Sulphates and sea water resistant Sea water resistant CEMENTC3A% C3A% + C4AF C3A% C3A% + C4AF

CEM I 5,0 22,0 5,0 22,0 CEM II 6,0 22,0 8,0 25,0 CEM III/A 8,0 25,0 10,0 25,0 CEM III/B ALWAYS RESISTANT ALWAYS RESISTANT CEM IV/A 6,0 22,0 8,0 25,0 CEM IV/B 8,0 52,0 10,0 25,0 CEM V/A 8,0 25,0 10,0 25,0

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The “Instruction for Structural Concrete” defines the level of intensity of sulphate attack by ground water as a function of sulphate ion concentration. The defined attack levels are:

200 – 600 mg SO4=/l low intensity attack

600 – 3.000 mg SO4=/l moderate attack

> 3.000 SO4=/l high intensity attack

The specifications limits and recommendations seem to be quite effective as it is not very common to find examples of field structures damaged by external sulphate attack in Spain. One of the cases known to the author is discussed below.

This particular case involves bottom part of concrete columns that were periodically in contact with industrial waste water with high sulphate content. At the time of the investigations the columns were no longer in direct contact with the waste water as its flow was diverted to a separate channel, away from the structure.

The typical appearance of a damaged column is shown in Figure 1 where substantial loss of paste from the area exposed to sulphate contaminated water and presence of white deposits can be observed. Greatest loss of material was occurred at the cement paste interface.

Figure 1. Concrete column damaged by external sulphate attack.

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The concrete collected from the deteriorated part of the column was analysed using scanning electron microscopy with energy dispersive x-ray capabilities. This investigation revealed the presence of large crystals of gypsum at the paste-aggregates interfaces (Figure 2) along with smaller crystals of potassium and sodium sulphates distributed throughout the microstructure (Figure 3).

Figure 2. Gypsum crystals (Y) at Figure 3. Potassium (SK) and sodium (SNA) aggregate interface. sulphates on the concrete surface.

2. Alkali-silica reactionIn the last ten years a lot of cases of structures affected by ASR have been found in Spain. Some factors that lead to this increase in the number of reported cases include: heat curing, change in composition of cements, aging of structures and, greater awareness regarding this kind of damage. Most of the structures affected by ASR eventually also develop deposits of ettringite.

Traditionally, Spanish aggregates were not considered to be susceptible to reactions with alkalis. However, some aggregates found in different parts of Spain are in fact reactive. The reactive aggregates are usually derived from polymorphic rocks that contain inclusions of deformed quartz with undulate extinction. Types of rocks that can yield a reactive aggregate typically include: granites, gneisses, quartzites, and volcanic rocks.

The main types of structures affected by alkali-silica reaction are typically: dams, bridges, canals, pavements, precast elements (beams, railway sleepers…), etc. Figure 4 shown some examples of structures affected by ASR.

The structures affected by ASR show the typical map cracking pattern, and sometimes, discoloration around the cracks. As expected, when samples taken from affected structures are studied in an optical or electronic microscope, deposits of gels and micro-crystals are found in the microstructure. These typical ASR products are observed both inside the particles of affected aggregates and in the surrounding cement paste.

Figure 5 shows a petrographic image of siliceous aggregate particles affected by ASR. The gel-filled cracks are visible inside the particle and in the surrounding paste. Figures 6 and 7 show typical examples of massive gel deposits and rosette like crystals.

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Figure 4. Concrete canal affected by ASR.

Figure 5. Gel vein in siliceous aggregate and in the surrounding paste.

Figure 6. Massive deposit of ASR gel. Figure 7. Rosette-like crystals.

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4. Alkali-silica reaction and ettringite formation

In some structures damage both due to ASR and ettringite formation was observed. These structures typically developed surface map cracking and, in some cases, discoloration of concrete around the cracks.

The structures where both alkali-silica reaction and ettringite formation symptoms are observed typically include dams, bridges and precast elements. Figure 8 show one of these structures.

The microstructure of these concretes contains typical ASR products (gel and microcrystalline deposits in pores and cracks of altered aggregates and cement paste) but, also substantial deposits of ettringite. This ettringite deposits are found inside pores, around aggregates, and in cracks. Examples of such deposits are given in Figures 9 and 10.

Figure 8. Dam showing surface map cracking.

Figure 9. Crystals of ettringite at aggregate-paste interface.

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Figure 10. Cracks filled with ettringite.

4.1 Railway sleepers An example of precast elements affected by both ASR and ettringite formation involves concrete railway sleepers, manufactured in Spain between 1998 and 1991.

Some of these sleepers showed external cracks and material loss and were studied in order to assess the potential for development of internal expansive reactions. During production these sleepers were heat cured, with temperature greater than 80ºC; and developed visible damage between 1 and 2 years after they have been manufactured.

In this case most of the damage was due to ASR as evidenced by the presence of gel and micro-crystals in pores, cracks, altered aggregates, etc. Ettringite deposits were found in addition to ASR products but, it was no possible to attribute this ettringite crystallization to delayed ettringite formation.

In general, there was no evidence of general gap formation around the aggregates except for some calcareous aggregates were partial gaps filled with ettringite deposits were observed. Based on these observations the ettringite observed in railway sleepers was not associated with DEF.

Figure 11 shows an image of a general area of concrete with no general gaps around aggregate particles. Figure 12 shows a partial gap around calcareous aggregate particlefilled with deposits of ettringite.

