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Factors influencing rheological properties of fresh cement asphalt emulsion paste
Construction and Building Materials 68 (2014) 611–617
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Factors influencing rheological properties of fresh cement asphalt emulsion paste Yiqiu Tan, Jian Ouyang ⇑, Yunliang Li School of Transportation Science and Engineering, Harbin Institute of Technology, Harbin, China
h i g h l i g h t s The factor of the rheological properties of fresh CA paste was studied. Viscosity modifying agent greatly improves the yield stress of CA paste. Asphalt emulsion stability affects the growth rate of plastic viscosity of CA paste. Superplasticizer reduces the yield stress of CA paste and its growth rate.
a r t i c l e
i n f o
Article history: Received 13 April 2014 Received in revised form 2 July 2014 Accepted 3 July 2014
Keywords: Cement asphalt emulsion paste Rheological properties Yield stress Plastic viscosity
a b s t r a c t As a grouting material, the flow ability and uniformity of fresh cement asphalt emulsion mortar (hereinafter abbreviated as CA mortar) are governed by its rheological properties. The rheological properties of fresh cement asphalt paste were investigated, in which influencing factors, such as type of asphalt emulsion, superplasticizer dosage, and water to cement ratio, were discussed. Results indicate that CA paste with plain asphalt emulsion has too low yield stress to resist segregation, while it has good yield stress when employing asphalt emulsion modified by viscosity modifying agent. Superplasticizer can reduce the initial yield stress and plastic viscosity, however, its water-reducing effect is not obvious for the water-reducing effect of emulsifier in asphalt emulsion. The stability of asphalt emulsion affects the growth rate of plastic viscosity of CA paste, and superplasticizer dosage affects the growth rate of yield stress, which both affects the retention ability of good fluidity. Therefore, a well stable asphalt emulsion modified with viscosity modifying agent, and high superplasticizer dosage (1% recommended in here) are preferred to produce CA mortar. Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
1. Introduction Cement–asphalt emulsion composite materials are advanced materials with the combined merits of cement and asphalt, which have found wide application in the road construction [1–4] and building industry [5,6]. In recent years, with the construction of high-speed railway in the world, there has been an increase in the application of cement–asphalt emulsion mortar (hereinafter abbreviated as CA mortar). CA mortar can not only act as a supporter, but also be used as an elastic layer for track slab, which is a key material influencing the service life of slab track. Its properties are gained more and more concerns [7–9]. In situ construction, CA mortar is grouted into a narrow preadjusted space with the size of 6450 2550 30 mm between the concrete roadbed and precast track slabs in order to ensure ⇑ Corresponding author. E-mail address: [email protected] (J. Ouyang). http://dx.doi.org/10.1016/j.conbuildmat.2014.07.020 0950-0618/Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
the smoothness of the track slabs. Thus, fresh CA mortar must possess adequate flow-ability and flow-ability retention for a certain period to ensure successful construction. Furthermore, a good segregation resistance is required for fresh CA mortar to prevent itself from segregation in and after construction. A good grouting result is the key of the successful application of CA mortar. Therefore, it is of great importance to deeply understand the rheological behavior of CA paste. Fresh CA paste is a kind of suspension in which cement grains and asphalt droplets are dispersed in aqueous phase, which is a little similar to cement paste. It was reported that rheological behavior could not only reveal the inherent properties of suspension system, such as colloidal forces, thixotropy, particle morphology and concentration [10], but was also relevant with flow ability and segregation resistance. Theoretical analysis and numerical simulation showed that the spread diameter of cement paste was a function of the yield stress and material volume, and the flow time of a cone flow test was related with plastic viscosity and yield
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stress [11–14]. Furthermore, non-settling critical diameter for sphere particles was governed by the yield stress and density of the cement paste matrix [15,16]. Therefore, the rheological parameters, yield stress and plastic viscosity, can reflect the rheological behavior of CA paste and determine whether the workability of CA mortar is good or not. Asphalt emulsion is unstable when mixed with cement. Thus, previous studies were more focused on the development of fresh CA paste status and the interaction of cement and asphalt emulsion. The rheological properties of fresh CA paste and its development were investigated [18,19]. The results showed that the growth of rheological properties with shelf time were dependent on the content and type of asphalt emulsion used in the CA paste. CA paste with more asphalt emulsion content and anionic asphalt emulsion had the smaller growth rate of rheological properties. The development of fresh CA paste status was considered as the adsorption behavior of asphalt emulsion on cement grains [18]. And it had some relationship with cement hydration according to a model linking yield stress and relative hydration degree [19]. The interaction of cement and asphalt emulsion can reveal the properties of fresh CA paste to some extent. Asphalt emulsion has retarding effect on cement hydration, and the retarding effect is dependent on the asphalt emulsion to cement ratio, asphalt emulsion types and emulsifier types [19,22,23]. However, the chemical stability of asphalt emulsion was degraded on the effect of Ca2+ concentration, PH change, and water loss induced by cement hydration [17]. Some methods were reported to improve the stability of asphalt emulsion, such as utilizing some non-ionic emulsifier to produce asphalt emulsion [20] or adding F-type superplasticizer to cement–asphalt emulsion mastic [21]. These studies are significantly meaningful to understand the development of fresh CA paste status, however, a systematic study about the rheological of CA mortar was seldom reported. Only a qualitative study about the rheological behavior of CA mortar was reported [24]. Therefore, further quantitative research on the rheological behavior of fresh CA paste and its effect on working ability are necessary. In this study, the rheological parameters, yield stress and plastic viscosity, were employed to study the rheological behavior of CA paste. Three influencing factors, such as asphalt emulsion type, superplasticizer dosage, and water–cement ratio, were used to discuss the rheological behavior of CA paste and demulsification of asphalt emulsion in the tests. At last, some fresh CA mortars were produced to further discuss the effect of the rheological behavior on the working properties of CA mortar. As a whole, this work was intended to reveal the effect of the main component materials on the rheological behavior of CA paste and study how to adjust and control the workability of fresh CA mortar. 2. Experimental program 2.1. Materials and specimens preparation Cement pastes were prepared with an ordinary Portland cement P.O.42.5. Two anionic asphalt emulsions (A1, A2) were employed in the preparation of the cement asphalt emulsion pastes, in which asphalt dosage was about 60%. Anionic asphalt emulsions were tested according to the Chinese standard [34] and their properties were shown in Table 1. The difference between A1 and A2 is that A2 is added with a viscosity modifying agent in its production process, therefore its Engler viscosity is much higher than A1. Besides, the emulsifier dosage is 3% in A1, while it is 4% in A2. Tap water (W), organic silicon defoamer (D), and polycarboxylate superplasticizer (SP) were utilized according to mix proportions in Table 2. In this table, all CA pastes had the same asphalt emulsion–cement mass ratio (A/C) at 0.4 and defoamerasphalt emulsion mass ratio at 0.25% to avoid the effect of air content on rheological properties of CA paste for the addition of asphalt emulsion would increase air content of pastes. CA pastes except samples 9 and 10 had the same W/C at 0.4. The mixed CA mortars were employed to investigate their flow-ability and separation rate, which proportions and results are shown in the following Table 4. In Table 4, all the CA mortars had the same sand–cement mass ratio at 1.4 and asphalt emulsion–cement mass ratio at 0.4. The W/C in all CA mortars was adjusted to meet the flowability requirements of CA mortar construction.
