Deterioration of Foundation Elements of Buildings, Due to Sulfates Attack

Fuad Carlos Zarzar Júnior 1 Romilde Almeida de Oliveira2

ABSTRACT The external attack of sulfate is not completely understood. It is defined as an action of sulfate ions involving deterioration owing to chemical reactions with cement hydrated mortar. When this occurs, a physical manifestation will tend to produce an expansive reaction, resulting in cracks or fissures easy to identify at site, generating damage or deterioration. Systematic study of chemical reactions produced by sulfate attack on concrete is being developed to provide support to the civil engineer whenever confronting this kind of problem, so, they can recognize it immediately at work. In this article, a case study of building foundation is presented and the attack by elements of sulfates in its foundations is confirmed.

KEYWORDS Attack by sulfate ions, Concrete, Pathology, Identification of attack, Repairing.

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Programa de Pós-Graduação em Engenharia Civil, Universidade Federal de Pernambuco (UFPE), Recife-PE, BRAZIL, [email protected] Programa de Pós-Graduação em Engenharia Civil, Universidade Federal de Pernambuco (UFPE), Recife-PE, BRAZIL, [email protected] Universidade Federal de Pernambuco (UFPE), Recife-PE, BRAZIL, [email protected]

Fuad. Carlos Zarzar Júnior, Romilde. Almeida de Oliveira

1 INTRODUCTION The objective of this article is to provide an understanding of the deterioration of the foundation of a structure and identify a set of techniques that facilitate the professionals who work with concrete pathologies in order to recognize the sulfate attack through the manifestation of the damage observed. A case study was carried out as a contribution to understanding the collapse of the structure of the Building Éricka, which was situated in Olinda - Recife Metropolitan Region (Brazil), in November 1999. 2 SULFATE AND SULFATE ATTACK Sulfates are salts in which the negatively charged ion SO42– forms a compound with a metal (positively charged ion), such as Ca2+. On the ground, the sulfates soluble in water are of particular concern and can therefore be readily transported to react with the concrete. Such sulfates include gypsum (calcium sulfate, CaSO4), epsomite (magnesium sulfate, MgSO4), and sodium sulfate decahydrate, Na2SO4.10H2O. For the sulfate attack to occur the sulfates must be carried to the interior of the concrete by interstitial water. Four sulfate compounds are soluble in water and common geological materials: calcium sulfate, magnesium sulfate, sodium sulfate and potassium sulfate. Calcium sulfate is the most predominant, but less soluble (giving a maximum dissolved concentration of about 1400mg SO4 per liter of water) and for this reason is less deleterious to concrete, [Communities and Local Government, 2008]. Of all these, the most damaging one is magnesium sulfate, since the cations of magnesium may on its own contributing to the destruction of the concrete. A feature of the magnesium ion attack on Portland cement paste is that the attack is eventually extended to the calcium silicate hydrate (C-H-S) which is the main cementitious component. Under prolonged contact with a solution of magnesium, the (C-H-S) in the cement paste hydrated gradually lose calcium ions, which are partially or sometimes completely replaced by magnesium ions. The final product of this substitution reaction is a magnesium silicate hydrate (Mg3H2Si4O12). The formation of this salt is associated with loss of cementitious characteristics (loss of strength and disintegration of concrete), Neville [2004]. The amount of soluble sulfates present in the material is a significant factor in determining the potential of sulfate attack. Unfortunately, it can be difficult to obtain representative values of soluble sulfates in some materials because of their inherent variability. 2.1 Portland cement. This type of cement is most vulnerable to sulfates attack, since it transfers to the concrete cement considerable doses of calcium aluminate hydrates and calcium silicate hydrates. Portland cement is a hydraulic binder obtained from clinker to which is added gypsum (CaSO4). The clinker is the product of heating to high temperatures from a mixture of clay and calcium. Thus, Portland cement is composed primarily of calcium and silica in its composition. Besides the type of cement, the vulnerability of concrete to be attacked by sulfates depends on the easiness of sulfates to migrate to the interior. What controls this is the permeability of concrete to water, a property that depends on the size and interconnectivity of pores in the concrete. The concrete, thoroughly mixed and compacted, and cured, has a content ranging from moderate to high of cement and a low mixing ratio of water to cement is less permeable, and therefore has the greatest resistance to attack by sulfates. The concrete that has become partially or totally "carbonated" is more resistant to sulfate attack. Carbonation of concrete results from the reaction with airborne carbon dioxide (CO2). The main reaction is with calcium hydroxide Ca(OH)2 in the matrix of concrete, the final products are calcite (CaCO3) and water. The reaction results in a loss of alkalinity that is associated with the presence of calcium hydroxide (pH can fall from 12 to less than 9) and as a result, the formation of expansive minerals, ettringite and thaumasite, that need a higher pH, is prevented. The carbonation of concrete proceeds more easily in permeable concrete that is exposed to the atmosphere with relative humidity, between 50-70 percent [BRE Digest 263, 1982], and is progressive with time, starting at the concrete surface. Humid conditions below 20% or above 95% delays or prevents carbonation to take place. Some cases of sulfate attack have been reported, in which the concrete has been contaminated with excess of sulfates as a result of impure cement production, or where there is contamination at the site with

