9. Degradation of Mortar and Concrete

This chapter covers aspects of how to model and measure the degradation of concrete. Most degradation processes are either physical mechanisms, like freeze-thaw attack, or a combination of chemical/physical mechanisms, like alkali-silica reaction, sulfate attack, or delayed ettringite attack. But whether physical, or chemico-mechanical, the mechanisms of concrete degradation involve the transport of water, or ions, or both through the pore space of the concrete. Therefore the study of transport properties is a necessary first step for understanding the mechanisms of degradation.

One important question that must also be dealt with is how transport properties change during degradation. One cannot simply assume some fixed value of ionic diffusivity, for example, that does not change during degradation. As cracks are formed, due to expansive growth of reactive inclusions, the pore space changes and therefore the transport properties change as well.

This chapter also includes some novel experimental data on degradation testing. As degradation/durability tests increase in sophistication and have a more solid basis in basic chemistry and physics, degradation models will become more useful in predicting test results.

This section describes one study of how the leaching of calcium hydroxide (CH) affects the diffusivity of cement paste. The removal of CH will also change the pore space, and therefore the ionic diffusivity as well. This study draws upon the C₃S model, and adds an algorithm for randomly removing CH from the model, similar to how CH would be removed in a real leaching process. The diffusivity is computed for a given pore structure using a conjugate gradient finite difference method, and the effects of leaching are understood in terms of percolation ideas. 

(1a) Modelling the leaching of calcium hydroxide from cement paste: Effects on pore space percolation and diffusivity

An updated study on real cement pastes (instead of C₃S only) including simulations of the actual calcium profiles in real specimens is found in the following.

(1b) Influence of Calcium Hydroxide Dissolution on the Transport Properties of Hydrated Cement Systems

An experimental study of the effect of the incorporation of fly ash on the leaching properties of pastes and mortars. The experiment include those necessary in order to be able to accurately model the hydration using CEMHYD3D.

(1c) Effect of the incorporation of Municipal Solid Waste Incineration fly ash in cement pastes and mortar I. Experimental study

The accompanying modeling study to go with the experimental results above. The models include the prediction, using CEMHYD3D, of hydration in the presence of fly ash and prediction of leaching.

(1d) Effect of the incorporation of Municipal Solid Waste Incineration fly ash in cement pastes and mortars II: Modeling

Since leaching affects the microstructure of the cement paste, it has an effect on all properties, including elastic properties. This study models the effect of leaching on the elastic properties of cement paste, using a combination of CEMHYD3D the finite element code elas3d.f, and includes some comparison to experimental results.

(1e) Hydrate dissolution influence on the Young's modulus of cement pastes

This section examines the effect, for a single aggregate surrounded by an arbitrary shell, of how different expansive conditions can affect any possible crack pattern that results from the stresses caused by the expansive forces arising from various degradation mechanisms. The case of a single spherical aggregate can be solved exactly by analytical expressions. The cases of the matrix expansive with no shell present (freeze-thaw), aggregate expansive with no shell (alkali- aggregate reaction), and thin shell expansive are all treated. Also, if cracking is assumed to occur such that the aggregate is separated from the matrix, the size of the displacement rim around the aggregate is calculated.

(2) Stress, displacement, and expansive cracking around a single spherical aggregate under different expansive conditions

The output of models is often 3-D images. Acquiring 3-D images of real systems in order to compare to model images is often difficult. This paper briefly describes how x-ray microtomography can be used to acquire a reasonably good image of a mortar, at about 10 micrometers per pixel resolution, which is enough for quantifying the aggregate/entrapped air system, but not enough to see details of the cement paste.

(3) X-ray microtomography of an ASTM C109 mortar exposed to sulfate attack

One concern for the durability and safety of high performance concretes is their susceptibility to catastrophic spalling. This paper examines this phenomena from a microstructural viewpoint, focusing on the role of percolation of the interfacial transition zones, and its modification by the incorporation of organic fibers.

(4) Fibers, Percolation, and Spalling of High Performance Concrete

Alkali-silica reaction is ubiquitous in the world of concrete, and is sometimes destructive. The measurement of how much alkali-silica reaction may have affected a particular material has been limited in the past to the measurement of length change or strain. This preliminary study shows how induced stresses, which should be a better indicator of what is actually happening in the material, can be measured and analyzed in the laboratory.

(5) Influence of Silica Fume on the Stresses Generated by Alkali-Silica Reaction

One of the most studies and measured degradation effect is due to the reaction of sulfate with cement. This section concentrates in measurements and methodologies to determine the resistance of concrete or cement to sulfate attack.

(6a) Sulfate Resistance of Concrete: A New Approach

(6b) Developing a More Rapid Test to Assess Sulfate Resistance of Hydraulic Cements

Go to Chapter 10: Aggregates: Shape and properties

Go back to Chapter 8. Curing and Autogeneous shrinkage of concrete

References

(1a) D.P. Bentz and E.J. Garboczi, Materials and Structures 25, 523-533 (1992).
(1b) J. Marchand, D.P. Bentz, E. Samson, and Y. Maltais, in Reactions of Calcium Hydroxide in Concrete (American Ceramic Society, Westerville, OH, 2001).
(1c) S. Remond, P. Pimienta and D.P. Bentz, Cement and Concrete Research 32 (2), 303-311 (2002).
(1d) S. Remond, D.P. Bentz, and P. Pimentia, Cement and Concrete Research 32 (4), 565-576 (2002).
(1e) S. Kamali, M. Moranville, E. Garboczi, S. Prené, B. Gérard, International Conference on Fracture Mechanics of Concrete and Concrete Structures (2004).
(2) E.J. Garboczi, Cement and Concrete Research 27, 495-500 (1997).
(3) D.P. Bentz, N.S. Martys, P.E. Stutzman, M.S. Levenson, E.J. Garboczi, J. Dunsmuir, and L.M. Schwartz, in Microstructure of Cement-Based Systems/Bonding and Interfaces in Cementitious Materials, edited by S. Diamond et al. (Materials Research Society Vol. 370, Pittsburgh, 1995), pp. 437-442.
(4) D.P. Bentz, ACI Materials Journal 97 (3), 351-359 (2000).
(5) C. Ferraris, E. Garboczi, P. Stutzman, J. Winpigler, and J. Clifton, Cement, Concrete, and Aggregates 22, 73-78 (2000).
(6a) C.F. Ferraris, P.E. Stutzman, and K.A. Snyder, Portland Cement Association, (2006).
(6b) C.F. Ferraris, P.E. Stutzman, M. Peltz, and J. Winpigler, Journal of Research of the National Institute of Standards and Technology, Vol.110, 5, 529-540 (2005).

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