2.2. Test methods 2.2.1. Rheological measurements The rheological behavior of CA pastes were measured immediately using a parallel plate rheometer in controlled shear rate mode at a room temperature about 25 °C. The diameter of the parallel plate employed in the test was 25 mm and its gap away from the lower plate was 1000 lm to minimize sedimentation effects on rheological measurements. For all pastes, as shown in Fig. 1, the rheological test consists of the following steps: (1) pre-shearing with 50 s1 for 60 s after mixing; (2) a linear decreasing shear rate from 50 s1 to 0 s1 for 10 s; (3) the shear rate kept at 0 s1 for 60 s and 300 s to study the rheological behavior of pastes with 1 min and 5 min resting time after mixing, the pre-shearing action was intended to cause structural breakdown of pastes and create uniform conditions before testing; (4) a first hysteresis cycle was drawn 2 min with the maximum shear rate at 50 s1; (5) a second hysteresis cycle was loaded again after the previous one. Data of shear stress and shear rate were recorded once per second. The up curves of the second hysteresis loop were adopted to analyze and discuss the rheological behavior of CA paste. 2.2.2. Flow ability and uniformity test of CA mortar The flow ability and uniformity of CA mortar was evaluated by the flow time, spread diameter, and separation rate, respectively. The specific steps of the three tests are described briefly in the following according to the Chinese Specification [35]. An EN445 cone flow test was performed to gain the flow time of CA mortar, which size was shown in Fig. 1. The funnel was loaded with 1000 ml of fresh CA mortar and the time required to flow out of the nozzle is the flow time of tested CA mortar. The spread test of CA mortar was measured by a steel cylinder which inner diameter is 50 ± 1 mm and height is 190 mm ± 2 mm. The cylinder was put on the center of a horizontal glass pane and filled with fresh CA mortar. The cylinder should be lifted rapidly to about 150 mm high. After the tested CA mortar became static, the diameter of CA mortar in the glass pane was measured to be the spread of the tested CA mortar. The test procedure of separation rate was as follows. Fresh CA mortars were poured into cylindrical moulds with the size of 50 mm inner diameter and 50 mm height. The cylindrical specimens were demoulded after hardening, and then they were cut into halves by handsaw. The density of each half was measured. According to Eq. (1), the separation rate was calculated by the difference in density between top and bottom halves.
Separation rate ¼
q 1 q2 100% 2ðq1 þ q2 Þ
ð1Þ
where q1 is density of bottom half, q2 is density of top half.
3. Results and discussion 3.1. Rheological parameters regression Yield stress and plastic viscosity are two fundamental parameters, which should be fitted to quantitatively study the rheological behavior of CA pastes. A modified Bingham law from [25] given by Eq. (2) can be adapted to acquire yield stress and plastic viscosity.
s ¼ s0 ½1 expð3c_ =c_ crit Þ þ gp c_
ð2Þ
where s is the shear stress (Pa); c_ is the shear strain rate (1/s); s0 is the yield shear stress (Pa) and gp is the plastic viscosity (Pa s). c_ crit is critical strain rate which determines the transition to a constant plastic viscosity. Though this model can give satisfactory regression results of strain rate from 0 to 50 s1, the experimental data of samples with A1 which have small yield stress may be not very fit with the regression results at low strain rate, as shown in Fig. 2a. Thus, to acquire more accurate fitted results, the yield stresses of CA pastes with A1 and cement paste were calculated by fitting the initial portion of the ascending branch (strain rate of 0–14 s1), as shown in Fig. 2b. For the yield stresses of samples with A2 had no differences by these two calculation methods, as shown in Fig. 2c and d, we adopted the first method to acquire yield stress. For all the samples, plastic viscosity was calculated by fitting the whole flow curve of strain rate from 0 to 50 s1. After such consideration, the fitted data are listed in Table 3.
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Y. Tan et al. / Construction and Building Materials 68 (2014) 611–617 Table 1 Properties of anionic asphalt emulsions. Test on emulsion
Value
Engler viscosity (25 °C, Pa s) Mean particle diameter (lm) Sieve test (1.18 mm, %) Storage stability (1d, 25 °C, %) Storage stability (7d, 25 °C, %)
Test on residue from distillation
A1
A2
7.6 3.336 0.01 0.4 1.1
22 2.62 0 0.02 0.6
Solid content (%) Penetration (25 °C, 100 g, 5 s, 0.1 mm) Softening point (R&B, °C) Ductility (25 °C, cm)
Table 2 Mix proportions and asphalt emulsion types of CA pastes. Sample
C
A
W
SP
D
1 2 3 4 5 6 7 8 9 10 11
1 1 1 1 1 1 1 1 1 1 1
0.4(A1) 0.4(A1) 0.4(A1) 0.4(A1) 0.4(A2) 0.4(A2) 0.4(A2) 0.4(A2) 0.4(A2) 0.4(A2) 0
0.24 0.24 0.24 0.24 0.24 0.24 0.24 0.24 0.27 0.29 0.4
0 0.002 0.004 0.01 0 0.002 0.004 0.01 0 0 0
0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0
A1
A2
60 66.3 46.7 >150
60.4 64.5 46.5 127
pure cement paste (sample 11 in Table 3). Segregation has direct relationship with yield stress [15,16], which indicates that CA mortars with plain asphalt emulsion (such as A1) has large probability to sediment. As can be seen from Fig. 3b, the plastic viscosity of CA pastes with A1 at both 1 min and 5 min resting time is much larger than them with A2. Also, the plastic viscosity increments in the first 5 min resting time are larger for CA pastes with A1 than these with A2. No obvious increment of plastic viscosity is observed about pure cement paste in Table 3. In the study of rheological behavior of suspensions, some viscosity models confirm clearly that viscosity is related with the maximum particle packing density and particle volume fraction [26,27], for an example like Eq. (3). Increase in the particle volume fraction and decrease in the maximum particle packing density can increase suspension viscosity. In here, the viscosity of CA pastes only alters with the maximum particle packing density for all CA pastes have the same particle volume fraction. The maximum particle packing density is proved to be related to the particle size distribution [10], and flocculation state for highly-concentrated suspensions [28]. It is some reasonable to assume that plastic viscosity is the viscosity at infinite strain rate which week flocculation can be ignored for plastic viscosity is the viscosity at infinite strain rate in the modified Bingham model. Therefore, plastic viscosity only differs with the particle size distribution in here. As shown in Table 1, the mean size of asphalt emulsion A1 is higher than A2, it is inferred that CA pastes with A1 should have lower viscosity for that increase in particle size can decrease viscosity. However, the actual results have the opposite trend. This is the results of asphalt droplets coalescence and strong flocculation when asphalt emulsion and cement are mixed together, which alter the particle size distribution and reduce free water content of CA paste system. When the stability of asphalt emulsion is good, the rate of asphalt droplets coalescence would be low, therefore CA pastes would have a low plastic viscosity and a low growth rate. For the emulsifier dosage is 3% in A1 and 4% in A2, it is definitely that the stability of A2 is better than A1, therefore, the rheological behavior of CA pastes with A2 is better.
gr ¼ 1 þ Fig. 1. EN445 flow cone size of flow time test (mm).
Value
0:75/=/m 1 /=/m
2 ð3Þ
where gr is the relative viscosity defined as a ratio of suspension viscosity to the viscosity of suspending medium, / is particle volume fraction, and /m is the maximum particle packing density.
