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A new test method to characterize the pressure-dependent shear behavior of fresh concrete
Construction and Building Materials 233 (2020) 117255
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
A new test method to characterize the pressure-dependent shear behavior of fresh concrete Tilo Proske, Moien Rezvani ⇑, Carl-Alexander Graubner Institute of Concrete and Masonry Structures, Technische Universität Darmstadt, Franziska-Braun-Str. 3, 64287 Darmstadt, Germany
h i g h l i g h t s The shear resistance of concrete is significantly influenced by the actual pressure level. The influence of pressure on the yield stress increases as the flowability of concrete decreases. Significant influence of pressure level on yield stress was observed at early state. The volume fraction of aggregate is the main governing parameter on the pressure-dependent shear behavior.
a r t i c l e
i n f o
Article history: Received 29 April 2019 Received in revised form 4 October 2019 Accepted 12 October 2019
Keywords: Yield stress Plastic viscosity Shear behavior Variable pressure Rheometer
a b s t r a c t The development of formwork pressure is essentially affected by the rheological behavior of fresh concrete, while the shear behavior itself might be influenced by the magnitude of normal pressure. A test set-up was developed in which the fresh mortar and concrete can be sheared in a pressure cell by a vane as a part of a concrete rheometer (ICAR). The concrete is directly loaded in the pressure cell by a hydraulic jack. Preliminary shear tests were conducted on concretes with moderate and high flowability at constant water/binder-ratio of 0.4 with the focus on the static yield stress. The additional pressure was applied up to 240 kPa. The consistency class of the concrete F4, F5 and F6 was varied by choosing the paste volume to 300, 315 and 330 l/m3, respectively. The results show the significant influence of applied pressure on the measured yield torque or yield stress in early concrete age especially for concretes with moderate flowability (F4) or with relatively high volume fraction of aggregates respectively. However, a significant increase of yield resistance by applied pressure was identified for all concretes after 90 min concrete age. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The rheology as one of the fundamental characteristics of the fresh concrete can influence the production, processing, formwork pressure, surface quality as well as the mechanical and durability properties of concrete significantly [1–6]. A detailed knowledge of the rheological behavior of fresh concrete at variable pressure levels is essential for an accurate calculation and prediction of formwork pressure and can help to improve current practice in formwork design [7]. Further on, the high-pressure processing (e.g. pumping, extrusion and printing) of fresh concrete can be improved by a better understanding of the pressure-dependent rheological behavior, in addition to other effects as shown e.g. in Refs. [2,8]. Fig. 1(c and d) illustrates the development of yield stress s0 of fresh concrete and concrete pressure rh depending on the ⇑ Corresponding author. E-mail address: [email protected] (M. Rezvani). https://doi.org/10.1016/j.conbuildmat.2019.117255 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
casting time t and vertical pressure rv(h) (due to concrete weight). The surface friction sw is neglected in the shown example. As plotted, an exponential evolution of yield stress versus time can be assumed for fresh concrete [9,10]. It is shown in Fig. 1(c) that the yield stress under vertical pressure (s0(rv), red line) can be higher than the yield stress under atmospheric pressure (s0(ratm), blue line). In this case an increased yield stress leads to a smaller but more realistic formwork pressure (Fig. 1(d) red line, rh(rv) < rh(ratm)). Several efforts were carried out in the literature to predict the lateral pressure of concrete on the formwork by considering the time-depending rheology, especially based on the yield stress of concrete [4,9,11–19]. Among them, several models [3,15,16,19] are based on the pressure-dependent rheology of concrete. They address that the increased pressure due to the concrete weight can influence the shear resistance of the fresh concrete. Extensive investigations have already been conducted on the rheological characteristics of concrete under atmospheric pressure
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Placing hose Yield stress
Concrete level
Pressure on formwork 0
Fresh concrete
Surface friction w= 0
Without surface friction
Example: casting rate x setting time formwork height
0
b
Aged concrete (setting)
0
(
atm)
c
h( atm)
h(t)
d
=
hydro
Influence of pressure
h(t)
a
v(h)
( v)
h( v)
Fig. 1. Influence of the pressure level on the yield stress of fresh concrete s0 and the effects on formwork pressure rh in tall formwork.
by many researchers [1,20–26]. In some cases the pumpability of concrete was tested in special pipe rheometers under slightly increased pressure [8,27]. Especially in tall formwork or in concrete pipes during pumping the impact of the pressure level on the rheology can be significant. Astonishingly, systematic investigations to analyze the influence of the pressure on the concrete rheology are missing so far. Therefore, the existing model approaches to describe the flow behavior of fresh concrete are primarily applicable for atmospheric state [25,28,29], but not necessarily for higher pressure. It is essential that further investigations on the influence of the pressure level on concrete rheology should focus on the early as well as the aging state of concrete up to the setting. Aim of the present study was to develop a new test set-up in which the concrete could be sheared directly by means of a vane rheometer under pressurized condition. In first measurements, the influence of variable pressure levels on the shear behavior of fresh concrete as a function of cement paste volume and consistency class was analysed. Knowledge from future direct measurement on concrete conducted by the developed test device can provide a more precise prediction of the rheological behavior and formwork pressure.
