ASR in Concrete — PART 3

CURRENT ASR PROBLEM AND FAILED SOLUTIONS
ASR gel is defined as an expansive gel that is created by the combination of reactive silica, soluble alkalis, and hydroxides in the cementitious solution [13]. The data in Figure 4 illustrates the polymerization of the ASR gel, rooted in the ITZ between the cement paste and the reactive aggregate, but also found in a polymerized state within the aggregate. Stresses develop as ASR gel expands in the confined space of the hydrating cementitious matrix of the concrete structure [14, 15]. Under normal conditions, this process takes several months, or even years, to evolve, but can be accelerated by harsh environments, availability of soluble alkalis, and availability of absorbed H2O from the environment. Also, diffusion of H2O into concrete can be accelerated by cracking in the HCM of concrete. Due to the porous structural network and brittle nature of the HCM, cracks gradually form throughout the concrete [16]. Failure will ensue, as depicted in Figure 4, as soluble alkalis in the cement paste combine with reactive silica in the aggregate to form an expansive gel at the ITZ. The expansive forces cause micro-stresses that compromise the concrete structure. These micro-stresses eventually lead to cracks, increasing the permeability of the concrete and exacerbating ASR gel expansion. Eventually, the composite structure of the concrete is compromised by the expansion, and the concrete structure becomes unserviceable [14].

Figure 4 — Time lapse of ASR failure of an aggregate within the HCM of concrete. The time scale can vary from months to years.
As stated earlier, chemical degradation of concrete structures nationally account for a substantial amount of taxpayer dollars. ASR gel is one of the chemical degradation mechanisms that contributes to the reduction of the structural integrity of concrete civil structures, which must then be repaired or replaced. Current ASR mitigation materials and the associated concrete mixtures are falling short of generating durable concrete infrastructure [3]. In 2009, the United States Bureau of Reclamation published a document to address new recommendations for ASR mitigation in reclamation concrete construction. The objective behind the publication was to recognize recent ASR degradation failures of concrete dams, due to the following causes: lack of availability of low-alkali-cement, increased usage of poor quality aggregates, increased alkali content of concrete, and reduction of availability of suitable Class F fly ash [17].
An example of excessive ASR gel damage is illustrated in Figure 5, showing a bridge that has been taken out of service due to ASR damage [18]. In the portion shown, the concrete, which makes up the main structural material of the bridge, has excessive map cracking that has compromised its structural integrity. The magnified cross-section highlights a portion of the bridge. There is a distinct diagonal boundary that separates the concrete that was open to the environment (upper triangular section) from the portion of the concrete that was covered with soil (lower triangular section). This upper portion was a continually exposed and wetted surface, which created conditions for excessive ASR gel expansion, causing the map-cracking shown [19]. Supplementary additives in the form of secondary cementitious and pozzolanic materials were used to fortify (but failed) concrete against this ASR gel expansion. These supplementary materials were used in standardized mitigation techniques, published by the Canadian Standards Association [20].