3.2. Effect of asphalt emulsion type 3.3. Effect of superplasticizer Based on the fitted data in Table 3, the effect of different asphalt emulsion on the rheological behavior of CA paste is illustrated in Fig. 3. As shown in Fig. 3a, CA pastes with A1 have very low yield stress, while the CA pastes with A2 have extreme high yield stress. This difference is mainly caused by a special thickening agent in asphalt emulsion A2, which makes the gel structure of CA paste more strong at low shear state. It should be noted that for free emulsifier and superplasticizer can prevent particles flocculation [19], the yield stress of CA pastes with A1 are also much lower than
The effect of superplasticizer dosage on the rheological behavior of CA paste is shown in Fig. 4. It can be seen from Fig. 4a that yield stress of CA pastes at 1 min resting time decreases slightly with the increase of superplasticizer no matter which asphalt emulsion is employed, however, a sharp drop is observed when CA pastes rest 5 min. This seems to be due to the flocculation of CA paste at rest for the flocculation state of suspension increases with the resting time [29]. When CA pastes are just stirred at a high shear rate,
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Shear stress (Pa)
Shear stress (Pa)
6
y=0.0803(1-exp(-3x/0.0021))+0.4925*x 2 R =0.9897
20 15 10 5 0
Data Fit 0
(b)
7
(a)
25
10
20
30
40
y=0.6673[1-exp(-3x/1.3368)]+0.4294x 2 R =0.9984
5 4 3
Yield stress 2
Critical strain rate
1 0
50
0
2
4
6
Data Fit 8
10
12
14
Strain rate (1/s)
Strain rate (1/s) 16
(c)
25
Yield stress
Shear stress (Pa)
Shear stress (Pa)
Yield stress
12
20 15
y=11.1148[1-exp(-3x/2.8741)]+0.2785x 2 R =0.9992
10
Critical strain rate 5 0
(d)
14
Data Fit 0
10
20
30
40
10
y=10.928[1-exp(-3x/2.8154)]+0.3257x 2 R =0.9988
8 6 4
Critical strain rate Data Fit
2 0
50
0
2
4
Strain rate (1/s)
6
8
10
12
Strain rate (1/s)
Fig. 2. Identifying yield stress and plastic viscosity by modified Bingham law. (a) and (b) CA pastes with asphalt emulsion A1, (c) and (d) CA pastes with asphalt emulsion A2.
Table 3 Regressive rheological parameters of CA pastes. Sample
1 2 3 4 5 6 7 8 9 10 11
Resting time = 1 min
and obtain a low flocculation strength of CA paste. Therefore, the yield stress increments of CA pastes can be reduced by increasing superplasticizer dosage. Especially no obvious change in yield stress is observed when adding 1% of superplasticizer. The effect of superplasticizer dosage on plastic viscosity of CA paste is shown in Fig. 4b. The plastic viscosity of CA pastes decrease with the increase of superplasticizer, and the slope of CA paste with A1 is larger than that with A2. As discussed in the Section 3.2, the plastic viscosity is related with the coalescence and strong flocculation state of asphalt droplets and cement grains when asphalt emulsion and cement are mixed together. The addition of superplasticizer can make the dispersed effect of CA particles better and prevent asphalt droplets coalescence. Therefore, a decline in plastic viscosity is acquired when adding superplasticizer, especially when superplasticizer dosage is lower than 0.4%. It should be noted that emulsifier can also act as surfactant to avoid flocculation and coalescence [19]. The emulsifier dosages of A2 is larger than A1, therefore the water-reducing effect of superplasticizer on CA paste with A1 is more obvious. From the results of yield stress and plastic viscosity, more superplasticizer (at least 0.4%) is recommended when producing CA mortar.
Resting time = 5 min
s0 (Pa)
c_ crit (s1)
gp (Pa s)
s0 (Pa)
c_ crit (s1)
gp (Pa s)
1.0861 0.7848 0.6673 0.4288 12.1919 11.1786 10.7737 11.1148 9.5352 7.9218 2.6383
6.3216 1.5161 1.3368 0.351 4.4813 3.3547 2.9853 2.8741 3.8922 3.429 3.0601
0.9918 0.5946 0.4925 0.4762 0.4734 0.3844 0.3502 0.2785 0.2989 0.1916 0.5518
14.932 4.6824 0.6677 0.674 22.9354 14.4565 12.9554 11.327 12.1919 10.3218 4.8868
8.435 3.5161 1.395 1.0567 16.8193 6.3547 2.9067 2.3342 5.4813 3.7714 3.285
1.132 0.9489 0.8213 0.7184 0.5318 0.4626 0.4069 0.3929 0.4734 0.2694 0.5919
the flocculation state is very low so that yield stress is very low. After a few resting minutes, the flocculation structure builds up to acquire a high yield stress. The function of superplasticizer is to prevent cement grains and asphalt droplets from flocculation
Table 4 Workability results of CA mortars. Sample
1 2 3 4 5 6 7 8
Emulsion
A1 A1 A1 A1 A2 A2 A2 A2
W/C
0.41 0.40 0.39 0.38 0.42 0.42 0.42 0.41
SP
0.01 0.01 0.01 0.01 0 0.005 0.01 0.01
Initial state
30 min rest
Separation rate (%)
Flow time (s)
Spread (mm)
Flow time (s)
Spread (mm)
81 96 107 119 117 104 97 106
360 345 335 320 280 285 290 285
103 115 127 167 – 138 112 118
360 340 335 320 265 280 290 285
6.3 5.6 4.1 1.8 0.4 0.5 0.4 0.3
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(a) 12.1919
1.2
A1 A2 10.7737
11.1786
Yield stress (Pa)
A1 at 1 min resting time A1 at 5 min resting time A2 at 1 min resting time A2 at 5 min resting time
1.0
10 8 6 4 2
(b)
11.1148
Plastic viscosity (Pa.s)
12
0.8 0.6 0.4 0.2
1.0861
0
0.7848
0.6673
0.2
0
0.4288
0.4
0.0
1.0
0
1.0
0.4
0.2
m(SP)/m(C) (%)
m(SP)/m(C)(%)
Fig. 3. Effect of different asphalt emulsion on (a) yield stress and (b) plastic viscosity of CA paste.
24 1.2
(a)
22 A1 at 1 min resting time A1 at 5 min resting time A2 at 1 min resting time A2 at 5 min resting time
12 10 8
20 18 16
6
14
4
12
2
10
0
0.0
0.2
0.4
0.6
0.8
1.0
(b)
1.1
Yield stress of A2 (Pa)
Yield stress of A1 (Pa)
14
Plastic viscosity (Pa.s)
16
A1 at 1 min resting time A1 at 5 min resting time A2 at 1 min resting time A2 at 5 min resting time
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3
8
0.0
0.2
m(SP)/m(C) (%)
0.4
0.6
0.8
1.0
m(SP)/m(C) (%)
Fig. 4. Effect of superplasticizer dosage on (a) yield stress and (b) plastic viscosity of CA paste.
(a)
0.5 0.4734
Yield stress (Pa)
20 16 12
12.1919
8
A2 at 1 min resting time A2 at 5 min resting time 12.4923 10.3218 9.5352 7.9218
A2 at 1 min resting time A2 at 5 min resting time
0.4
0.3734 0.2989
0.3
0.2694 0.1916
0.2 0.1
4 0
0.5318
(b)
22.9354
Plastic viscosity (Pa.s)
24
0.4
0.43
0.45
W/C
0.0
0.43
0.4
0.45
W/C
Fig. 5. Effect of water–cement ratio on (a) yield stress and (b) plastic viscosity of CA paste.
