Behavior of High Performance Concrete Exposed to

Behavior of High Performance Concrete Exposed to Internal Sulfate Attack (Gypsum-Contaminated Aggregate) Tariq S. Al-Attar1, Alaa M. Al-Khateeb2, and Abid H. Bachai3 1

PhD, Lecturer, Building & Construction Engineering Department, University of Technology, Iraq; email: [email protected]. 2 MSc, Lecturer, Building & Construction Engineering Department, University of Technology, Iraq; email: [email protected]. 3 MSc, Researcher, Ministry of Science & Technology, Iraq; email: [email protected].

Abstract Contamination of aggregates with sulfate salts (especially gypsum) is a major problem in Middle East concrete construction. Fine aggregate has more detrimental effect due to its large surface area (fineness). An experimental work had carried out to investigate the behavior of high performance concrete exposed to internal sulfate attack in comparison with traditional concrete. Four sulfate contents (0.5, 1.5, 2.0, and 2.5 %) in fine aggregate were studied. Metakaolin was used as a pozzolanic material, which has proved itself as a good active one. Teasing program included compression, splitting, and ultrasonic pulse velocity tests. 168 cubes and 216 cylinders were cast throughout this program. The test period was extended to 210 days. The harmful effect (reduction in strength) of sulfate was obvious in early ages in contrast to external attack (as early as 7 days). The reduction in strength was continuous and higher in later ages and for higher SO3 contents with reference to the mix with 0.5% SO3. Test results showed that high performance concrete had lost strength, in such an environment, but it was more resistant than normal concrete and it may gain strength with age due to the pozzolanic action of metakaolin. The ultrasonic velocity results were positively proportional to strength development but with a slower rate of variation. Introduction The primary objective in developing high performance concrete (HPC) is, mostly, to have adequate resistance to aggressive environments (Babu-1993).Improved resistance to sulfate attack is obtained by the addition of, or even by partial replacement of the cement by, pozzolana. They remove free Ca(OH)2 and render the alumina-bearing phases inactive , but sufficient time must allowed for the pozzolanic activity to be developed before the concrete is exposed to the sulfates . Many pozzolanas have been found very effective in making concrete resistance to sulfate attack (Neville-1995).In this study, the high reactivity metakaolin (HRM) -local cheep material - is used in conjunction with high range water reducing agent to produce high performance concrete with special feature in both fresh and hardened

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states. Limited amount of work has been undertaken to investigate the combined effect of mineral and chemical admixture on the resistance of high performance concrete to internal sulfate attack. Literature Review Internal sulfate attack. Internal sulfate attack results from the reaction between sulfate in concrete ingredients ( water , cement , sand , gravel ) and cement paste, which had calcium aluminates, and water to form calcium sulphoaluminate. The hazard is illustrated in the materials which cause high tensile stresses lead to expansion and disruption of concrete. Most of the research work on the internal sulfate attack was done on investigating resistance of normal concrete to internal sulfate attack. Limited amount of published litterateur are available on the internal sulfate attack of high performance concrete. Lerch (1946) was, perhaps, the first to define the "optimum gypsum content" of cement. It was defined as the content which gives the highest compressive strength, lowest drying shrinkage and little expansion in water. Al-Rawi (1985) stated that, a major cause of failure of concrete structures in the Middle East was the contamination of sand with sulfates in the form of gypsum. He pointed out that gypsum is normally added to cement to retard early hydration and prevent quick set, thus total sulfate in concrete may be high enough to cause internal sulfate attack. It was concluded that it is possible to reduce the gypsum added to cement and consequently raise the upper limit of sulfate content in aggregate. Ahmed (1980) studied the effect of natural pozzolana (Tuff) and rice husk ash on resistance of concrete to sulfate attack and hot atmosphere. One of the main conclusions was that using natural pozzolana in 30% as partial replacement of cement had improved the resistance of concrete and mortar to the sulfate attack. Ahmed had suggested making use of this conclusion in rising the allowable percentage of sulfate in concrete. High reactivity metakaoline. Kaolin is a fine white clay mineral that has been traditionally used on the manufacture of porcelain. Kaolinite is the mineralogical term that is applicaple to kaoline clays. Hydrated aluminum disilicate, Al2Si2O5(OH)4, is the common constituent of Kaolin . For metakaolin the meta prefix in the term is used to denote change. The change that is taking place is dehydroxylization, which is brought by the application of heat over a defined period of time. The key in producing metakaolin, for use as a supplementary cementing materials or pozzolana, is to achieve as near to complete dehydroxylization as possible without over heating. Successful processing results in a disordered, amorphous state which is highly pozzolanic. Thermal exposure beyond a defined point will result in sintering and the formation of mullite, which is dead burnt and not reactive. In other words, for Kaolinite, to be optimally altered to metakaolin state, requires that it is thoroughly roasted but never burn (Advanced Cement Technologies- 2002).