2. Pressure dependency of concrete yield stress 2.1. Mechanical background and assumptions The shear resistance of a material at a very small shear rate under atmospheric pressure is called cohesion, which is a result of inter-particle interactions, namely friction and adhesion. The shear resistance can increase under an additional normal pressure. Such growth in the yield stress can be attributed to the increase of friction between solid particles s due to higher effective stress r0n arising from the applied normal stress rn (see Fig. 2). The intensity of the pressure-dependent friction is often connected to the dilatation of the particles system during shearing. Recent studies on fresh mortar and concrete show that the dilatancy of solid particles can result in an increase of the yield stress as the concrete suspension has a quasi-granular behavior [30]. It was observed, that two phenomena control the pressure-dependency during shearing; 1) the dilatancy of the interconnected particle network which will increase the shear resistance and 2) the collapse of cement colloidal network which decreases the friction [30]. However, for high flowable concretes like SCC, the dilatancy will increase the internal friction especially in aging state due to the
hydration and formation of the particle network. In this case they will behave as ‘‘hard” suspensions as defined in [31]. The stress-dependent yield stress of the fresh concrete s0(rn) can be characterized by specific material models which are applied in the soil mechanics. One option could be the Mohr-Coulomb failure criterion which is already used by several researchers (see e.g. [3,32–37]) to describe the pressure-dependent shear behavior of fresh concrete, with the material parameters cohesion c (stressindependent term) and friction angle u (stress-dependent term). The yield condition is represented by the Mohr’s failure envelope. Hereby the shear stress s0 is a function of cohesion c, normal stress rn and friction angle u:
s0 ¼ c þ rn tanu
ð1Þ
It must be kept in mind that the yield stress s0 can only be considered for the design of formwork if the concrete is deformed, at least with a shear angle at which the maximum shear stress is reached. Otherwise the formwork pressure would be underestimated. However, in the design of conventional formwork, a certain formwork deformation is allowed for the limit state (for both serviceability and ultimate limit state). In this cases, the concrete should be sheared sufficiently. It is further to notice that the internal friction (adherence) represents the maximum achievable value of the surface friction sw between formwork surface and fresh concrete. 2.2. Measurements regarding the pressure-dependency of concrete shear resistance The friction angle of vibrated concrete was determined experimentally in several studies [33,38]. Ritchie [33] conducted triaxial tests on the fresh concrete, where aggregate and cement content, w/c-ratio as well as slump value of fresh concrete (according to DIN EN 12350-2) were varied. l’Hermite&Tournon [38] investigated the influence of vibration on the friction angle and cohesion. They concluded a significant reduction of shear stress during vibration especially at low normal pressure levels. Only a few studies are conducted concerning the influence of pressure on the flow behavior of highly flowable cement paste and concrete (SCC) in fresh state. The yield stress of fresh concretes was investigated by Li et al. [37,39]. The results showed that yield stress of all used materials increases linearly by growth of normal stress. However, the influence of normal stress on concrete was more significant compared to mortar and cement paste mixtures due to a larger friction angle of concrete with u 3:0 . Fig. 3 (left) shows the principle effect of the normal stress rn on the shear
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Cement particles
n
=
Aggregates
n'
+u ( n) Transfer of normal load by granular skeleton (effective stresses n') and shear component (
Water and Air
Residual load transfer by the liquid phase (pore water pressure u) More solid-solid contact by: - Hindered expansions (dilatation) of the particle system - Compression of the liquid/gaseous phase - Loss of water (e. g. hydration)
( n
n)
=
n'
Increased friction between solid particles (e.g. aggregates and hydration products)
+u
Fig. 2. Contribution of normal pressure on the shear resistance of fresh concrete.
t = constant
0
n2
0,atm
n1
n2 n1
0
0
n2
n1 0
Shear rate [1/s]
c
c F1 cF 6 cSCC 0
n
tan n
tan
F1
Friction angle F6
Consistency class according to EN 206-1 and DIN EN 12350-5
0
SCC Normal pressure
n
Fig. 3. Influence of normal pressure rn and shear rate c_ on shear resistance of SCC [38] (left); influence of normal pressure rn on yield stress s0 and internal friction angle u respective of concretes with different consistency class [3] (right).