3.4. Effect of water–cement ratio The effect of water–cement ratio on the rheological behavior of CA paste is shown in Fig. 5. The yield stress and plastic viscosity of CA pastes decrease sharply by increasing just small water–cement ratio compared to the effect of superplasticizer dosage. It seems to be due to an increase in free water content and a decrease in the particle volume fraction of CA paste when increasing water. To the contrast, the water-reducing function of asphalt emulsion leads to no obvious water-reducing effect of superplasticizer, as shown in Fig. 4. However, it does not mean water–cement ratio is the best way to adjust the flow properties of CA mortar. Apart from durability degradation with high W/C, the yield stress of CA paste with high W/C but no superplasticizer increases a lot after 5 min resting time. The workability time of CA paste may be not long if no
superplasticizer to prevent particles from flocculation. Therefore, the right approach is to determine a high superplasticizer dosage (for example 1%) firstly, and then adjust W/C to get a proper workability of CA mortar. 3.5. Flow ability and uniformity of CA mortar CA mortars which were mixed with the proportions of Table 4 were employed to investigate the flow ability and uniformity of CA mortar. The results are shown in Table 4. It can be seen that the flow-ability of CA mortar with A1 decreases with the declining W/C, which can be well coherent with the above analysis of the effect of water–cement ratio on rheological properties of CA pastes as mentioned above. Meanwhile, CA mortars with A1 have better initial fluidity than those with A2 even though lower W/C is
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employed, and their spreading diameters are larger. This seems to be due to low yield stress and high plastic viscosity of CA paste with A1. Previous studies pointed out that flow time was determined by yield stress and plastic viscosity [13,14], to be more accurate, by apparent viscosity at actual operation shear-rate, which is between 1 s1 and 100 s1 for cement materials’ pumping [14,30]. In this shear-rate internal, CA pastes with A1 seem to have a lower apparent viscosity for their low yield stress, therefore, the CA mortars with A2 should have higher W/C to meet the flow-ability requirements in construction. The spreading diameter has a direct relationship with yield stress [11,12], but no relationship with plastic viscosity [31–33]. Therefore, the spread of CA mortars with A1 are larger than those with A2. Furthermore, the initial spread of CA mortars with A2 differs little with superplasticizer dosage, which is due to no obvious difference of initial yield stress as observed previously in rheology test. It is very conspicuous in Table 4 that the separation rate of CA mortar with A1 is much higher than that with A2, which indicates that obvious segregation has already happened in CA mortar with A1. The non-settling critical diameter for sphere particles is governed by the yield stress and density of the cement paste matrix [15,16], which relationship is expressed as Eq. (4). In CA mortar system, the maximum fine sand diameter is 1.18 mm. The authors supposed the density of sand and CA paste was 2.6 g/cm3 and 1.7 g/ cm3, respectively. Therefore, a minimum yield stress of CA paste to resist segregation is 0.59 Pa based on Eq. (4). It can be seen from Table 3 that the yield stress of CA paste with A1 at 1.0% superplasticizer is less than this value, and yield stress in other pastes only exceeds a little. It should be noted that the yield stress is measured at 1 min resting time, not just after stirring. In that condition, the yield stress may be lower and cannot be enough to resist segregation. Therefore, asphalt emulsion with no viscosity modifying agent (such as A1) is not suitable to producing CA mortar. If we have to adopt this asphalt emulsion, some additives for improving yield stress should be required.
dc ¼
18s0 jqs qf jg
ð4Þ
where dc is the non-settling critical diameter, qs is the density of sand particles, qf is the density of paste. It can be seen that the fluidity of CA mortar with A1 degrades more quickly than CA mortar with A2 when compared sample 2 with sample 7 and compared sample 3 with sample 8 in Table 4, which have almost the same initial flow time. From the flow time loss of samples 5, 6, and 7, it can be also seen that the fluidity of CA mortar with more superplasticizer degrades less. It is confirmed again that the rheological properties of CA paste lose more slowly when A2 and more superplasticizer are employed in CA mortar. It is very interesting that the spread of CA mortar with 1% superplasticizer changes little in 30 min rest, but flow time increases a lot. It can be due to no obvious change of yield stress but a continually increasing plastic viscosity with time as shown in previous rheology test. 4. Conclusions In this paper, a modified Bingham model was employed to study the rheological properties of fresh cement asphalt paste. The influencing factors, such as asphalt emulsion type, superplasticizer dosage and water cement ratio, were discussed. The following conclusions can be drawn from the presented results: (1) Yield stress and plastic viscosity of CA paste are affected with asphalt emulsion type. CA paste with plain asphalt emulsion has too low yield stress to resist segregation, while
it has tremendous yield stress when employing asphalt emulsion modified by viscosity modifying agent. The plastic viscosity and its growth rate are relevant with the stability of asphalt emulsion. With asphalt emulsion of good stability, the plastic viscosity of CA paste and its growth rate is low. (2) Superplasticizer can improve the rheological properties of CA mortar, which reduces the initial yield stress and plastic viscosity. Furthermore, the main function of superplasticizer can prevent CA paste from flocculation in order to reducing the yield stress growth rate. With addition of more superplasticizer, the yield stress of CA paste increases less with time. Especially no obvious change in yield stress is observed with time when adding 1% of superplasticizer. However, superplasticizer is not related with the growth rate of plastic viscosity. (3) Water–cement ratio can remarkably reduce the yield stress and plastic viscosity of CA paste. However, the low yield stress cannot keep stable when no superplasticizer is added. (4) Combined with workability tested results of CA mortar and rheology test results of CA paste, it can be inferred that the stability of asphalt emulsion affects the growth rate of plastic viscosity of CA paste, and superplasticizer dosage affects the growth rate of yield stress, which both affect the retention ability of good fluidity. Asphalt emulsion modified with viscosity modifying agent improve the uniformity of CA mortar. Therefore, a well stable asphalt emulsion modified with viscosity modifying agent, and high superplasticizer dosage (1% recommended in here) are preferred to produce CA mortar.
Acknowledgements The authors would like to thank research support by the National Natural Science Foundation (No. 51078114) of China. The corresponding author acknowledges the support of China Scholarship Council and the suggestion of Prof. Surendra P. Shah in Northwestern University. References [1] Schmidt RJ, Santucci LE, Coyne LD. Performance characteristics of cement modified asphalt emulsion mixes. Proc AAPT 1973;42:300–19. [2] Brown SF, Needham D. A study of cement modified bitumen emulsion mixtures. Proc AAPT 2000:69. [3] Bocci M, Grilli A, Cardone F, et al. A study on the mechanical behaviour of cement–bitumen treated materials. Constr Build Mater 2011;25(2):773–8. [4] Oruc Seref, Celik Fazil, Vefa Akpinar M. Effect of cement on emulsified asphalt mixtures. J Mater Eng Perform 2006;16(5):578–83. [5] Li G, Zhao Y, Pang S, et al. Experimental study of cement-asphalt emulsion composite. Cem Concr Res 1998;28(5):635–41. [6] Song Hun, Do Jeongyun, Soh Yangseob. Feasibility study of asphalt-modified mortars using asphalt emulsion. Constr Build Mater 2006;20(5):332–7. [7] Shuguang Hu, Yunhua Zhang, Fazhou Wang. Effect of temperature and pressure on the degradation of cement asphalt mortar exposed to water. Constr Build Mater 2012;34:570–4. [8] Qiang W, Peiyu Y, Ruhan A, et al. Strength mechanism of cement-asphalt mortar. J Mater Civ Eng 2011;23(9):1353–9. [9] Wang FZ, Liu ZC, Wang T, Hu SG. Temperature stability of compressive strength of cement asphalt mortar. ACI Mater J 2010;107(1):27–30. [10] Barnes HA, Hutton JF, Walters K. An introduction to rheology. Amsterdam: Elsevier; 1989. [11] Roussel N, Coussot P. Fifty cent rheometer for yield stress measurements: from slump to spreading flow test. J. Rheolo. 2005;49:705–18. [12] Bouveta Adrien, Ghorbela Elhem, Bennacer Rachid. The mini-conical slump flow test: analysis and numerical study. Cem Concr Res 2010;40(10):1517–23. [13] Cremonesi M, Ferrara L, Frangi A, et al. Simulation of the flow of fresh cement suspensions by a Lagrangian finite element approach. J Non-Newton Fluid 2010;165(23–24):1555–63. [14] Ferrara L, Cremonesi M, Tregger N. On the identification of rheological properties of cement suspensions: rheometry, computational fluid dynamics modeling and field test measurements. Cem Concr Res 2012;42(8):1134–46. [15] Saak Aaron W, Jennings Hamlin M, Shah Surendra P. New methodology for designing self-compacting concrete. ACI Mater J 2001;98(6):429–39.