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gypsum plaster. 3 SULFATE MIGRATION Sulfates in the hardcore or soil will not be damaging the concrete if they remain dry. Water is necessary to dissolve the sulfates and carry their anions into the concrete. The amount of water needed is not great. The surrounding soil needs not to be saturated, just be humid. In some cases the moisture content in the soil is just between 12-14 percent. A common source is water from the sub-soil that is carried up through the finer fraction of the hardcore due to capillary action. Once in contact with concrete, water is drawn by capillary action due to evaporation at the surface. 4 TYPES OF MECHANISMS OF ATTACK BY SULFATE 4.1 Conventional sulfate attack or in form of ettringite In the conventional sulphate attack, sulfates and water react with the tricalcium aluminate-found in Portland cement to form a hydrate sulfoaluminate calcium (3CaO.Al2O3.3CaSO4.31H2O) known as ettringite. In the manufacture of portland cement 5% of sulfate in the form of gypsum to inhibit the instant setting time of clinker is added. This type of reaction, long known due to numerous publications on attack by sulfates, has been attributed to this modality. The formation of ettringite can be destructively expansive due to the fact that it has a solid volume larger than the original constituents and typically grows as acicular crystals which collectively can generate high internal stress in the concrete. To produce deleterious amounts of ettringite, the reaction requires the presence of: a) a significant concentration of sulfate soluble in water; b) concrete containing a substantial content of calcium aluminate hydrates, such as the concrete produced with most Portland cement; c) presence of humidity. Sulfate SO42- ions, which enter the cementitious component may react with calcium hydroxide Ca(OH)2 in the matrix of cement of the concrete forming gypsum (calcium sulfate dihydrate, CaSO4.2H2O). Ca(OH) 2 + C-S-H + SO42- + H20
CSH2 (gypsum) This reaction product also has a solid volume larger than the original constituents and in some cases may contribute to the degradation of concrete. The ions sulfate may also bond with other ions to form the various reactants that result in sulfate attack as, sodium sulfate (NaSO4), and magnesium sulfate (MgSO4). If the magnesium ions accompany the sulfates, they may also react with calcium hydroxide, producing brucite (magnesium hydroxide, Mg(OH)2) that due to its low solubility precipitates out of solution, leading to a increase in solid volume. The magnesium ions can also attack calcium silicate hydrates, the primary bonding material in concrete [Communities & Local Goverment, 2008]. Byproducts caused by these reactions can create expansion that disrupts paste cohesion and leads to loss of strength in the concrete. Laboratory tests demonstrate that the first effect of the conventional form of sulfate attack is the increased strength and density of concrete as the reaction products fill the pores. When the pores are filled, future formations of ettringite induce expansive internal tensions that, if greater than the tensile strength of concrete, will produce the rupture of the affected region. These cracks with crystalline accumulations are characteristic signs of the conventional form of sulfate attack. According to Neville [2004],''When the analysis of the concrete demonstrate a high content of sulfate it does not necessarily mean a deterioration, although, inversely, the loss of strength or visual deterioration accompanied by a high content of sulfate would be evidence of attack by sulfate. He further emphasizes that "the presence of ettringite by itself is not a sign of sulfate attack." 4.2 Sulfate attack with formation of thaumasite XII DBMC, Porto, PORTUGAL, 2011

