Sarutt Sripetchr 1* and Surachai Thachepan 2

Transcription

1 Characterization of Potential Reactivity to Alkalis of Thailand Aggregates by Chemical Method Sarutt Sripetchr 1* and Surachai Thachepan 2 ABSTRACT The best way to prevent Alkali-Silica Reaction (ASR) is to take appropriate precautions before concrete is placed. As so, the potential of aggregate for ASR can be tested by various standard laboratory tests. This study is focused on the alkali-silica reactivity of three aggregates from different sources in Thailand. It has been investigated using ASTM C 289: Standard Test Method for Potential Alkali-Silica Reactivity of Aggregates (chemical method). The rapid method is applicable to the determinate the dissolved silica (S c ) and the reduction in alkalinity (R c ) in a representative aggregate samples. Results indicate large distinction between the ASR potential of the three aggregates of Thailand. Aggregates CNCM and RSBB was classified as innocuous whereas aggregate KPSA was classified as deleterious.. Key words: aggregates, alkali-silica reactivity, chemical method, ASTM C 289. *Corresponding author; address: sarutt8nanomaterials@gmail.com 1 Department of Materials Science, Faculty of Science, Kasetsart University, Bangkok, Department of Chemistry, Faculty of Science, Kasetsart University, Bangkok, 10900

2 INTRODUCTION Alkali-silica reaction (ASR) is one of the most recognized deleterious phenomena in concrete. This phenomenon has been observed since the 1930s in California (USA) and later documented by Stanton (1940). The ASR (Dent Glasser, L.S. and Kataoka, N., 1981) is a chemical reaction between the reactive silica contained in the aggregates and the alkalis (Na 2 O and K 2 O) within the cement paste. The result is an alkali-silicate hydrate gel (Davies, G. and Oberholster, R.E. 1986). that absorbs water and increases in volume (Zollinger et al., 2009). If the gel is confined by the cement paste, it is built up pressure causing internal stresses that eventually could crack the concrete (BRE Digest 330, 2004). Due to the aggregates generally occupy 60% to 75% of the concrete volume. The aggregates are less expansion than cement paste and their usage should be maximized. One could therefore expect that the properties of the concrete would significantly affected by the properties of the aggregate. For this reason, there is a need for rapid and reliable test methods which can distinguish between innocuous and potentially alkali-reactive aggregates. Based on Stanton s work, the U.S. Bureau of Reclamation (Meissner, 1941) conducted investigations of abnormal cracking in concrete dams. Meissner s findings generally corroborated those of Stanton, and lent further credence to the importance of cement alkali level, aggregate composition, and environmental requirements in the development of expansion due to ASR. One outcome of this work was the development of the quick chemical test, ASTM C 289 (Mielenz et al., 1948). A rapid chemical test is in order to determine the potential reactivity of silica-bearing aggregates. This method was subsequently standardized as ASTM C 289 in 1952 and has undergone several minor revisions until the current version in 2007 (ASTM C 289, 2007). This study aims to gain the rapid data and reliable results from the chemical method. Therefore, before using aggregates in concrete is very important practices for prevent damage.. MATERIALS AND METHODS 1. Selection and Preparation of Aggregate Samples. Three aggregates including both coarse and fine aggregates were collected from different sources in Thailand. The coarse (rocks) aggregates CNCM and RSBB were collected from Nakhonsawan and Saraburi provinces. The fine (sand) aggregate KPSA was collected from Chonburi province. Three aggregate was powdered and distributed to the average particle size showing two maxima, at 150 µm and 300 µm. The samples were cleaned by water until clearest liquid and dried the washed sample at 105 ± 5ºC for 24 hours.

3 2. Reaction Procedure. The chemical method assesses the reactive potential of a previously prepared aggregate, of a large enough size to pass through the 0.30-mm sieve and be retained in the 0.15-mm sieve. Subsequently, 25 g of this material was set aside and placed in each of the three of plastic polypropylene container along with 25 ml of NaOH at a concentration of 1 N. The four reaction containers, by means of a pipet, add 25 ml of the same NaOH solution to serve as a blank. The samples were placed in a water bath for 24 hours and at 80ºC. After this phase, the solutions were immediately filtered then stir the filtrate to assure homogeneity. The pipet was used for an aliquot of 10 ml of the filtrate and diluted with water to 200 ml in a volumetric flask. This diluted solution was reserved in the plastic container for determination the dissolved SiO 2 and the reduction in alkalinity. 3. Determination of Dissolved Silica by the Gravimetric method The dilute solutions was transferred by pipet 100 ml to an evaporating dish and added 10 ml of HCl (1.19 kg/l) then evaporated for dryness on a hot plate. The adding of 10 ml of HCl (1.19 kg/l) to the residue further was treated without heating before water filtering. The dish was covered and digested for 10 min on a hot plate. The solution was diluted with an equal volume of hot water and immediately filtered through quantitative grade, low ash filter paper, and washed the separated silica (SiO 2 ) thoroughly with hot water and reserved for the residue. The containing residue paper was placed in an alumina crucible for drying and igniting at 950ºC for 3 hours until the carbon of the filter paper is completely consumed without inflaming. The mass must be constantly remained. The SiO 2 was treated by transferring the small amounts of impurities, in the plastic container with a few drops of water, about 10 ml of HF, and one drop of H 2 SO 4, and evaporated cautiously for dryness on the steam bath. Finally, the small residue was transferred to an alumina crucible and heated at 960 C for 10 minutes then cooled down the temperature before mass determining. The different between this determination and that previously obtained represents the amount of SiO Determination of Dissolved Silica by the Photometric Method The preparation of a solution series known as silica concentration were varied from 0.0 to 0.5 mmol/l by diluting portions of the stock solution of sodium metasilicate (10 mmol/l). The portions of sodium metasilicate solution were placed to 100-mL volumetric flasks about half filled with water. The 2 ml of the ammonium molybdate solution was added before 1 ml of HCl (1 + 1) and agitated by swirling the flask. The solution was kept for 15 min at room temperature. After that, adding 1.5 ml of the oxalic acid solution, fill the flask to the mark with water, were thoroughly mixed then kept 5 minutes. The uv-vis spectrophotometer at 410 nm (V-670, JASCO) was selected to study