Y. Tan et al. / Construction and Building Materials 68 (2014) 611–617 [16] Roussel N. A theoretical frame to study stability of fresh concrete. RILEM Mater Struct 2006;39(1):75–83. [17] Wang Fazhou, Liu Yunpeng, Shuguang Hu. Effect of early cement hydration on the chemical stability of asphalt emulsion. Constr Build Mater 2013;42:146–51. [18] Hu S-G, Wang T, Wang F-Z, et al. Adsorption behaviour between cement and asphalt emulsion in CA mortar. Adv Cem Res 2009;21:11–4. [19] Zhang Y, Kong X, Hou S. Study on the rheological properties of fresh cement asphalt paste. Constr Build Mater 2012;27(1):534–44. [20] Wang Fazhou, Liu Yunpeng, Zhang Yunhua. Experimental study on the stability of asphalt emulsion for CA mortar by laser diffraction technique. Constr Build Mater 2012;28(1):117–21. [21] Lua Cheng-Tsung, Kuob Ming-Feng, Shen Der-Hsien. Composition and reaction mechanism of cement–asphalt mastic. Constr Build Mater 2009;23(7): 2580–5. [22] Yang JB, Yan PY, Kong XM, et al. Study on the hardening mechanism of cement asphalt binder. Science China 2010;53(5):1406–12. [23] Tan Y, Ouyang J, Lv J, et al. Effect of emulsifier on cement hydration in cement asphalt mortar. Constr Build Mater 2013;47:159–64. [24] Liu Z, Hong J, Liu J. Rheological behavior of fresh cement asphalt mortar. ASCE Sustain Construct Mater 2012:54–63. [25] Tregger N, Ferrara L, Shah SP. Identifying cement rheological properties from the mini-slump flow test. ACI Mater J 2008;105(6):556–8.
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[26] Chong JS, Christiansen EB, Baer AD. Rheology of concentrated suspensions. J Appl Polym Sci 1971;15(8):2007–21. [27] Liu Dean-Mo. Particle packing and rheological property of highly-concentrated ceramic suspensions: U m determination and viscosity prediction. J Mater Sci 2000;35:5503–7. [28] Liu Dean-Mo. Theoretical determination of floc size in highly-concentrated zirconia-wax suspensions. Acta Mater 2002;50:1927–35. [29] Roussel Nicolas. A thixotropy model for fresh fluid concretes: theory, validation and applications. Cem Concr Res 2006;36(10):1797–806. [30] Reed JS. Principles of ceramic processing. 2nd ed. NY: J. Wiley & Sons; 1995. [31] Beaupré D, Mindess S. Rheology of fresh concrete: principles, measurement, and applications. In: Skalny J, Mindess S, editors. Materials science of concrete V. Acers; 1998. p. 149–190. [32] Tattersall G, Banfill P. The rheology of fresh concrete. London: Pitman Advanced Pub. Program; 1983. [33] Wallevik JE. Relationship between the Bingham parameters and slump. Cem Concr Res 2006;36(7):1214–21. [34] Ministry of Communications of PR China. JTG E20–2011: standard test methods of bitumen and bituminous mixtures for highway engineering. China: China Communications Press; 2011 [in Chinese]. [35] Chinese Railway Specification. Technical specification of CA mortar for CRTS non-ballast slab track. China: China Railway Publishing House; 2008 [in Chinese].
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Factors influencing rheological properties of fresh cement asphalt emulsion paste Yiqiu Tan, Jian Ouyang ⇑, Yunliang Li School of Transportation Science and Engineering, Harbin Institute of Technology, Harbin, China
h i g h l i g h t s The factor of the rheological properties of fresh CA paste was studied. Viscosity modifying agent greatly improves the yield stress of CA paste. Asphalt emulsion stability affects the growth rate of plastic viscosity of CA paste. Superplasticizer reduces the yield stress of CA paste and its growth rate.
a r t i c l e
i n f o
Article history: Received 13 April 2014 Received in revised form 2 July 2014 Accepted 3 July 2014
Keywords: Cement asphalt emulsion paste Rheological properties Yield stress Plastic viscosity
a b s t r a c t As a grouting material, the flow ability and uniformity of fresh cement asphalt emulsion mortar (hereinafter abbreviated as CA mortar) are governed by its rheological properties. The rheological properties of fresh cement asphalt paste were investigated, in which influencing factors, such as type of asphalt emulsion, superplasticizer dosage, and water to cement ratio, were discussed. Results indicate that CA paste with plain asphalt emulsion has too low yield stress to resist segregation, while it has good yield stress when employing asphalt emulsion modified by viscosity modifying agent. Superplasticizer can reduce the initial yield stress and plastic viscosity, however, its water-reducing effect is not obvious for the water-reducing effect of emulsifier in asphalt emulsion. The stability of asphalt emulsion affects the growth rate of plastic viscosity of CA paste, and superplasticizer dosage affects the growth rate of yield stress, which both affects the retention ability of good fluidity. Therefore, a well stable asphalt emulsion modified with viscosity modifying agent, and high superplasticizer dosage (1% recommended in here) are preferred to produce CA mortar. Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
1. Introduction Cement–asphalt emulsion composite materials are advanced materials with the combined merits of cement and asphalt, which have found wide application in the road construction [1–4] and building industry [5,6]. In recent years, with the construction of high-speed railway in the world, there has been an increase in the application of cement–asphalt emulsion mortar (hereinafter abbreviated as CA mortar). CA mortar can not only act as a supporter, but also be used as an elastic layer for track slab, which is a key material influencing the service life of slab track. Its properties are gained more and more concerns [7–9]. In situ construction, CA mortar is grouted into a narrow preadjusted space with the size of 6450 2550 30 mm between the concrete roadbed and precast track slabs in order to ensure ⇑ Corresponding author. E-mail address: [email protected] (J. Ouyang). http://dx.doi.org/10.1016/j.conbuildmat.2014.07.020 0950-0618/Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
the smoothness of the track slabs. Thus, fresh CA mortar must possess adequate flow-ability and flow-ability retention for a certain period to ensure successful construction. Furthermore, a good segregation resistance is required for fresh CA mortar to prevent itself from segregation in and after construction. A good grouting result is the key of the successful application of CA mortar. Therefore, it is of great importance to deeply understand the rheological behavior of CA paste. Fresh CA paste is a kind of suspension in which cement grains and asphalt droplets are dispersed in aqueous phase, which is a little similar to cement paste. It was reported that rheological behavior could not only reveal the inherent properties of suspension system, such as colloidal forces, thixotropy, particle morphology and concentration [10], but was also relevant with flow ability and segregation resistance. Theoretical analysis and numerical simulation showed that the spread diameter of cement paste was a function of the yield stress and material volume, and the flow time of a cone flow test was related with plastic viscosity and yield