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This form of sulfate attack was recognized initially in the UK in the 80s and early 90s and has been found in several cases of deterioration of buried concrete in which the cementitious matrix had been completely replaced by thaumasite transforming the concrete material in a soft material without cohesion. The reaction product is the mineral thaumasite which is a mixture of carbonate, sulfate and calcium silicate hydrate (CaSiO3.CaCO3.CaSO4.15H2O), [Communities & Local Government, 2008]. Sulfates + CSH
thaumasite Deleterious levels require: a. significant concentration of sulfates soluble in water; b. concrete containing calcium silicate hydrate, C-S-H (which looks like fibrous structures, is the component responsible for the mechanical strength of cement used in construction. C-S-H presents a partially crystalline phase resulting from hydration of silica-alumina with pozzolanic activity such as volcanic ash, silica, etc ...), as in most phases of cementitious calcium silicate in Portland cement; c. a source of carbonate, usually from a limestone aggregate; d. a pH of 10 or higher, as is found in non-carbonated concrete; e. continuous wetting; f. low temperature (generally below 15° C). Since the hydrated calcium silicates provide the main bonding agents in Portland cement, this form of attack weakens the concrete, as well, causing some expansions and in advanced cases, the matrix of cement paste is reduced to a friable mass without cohesion. Therefore, due to tropical temperatures existing in the metropolitan area of Recife, it is unlikely that the thaumasite turns into a threat. According to the durable concrete guide ACI [1992], there are two mechanisms which may be sulfate attack: formation of gypsum and ettringite formation. It is believed that both products of these reactions deteriorate the concrete by the increase in solid volume in general. From the point of view of the engineers, what matters is what happened to the concrete: an action that does not result in deterioration or loss of durability is not considered an attack, [Neville 2004]. As Neville [2004] argues, the forms of external or internal attack are major causes of deterioration of concrete structures, [Skalny et al, 2002]. Overall, the deterioration related to sulfates are usually insignificant compared to the damage by corrosion of reinforcement or freezing and thawing. 5 TYPICAL DAMAGE OF THE SULFATE ATTACK 5.1 Mechanisms of damage The reaction of sulfates in the concrete tends to produce expansion in the surrounding where the concrete is contained in the case of slabs in contact with sulfate soil, the concrete will tend to produce a horizontal expansion in the slab or in the concrete oversite. Generally, these concrete slabs are restrained by external and internal walls. As a result the slab or the concrete oversite is uplifted into a dome shape that over time can cause a deflection of several centimeters. The dome will produce traction on the top of the concrete, leading to a map pattern of cracking, see ‘Fig 1’. In the case of magnesium sulfate, a low content of calcium hydroxide in hydrated cement is undesirable because it encourages the reaction of sulfates with calcium silicate hydrate C-S-H, leading to strength reduction of the cement matrix and mass loss, Neville [2004]. 5.2 Visual appearance The first visible sign of sulfate attack on a concrete slab is usually an unevenness surface in which can be accompanied by the appearance of cracks in the concrete oversite on the slab, initially narrow, but widens over time. Similarly the reaction of sulfates in blocks or concrete foundations, where there is any resistance to the expansion causes cracks in the form of map allowing more water to penetrate. This increases the degradation of concrete and consequent depassivation of the reinforcement, see

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‘Fig. 2’. The cracking frame is similar to that produced by alkali-aggregate reaction. The formation of ettringite in the concrete causes an increase in volume. This action requires moisture, sulfate in the environment or in the cement, pre-existing cracks in the structure and high porosity.

Figure 1. Sulfate attack on concrete oversite [Communities & Local Government, 2008].

Figure 2. Cracking in reinforced concrete base of a power grid tower [Thomaz 2008].