Figure 11. Concrete with no general gaps at aggregates.

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Figure 12. Partial gap around calcareous aggregate filled with ettringite deposits

5. Internal sulphate attack

To date, only one case of internal sulphate attack without associated ASR damage was encountered in the filed concrete in Spain.

The structure was a 30 years old bridge where extensive cracking of concrete piers was observed. The detail of the affected pier of this bridge is shown in Figure 13. Environmental conditions at the site are characterized by high relative humidity (about 80-90%), and temperature between 2ºC and 35ºC.

Figure 13. Bridge pier affected by internal sulphate attack.

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The concrete used in the construction of the pier was made with clinker that contained high amounts of alkali sulphates. During the microstructural examination some large unhydrated grains of cement were found.

Although there was no evidence of ASR damage in this concrete, numerous ettringite deposits were found in different locations (cracks, aggregate interfaces, inside and around unhydrated cement grains, etc). Figures 14 and 15 show, respectively, ettringite deposits inside unhydrated cement grain, and at aggregate interface.

In the study by electron microscopy a lot of grains with alkaline sulphates were found along with the evidence of sulphate being released from unhydrated cement grains. Different steps involved in the process of the sulphate release from the cement grains were observed. In one step the alkalis remaining in the interstitial phase formed a rim while the sulphates were released from the interstitial phase, going into the cement paste (Figure 16). In the next step the sulphates are completely released from the anhydrous cement grains and form ettringite in the paste while the alkalis remaining in the cement grains forming a rim (Figure 17).

Figure 14. Ettringite deposits inside unhydrated cement grain.

Figure 15. Ettringite at aggregate interface.

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Figure 16. Release of sulphates from the unhydrated grain while the alkalis remain forming a rim in the interstitial phase.

Figure 17. Alkalis remaining into the unhydrated grain while the sulphates have been released from it.

6. Conclusions Until now it has been not possible to find structures of field concrete affected by delayed ettringite formation in Spain.

Although, there is a very long experience of external sulphate attack in field structures, due to high level of sulphates present in soils and in ground water. In these cases, surface alteration and loss of material have been observed.

With respect to internal expansive reactions there is a lot of experience of field structures affected by alkali-silica reactions. In some structures damage due to both ASR and ettringite formation was observed. However, it is not possible to associate this ettringite with the DEF process.

In the case of railway sleepers heat cured most of the damage observed was due to ASR, although ettringite deposits were found in addition to ASR products. In general, there

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was no evidence of general gap formation around the aggregates except for some calcareous aggregates were partial gaps filled with ettringite deposits. It was not possible to attribute this ettringite crystallization to DEF.

Finally, only one case of internal sulphate attack without associated ASR damage was encountered in field concrete in Spain. The structure was a bridge made using a concrete with clinker that contained high amounts of alkali sulphates. During microstructural examination a lot of grains with alkaline sulphates were found along with evidence of sulphate being released from unhydrated cement grains and forming ettringite in the paste, while the alkalis remaining in the interstitial phase formed a rim.

7. References 1 Gartner, E.M. and Tang, F.J., ‘Formation and properties of high sulfate Portland

cement clinkers’, Cement. 84 (2) (1987) 141-164. 2 Harrison, A., ‘Some effects of recirculating alkali loads on clinker microstructure

and properties’, World Cement. 22 (12) (1991) 24-26. 3 Taylor, H.F.W., ‘Ettringite in cement paste and concrete’, Conference on Beton: du

Materiau a la Structure. Arles, France (1994). 4 Bonatous, L., Bessada, C., Massiot, D., Coutures, J.P., Le Rolland, B. and

Colombet, P. ‘29Si MAS MRN study of dicalcium silicate: the structural influence of sulfate and alumina stabilizers’, Journal American Ceramic Society. 78 (10) (1995) 2603-2608.

5 Menéndez, E., ‘Deterioro del hormigón debido a reacciones expansivas’. Curso sobre Patologías del Hormigón. Barcelona (Spain) (1996).

6 Helfort, D., Soerense, J. and Coulthard, E., ‘Mineralogy of sulphate rich clinker and the potential for internal sulphate attack’, Proceedings of the Nineteenth International Conference on Cement Microscopy. International Cement Microscopy Association. Cincinnati, Ohio (USA). (1997).

7 Skanly, J.P., Diamond, S. and Lee, R.J., ‘Sulfate attack, interfaces and concrete deterioration’. The Interfacial Transition Zone in Cementitious Composites. E& FN Spon, London. UK (1998).

8 Diamond, S. and Lee, R.J., ‘Microstructural alterations associated with sulfate attack in permeable concretes’. Seminar on Sulfate Attack Mechanisms. Quebec, Canada, (1998).

9 Menéndez, E., ‘Aplicación de Técnicas Instrumentales a la Evaluación de la Durabilidad’. Curso CEMCO-98 Seminario sobre Durabilidad de Estructuras de Hormigón: Vida Útil, Refuerzo y Reparación. Madrid (Spain) (1998).

10 Menéndez, E,, ‘Electron microscopy: a tool to study degradation process in field concretes like sulfate attack and ettringita formation’, in ‘7th Euroseminar on Microscopy Applied to Building Materials’, Proceedings of an International Conference, Delf (Netherland), June, 1999, 171-180.