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stress [11–14]. Furthermore, non-settling critical diameter for sphere particles was governed by the yield stress and density of the cement paste matrix [15,16]. Therefore, the rheological parameters, yield stress and plastic viscosity, can reflect the rheological behavior of CA paste and determine whether the workability of CA mortar is good or not. Asphalt emulsion is unstable when mixed with cement. Thus, previous studies were more focused on the development of fresh CA paste status and the interaction of cement and asphalt emulsion. The rheological properties of fresh CA paste and its development were investigated [18,19]. The results showed that the growth of rheological properties with shelf time were dependent on the content and type of asphalt emulsion used in the CA paste. CA paste with more asphalt emulsion content and anionic asphalt emulsion had the smaller growth rate of rheological properties. The development of fresh CA paste status was considered as the adsorption behavior of asphalt emulsion on cement grains [18]. And it had some relationship with cement hydration according to a model linking yield stress and relative hydration degree [19]. The interaction of cement and asphalt emulsion can reveal the properties of fresh CA paste to some extent. Asphalt emulsion has retarding effect on cement hydration, and the retarding effect is dependent on the asphalt emulsion to cement ratio, asphalt emulsion types and emulsifier types [19,22,23]. However, the chemical stability of asphalt emulsion was degraded on the effect of Ca2+ concentration, PH change, and water loss induced by cement hydration [17]. Some methods were reported to improve the stability of asphalt emulsion, such as utilizing some non-ionic emulsifier to produce asphalt emulsion [20] or adding F-type superplasticizer to cement–asphalt emulsion mastic [21]. These studies are significantly meaningful to understand the development of fresh CA paste status, however, a systematic study about the rheological of CA mortar was seldom reported. Only a qualitative study about the rheological behavior of CA mortar was reported [24]. Therefore, further quantitative research on the rheological behavior of fresh CA paste and its effect on working ability are necessary. In this study, the rheological parameters, yield stress and plastic viscosity, were employed to study the rheological behavior of CA paste. Three influencing factors, such as asphalt emulsion type, superplasticizer dosage, and water–cement ratio, were used to discuss the rheological behavior of CA paste and demulsification of asphalt emulsion in the tests. At last, some fresh CA mortars were produced to further discuss the effect of the rheological behavior on the working properties of CA mortar. As a whole, this work was intended to reveal the effect of the main component materials on the rheological behavior of CA paste and study how to adjust and control the workability of fresh CA mortar. 2. Experimental program 2.1. Materials and specimens preparation Cement pastes were prepared with an ordinary Portland cement P.O.42.5. Two anionic asphalt emulsions (A1, A2) were employed in the preparation of the cement asphalt emulsion pastes, in which asphalt dosage was about 60%. Anionic asphalt emulsions were tested according to the Chinese standard [34] and their properties were shown in Table 1. The difference between A1 and A2 is that A2 is added with a viscosity modifying agent in its production process, therefore its Engler viscosity is much higher than A1. Besides, the emulsifier dosage is 3% in A1, while it is 4% in A2. Tap water (W), organic silicon defoamer (D), and polycarboxylate superplasticizer (SP) were utilized according to mix proportions in Table 2. In this table, all CA pastes had the same asphalt emulsion–cement mass ratio (A/C) at 0.4 and defoamerasphalt emulsion mass ratio at 0.25% to avoid the effect of air content on rheological properties of CA paste for the addition of asphalt emulsion would increase air content of pastes. CA pastes except samples 9 and 10 had the same W/C at 0.4. The mixed CA mortars were employed to investigate their flow-ability and separation rate, which proportions and results are shown in the following Table 4. In Table 4, all the CA mortars had the same sand–cement mass ratio at 1.4 and asphalt emulsion–cement mass ratio at 0.4. The W/C in all CA mortars was adjusted to meet the flowability requirements of CA mortar construction.
2.2. Test methods 2.2.1. Rheological measurements The rheological behavior of CA pastes were measured immediately using a parallel plate rheometer in controlled shear rate mode at a room temperature about 25 °C. The diameter of the parallel plate employed in the test was 25 mm and its gap away from the lower plate was 1000 lm to minimize sedimentation effects on rheological measurements. For all pastes, as shown in Fig. 1, the rheological test consists of the following steps: (1) pre-shearing with 50 s1 for 60 s after mixing; (2) a linear decreasing shear rate from 50 s1 to 0 s1 for 10 s; (3) the shear rate kept at 0 s1 for 60 s and 300 s to study the rheological behavior of pastes with 1 min and 5 min resting time after mixing, the pre-shearing action was intended to cause structural breakdown of pastes and create uniform conditions before testing; (4) a first hysteresis cycle was drawn 2 min with the maximum shear rate at 50 s1; (5) a second hysteresis cycle was loaded again after the previous one. Data of shear stress and shear rate were recorded once per second. The up curves of the second hysteresis loop were adopted to analyze and discuss the rheological behavior of CA paste. 2.2.2. Flow ability and uniformity test of CA mortar The flow ability and uniformity of CA mortar was evaluated by the flow time, spread diameter, and separation rate, respectively. The specific steps of the three tests are described briefly in the following according to the Chinese Specification [35]. An EN445 cone flow test was performed to gain the flow time of CA mortar, which size was shown in Fig. 1. The funnel was loaded with 1000 ml of fresh CA mortar and the time required to flow out of the nozzle is the flow time of tested CA mortar. The spread test of CA mortar was measured by a steel cylinder which inner diameter is 50 ± 1 mm and height is 190 mm ± 2 mm. The cylinder was put on the center of a horizontal glass pane and filled with fresh CA mortar. The cylinder should be lifted rapidly to about 150 mm high. After the tested CA mortar became static, the diameter of CA mortar in the glass pane was measured to be the spread of the tested CA mortar. The test procedure of separation rate was as follows. Fresh CA mortars were poured into cylindrical moulds with the size of 50 mm inner diameter and 50 mm height. The cylindrical specimens were demoulded after hardening, and then they were cut into halves by handsaw. The density of each half was measured. According to Eq. (1), the separation rate was calculated by the difference in density between top and bottom halves.
Separation rate ¼
q 1 q2 100% 2ðq1 þ q2 Þ
ð1Þ
where q1 is density of bottom half, q2 is density of top half.
3. Results and discussion 3.1. Rheological parameters regression Yield stress and plastic viscosity are two fundamental parameters, which should be fitted to quantitatively study the rheological behavior of CA pastes. A modified Bingham law from [25] given by Eq. (2) can be adapted to acquire yield stress and plastic viscosity.
s ¼ s0 ½1 expð3c_ =c_ crit Þ þ gp c_
ð2Þ
where s is the shear stress (Pa); c_ is the shear strain rate (1/s); s0 is the yield shear stress (Pa) and gp is the plastic viscosity (Pa s). c_ crit is critical strain rate which determines the transition to a constant plastic viscosity. Though this model can give satisfactory regression results of strain rate from 0 to 50 s1, the experimental data of samples with A1 which have small yield stress may be not very fit with the regression results at low strain rate, as shown in Fig. 2a. Thus, to acquire more accurate fitted results, the yield stresses of CA pastes with A1 and cement paste were calculated by fitting the initial portion of the ascending branch (strain rate of 0–14 s1), as shown in Fig. 2b. For the yield stresses of samples with A2 had no differences by these two calculation methods, as shown in Fig. 2c and d, we adopted the first method to acquire yield stress. For all the samples, plastic viscosity was calculated by fitting the whole flow curve of strain rate from 0 to 50 s1. After such consideration, the fitted data are listed in Table 3.