5.3 Influence of pH The pH of the attacking solution also influences the formation of deleterious compounds to the cementitious structure, since the ettringite is stable at pH between 10.5 and 13.0 - Souza [2006]. 5.4 Influence of the compressive strength If there are registry values of the compressive strength of the concrete early in the beginning of construction, one can compare the results of tests performed at one moment with those of the beginning of construction and make a diagnosis of the deterioration. If the compressive strength at this moment is smaller than that of the structural design, this may be an indication of sulfate attack. 5.5 Sulfate-resistant Portland cement (SR-PC) – type V According to the ABCP (Brazilian Association of Portland cement), Portland cement SR offers resistance to aggressive sulfated environment, as sewage or industrial wastewater, seawater and some types of soils. The SR-PC can be used in concrete mixing plans, high performance concrete, structural rehabilitation works, shotcrete, reinforced and prestressed, precast elements of concrete, industrial floors, pavements, reinforced mortar, mortar as well as in concrete subjected to attack of aggressive environment, such as water and sewage treatment plants, works in coastal regions, groundwater and sea. According to NBR 5737, five basic types of cement - CP I, CP II, CP III, CP IV and CP V-ARI may be resistant to sulfates, since it suits in one of the following conditions: a) tricalcium aluminate content (C3A) of clinker and additions of carbonate content with a maximum of 8% and 5% by mass, respectively; b) furnace cement that contains between 60% and 70% of granulated slag by mass; c) pozzolanic cements that contain between 25% and 40% of pozzolanic material, by mass; d) cements with long term test results or work that confirm resistance to sulfates. 6 REPAIR Corrective Adequacy depends on the severity of the damage, the perceived risk of future damage and the degree of security required. It is for professionals involved in a specific property, to decide which techniques to use. If the property has a slight damage attributed to sulfate attack, then it is not necessary to perform immediate correcting work, due to the fact that the process of sulfate attack is generally slow. Periodic inspections of the property including the existence of cracks may be carried out to monitor the progress of damage, and make a report of findings. Ensure complete removal and a

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new concrete only when there is expansion and risk of sulfate attack by others components of the building. Sulfates, water and cracks should be avoided, Thomaz [2008]. For this, one must: a) avoid the use of cement with excess of sulfates. The cement type CPII-E (with the addition of granulated blast furnace), the CPII - (with slag-NBR5735) and SR-PC, are the best alternative to eliminate sulfates; b) avoid access of sulfates from environment to the concrete, as occurs in silos of coal with high content of sulfates; c) avoid contact of concrete with water in which the content of sulfate SO42– is greater than 3.000 mg/liter, BS EN 196-2 [2005]. If unavoidable, one solution is to waterproof the concrete; d) If the sulfate content in water is between 600 mg/lt < SO42– 38MPa and concentration of sulfate-resistant Portland cement (SR_PC) > 340 kg/m3 of concrete, microsilica content = 17kg/m3 and super-plasticizer. Use water/(cement + microsilica) less than 0.38, Thomaz [2008]; e) have a concrete cover of reinforcement not below 5cm; f) avoid exposure to rain and moisture in industrial environments with high concentrations of sulfates, as the area of oil refineries; g) use sealing. Avoid also cracks caused by: a) thermal shrinkage; b) shrinkage due to cure defective; c) steam curing temperature above 65 degrees centigrade. 7 CASE STUDY 7.1 Building Ericka The collapse of the building Ericka happened by smashing of the masonry foundation, starting in the area bounded by walls corresponding to the two-bedroom of the ground floor apartment situated in the back of the building. Inspections were carried out after the collapse and the following aspects observed: a) the building was located between two canals: Matadouro and the Rio Morto; b) due to the influence of the local hydrographic grid the water level is high and it is common the presence of wetlands, especially in periods of intense rainfall; c) the area has been occupied for over thirty years and has received aggregate for the construction of access roads and housing. The smashing happened due to the loss of strength of concrete blocks from the external foundation that have deteriorated over time due to the attack of sulfates present in groundwater, the adoption of an inadequate constructive system and the use of the following bad points: a) external walls built in a single layer of concrete blocks with a thickness of 14cm functioning as retaining walls of a high hardcore that accelerated the deterioration of these blocks; b) the permanent contact of the hardcore with the concrete blocks of high porosity used in external foundation, where one side was subjected to moisture and having the other side free to evaporation, can help the chemical process resulting in leaching, Neville [2004]; c) the use of various types of concrete blocks and ceramic blocks with inadequate assembly; d) It was also confirmed the phenomenon of moisture expansion, with loss of strength of the ceramic blocks present in small proportion in the foundations.

