1、1ACI 238.2T-14ConCrete thixotropyKeywords: aggregate segregation; formwork pressure; plastic viscosity; pumpability; rheology; rheometer; static yield stress; thixotropy; yield stress.IntroductionFresh concrete can exhibit different rheological behavior when at rest than when flowing. This differenc
2、e is due to thixotropy, which can have important consequences for formwork pressure, multi-lift casting, slip-form paving, pumping, and segregation resistance. This TechNote defines thixotropy and distinguishes it from other changes in rheological properties; discusses the origins of, test methods f
3、or measuring, and factors affecting thixotropy; and concludes with its applications.Description of thixotropyRheology is the science dealing with the deformation and flow of materials and is typically described based on the relationship between shear stress and shear rate, as shown in Fig. 1. Viscos
4、ity () is the ratio of shear stress to shear rate at a given shear rate.Concrete rheology is most commonly characterized in terms of the Bingham model (Fig. 1), which is defined in terms of yield stress (t0) and plastic viscosity (mpl) (ACI 238.1R).Two measures of yield stress are typically reported
5、. Static yield stress (t0-static) is the minimum shear stress to initiate flow from rest. Dynamic yield stress (t0-dyanmic) is the minimum shear stress to maintain flow.Plastic viscosity is the change in shear stress divided by the change in shear rate, for the shear stress greater than the dynamic
6、yield stress. For Bingham materials, plastic viscosity is independent of shear rate, but the viscosity depends on the shear rate and is typically referred to as apparent viscosity (apparent).Thixotropy is a reversible, isothermal, time-dependent decrease in viscosity when a fluid is subjected to inc
7、reased shear stress or shear rate (Mewis 1979). The change in viscosity is considered reversible because the viscosity will increase to its original value when the shear stress or shear rate is decreased to its original value. The change in viscosity due to thixotropy is considered to be isothermal
8、to distinguish changes in viscosity due to thixotropy from changes in viscosity due to changes in temperature. The change in viscosity is considered to be time-dependent because the change in viscosity occurs over a period of time, rather than instantaneously when the shear is applied or removed.Thi
9、xotropy should be distinguished from irreversible changes in viscosity. Such changes may be caused by hydration, as manifested by slump loss and setting. In addition, segregation can result in a more paste-rich region of concrete with lower viscosity. Although the original nonsegregated composition
10、could be restored by remixing, changes in viscosity due to segregation should not be considered thixotropy.Therefore, changes in viscosity can be attributed to both reversible and nonreversible phenomena. In this way, the effects due to thixotropy can be separated from effects due to other phenomena
11、, such as setting. In practice, there is often no relevance in differentiating the reversible and irreversible processes because it is the overall increase in viscosity when concrete is at rest over time that will influence the fresh state performance, such as concrete segregation resistance or form
12、work pressure (Billberg 2009).Thixotropy should be distinguished from shear-thinning characteristics of a fluid, wherein the viscosity decreases with increasing shear rate. Thixotropy is time-dependent and can be observed at a constant shear rate, whereas shear thinning characteristics are shear-rat
13、e-dependent, not time-dependent.Origins of thixotropyThixotropy is due to two main aspects: 1) structural build-up when concrete is at rest; and 2) structural break-down when concrete is under a shear or flow.When a concrete mixture is at rest, particles in the cement paste flocculate and move toget
14、her with time, forming a Fig. 1Bingham rheological model.TechNote2CONCRETE THIXOTROPY (ACI 238.2T-14)three-dimensional, networked internal structure. This internal structure results in an increase in concrete viscosity. Flocculation is the result of Van der Waals attraction and Brownian motion (Shaw
15、 1992).If this same concrete is subjected to shearing, the flow can result in breaking up of flocs due to rupture of interparticle links and flocculation due to particle contacts (Wallevik 2003). At a constant rate of flow, a balance between breakup of flocs and flocculation will be reached with tim
16、e. The internal structure will be reduced compared to the at-rest state, resulting in a decrease in concrete viscosity.Measuring thixotropyMultiple methods are available for measuring thixotropy of cement paste, mortar, or concrete. As discussed previously, concrete experiences both reversible and i
17、rreversible changes in viscosity. Thus, directly measuring the reversible change due to thixotropy is challenging (Ferron et al. 2007). Thixotropy can be characterized based on the degree of material structural build-up or breakdown.The hysteresis loop test and constant shear rate tests require the