4 the absorbance of the various solutions then compared with DI water. A calibration curve was prepared by plotting the absorbance readings against the known concentrations of silica in each solution. The diluted solution of aggregates was transferred by pipet a 10-mL aliquot to a 100-mL volumetric flask half filled with water and proceeded as a previously condition. The concentration of silica in the diluted solution of aggregates was directly read from the previously condition and compared with calibration curve correlating absorption of light of this wave length with silica concentration. 5. Reduction in Alkalinity The dilute solution was transferred by pipet a 20-mL aliquot to a 125-mL Erlenmeyer flask, added 2 or 3 drops of phenolphthalein solution, and titrated with 0.05-N HCl to the phenolphthalein end point. RESULTS AND DISCUSSION 1. Determination of Dissolved Silica and Reduction in Alkalinity The average results (from the triplicates) of the dissolved silica (S c ) and of reduction in alkalinity (R c ) were obtained by the chemical method for the different aggregates tested were shown in Table 1. The S c of KPSA and RSBB aggregates has high values of dissolved silica, which means that the KPSA and RSBB aggregates has high content of silica mineral in amorphous structure. The amorphous silica mineral has a highly disordered structure which performs it unstable at high ph condition. On the other hand, the CNCM aggregate has low values of dissolved silica, which means that the CNCM aggregate has low content of an amorphous silica mineral. The R c value indicates the concentration of NaOH solution to reduce due to it reacts with an amorphous silica mineral in the aggregates. The CNCM and RSBB aggregates have positive value which means that the CNCM and RSBB aggregates have an amorphous silica component. The RSBB aggregate has the R c value more than the CNCM aggregates which performs that the RSBB aggregate has an amorphous silica component more than the CNCM aggregate. On the other hand the KPSA aggregate has a negative value. This abnormal behavior is due to the reaction involving the ions of certain sodium and potassium which increase the ph in solution.

5 Table 1 Chemical test result of Thailand aggregates. Sample CNCM KPSA RSBB Silica content in the original filtrate (S c ) (mmol/l) Reduction in alkalinity (R c ) (mmol/l) Results Mean Results Mean Interpretation of Results The mean S c and R c values of three aggregates were plotted in Figure 1. This graph was created by Microsoft Excel 2010 in accordance to ASTM C 289 chemical test (adapted from ASTM C 289, 2007 and Fernandes, 2005) which illustrate the division between innocuous and deleterious aggregates (blue solid curve). The potentially deleterious aggregates represented by area above the solid line. Correlations between S c and R c were performed an innocuous behavior was found for aggregates CNCM and RSBB, whereas aggregate KPSA plotted in the deleterious field. Aggregate RSBB presents the highest value of R c (67.9 mmol/l) and aggregate KPSA shows the lowest value ( mmol/l). The S c ranging from 7.40 mmol/l to 57.9 mmol/l was determined with the highest value belonging to KPSA and the lowest to CNCM. After the tests were performed, only samples KPSA were considered deleterious for their reactivity to alkalis through this method. The remaining samples were classified as innocuous.

6 Figure 1 Distribution of the Thailand aggregates in ASTM C 289 plot (adapted from ASTM C 289, 2007 and Fernandes, 2005). CONCLUSION The ASTM C 289 chemical test results presented the large distinction between the ASR potential of the three aggregates of Thailand. Aggregates CNCM and RSBB was classified as innocuous, whereas aggregate KPSA was classified as deleterious. ACKNOWLEDGEMENTS Financial support from My Adviser Dr. Surachai Thachepan Fund and Department of Chemistry Faculty of Science Kasetsart University were gratefully acknowledged.

7 REFERENCES ASTM C Standard test method for potential alkali silica reactivity of aggregates (Chemical method). The American Society for Testing and Materials Philadelphia, USA: 7 pp. BRE Digest Alkali-silica reaction in concrete. Building Research Establishment Centre for Concrete Construction UK: 32 pp. Davies, G. and Oberholster, R.E The alkali-silica reaction product: a mineralogical and an electron microscopic study. pp In Proceedings of the 8 th International Conference on Cement Microscopy, Duncanville, Texas, USA. Dent Glasser, L.S. and Kataoka, N The chemistry of alkali-aggregate reaction. Cement and Concrete Research (11): 1-9. Fernandes, I Caracterização petrográfica, química e física de agregados graníticos em Betões, Estudo de casos de obra. Ph.D. thesis, Universidade do Porto. Meissner, H. S Cracking in Concrete Due to Expansive Reaction between Aggregate and High-Alkali Cement as Evidenced in Parker Dam, pp ACI JOURNAL, Proceedings Vol. 37, Apr. Mielenz, R. C., Greene, K. T. and Benton, E. J Chemical Test for the Reactivity of Aggregates with Cement Alkalis: Chemical Processes in Cement-Aggregate Reaction, pp ACI JOURNAL, Proceedings Vol. 44. Stanton, T Expansion of concrete through reaction between cement and aggregates. pp In Proceedings of the American Society of Civil Engineering. Vol. 66, No. 10.