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Experimental Work The materials used for making concrete mixes were: Type I Portland cement (ASTM C 150), a natural siliceous sand with 2.47 fineness modulus as fine aggregate and crushed gravel of 14 mm maximum size as coarse aggregate (ASTM C33). The original sulfate contents were 0.47% and 0.041% in sand and gravel respectively. A superplasticizer which is known commercially as Sikament R-2002 is used as a high range water-reducing agent (ASTM C494 type G). It was found that 2.5 % by weight of cement is the optimum dosage (maximum water reduction) by trial mixes. Eight percent replacement, by weight of cement, of high reactivity metakaolin, HRM, was used. HRM had 114 pozzolanic reactivity index (ASTM C 311). Table (1) shows chemical composition and physical properties of used cement and metakaolin. It was decided to investigate four percentages (0.5, 1.5, 2.0, and 2.5 % by weight of sand) of sulfate in sand only due its large surface area .The aimed percentages were reached by using natural gypsum, consisting of 23.05 percent SO3 and 54.5% CaSO4. Table (1): Chemical compositions and physical properties of used Portland cement and metakaolin. 1. Chemical Composition, %: CaO SiO2 Al2O3 Fe2O3 SO3 MgO Loss on ignition (LOI) 2. Physical Properties: Fineness (Blaine), m2/kg. Specific Gravity Color Setting Time, hr: Initial Final Compressive Strength, MPa: 3- Days. 7- Days.

Portland Cement 64 20 5 3.90 1.82 1.36 2.48

Metakaolin 0.9 58 36 1.3 0.07 1.4

306 3.14 gray

656 2.64 off-weight

2.16 3.00

-

27.00 37.00

-

Two concrete mixes were used. These were representing normal and high performance concrete. The proportioning was done according to the ACI 211.1-91 recommendations. Table (2) displays the details of used mixes. Table (2): Details of concrete mixes. Cementitious Materials, kg/m3

Aggregate, kg/m3

Cement

HRM

Fine

Coarse

W/CM Ratio by Weight

550 506

0 44

638 638

956 956

0.42 0.30

Mix

1 2

Superplasticizer Percent by Weight of Cement 0 2.5

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Standard ASTM procedures were used for casting, curing, and testing concrete specimens (100*100*100 mm cubes, and 150*300 mm cylinders). The testing program was extended to 210-days age. The conducted tests were as follows: 1. Compressive strength (ASTM C39). 2. Splitting tensile strength (ASTM C 496). 3. Ultra pulse velocity of concrete (ASTM C 597). Results and Discussion Effect of SO3 content on concrete behavior. Table (3) shows all the results of the carried out tests. Test results indicated that normal and high performance concrete specimens exhibited reduction in compressive, splitting tensile strength and pulse velocity with the increase of SO3 content in sand as partial replacement by weight of sand, but high performance concrete showed more resistance to internal sulfate attack than normal concrete. According to these results the reduction in strength and pulse velocity have started early and it is obvious at 7-days age. With reference to Figures (1) and (2), at early ages (lower than 28-days), there was a reduction in strength for both normal and high performance concrete and this reduction depends on the SO3 content in sand. The relation between these two variables is positive. At later ages (more than 28-days) the behavior of normal and high performance concrete are different. The reduction in strength continued increasing for normal concrete, meanwhile, for HPC the reduction decreased. This could be resulted from the pozzolanic reaction which increases the amount of hydration product and consumes Ca(OH)2. Low permeability could be another reason for that. Table (3): Tests Results. Mix SO3 in Sand, %. Property Age, days Compressive 7 Strength, 28 MPa. 90 210 Splitting 7 Tensile 28 Strength, 90 MPa. 210 Ultra-Pulse 7 Velocity, 28 km/s. 90 210