18、use of rheometers, which may include concentric cylinder geometry (Billberg 2009), vane geometry (Khayat et al. 2008; Koehler and Fowler 2008), or parallel plate geometry (de Larrard et al. 1997).Hysteresis loop test methodThe hysteresis loop test (Ferron et al. 2007) is conducted by using a rheomet
19、er to apply a range of shear rates to concrete initially at rest. To perform the test, concrete is placed in a rheometer, and left at rest for a defined period of time to allow structural build-up to occur. Rest time, typically on the order of minutes, could change from test to test depending on the
20、 materials, equipment, and application considered. The shear rate in the rheometer is increased from zero to a predetermined maximum level and then decreased back to zero. Typically, the time period over which the shear rate is increased (up-curve) is equal to the time period over which it is decrea
21、sed (down-curve). The response in terms of material shear stress is recorded and plotted against the shear rate, resulting in flow curves as shown in Fig. 2. Any difference between the up- and down-curves is termed hysteresis.When the material is sheared at an increasing rate, the structure of the m
22、aterial gradually becomes more and more broken. When the material is then sheared with a decreasing rate, the gradual building up of the structure begins again; however, the rate of the structural build-up is slower than the rate of the breakdown that has occurred during the up-curve. Thus, a hyster
23、esis loop is formed by the up-curve and the down-curve as shown in Fig. 2.Thixotropy of the material is quantified by the area enclosed between the up- and down-curves of the hysteresis loop, which relates to the energy needed to break down the reversible microstructure of the tested material. Thus,
24、 a highly thixotropic mixture is typically characterized by having a large area within the hysteresis loop. If the up- and down-curve coincide perfectly (that is, the enclosed area equals zero), the material is considered nonthixotropic. The hysteresis loop test is greatly dependent on the testing p
25、rocedures and equipment. The test should merely be used as a preliminary indicator of thixotropic behavior and not for quantitative measurements. This is because the influences of shear rate and time are coupled; that is, the shape of the hysteresis loop can be affected by the rate at which the shea
26、r rate is increased or decreased.Constant shear rate testThe constant shear rate test is conducted by measuring the shear stress of a material initially at rest and then sheared at a constant rate. At the start of the test, for a material that has been left at rest for some time, the flocculated par
27、ticle Fig. 2Typical hysteresis loop for measuring thixotropy.Fig. 3Shear stress decay at constant shear rate.Fig. 4Static yield stress measurements (Billberg 2009).3CONCRETE THIXOTROPY (ACI 238.2T-14)structure is reflected by the high initial shear stress (ti). With time, the structure gradually bre
28、aks down and the shear stress decreases. Eventually, the equilibrium shear stress (teq) is reached. Figure 3 shows the establishment of the equilibrium condition, where the initial shear stress (ti) is the shear stress measured right after the tested material is first sheared, and the equilibrium sh
29、ear stress (teq) is the shear stress obtained when a steady-state (or constant) shear stress is reached. The difference between the initial shear stress (ti) and the equilibrium shear stress (teq) is used as a measure to characterize the energy needed to break down the microstructure of the tested m
30、aterial, which is an indication of thixotropy.Static yield stress testThis method involves shearing a material initially at rest by increasing the shear strain of the sample very slowly and measuring the shear stress to create a shear stress-shear strain relation. The shear stress in the material sl
31、owly increases due to the applied shear strain until the structure breaks. The material structure is characterized by the peak stress, which is the static yield stress t0-static. Figure 4 shows such shear stress-shear strain measurements of self-consolidating concrete (SCC) performed after various t
32、imes at rest (Billberg 2009). The test results can be affected by the rate at which shear strain is applied.Portable vane testThixotropy can also be characterized by measuring the increase of static yield stress at rest without a rheometer (Khayat and Omran 2010, 2011; Billberg and sterberg 2001; Kh
33、ayat et al. 2008; and Roussel and Cussigh 2008). The portable vane (PV) test (Khayat and Omran 2010, 2011), which is shown in Fig. 5, consists of four-bladed vanes measuring 1.47 in. (37.5 mm) in radius and various heights of 9.8, 7.9, 5.9, 3.9 in. (250, 200, 150, and 100 mm). A dial indicator-type