resistance s of SCC in a shear test. It indicates that the shear stress increases as the normal stress grows. Yim et al. [40] investigated the effects of different supplementary cementitious materials (SCM) and admixtures on the rheological properties of different cement pastes at two pressure levels. It was concluded that the influence of hydrostatic pressure on plastic viscosity of cement paste is negligible while the yield stress may be affected by the amount and type of used SCM. The results of Kim et al. [41] have shown only limited influence of the pressure on the rheology of cement paste. Triaxial compression tests on mortar mixtures with different binder types and water-cementitious material ratios (w/cm) were presented by Khayat et al. [32]. The results show friction angles between 25 and 31° for low flowable mortars with flow diameter of approx. 190 mm following 25 drops according to ASTM C 1437. The reason to use such mortars with relatively low workability compared to self-compacting properties was to ensure sufficient cohesion to prevent the specimens collapse during its mounting in a vertical position in the flexible membrane of the triaxial cell. Therefore, the application of the triaxial test is not suitable to investigate the pressure-dependent rheology of highly-flowable concrete like SCC. Studies on the influence of pressure on the flow behavior of SCC in aging state are very limited in the literature. Mettler et al. [42] investigated the strength and failure development of SCC and calculated the cohesion and friction angle based on compressive and
tensile strength (e.g. for the Mohr-Coulomb and Drucker-Prager model). It is shown that the friction angle rises from 0 to 48° during the transition of concrete from a non-Newtonian fluid to a cohesive material. Indirect measurements on pressure-dependent shear behavior of concrete were also conducted by the current authors using a self-developed test apparatus described in Refs. [3,43–45]. The concretes were placed in a hard-plastic cubic cell (25 25 25 cm3) in which the concrete sample was loaded directly in vertical direction. During the application of vertically pressure the resulting horizontal stresses was measured. These tests were conducted on highly workable concretes (SCC, F6, F5) as well as on low and moderate workable concretes (consistency class F1 to F4), just after mixing up to the final setting time of the concrete (maximum 10 h), early to aging state, respectively. The shear behavior of the fresh concrete was evaluated indirectly using appropriate mechanic models (e.g. Mohr’s model). The material parameters friction angle u and cohesion c (or yield stress s0) were calculated based on the ratio of horizontal pressure rh to vertical pressure rv (lateral pressure factor k) for an active soil state. The experiments were performed with both rigid and deformable formwork. The results showed that the friction angle u of SCC is about zero at early concrete age but increases significantly at aging state [3,44]. Fig. 3 (right) reveals further that the consistency class of concrete might be an indicator for the pressure-dependent yield stress. The friction angle u increased
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with the decrease of workability even at early state. The effect of time (aging state) on the pressure-dependent shear stress s and the friction angle u was also remarkable for all consistencies. 2.3. Conclusion based on the existing measurements For fresh cement pastes at early state which are watersaturated, the friction angle can be assumed with u = 0 [20]. On concrete level, it is assumed that in the early state, the friction angle of a very flowable concrete (e.g. SCC) tends to about zero as all solid particles are highly dispersed by water and superplasticizers [3]. This behavior should be independent of the volume of the suspended solids if the paste volume is high enough to prevent a dilatation effect of the coarser particles. However, in the aging state the solid particles will be increasingly connected by agglomeration and loss of water due to hydration. This will lead to an increase of the friction angle. Stiffer concrete (e.g. consistency classes F1–F4) exhibits larger friction angles even in early state, due to a higher content of solid particles and/or lower dispersion. In aging state the increase of yield stress may be more significant compared to SCC because of the higher volume fraction of coarse aggregates (see also Ref. [46]). 3. Preliminary observations of aggregate displacement during shearing To understand the aggregate displacement during shearing which may control the pressure dependent shear behavior of concrete (see Fig. 4), exemplary analysis were conducted by the authors on model concrete samples with X-ray computed tomography technique. For this purpose, a versatile high-resolution 3D computed tomography General Electric v|tome|x s 240 d (micro CT and nano CT) and 3D metrology was used. To figure out the relative particle displacement Dla and rotation of the particle da in the system by a shear process, the scans were conducted on samples before and after applying a shear displacement. In this observation, the maximum shear angle c was about 30° and applied in a special mold with acrylic glass walls. The two smaller opposite walls were rotatable, whereas the upper and bottom surfaces were profiled. To achieve a sufficient resolution and minimize the wall-effect, the dimension of concrete sample and the maximum aggregate size
Before shearing
a
Δla
After shearing
Fig. 4. Computer tomography images of a concrete sample with w/c = 0.40 at early age before and after shearing.
