1Concrete BiodeteriorationCEE 8813April 25, 2006Jonah KurthSlide 2Lecture Overview• History of Research • Types of Microbes• Methods of Deterioration• Methods of Remediation2Slide 3History of Research• Degredation of sewers first detected in 1900• Original explanation was purely chemical• Australian research in 1945 first to correlate bacteria and deterioration[9] Gu, J.D. et al (1998)Slide 4Early Australian Research• First to link bacteria to deterioration• Showed that Thiobacilli concretivorus, not H2S, caused most deterioration• Proposed multiple species are responsible for deteriorationFig. 1 - Blocks after removal of corroded material[18] Parker, C.D. (1945)3Slide 5Research in Germany• Identified three strains in Hamburg sewer– T. intermedius/novellus (colonizer)– T. neapolitanus (pH lowerer)– T. thiooxidans (destroyer)• Performed experiment before demolition– Turned on oxygen supply– pH increased, T. Thiooxidans diminished[16] Milde, K. et al (1983)Slide 6Current Research• Fungi considered for biodeterioration[9],[23]• Nitrifying bacteria discovered on building exteriors[23],[22]– No longer confines research to sewers– Concrete parallels stone and wood deterioration• Use of modern techniques to study microbes– SEM, LVSEM to photograph stains [5]– pCR analysis to identify species[27]4Slide 7Types of MicrobesMicrobes (Fungi, bacteria, algae)Autotrophs (light & air) Heterotrophs (need organics)SewersOutsideOutsideSlide 8Sewer Microbes• Environment– Anaerobic (oxygen poor)– Low light conditions– Sulfur Rich• Type of Deterioration– Dissolves cement compounds– Reduces reinforcing cover– Reduces section thickness, leading to collapse[19] Parker & Prisk, 1953Fig. 1: Metabolic reactions of Thiobacilli5Slide 9Atmospheric Microbes• Environment– “Local” site characteristics• Intensity of light exposure• Prevailing wind direction• Specific moisture conditions• Mineral/Physical/Chemical conditions of the surface– “Global” site characteristics• General climate (N. America vs. Europe, etc.)• Humidity and elevation• Ocean vs. Inland (specifically, salt content)[13]Slide 10Atmospheric Microbes• Environment• Types of Organisms– Cyanobacteria• Major contributor to biofilm color• Can grow in high UV, low moisture• Usually dominant biomass on concrete [6]– Fungi• Major contributor to physical biodeterioration• May work symbiotically with bacteria [9]– Algae• Highly dependent on water content, porosity• Traps water through biofilm “sheets”6Slide 11Atmospheric Microbes[5] Dubosc, A. et al (2001)Slide 12Atmospheric Microbes• Environment• Types of Organisms• What do they do? – Microbes are not benign– Cause premature weathering through:• Chemical attack• Active physical attack• Passive physical attack– ~30% of weathering is biological[3]7Slide 13Chemical Attack• Reactants for natural metabolism– Utilize ions present in cement– Minerals solubolized by metabolites– Enzymes break down mortar[8] Gaylarde, C.C. et al (2003)Slide 14Chemical Attack (2)• Reactants for natural metabolism• Products of natural metabolism– Nitric and organic acids– Release polyols, glycerols, disrupting silica[8] Gaylarde, C.C. et al (2003)8Slide 15Chemical Attack (3)Chart of porosity vs. depth after exposure to bacteria[3] DeGraef, B. et al (2005)Slide 16Active Physical AttackFilamentous growths– Can be from fungi, cyanobacteria, algae– Physically pry away components– Filaments are < 10 µm [8] Gaylarde, C.C. et al (2003)9Slide 17Passive Physical AttackBiofilms and slime!– Discoloration– Colonizer: enables other growth– Temperature: changes thermal behavior– Hydrophilic: traps moisture[25] Warscheid, T., & Krumbein, W. E. (1994). Kurth, J. (2007). North Ave Underpass.Slide 18Mitigating Growth• Active measures– Photocatalysts– Biocides• Passive Measures– Cement properties– Reducing favorable conditions10Slide 19Biocides• Paints and Coatings– Cover exposed surface with biocidal coating– Chemically disrupt cell growth– Limited effectiveness as time increases– Increase resistance to biocide by microbes• Disinfection and washing– Soaked with hypochlorite (bleach) and pressure washed– No post-application biocidal effects[23] Shirakawa, M.A. et al (2004)Slide 20Biocides (2)[23] Shirakawa, M.A. et al (2004)11Slide 21Photocatalysts• Light energy causes a catalytic reaction to break down cells• Possible catalysts: Metal Oxides (TiO2 and WO3) with noble metal cocatalysts (Pt & Ir)[1][15] Linkous, C.A. et al. (2000). Slide 22Titanium Dioxide• Importance of structure and dispersion– Only anatase is photocatalytic– Mixing with cement is not as efficient, but still effective• Commercially available product (Tx Active, Tx Aria)– Portland cement mixed with TiO2– ~3x as expensive as regular cement[21] Rachel, A. et al (2002)12Slide 23Applying Expensive Cement•TiO2layer shows no increase in effectiveness beyond ½”[*]• Existing practice fills “bugholes”, surface blemishes with cement• Material costs are only a fraction of total cost* Christopher Eagon, Essroc, personal communication (April 2007).Slide 24Reduce favorable conditions• Environmental– Reduce exposure to rain– Reduce airborne pollutants and nitrates– Keep birds away from structure• Construction Practices– Clean form releases from concrete– Smooth surfaces to reduce roughness13Slide 25Changing Cement Properties• Polymers– Reduce permeability of concrete to acid infiltration– Enable concrete to bridge microcracks– Some polymers work, others do not• Silica Fume and SCMs– Reduce porosity and moisture retention– Silica fume shown to inhibit growth [5]– Silica fume also shows reduced resistance to acid attack [24]• Increase w/c ratio– Decrease permeability, increase paste density [5]– MDOT study shows growth occurs mainly on paste [10]Slide 26Favorable Conditions Example14Slide 27Conclusions• Microbial growth is affected by many variables• Microbial activity significantly affects durability• More research is needed for a “Best Practice”recommendation for mitigation of growthSlide 28Future research• Examine role of SCMs in cement susceptibility– No data on fly ash or slag replacement– Silica fume data is limited– No data on cement with 5% limestone• Examine effect of concrete practice on growth– Form releases are