34、torque wrench is attached to the vane. After the given rest time, the torque wrench is slowly turned to measure the peak torque on the vane to break the material structure. The torque is then converted to static yield stress based on the vanes geometry (Khayat and Omran 2010). The different vane siz
35、es allow measurements of concrete after four rest periods (typically 15, 30, 45, and 60 minutes) with the same torque wrench. Thus, the shorter the rest time, the larger the vane used.Inclined plate testThe inclined plate (IP) test involves casting concrete in a cylindrical mold resting on a horizon
36、tal plate of a given roughness, followed by lifting the mold to allow the concrete to spread (Khayat and Omran 2010, 2011). Figure 6 shows this equipment used for a mortar test. After different times at rest, the plate with the spread sample is inclined until flow starts. The critical angle required
37、 for initiating the flow, the sample density, and the average height of the spread are used to determine the static yield stress. Typically, four IP tests are performed after various rest periods to evaluate the rate of increase in static yield stress with time.Effects of concrete materials and temp
38、erature on thixotropyThixotropy is affected by cement type, chemical admixtures, supplementary cementitious materials and fillers, water-to-powder ratio (w/p), total water content, and temperature. Table 1 summarizes the influence of these factors on thixotropy.High-range water-reducing admixtures (
39、HRWRA) and retarders generally lead to a reduction in flocculation, which reduces the amount of thixotropy (Urev et al. 1997). Viscosity-modifying agents (VMA) typically increase thixotropy, depending on the chemistry and mode of action of the VMA. For instance, Phan et al. (2006) revealed that a VM
40、A stabilizes concrete by increasing both the viscosity and thixotropy. Some VMA can actually increase the bridging Fig. 5Portable vane test method: (a) square bucket; and (b) vanes (Khayat and Omran 2010, 2011).Fig. 6Inclined plate test conducted with mortar (Khayat and Omran 2010, 2011).4CONCRETE T
41、HIXOTROPY (ACI 238.2T-14)between particles and positively influence the building up of a structure.The use of an accelerator results in a greater degree of structural build-up and, therefore, higher thixotropy.Slag, silica fume, and metakaolin, when used as a replacement for cement, generally increa
42、se thixotropy. Petkova and Samichkov (2007) suggested that the increase in thixotropy is attributable to the increased specific particle surface in the paste system. Salem (2002) proposed that this effect is due to the pozzolanic reaction as well as the rapid transformation of ettringite to monosufl
43、ate.It has also been shown that lowering the w/p of SCC by adding limestone fillers to cement increases the structural build-up (Billberg 2009).Tregger et al. (2010) used nano-clay at very small dosages to increase the thixotropy of semi-flowable SCC, which is concrete with workability between that
44、of the typical slump range and the typical slump flow range. The shape stability of the semi-flowable SCC was improved significantly with the nano-clay addition after demolding. Recent research also found that addition of clay materials in concrete can increase both the level of yield stress recover
45、ed after shear and the kinetics of the rebuilding of the microstructure (Kaci et al. 2011).Assaad and Khayat (2004) studied the thixotropy of five SCC mixtures with different combinations of cementitious materials. They showed that the concrete exhibited a high thixotropy when mixed with ternary cem
46、ent containing 6 percent silica fume and 22 percent fly ash in comparison with similar concrete made with 4 percent silica fume and no fly ash, with all percentages expressed by mass of cementitious materials.For a given cementitious material content, the structural build-up at rest increases with a
47、n increase in environmental temperature. This is related to the rapid change in cement paste structure due to the acceleration of hydration reactions. If the cement paste is more structured, more time is needed to recover the breakdown in its structure after it has been sheared and, therefore, a lar
48、ger thixotropy is measured (Coussot et al. 2002).Applications of thixotropyThixotropy and structural build-up behavior is important in modern concrete construction. Applications of the thixotropy of concrete can be found in the following:Control of concrete formwork pressureDuring placement, fresh c
49、oncrete, especially SCC, applies a pressure on formwork. If cast slowly enough, the structure of the concrete can build up and attain the ability to withstand some of the load from the concrete cast above. Therefore, concrete that sufficiently builds up the structure at rest due to thixotropy can significantly reduce the formwork pressure (Ovarlez and Roussel 2006; Beitzel 2009; Khayat and Omran 2010, 2011).Multiple-lift castingDuring multiple-lift placing, and to avoid structural defects, consider that a layer of thixotropic concrete has a limited time (depending on the degr