were selected 40 40 80 mm3 and 8 mm, respectively. The model concrete had w/c = 0.40 with a paste volume of 330 l/m3 (consistency class of F4). Test was conducted at quasi-early state (scan after approx. 15 min) as well as at aging state. Images captured from concrete sample before and after shearing is shown in Fig. 4. As Fig. 4 reveals, a significant relative particle displacement induced by shearing, including a rotation of several aggregates. Applying a shear to the specimen results in a rearrangement of the solid skeleton. In regions where solid-solid contacts exist, it is expected that the presence of a normal pressure will increase the required shear stress for the given deformation of c. It is to keep in mind that a direct contact of the aggregates is not necessary in all cases as the other finer solid particles are also included in the transfer of stresses in the overall skeleton. These mechanisms will be discussed specifically in Section 5.2. Due to the small particle size of the sand, these particles are not well to distinguish. However, it should be mentioned that the target of this analysis was to verify the displacement and rotation of the coarse aggregates during shear.
4. Development of a special testing device for rheological measurements on freh concrete under variable pressure A special test set-up was developed to enable a direct measurement of mortar and concrete yield stress under variable pressure level (see Fig. 5). The test apparatus is based on an existing test machine which is described in [3,43,44]. The fresh concrete is placed in a hard-plastic cubic cell (25 25 25 cm3). To prevent significant errors in the experiments, e. g. due to the ‘‘wall effect”, the maximum aggregate diameter of the concrete should not exceed 8 mm. The cell is confined in the horizontal direction by a rigid steel frame. The fresh concrete sample can be loaded vertically by a hydraulic jack. Two pressure sensors in the vertical walls measure the resulting horizontal pressure. The 4-blade vane of a rheometer (ICAR) is positioned in the testing cell. The ICARRheometer is fixed on the bottom of the steel frame. A sealed ball bearing at the entering point to the cell ensures a low friction during rotation. The drainage of paste and water is hindered additionally by special sealing grease with sufficient viscosity. The complete-sealing and suitability of the developed test set-up were verified by using wallpaper adhesive and fine mortar, respectively. Compared to test set-ups in which the pressure is applied with air or liquid (for example, [40,41,47]), the developed method may have the advantage to transfer the load to concrete in a more realistic way, especially if the processing of concrete in tall formwork systems is simulated by application of external pressure. In the developed test set-up, both the liquid phase and the granular phase are loaded directly. This probably leads to higher effective stresses in the granular system and therefore a higher shear resistance compared to a loading with air or liquid. Furthermore, during shearing the granular particles have no room for a dilatation at the surface as it might happen in set-ups where the pressure is applied with air or liquid. Compared to triaxial tests, the developed test set-up has the advantage of a directly and precise measurements especially for concretes with low friction angles. A further advantage is a fast preparation and execution of tests. Furthermore, the triaxial testing is not suitable to investigate the shear resistance of highlyflowable concrete because of the impossible installation of the workable concrete in the cell [30,32]. Based on the obtained maximum torque, the corresponding yield stress can be calculated by a suitable approach [48,49]. Assuming a uniform distribution of the shear stress in the shearing zone, calculation of yield stress using Eq. (2) is possible based on the measured maximum torque Tmax, the vane height H and the
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Side view
Section A-A
Vertical pressure Rigid frame
4 60
127
60
Pressure sensor
60
250
A
60
250
Vane
250
250
20
A
127
Fresh concrete
Ball bearing with seal Rigid subframe
ICAR
Dimensions in mm
ICARRheometer
Fig. 5. Developed test set-up to measure the pressure-dependent shear behavior of fresh concrete.
vane diameter D. The used vane has H = 127 mm and D = 127 mm. Therefore, Eq. (2) can be simplified as Eq. (3).
s0 ¼ s0 ¼
2 T max
D p 3
H D
ð2Þ
þ 13
2 T max 0:1273 p 1 þ 13
¼ 233 T max
ð3Þ
Tmax in Nm s0 in Pa After the maximum yield torque Tmax is reached during a test on concrete and mortar, a plug zone may be formed directly at the surrounding (cylindrical shape) of the vane. Outside this cylinder the concrete will be quasi at rest. Therefore, the cubic container geometry is suitable for the measurement of yield stress but less for the measurement of plastic viscosity.
