Damping Properties of Polymer-Modified Concrete

The paper presents selected results from a broad study undertaken to explore means by which one might increase damping in concrete, which is relatively low—except for when the concrete is relatively young—and relatively independent of temperature and frequency. The most easily implemented approach involves the addition of latex admixtures to the concrete, resulting in a product generically known as polymer-modified concrete (PMC). The resulting concrete takes on some of the characteristics of the polymer involved: damping and elastic moduli become temperature and frequency dependent, and damping performance is a function of the polymer’s glass transition temperature. INTRODUCTION Concrete is an extremely popular structural material due to its low cost and easy fabrication. However, it has low damping and many engineering applications would be enhanced if the concrete technologist could design concrete mixtures with increased damping. The development of such improved concrete has received much impetus because of challenges presented in the design of facilities engaged in research and fabrication at the nanometer scale [1, 2]. This paper summarizes a portion of the results of a research project examining means by which the damping properties of concrete might be modified as part of the structural design process. The most efficacious means to achieve this objective appears to be through the introduction of latex polymers at the time of mixing. Several polymers were examined in the course of this study. Experimental modal analysis (EMA) was used to measure modal damping and resonance frequencies in beams of rectangular cross section, supported in wire slings, and excited in free-free modes. By using multiple bending modes, data were obtained over a frequency range between about 150 Hz and 3000 Hz. The specimens were tested at several temperatures, and the loss factor and elastic moduli were of found to be dependent upon both frequency and temperature. The method of reduced variables was used to combine the effects of frequency and temperature, leading to the observation that polymer-modified concrete exhibited behavior similar to polymers. It is feasible to use polymer additives to significantly enhance concrete’s damping but, as with polymers used for applications such as constrained layer damping, the designer must give consideration to the ambient temperature and the frequency range of concern. MATERIALS EXAMINED The coarse and fine aggregates (e.g., gravel and sand), as well as the ASTM Type I-II portland cement, were commercially available bagged products. The coarse aggregate had 10 mm (3/8 in.) maximum size aggregate. Many polymers—particularly those characterized as “viscoelastic”—are known to possess desirable damping properties. Many of these are well documented, and a variety of methods exist in which viscoelastic polymers may be incorporated into plate and beam components of several types of structures [3]. Presented at 75 th Shock and Vibration Symposium, October 17-22, 2004, Virginia Beach, VA HA S&V 2004 Page 2 of 9 Viscoelastic polymers may be incorporated into concrete in a number of ways. (Some polymer treatments of concrete involve polymers that are not traditionally considered “damping materials,” including epoxy, though the resulting concrete will take on some of the damping properties of the polymer. However, these mix designs, many of which do not involve portland cement, not included in this study.) Prior studies have examined effects on damping when using asphalt-coated aggregate [4], ground rubber from tires [5], and other forms of latex [6, 7]. In each case, the studies were limited to observations that damping increased with the concentration of the polymer. None of these studies examined the role of temperature and frequency. A number of polymer admixtures were used in the study. The dynamic properties of concrete with these admixtures had not been examined previously. Three were liquid latex suspensions of styrene-butadiene rubber (SBR) in water. Some of the properties are given in Table 1, including the glass transition temperatures, Tg [8]. SBR-A is popular for use in concrete pavements, in part because of its improvement of durability. The other two are not used extensively. The specific gravity of these admixtures is approximately 1.02. A fourth polymer additive included in the study was ethyl-vinyl-acetate (EVA) powder, popular as an additive to commercially prepackaged tile grout. It has a Tg of 11.2°C [9]. In this study, it was mixed with the dry cement prior to mi xing. The fifth polymer additive is an emulsion of vegetable gum suspended in SBR-A liquid latex suspension and denoted herein as SBR/g. The gum results from soybean oil reacted with sulfur monochloride. The gum and latex components are mixed at a gum:latex ratio of 6:44. The resulting latex has a specific gravity of 1.036 and a pH of 0.9, and it consists of 48% polymer solids. The important concrete mix parameters, including concrete compressive strength, are summarized in Table 2. The term W/C represents the weight ratio of water to cement solids, the primary determinant of concrete strength. The term P/C represents the weight ratio of polymer solids to cement. Shown are the upper-bound P/C values used for all experiments except those in which concentration was varied. TEST CONFIGURATION EMA was performed on beams of rectangular cross section, 89 mm x 114 mm (3.5 in. x 4.5 in.), with a length of 1400 mm (55 in.). They were supported in wire slings and tapped with a small modal hammer having a mass of 235 g. A 100mV/g accelerometer, attached using a threaded stud to mounts cast in the concrete, was used as a vibration sensor. Figure 1 shows the measurement locations. Locations 2 and 3 were excited in the orientation shown; the specimens were rotated 90° (such that axis 2 was horizontal) for excitation of Location 1. Bending about axis 1 and axis 2 were denoted by B1 and B2, respectively. Signal processing was carried out using a commercial laptop-based two-channel spectrum analyzer, and commercial modal analysis software was used to extract frequencies and modal damping from accelerance FRF spectra using curve fitting. Table 1. Styrene-butadiene rubber (SBR) latex suspensions used in the study; S/B, Tg, and Percent Solids from Barclay (2004) [8] Designator S/B Tg (°C) Percent Solids pH SBR-A 60/40 10 48% 9.0 SBR-447 55/45 -17 47.5% 10.3 SBR-813 40/60 -41 47% 10.1 Table 2. Summary of compressive strengths at ~28 days Polymer W/C P/C fc’, MPa none 0.53 33.0 SBR-A 0.2 0.2 39.0 SBR-447 0.2 0.2 32.5 SBR-813 0.2 0.2 26.7 EVA 0.42 0.2 18.7 SBR/g 0.63 0.03 18.6