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Y. Tan et al. / Construction and Building Materials 68 (2014) 611–617 Table 1 Properties of anionic asphalt emulsions. Test on emulsion
Value
Engler viscosity (25 °C, Pa s) Mean particle diameter (lm) Sieve test (1.18 mm, %) Storage stability (1d, 25 °C, %) Storage stability (7d, 25 °C, %)
Test on residue from distillation
A1
A2
7.6 3.336 0.01 0.4 1.1
22 2.62 0 0.02 0.6
Solid content (%) Penetration (25 °C, 100 g, 5 s, 0.1 mm) Softening point (R&B, °C) Ductility (25 °C, cm)
Table 2 Mix proportions and asphalt emulsion types of CA pastes. Sample
C
A
W
SP
D
1 2 3 4 5 6 7 8 9 10 11
1 1 1 1 1 1 1 1 1 1 1
0.4(A1) 0.4(A1) 0.4(A1) 0.4(A1) 0.4(A2) 0.4(A2) 0.4(A2) 0.4(A2) 0.4(A2) 0.4(A2) 0
0.24 0.24 0.24 0.24 0.24 0.24 0.24 0.24 0.27 0.29 0.4
0 0.002 0.004 0.01 0 0.002 0.004 0.01 0 0 0
0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0
A1
A2
60 66.3 46.7 >150
60.4 64.5 46.5 127
pure cement paste (sample 11 in Table 3). Segregation has direct relationship with yield stress [15,16], which indicates that CA mortars with plain asphalt emulsion (such as A1) has large probability to sediment. As can be seen from Fig. 3b, the plastic viscosity of CA pastes with A1 at both 1 min and 5 min resting time is much larger than them with A2. Also, the plastic viscosity increments in the first 5 min resting time are larger for CA pastes with A1 than these with A2. No obvious increment of plastic viscosity is observed about pure cement paste in Table 3. In the study of rheological behavior of suspensions, some viscosity models confirm clearly that viscosity is related with the maximum particle packing density and particle volume fraction [26,27], for an example like Eq. (3). Increase in the particle volume fraction and decrease in the maximum particle packing density can increase suspension viscosity. In here, the viscosity of CA pastes only alters with the maximum particle packing density for all CA pastes have the same particle volume fraction. The maximum particle packing density is proved to be related to the particle size distribution [10], and flocculation state for highly-concentrated suspensions [28]. It is some reasonable to assume that plastic viscosity is the viscosity at infinite strain rate which week flocculation can be ignored for plastic viscosity is the viscosity at infinite strain rate in the modified Bingham model. Therefore, plastic viscosity only differs with the particle size distribution in here. As shown in Table 1, the mean size of asphalt emulsion A1 is higher than A2, it is inferred that CA pastes with A1 should have lower viscosity for that increase in particle size can decrease viscosity. However, the actual results have the opposite trend. This is the results of asphalt droplets coalescence and strong flocculation when asphalt emulsion and cement are mixed together, which alter the particle size distribution and reduce free water content of CA paste system. When the stability of asphalt emulsion is good, the rate of asphalt droplets coalescence would be low, therefore CA pastes would have a low plastic viscosity and a low growth rate. For the emulsifier dosage is 3% in A1 and 4% in A2, it is definitely that the stability of A2 is better than A1, therefore, the rheological behavior of CA pastes with A2 is better.
gr ¼ 1 þ Fig. 1. EN445 flow cone size of flow time test (mm).
Value
0:75/=/m 1 /=/m
2 ð3Þ
where gr is the relative viscosity defined as a ratio of suspension viscosity to the viscosity of suspending medium, / is particle volume fraction, and /m is the maximum particle packing density.
3.2. Effect of asphalt emulsion type 3.3. Effect of superplasticizer Based on the fitted data in Table 3, the effect of different asphalt emulsion on the rheological behavior of CA paste is illustrated in Fig. 3. As shown in Fig. 3a, CA pastes with A1 have very low yield stress, while the CA pastes with A2 have extreme high yield stress. This difference is mainly caused by a special thickening agent in asphalt emulsion A2, which makes the gel structure of CA paste more strong at low shear state. It should be noted that for free emulsifier and superplasticizer can prevent particles flocculation [19], the yield stress of CA pastes with A1 are also much lower than
The effect of superplasticizer dosage on the rheological behavior of CA paste is shown in Fig. 4. It can be seen from Fig. 4a that yield stress of CA pastes at 1 min resting time decreases slightly with the increase of superplasticizer no matter which asphalt emulsion is employed, however, a sharp drop is observed when CA pastes rest 5 min. This seems to be due to the flocculation of CA paste at rest for the flocculation state of suspension increases with the resting time [29]. When CA pastes are just stirred at a high shear rate,
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Shear stress (Pa)
Shear stress (Pa)
6
y=0.0803(1-exp(-3x/0.0021))+0.4925*x 2 R =0.9897
20 15 10 5 0
Data Fit 0
(b)
7
(a)
25
10
20
30
40
y=0.6673[1-exp(-3x/1.3368)]+0.4294x 2 R =0.9984
5 4 3
Yield stress 2
Critical strain rate
1 0
50
0
2
4
6
Data Fit 8
10
12
14
Strain rate (1/s)
Strain rate (1/s) 16
(c)
25
Yield stress
Shear stress (Pa)
Shear stress (Pa)
Yield stress
12
20 15
y=11.1148[1-exp(-3x/2.8741)]+0.2785x 2 R =0.9992
10
Critical strain rate 5 0
(d)
14
Data Fit 0
10
20
30
40
10
y=10.928[1-exp(-3x/2.8154)]+0.3257x 2 R =0.9988
8 6 4
Critical strain rate Data Fit
2 0
50
0
2
4
Strain rate (1/s)
6
8
10
12
Strain rate (1/s)
Fig. 2. Identifying yield stress and plastic viscosity by modified Bingham law. (a) and (b) CA pastes with asphalt emulsion A1, (c) and (d) CA pastes with asphalt emulsion A2.
Table 3 Regressive rheological parameters of CA pastes. Sample
1 2 3 4 5 6 7 8 9 10 11
Resting time = 1 min
and obtain a low flocculation strength of CA paste. Therefore, the yield stress increments of CA pastes can be reduced by increasing superplasticizer dosage. Especially no obvious change in yield stress is observed when adding 1% of superplasticizer. The effect of superplasticizer dosage on plastic viscosity of CA paste is shown in Fig. 4b. The plastic viscosity of CA pastes decrease with the increase of superplasticizer, and the slope of CA paste with A1 is larger than that with A2. As discussed in the Section 3.2, the plastic viscosity is related with the coalescence and strong flocculation state of asphalt droplets and cement grains when asphalt emulsion and cement are mixed together. The addition of superplasticizer can make the dispersed effect of CA particles better and prevent asphalt droplets coalescence. Therefore, a decline in plastic viscosity is acquired when adding superplasticizer, especially when superplasticizer dosage is lower than 0.4%. It should be noted that emulsifier can also act as surfactant to avoid flocculation and coalescence [19]. The emulsifier dosages of A2 is larger than A1, therefore the water-reducing effect of superplasticizer on CA paste with A1 is more obvious. From the results of yield stress and plastic viscosity, more superplasticizer (at least 0.4%) is recommended when producing CA mortar.
Resting time = 5 min
s0 (Pa)
c_ crit (s1)
gp (Pa s)
s0 (Pa)
c_ crit (s1)
gp (Pa s)
1.0861 0.7848 0.6673 0.4288 12.1919 11.1786 10.7737 11.1148 9.5352 7.9218 2.6383
6.3216 1.5161 1.3368 0.351 4.4813 3.3547 2.9853 2.8741 3.8922 3.429 3.0601
0.9918 0.5946 0.4925 0.4762 0.4734 0.3844 0.3502 0.2785 0.2989 0.1916 0.5518
14.932 4.6824 0.6677 0.674 22.9354 14.4565 12.9554 11.327 12.1919 10.3218 4.8868
8.435 3.5161 1.395 1.0567 16.8193 6.3547 2.9067 2.3342 5.4813 3.7714 3.285
1.132 0.9489 0.8213 0.7184 0.5318 0.4626 0.4069 0.3929 0.4734 0.2694 0.5919
the flocculation state is very low so that yield stress is very low. After a few resting minutes, the flocculation structure builds up to acquire a high yield stress. The function of superplasticizer is to prevent cement grains and asphalt droplets from flocculation
Table 4 Workability results of CA mortars. Sample
1 2 3 4 5 6 7 8
Emulsion
A1 A1 A1 A1 A2 A2 A2 A2
W/C
0.41 0.40 0.39 0.38 0.42 0.42 0.42 0.41
SP
0.01 0.01 0.01 0.01 0 0.005 0.01 0.01
Initial state
30 min rest
Separation rate (%)
Flow time (s)
Spread (mm)
Flow time (s)
Spread (mm)
81 96 107 119 117 104 97 106
360 345 335 320 280 285 290 285
103 115 127 167 – 138 112 118
360 340 335 320 265 280 290 285
6.3 5.6 4.1 1.8 0.4 0.5 0.4 0.3
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(a) 12.1919
1.2
A1 A2 10.7737
11.1786
Yield stress (Pa)
A1 at 1 min resting time A1 at 5 min resting time A2 at 1 min resting time A2 at 5 min resting time
1.0
10 8 6 4 2
(b)
11.1148
Plastic viscosity (Pa.s)
12
0.8 0.6 0.4 0.2
1.0861
0
0.7848
0.6673
0.2
0
0.4288
0.4
0.0
1.0
0
1.0
0.4
0.2
m(SP)/m(C) (%)
m(SP)/m(C)(%)
Fig. 3. Effect of different asphalt emulsion on (a) yield stress and (b) plastic viscosity of CA paste.