5. First studies on static yield stress of fresh concrete under pressure 5.1. Materials and methods The tested concretes had a w/b-ratio of 0.4 and different consistency classes (F4, F5 and F6 according to EN 206-1 and DIN EN 12350-5). The binder consists of 70% Portland cement CEM I 52,5 R and 30% limestone powder (LS). The paste volume was approx. 300 l/m3 for concrete with consistency class F4, 310 l/m3 for class F5 and 330 l/m3 for class F6. The water to binder ratio (CEM I + LS) was 0.40. Aggregates with a maximum diameter of 8 mm were used. To achieve the aimed flow value according to DIN EN 12350–5 a superplasticizer (PCE) was applied. The concrete temperature was approx. 22 °C. Table 1 shows the mix proportion and the target workability. A concrete batch of 35 l for each test was mixed in a single shaft mixer. Firstly, the air dried aggregates were mixed together with
Table 1 Mix proportion and the consistency of the tested concretes. Concrete characteristics
Concrete F4
Concrete F5
Concrete F6
284 130 159 894 894 2.5 30.5 2 67.5 81.4 F4 70 30 0.4 51 43 45 38
294 132 164 883 883 2.1 31.3 2 66.6 80.9 F5 70 30
314 135 176 859 859 1.4 33.1 2 64.9 79.9 F6 70 30
58 44 42 39
69 55 46 44
Mix design CEM I 52,5 R Limestone powder Water (excluding sorption water by aggregates) Aggregates 0–2 mm, naturally rounded Aggregates 2–8 mm, naturally rounded Superplasticizer Paste volume (binder, water and SP) Air content Volume of aggregates Volume of solids Consistency class (DIN EN 206-1) Binder CEM I 52,5 R Limestone powder w/b-ratio Consistency f (t = 0 min) f (t = 30 min) f (t = 60 min) f (t = 90 min)
kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 wt% of CEM I Vol% Vol% Vol% Vol% – wt% wt% – cm
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cement and limestone powder for 1 min. Afterwards water and superplasticizer were added and mixed for 5 min with 30 rpm. The flow values measured with the flow table test according to DIN EN 12350-5 are presented in Table 1. The flow value f was measured for all concretes at different times after mixing. Fresh concrete which was at rest was filled in the cone and compacted slightly with a rod for each table test. The cone was lifted and the concrete was dropped 15 times according to the standard test method. In this studies, the concrete samples were tested at applied pressure levels of 0 kPa (only concretes self-weight), 24, 80 and 240 kPa. The vertical pressure was raised stepwise from 0 to 240 kPa followed by unloading to 0 kPa. The concrete samples were sheared at each target pressure level under a slow rotational speed of 0.001 rps for approx. 30 s. The applied shear profile is plotted in Fig. 6. For each measurement the torque evolution was recorded and the maximum torque Tmax (static yield value s0) was determined. The load cycle and measurement procedure was conducted at 10 min after concrete placement and repeated at 30, 60 and 90 min. At each concrete age, all pressure levels were applied within a very short time (approx. 5 min in total). This short time ensures that the consistency of concrete does not change significantly. 5.2. Experimental results The results of the maximum yield torque and yield stress calculated with Eq. (3) respectively are shown exemplarily for concrete F4 in Fig. 7. Evident is a significant influence of the
applied pressure on the measured yield stress. For an applied pressure of approx. 30 kPa the yield increased up to about 100%. However, the increase is lower at higher pressure levels but still significant. Results for the concrete F5 are plotted in Fig. 8. As it reveals, three effects can be detected to influence the yield stress under variable pressure state: 1) the pressure effect, 2) the effect of pre-shearing and 3) the aging effect. The influence of the applied pressure can be identified as the difference between the measurements at 240 kPa and unloaded state at one cycle. In this case the influence of pre-sharing and aging is almost negligible (pure pressure effect). The hysteresis within the shearing in one cycle shows the pre-shearing effect. On the other hand, the difference between cycles at different concrete ages shows the aging effect. Both effects are influenced by thixotropy and structural build-up of the concrete which depend on the degree of disturbance during the test. Probably a certain plug zone is formed at the surrounding of the vane, but limited by the very small applied shear angle. The increase of shear resistance by the growing pressure level is clearly visible in Fig. 8. However, this effect is not very significant compared to concrete F4. The difference between the measured yield stresses at unloaded samples represents the impact of preshearing. It is visible that the pre-shearing history affects the magnitude of the yield stress notably. Meanwhile, it can be seen that a rise in the yield values is influenced by the aging effect. It should be mentioned that the magnitude of these effects depends on the mix proportion of concrete. Fig. 9 illustrates results for concrete F6. A relatively low influence of pressure on the yield stress at the early age of 30 min is visible. A high impact of applied pressure on the yield stress was observed at the age of 90 min. The increase of static yield stress at atmospheric pressure s0,atm for the concrete F6 – also expressed as thixotropy Athix – shows a value of approx. Athix = 0.3 Pa/s which is in the range of conventional SCC. The influence of the consistency class on the pressure dependent yield stress is shown in Fig. 10. Interestingly, the concretes F6 and F5 show no significant increase of yield stress with increasing pressure in early age. However, a high impact of pressure on the yield stress was measured during the test with concrete F4. Due to the comparable low internal friction angles and the high stiffness of the container, the horizontal pressure was measured in the same magnitude as the vertical pressure at early stages for all concrete (hydrostatic pressure plus surcharge load).