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8 ACCOMPLISHED TESTS 8.1 Groundwater analysis Aiming to identify the aggressiveness of the water found in the foundation quota, three water samples for chemical analysis were collected, and two samples collected in the foundation pit and collected in a borehole. The methodology for the determination of chemical parameters was recommended by "Standard Methods for Examination of Water and Wastewater, 1st Edition". 8.2 Analysis of ground water Some results of the ground water were accomplished in three samples showed on Table 1. Table 1. Results of ground water. Determination

Sample Sample Sample 01 02 03 Ph 8,7 8,3 7,7 Free carbon dioxide (CO2) - mg/L 0,0 0,0 7,7 Chloride (Cl) – mg/L 92,2 193,2 65,6 Sulfates (SO4) – mg/L 460,5 815,0 758,5 Bicarbonate alkalinity (CaCO3) - mg/L 74,1 51,5 121,9 Magnesium - mg/L 9,51 2,16 21,94 Calcium (Ca) – mg/L 92,47 63,4 282,2 According to the results presented in Table 1, we can make the following observations: a. the water has a high concentration of calcium; b. the results were analyzed according to Standard L1007 CETESB, revised in 1988 indicating that this is a water with high pH, average of 8.3, and by this aspect classified as a salinealkaline water of low to medium aggressiveness in which the leading phenomenon of aggressiveness is the carbonation accompanied by leaching. According to the sulfate content (average 678mg/lt), and in accordance with the same standard, it was a brackish water with high degree of aggressiveness, where the leading phenomenon of aggressiveness is the expansion by the formation of gypsum and/or ettringite accompanied by leaching. In this particular case, it was found the presence of ettringite accompanied by leaching in the mineralogical characterization test by X-ray diffraction carried out by the Brazilian Association of Portland Cement - SP. 8.2 X-ray diffraction, mix proportion determination and compressive strength tests. The mix-proportion determination tests indicated a sand and cement ratio of about 1:20, when it would be expected at most 1:10, according to local practice. X-ray diffraction tests revealed the mineral ettringite, resulting from the chemical reaction between sulfates and hydrated cement components found in whitish fragments of concrete blocks during visual inspections. This reaction has deleterious features causing the disintegration of the concrete. 6 concrete blocks were tested with dimensions 14cm x 19cm x 19cm taken from the base foundation of the building provided a mean compressive strength of 1.7Mpa. 9 CONCLUSIONS It was proved the action of sulfate in the building Ericka, but the repairs were not made on time. Several chemical tests of water/soil were carried out, as well as X-ray diffraction, compressive strength, etc.. Soil drainage near the foundations is important, due to the fact that the dry salt does not attack concrete. The effectiveness of cement type SR-PC, as well as low w/c content, favors the

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reduction of sulfate attack in concrete. The argument that only the test of compressive strength is sufficient is unfounded. Moreover, evaluating the extent of the damage by determining the change in compressive strength seems to be reliable, since one becomes aware of the values of the compressive strength at the beginning of construction project. Furthermore, the damage caused by sulfates can be easily avoided (or at least limited) with simple measures as those recommended in this article. ACKNOWLEDGMENT The authors would like to thank Karina Moraes Zarzar, PhD and Professor at the Delft Technology University, The Nederlands, for her important participation in the translation and understanding of this article. REFERENCES Communities and Local Government. 2008, Sulfate damage to concrete floors on sulfate-bearing hardcore- identification and remediation, February. BS EN 196-2. 2005, Methods of testing. Chemical analysis of cement, publication date: 31/03/05. British Research Establishment (BRE). 1982, The durability of steel in concrete: Part 1, Mechanism of protection and corrosion. BRE Digest 263 J.P. Skalny, I. Odler, J. Marchand. 2002, Sulfate Attack on Concrete, (Modern Concrete Technology Series, Volume 10), London. Mehta P.K.; Monteiro, P.J.M. 2008, Concreto - Microestrutura, Propriedades e Materiais. Editora IBRACON. São Paulo Neville, A. 2004, Review article, The confused world of sulfate attack on concrete, 8 April. P.K. Mehta. 1993, Sulfate attack on concrete: a critical review, Materials Science of Concrete, vol. III, Amer. Ceramic Society, Ohio, 105– 130. Souza, R. B. 2006, Suscetibilidade de pastas de cimento ao ataque por sulfatos – método de ensaio acelerado. 139p. Dissertação (mestrado) – Escola Politécnica da Universidade de São Paulo. São Paulo. Thomaz, E. C. S. 2008, Etringita, Exemplo Nº116; site: (consultado em: setembro) http://www.ime.eb.br/~webde2/prof/ethomaz/fissuracao/exemplo116.pdf – IME

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