Figure 5 — A bridge outside of Quebec City failed due to ASR damage. The magnified cross-section identifies the extreme map cracking by the ASR Gel expansion [19].
Current ASR gel mitigation materials (such as Class F fly ash) have been found by Thomas and Taylor to enhance concrete through a two-fold process: (1) replacement of OPC with secondary cementitious and pozzolanic materials to breach the pessimum of ASR gel expansion by reducing the amount of soluble alkalis brought by the OPC to the HCM and (2) providing excess free silica that increases the production of C-S-H, chemically binding H2O and alkalis and thus reducing their propensity to feed ASR gel expanion [19]. The ASR gel pessimum is defined as the chemical environment where all of the constituents are within the range of concentrations that promote the polymerization and expansion of ASR, largely dependent on the alkali content and silica concentration [14, 21]. At low concentrations, increasing the amount of reactive silica has been found to increase the expansion due to ASR gel polymerization. As the concentration of silica increases, at some point, the expansion peaks. At higher concentrations of reactive silica, potential ASR gel expansion decreases [21].
As noted, a pozzolanic material by itself will not gain strength when mixed with H2O. The pozzolanic material assists the cementitious compounds in reaching higher degrees of hydration, increasing the proportion of C-S-H and the hardened strengths of cement composites and concrete [11]. The pozzolanic material is usually associated with an amorphous form of silica collected from coal combustion residue, a ferrosilicon alloy product, or a substance sourced from naturally occurring clays and minerals [4, 11]. The impact of nano silica size and surface area on ASR gel polymerization and expansion can be compared to the Class F fly ash, a popular ASR gel mitigation material used in the concrete industry as a replacement by mass for OPC. Fly ash is a recycled coal combustion material that has a low purity of free silica (when compared to nano silica) and thus is designated as a pozzolanic material for concrete use [17, 22].
References
1. US-DOI. Materials in Use in the U.S. Interstate Highways. USGS Department of the Interior, 2006.
2. Thomas, M., B. Fournier, and K. Folliard. Report on Determining Reactivity of Concrete Aggregates and Selecting Appropriate Measures for Preventing Deleterious Expansion in Concrete Construction. Departmant of Transportation – FHWA, 2009. p. 1–22.
3. Kuennen, T. FHWA Research Zeroes in on Concrete Nanotechnology. Concrete Products, 2010. www.concreteproducts.com/mag/concrete_fhwa_research_zero.
4. Mindess, S., J. Young, and D. Darwin. Concrete. Prentice Hall, 2003. 2: p. 57–120.
5. Taylor, H.F.W. Cement Chemistry. Thomas Telford Books, 1997. 2: p. 89–226.
6. Belkowitz, J., and D. Armentrout. An Analysis on Nano Silica in Cement Hydration. National Ready-Mixed Concrete Association Conference Proceedings on Concrete Sustainability, 2010: p. 1–15.
7. Richardson, I. The Calcium Silicate Hydrates. Cement and Concrete Research, 2007. 38: p. 137–158.
8. Allen, A., J. Thomas, and H. Hennings. Composition and Density of Nanoscale Calcium-Silicate-Hydrate in Cement. Nature Materials, 2007. 6: p. 311–316.
9. Bentz, D. and Garboczi, E. Simulation Studies of the Effects of Mineral Admixtures on the Cement Paste-Aggregate Interfacial Zone. ACI Materials Journal, 1991: p. 518–529.
10. Bentz, D. and Stutzman, P. Evolution of Porosity and Caclium Hydroxide in Laboratory Concretes Containing Silica Fume. Cement and Concrete Research, 1994. 24: p. 1044–1050.
11. Holland, T. Silica Fume User’s Manual. Department of Transpotation — FHWA, 2005: p. 8–13.
12. Manzano, H., J. Dolado, A. Guerrero, and A. Ayuela. Mechanical Properties of Crystalline Calcium-Silicate-Hydrates: Comparison with Cementitious C-S-H Gels. Physical State Solutions, 2007. 204: p. 1775–1780.
13. Kurtis, K., P. Montiero, J. Brown, and W. Meyer-Ilse. Imaging of ASR Gel by Soft X-Ray Microscopy. Cement and Concrete Research, 1998. 28: p. 411–421.
14. Ichikawa, T. Alkali-Silica Reaction, Pessimum and Pozzolanic Effect. Cement and Concrete Research, 2009. 39: p. 716–726.
15. Rodrigues, A., P. Montiero, and G. Sposito. The Alkali-Silica Reaction:The Surface Charge Density of Silica and its Effect in Expansion. Cement and Concrete Research, 1999. 29: p. 527–530.
16. Chen, J., J. Thomas, H. Taylor, and H. Jennnings. Solubility and Structure of Calcium-Silicate-Hydrate. Cement and Concrete Research, 2004. 34: p. 1499–1519.
17. Hurcomb, D., K. Bartojay, and K. Fay. New Recommendation for ASR Mitigation in Reclamation Concrete Construction. U.S. Department of Interior MERL Laboratory, 2009.
18. Thomas, M. The Effect of Supplementary Cement Materials on Alkali-Silica Reaction: A Review. Cement and Cocnrete Research, 2011. 41: p. 1224–1231.
19. Thomas, M. Mitigating ASR in Maine Bridges – Reactive Solutions. Departmant of Transportation – FHWA, 2011. 3: p. 1–3.
20. CSA A23.2–27A. Use of Supplementary Materials for Counteracting Alkali-Silica Reaction. CSA, 2000. www.csa.ca.
21. Garcia-Diaz, E., D. Bulteel, Y. Monnin, and P. Fasseu. ASR Pessimum Behavior of Siliceous Limestone Aggregates. Cement and Concrete Research, 2010. 40: p. 546–549.
22. Helmuth, R. Fly Ash in Cement and Concrete. U.S.A.: Portland Cement Association, 1987: p. 85–138.
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