Fig. 6. Shear profile applied for the measurements.
Fig. 7. Measured yield torque and stress at different times and pressure levels for consistency class F4.
Fig. 8. Measured yield torque and stress at different times and pressure levels for consistency class F5.
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5.3. Discussion
Fig. 9. Measured yield torque and stress at different times and pressure levels for consistency class F6.
It can be figured out that at early state the influence of pressure on yield stress is only considerable for concrete with low or moderate workability. In this case, the cement particles are not well dispersed by superplasticizers (see [32]) or the paste volume is so low that the solid particles stay in contact (Fig. 11(a)) and are able to transfer effective stresses in the solid system (Fig. 11 left). An increase of paste volumes in concretes with higher workability (F5 and F6) reduces the pressure-dependency at early age significantly. The distance between the aggregates is increasing and the direct contact and therefore the friction resistance is very limited (Fig. 11(c)). Also the better dispersion of the smaller particles reduces the contacts between the solid particles. This reduces the load transfer and friction resistance of the solids accordingly. The internal friction angle u of the concrete was calculated based on Eqs. (1) and (4). The cohesion c is assumed equal to the yield stress at atmospheric pressure s0;atm . This gives:
u ¼ tan1
Fig. 10. Yield torque for consistency class F4, F5 and F6 at concrete age of 10 min versus applied pressure.
Aging state t1 < tset
Unhydrated cement particle dispersed by Fine aggregates superplasticizers Water
(a)
s0 c s0 s0;atm ¼ tan1 Dr n Dr n
Cement agglomeration
Partially hydrated cement
(b)
Low/moderate workable concrete
>> 0
>0 (c)
(d)
Highly workable concrete (SCC)
0
ð4Þ
At early age, the concrete F4 shows already a considerable friction angle of approx. u10 min = 1.5° (for Drn = 80 kPa). During time evolution the friction angle increases for F4 to approx. u30 min = 3.5° (for Drn = 24 kPa). The friction angle of the concretes F5 and F6 in early age is negligible. It is outlined that in highly dispersed systems, the volume fraction of aggregate is the main governing parameter on the pressuredependent shear behavior. However, it is ascertained that the volume fraction of aggregates at which the pressure has a significant influence on the yield stress depends on a number of parameters, like the maximum aggregate diameter, the dilatation behavior of the aggregates (angle of dilatation), the packing density of aggregates or the maximum particle volume fraction as described in Ref. [50]. Also the properties of the cement paste will be significant as there exists an interaction of all solid particles regarding the packing density including the wall and loosening effect (see [51]). In addition to the above described factors, a low dispersion of the colloid particles or a relatively low content of superplasticizer
Early state t0 Coarse aggregate
>0
Fig. 11. Illustration of the shear resistance depending on the solid particles in fresh concrete at early and aging state.
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T. Proske et al. / Construction and Building Materials 233 (2020) 117255
will provoke higher internal friction angles (as shown in Ref. [32]). This can be attributed to the formation of colloidal agglomerates. It is further to mention that the air content of the concrete may be relevant as compression reduces the air volume and therefore increases the solid volume ratio in the concrete at higher pressure levels. This will be investigated in the future studies specifically. At aging state the influence of pressure on the yield stress increases continuously as the contact between the solid particles increases do to colloidal build-up and hydration (Fig. 11(b) + (d)). The solid-solid load transfer increases accordingly and at the same time the friction resistance of the solid skeleton. This is more significant for concretes with low or moderate workability. At aging state, the friction angle increased faster for concrete F5 with approx. u30 min = 0.3° whereas concrete F6 showed this friction angle later at 90 min. It is to mention that the pre-shearing effect presumably reduced the friction angle significantly. Future measurements should be conducted on undisturbed samples for every pressure level and concrete age. It is to mention that the present studies are only first experiments on selected concrete mixes with specific additives as the cement and superplasticizer which can have a significant influence on the workability loss. To give general explanations and to improve the existing rheological models, more investigations on the pressure-dependency are necessary.