0.5 42.0 51.20 60.30 67.50 3.88 4.20 4.39 4.62 4.558 4.651 4.708 4.761

1. Normal Concrete 1.5 2.0 41.90 52.40 58.60 52.10 3.82 4.04 4.22 3.76 4.556 4.642 4.701 4.699

41.60 48.50 56.80 45.20 3.50 3.76 4.14 3.30 4.554 4.616 4.687 4.657

2.5 38.70 46.80 53.20 41.20 3.35 3.70 3.99 3.14 4.520 4.603 4.659 4.616

2. High Performance Concrete 0.5 1.5 2.0 2.5 48.0 67.4 74.0 83.60 4.63 4.88 5.06 5.50 4.616 4.770 4.803 4.854

47.0 58.30 68.90 72.60 4.56 4.75 4.98 5.32 4.608 4.699 4.768 4.793

45.20 54.60 61.30 68.30 4.45 4.62 4.85 5.11 4.590 4.672 4.719 4.766

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43.30 52.70 60.10 65.60 4.32 4.49 4.71 4.58 4.570 4.655 4.712 4.750


Reduction in Compressive Strength, %.

45 40

SO3-0.5 SO3-1.5 SO3-2 SO3-2.5

35 30 25 20 15 10 5 0 0

50

100

150

200

250

Age, Days.

Figure (1): The reduction in compressive of normal concrete with different SO3 contents.

Reduction in Compressive Strength, %.

30

SO3-0.5 SO3-1.5 SO3-2 SO3-2.5

25

20

15

10

5

0 0

50

100

150

200

250

Age, Days.

Figure (2): The reduction in compressive of high performance concrete with different SO3 contents. General Relationships. Generally speaking, results displayed in Table (3) could be used in constructing a relationship between compressive and splitting tensile strength. Results indicated that the compressive and splitting tensile strength is related to each other, but there is no direct proportionality between them. Consequently, as the compressive strength increases, the tensile strength also increases but with different rates. Another relationship could be drawn between strength and ultra pulse velocity. The later is often used in assessing damaged concrete structures; therefore, such a relation would

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enhance engineering judgment. The ultrasonic velocity results were positively proportional to strength development but with a slower rate of variation. Conclusions 1. High reactive metakaolin (HRM) has proved itself as an active pozzolanic material (pozzolanic activity was equal to 114). It could be used in producing high performance concrete with 28-days compressive strength approaches (70) MPa. 2. High performance concrete showed better resistance to internal sulfate attack than normal concrete. 3. There was a reduction in strength at early ages (less than 28 days) for normal and high performance concrete. The reduction was positively correlated to the SO3 presented in sand. At later ages (more than 28 days) in high performance concrete, the reduction in strength decreased while in normal concrete it increased continuously. The low permeability and pozzolanic action of HRM could be the cause of strength improvement. 4. The relationships between compressive, splitting strength and pulse velocity for the present work were in agreement with international literature. References ACI Committee 211. (1997)."Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete- ACI 211.1-91 (Reapproved 1997)". ACI Manual of Concrete Practice- 1999, Michigan, 38 pp. Advanced Cement Technologies. (2002)."High Reactivity Metakaolin, HRM, Engineered Mineral Admixture for Use with Portland Cement", http://www.sales@ metakaolin.com,1-7. Ahmed, H. K. (1980). Effect of Pozzolan on Cement and Concrete Resistance to Sulfate Attack and Hot Weather. MSc.Thesis, University of Technology, Iraq, 115 pp.( in Arabic). Al-Rawi, R. S. (1985)." Internal Sulfate Attack in Concrete Related to Gypsum Content of Cement with Pozzolan Addition". ACI-Rilemm Symposium," Technology of Concrete when Pozzolans, Slag, and Chemical Admixtures are Used". 543-556, Monterrey, N. L., Mexico. Babu, K.G. (1993)." High Performance Concrete". International Symposium on Innovative World of Concrete. (ICI-IWC-93), 2, 169-180. Lerch, W. (1946)." The Influence of Gypsum on the Hydration and Properties of Portland Cement Paste". ASTM Proceedings, 46, 1252-1292. Neville, A. M. (1995). Properties of Concrete. Longman, Essex, 844 pp.

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