Concrete inFocus - Spring 2015 - (Page OC3)

Infocus online connections: materials www.nrmca.org/news/connections Why is the Air There? Thinking about Freeze-Thaw in Terms of Saturation H. Todak, C. Lucero, W.J. Weiss Introduction The discovery of the benefits of air entraining admixtures was quite accidental in the early 1930's. It was observed that concrete pavements containing 'crusher oil' showed improved performance in cold environments due to the additional void space it produced within the concrete (Gonnerman, 1944). Since this time the use of air entraining admixtures has become a prominent feature in the design of concrete mixtures. Today, it is generally accepted that entrained air is a nearly essential part of a concrete mixture design to improve freezethaw durability, but what role does the airentrainment actually play to accomplish this task? The answer is multi-faceted. Air entraining admixtures stabilize air bubbles generated during mixing to add large (approximately 0.05 to 1.25 mm), stable, air-filled voids into the paste portion of the concrete (Mindess et al., 2002). These entrained air voids provide additional space to accommodate the volumetric expansion of water (and associated pressure) that occurs during freezing. While we frequently think of the volume of air being designed for a mixture as meeting a specific volume of the mixture (e.g. 5.5%) or meeting a critical spacing factor (e.g. 0.008 in); this paper examines the influence of the air in a slightly different way. The role of air is discussed in terms of the degree of saturation (S) and the rate at which this degree of saturation changes. Degree of Saturation and Damage in Cementitious Materials The degree of saturation (S) is most easily defined as the ratio of the volume of liquid in the pores (Vfilled ) and the total pore volume (Vtotal) as shown in Equation 1. Vfilled S = Vtotal Equation 1 There is a strong correlation between the degree of saturation and freeze-thaw damage that can occur in concrete (Fagerlund, 1977; Yang et al., 2007; Li et al., 2012). It has been proposed that a critical degree of saturation exists which is required to cause damage in both concrete and mortar. Once this critical value is reached, estimated to be in a range from 75 to 91%, concrete becomes susceptible to damage when exposed to even a single freeze-thaw cycle. Figure 1 provides data from recent studies (Li et al., 2012) to illustrate the influence of the degree of saturation in concrete on the damage that forms. It can be seen that for materials with a low degree of saturation, very little, if any damage occurs during freezing. However, as the degree of saturation increases, the damage occurs more rapidly and the damage is more severe. It is also interesting to note that this occurs in both non-air entrained concrete (4% air in Figure 1b) as well as airentrained concrete (9% air in Figure 1b). (a) (b) Figure 1: Influence of Degree of Saturation on Damage in Concrete Undergoing Freeze-Thaw Damage (a) Damage vs Degree of Saturation and (b) Rate of Damage vs Degree of Saturation (Li et al., 2012) concrete INFOCUS ı OC3 http://www.nrmca.org/news/connections

Table of Contents for the Digital Edition of Concrete inFocus - Spring 2015

Ready Mixed Plant Innovations
Data Security of Credit Card Processing in the Concrete Industry
Oldcastle Material Group
How the Concrete Paving Industry is Incorporating Sustainability into Our Practices
Index of Advertisers
The Trail to Your Future Business Should be Paved with Concrete
Your Biggest Environmental Threat in 2015 is NOT Who You Think!
Why is the Air There? Thinking about Freeze-Thaw in Terms of Saturation

Concrete inFocus - Spring 2015

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