6. Conclusion and outlook The shear resistance of concrete may be significantly influenced by the actual pressure level. A testing device was developed in which the fresh concrete is sheared in a cubic pressure cell by an inserted vane as a part of a concrete rheometer (ICAR). The concrete is directly loaded in the pressure cell by a hydraulic jack. Preliminary tests on concretes with different consistency classes have revealed a notable influence of the applied pressure on the yield torque or yield stress. The results obtained for concrete with moderate flowability (consistency class F4) showed that the yield torque is significantly affected by the pressure, while this effect was insignificant for concretes with a larger table flow and a higher paste volume. However, a significant increase in yield resistance was identified for all concretes under higher pressure at aging state. Considerable friction angles of approx. u = 1.5° and 3.5° were observed for concrete with consistency class of F4 at early and aging states, respectively. Whereas the friction angles of the concretes F5 and F6 in early age were negligible. At aging state, the friction angles increased for concrete F5 with approx. u = 0.3° after 30 min while concrete F6 showed a similar friction angle later at 90 min. It is outlined that in highly dispersed systems, the volume fraction of aggregate is the main governing parameter on the pressuredependent shear behavior. At early state, a significant influence of pressure level on yield stress was observed for an aggregate volume / above its critical value /c,pressure = 0.67. Furthermore, it was figured out that this critical aggregate volume is about 85% of the packing density of highly compacted aggregates /m. Furthermore, the present study and the literature show that the interlocking effect of all solids (aggregates and finesses) in combination with the degree of dispersion (by superplasticizers) plays an important role regarding the pressure-dependent behavior. It should be mentioned that further systematic measurements to identify the influence of pressure level on the yield stress should be conducted on samples which are not sheared previously by the vane (without pre-shearing) which do not show a disturbed shear and plug zone. An improved testing device with a cylindrical pressure cell will allow a more accurate study on both yield stress and viscosity. In the future, such measurements on moderate and
highly flowable concretes as well as SCC are planned. The influence of air content will be studied specifically. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] N. Roussel, Understanding the Rheology of Concrete: Ed. by N. Roussel, 2012. [2] N. Roussel, Rheological requirements for printable concretes, Cem. Concr. Res. 112 (2018) 76–85. [3] T. Proske, Frischbetondruck bei Verwendung von Selbstverdichtendem Beton (Formwork Pressure using Self-Compacting Concrete) (Dissertation), Technische Universität Darmstadt, 2007. [4] P.H. Billberg, N. Roussel, S. Amziane, M. Beitzel, G. Charitou, B. Freund, J.N. Gardner, G. Grampeix, C.-A. Graubner, L. Keller, K.H. Khayat, D.A. Lange, A.F. Omran, A. Perrot, T. Proske, R. Quattrociocchi, Y. Vanhove, Field validation of models for predicting lateral form pressure exerted by SCC, Cem. Concr. Compos. (2014). [5] N. Roussel, Rheology of fresh concrete: from measurements to predictions of casting processes, Mater. Struct. 40 (2007) 1001–1012. [6] D. Feys, G. de Schutter, R. Verhoeven, Parameters influencing pressure during pumping of self-compacting concrete, Mater. Struct. 46 (2013) 533–555. [7] T. Proske, K.H. Khayat, A. Omran, O. Leitzbach, Form pressure generated by fresh concrete: a review about practice in formwork design, Mater. Struct. 47 (2014) 1099–1113. [8] V. Mechtcherine, V.N. Nerella, K. Kasten, Testing pumpability of concrete using sliding pipe rheometer, Constr. Build. Mater. 53 (2014) 312–323. [9] A. Perrot, A. Pierre, S. Vitaloni, V. Picandet, Prediction of lateral form pressure exerted by concrete at low casting rates, Mater. Struct. 