24 1.2
(a)
22 A1 at 1 min resting time A1 at 5 min resting time A2 at 1 min resting time A2 at 5 min resting time
12 10 8
20 18 16
6
14
4
12
2
10
0
0.0
0.2
0.4
0.6
0.8
1.0
(b)
1.1
Yield stress of A2 (Pa)
Yield stress of A1 (Pa)
14
Plastic viscosity (Pa.s)
16
A1 at 1 min resting time A1 at 5 min resting time A2 at 1 min resting time A2 at 5 min resting time
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3
8
0.0
0.2
m(SP)/m(C) (%)
0.4
0.6
0.8
1.0
m(SP)/m(C) (%)
Fig. 4. Effect of superplasticizer dosage on (a) yield stress and (b) plastic viscosity of CA paste.
(a)
0.5 0.4734
Yield stress (Pa)
20 16 12
12.1919
8
A2 at 1 min resting time A2 at 5 min resting time 12.4923 10.3218 9.5352 7.9218
A2 at 1 min resting time A2 at 5 min resting time
0.4
0.3734 0.2989
0.3
0.2694 0.1916
0.2 0.1
4 0
0.5318
(b)
22.9354
Plastic viscosity (Pa.s)
24
0.4
0.43
0.45
W/C
0.0
0.43
0.4
0.45
W/C
Fig. 5. Effect of water–cement ratio on (a) yield stress and (b) plastic viscosity of CA paste.
3.4. Effect of water–cement ratio The effect of water–cement ratio on the rheological behavior of CA paste is shown in Fig. 5. The yield stress and plastic viscosity of CA pastes decrease sharply by increasing just small water–cement ratio compared to the effect of superplasticizer dosage. It seems to be due to an increase in free water content and a decrease in the particle volume fraction of CA paste when increasing water. To the contrast, the water-reducing function of asphalt emulsion leads to no obvious water-reducing effect of superplasticizer, as shown in Fig. 4. However, it does not mean water–cement ratio is the best way to adjust the flow properties of CA mortar. Apart from durability degradation with high W/C, the yield stress of CA paste with high W/C but no superplasticizer increases a lot after 5 min resting time. The workability time of CA paste may be not long if no
superplasticizer to prevent particles from flocculation. Therefore, the right approach is to determine a high superplasticizer dosage (for example 1%) firstly, and then adjust W/C to get a proper workability of CA mortar. 3.5. Flow ability and uniformity of CA mortar CA mortars which were mixed with the proportions of Table 4 were employed to investigate the flow ability and uniformity of CA mortar. The results are shown in Table 4. It can be seen that the flow-ability of CA mortar with A1 decreases with the declining W/C, which can be well coherent with the above analysis of the effect of water–cement ratio on rheological properties of CA pastes as mentioned above. Meanwhile, CA mortars with A1 have better initial fluidity than those with A2 even though lower W/C is
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employed, and their spreading diameters are larger. This seems to be due to low yield stress and high plastic viscosity of CA paste with A1. Previous studies pointed out that flow time was determined by yield stress and plastic viscosity [13,14], to be more accurate, by apparent viscosity at actual operation shear-rate, which is between 1 s1 and 100 s1 for cement materials’ pumping [14,30]. In this shear-rate internal, CA pastes with A1 seem to have a lower apparent viscosity for their low yield stress, therefore, the CA mortars with A2 should have higher W/C to meet the flow-ability requirements in construction. The spreading diameter has a direct relationship with yield stress [11,12], but no relationship with plastic viscosity [31–33]. Therefore, the spread of CA mortars with A1 are larger than those with A2. Furthermore, the initial spread of CA mortars with A2 differs little with superplasticizer dosage, which is due to no obvious difference of initial yield stress as observed previously in rheology test. It is very conspicuous in Table 4 that the separation rate of CA mortar with A1 is much higher than that with A2, which indicates that obvious segregation has already happened in CA mortar with A1. The non-settling critical diameter for sphere particles is governed by the yield stress and density of the cement paste matrix [15,16], which relationship is expressed as Eq. (4). In CA mortar system, the maximum fine sand diameter is 1.18 mm. The authors supposed the density of sand and CA paste was 2.6 g/cm3 and 1.7 g/ cm3, respectively. Therefore, a minimum yield stress of CA paste to resist segregation is 0.59 Pa based on Eq. (4). It can be seen from Table 3 that the yield stress of CA paste with A1 at 1.0% superplasticizer is less than this value, and yield stress in other pastes only exceeds a little. It should be noted that the yield stress is measured at 1 min resting time, not just after stirring. In that condition, the yield stress may be lower and cannot be enough to resist segregation. Therefore, asphalt emulsion with no viscosity modifying agent (such as A1) is not suitable to producing CA mortar. If we have to adopt this asphalt emulsion, some additives for improving yield stress should be required.
dc ¼
18s0 jqs qf jg
ð4Þ
where dc is the non-settling critical diameter, qs is the density of sand particles, qf is the density of paste. It can be seen that the fluidity of CA mortar with A1 degrades more quickly than CA mortar with A2 when compared sample 2 with sample 7 and compared sample 3 with sample 8 in Table 4, which have almost the same initial flow time. From the flow time loss of samples 5, 6, and 7, it can be also seen that the fluidity of CA mortar with more superplasticizer degrades less. It is confirmed again that the rheological properties of CA paste lose more slowly when A2 and more superplasticizer are employed in CA mortar. It is very interesting that the spread of CA mortar with 1% superplasticizer changes little in 30 min rest, but flow time increases a lot. It can be due to no obvious change of yield stress but a continually increasing plastic viscosity with time as shown in previous rheology test. 4. Conclusions In this paper, a modified Bingham model was employed to study the rheological properties of fresh cement asphalt paste. The influencing factors, such as asphalt emulsion type, superplasticizer dosage and water cement ratio, were discussed. The following conclusions can be drawn from the presented results: (1) Yield stress and plastic viscosity of CA paste are affected with asphalt emulsion type. CA paste with plain asphalt emulsion has too low yield stress to resist segregation, while
it has tremendous yield stress when employing asphalt emulsion modified by viscosity modifying agent. The plastic viscosity and its growth rate are relevant with the stability of asphalt emulsion. With asphalt emulsion of good stability, the plastic viscosity of CA paste and its growth rate is low. (2) Superplasticizer can improve the rheological properties of CA mortar, which reduces the initial yield stress and plastic viscosity. Furthermore, the main function of superplasticizer can prevent CA paste from flocculation in order to reducing the yield stress growth rate. With addition of more superplasticizer, the yield stress of CA paste increases less with time. Especially no obvious change in yield stress is observed with time when adding 1% of superplasticizer. However, superplasticizer is not related with the growth rate of plastic viscosity. (3) Water–cement ratio can remarkably reduce the yield stress and plastic viscosity of CA paste. However, the low yield stress cannot keep stable when no superplasticizer is added. (4) Combined with workability tested results of CA mortar and rheology test results of CA paste, it can be inferred that the stability of asphalt emulsion affects the growth rate of plastic viscosity of CA paste, and superplasticizer dosage affects the growth rate of yield stress, which both affect the retention ability of good fluidity. Asphalt emulsion modified with viscosity modifying agent improve the uniformity of CA mortar. Therefore, a well stable asphalt emulsion modified with viscosity modifying agent, and high superplasticizer dosage (1% recommended in here) are preferred to produce CA mortar.
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