48 (2015) 2315–2322. [10] T. Lecompte, A. Perrot, Non-linear modeling of yield stress increase due to SCC structural build-up at rest, Cem. Concr. Res. 92 (2017) 92–97. [11] G. Ovarlez, N. Roussel, A physical model for the prediction of lateral stress exerted by self-compacting concrete on formwork, Mater. Struct. 39 (2007) 269–279. [12] Y. Vanhove, C. Djelal, A. Magnin, Prediction of the lateral pressure exerted by self-compacting concrete on formwork, Mag. Concr. Res. 56 (2004) 55–62. [13] D.A. Lange, B. Birch, J. Henchen, F. Tejeda-Dominguez, L. Struble, Modeling formwork pressure of SCC, in: S.P. Shah (Ed.), Proceedings of the 3rd North American Conference on the Design and Use of Self-Consolidating Concrete, Chicago, USA, 2008, pp. 295–300. [14] K.H. Khayat, J. Assaad, Use of rheological properties of SCC to predict formwork pressure, Proceedings of the Second North American Conference on the Design and Use of Self-Consolidating Concrete and the Fourth RILEM International Symposium on Self-Compacting Concrete, Chicago, 2005. [15] B. Freund, Frischbetondruck lotrechter, geneigter und gekrümmter Betonbauteile bei Verwendung von Betonen mit hoher Fließfähigkeit, 2017 (Dissertation). [16] T. Proske, C.-A. Graubner, Pressure on formwork using SCC-experimental studies and modelling, Proceedings of the Fifth RILEM International Symposium on Self-Compacting Concrete, Ghent, 2007. [17] DIN 18218:2010-01, Frischbetondruck auf lotrechte Schalungen (Pressure of Fresh Concrete on Vertical Formwork), Beuth Verlag, 2010. [18] C.-A. Graubner, E. Boska, C. Motzko, T. Proske, F. Dehn, Formwork pressure induced by highly flowable concretes – design approach and transfer into practice, Struct. Concr. 13 (2012) 51–60. [19] M. Specht, Die Belastung von Schalung und Rüstung durch Frischbeton, 1973. [20] M. Haist, Zur Rheologie und den physikalischen Wechselwirkungen bei Zementsuspensionen (Dissertation), Karlsruhe Institute of Technology, 2009. [21] S. Shyshko, Numerical simulation of the rheological behavior of fresh concrete (Dissertation), Technische Universität Dresden, 2013. [22] C.F. Ferraris, L. Brower, D. Beaupre, F. Chapdelaine, P. Domone, E. Koehler, L. Shen, M. Sonebi, L. Struble, D. Tepke, O. Wallevik, J. Wallevik, Comparison of Concrete Rheometers, International Tests at LCPC (Nantes, France) in October 2000, 2000. [23] O.H. Wallevik, Compendium ‘‘Rheology of Coarse Particle Suspensions, such as Cement, Paste, Mortar and Concrete, The Iceland Building Research Institute, 2002. [24] J.E. Wallevik, Rheological properties of cement paste: thixotropic behavior and structural breakdown, Cem. Concr. Res. 39 (2009) 14–29. [25] O.H. Wallevik, J.E. Wallevik, Rheology as a tool in concrete science: the use of rheographs and workability boxes, Cem. Concr. Res. 41 (2011) 1279–1288. [26] D. Feys, R. Verhoeven, G. de Schutter, Fresh self compacting concrete, a shear thickening material, Cem. Concr. Res. 38 (2008) 920–929. [27] E. Secrieru, M. Butler, V. Mechtcherine, Prüfen der Pumpbarkeit von Beton – Vom Labor in die Praxis, Bautechnik 91 (2014) 797–811. [28] O.H. Wallevik, D. Feys, J.E. Wallevik, K.H. Khayat, Avoiding inaccurate interpretations of rheological measurements for cement-based materials, Cem. Concr. Res. 78 (2015) 100–109.
T. Proske et al. / Construction and Building Materials 233 (2020) 117255 [29] J. Yammine, M. Chaouche, M. Guerinet, M. Moranville, N. Roussel, From ordinary rheology concrete to self compacting concrete: a transition between frictional and hydrodynamic interactions, Cem. Concr. Res. 38 (2008) 890–896. [30] T. Lecompte, A. Perrot, V. Picandet, H. Bellegou, S. Amziane, Cement-based mixes: shearing properties and pore pressure, Cem. Concr. Res. 42 (2012) 139– 147. [31] P. Coussot, C. Ancey, Rheophysical classification of concentrated suspensions and granular pastes, Phys. Rev. E 59 (1999) 4445–4457. [32] J.J. Assaad, J. Harb, K.H. Khayat, Use of triaxial compression test on mortars to evaluate formwork pressure of self-consolidating concrete, MJ 106 (2009) 439–447. [33] A.G.B. Ritchie, The triaxial testing of fresh concrete, Mag. Concr. Res. 14 (1962). [34] P.F.G. Banfill, Rheology of fresh cement and concrete, Proceedings of an International Conference, Liverpool, 1990, CRC Press, 2014. [35] A. Mahboubi, A. Ajorloo, Experimental study of the mechanical behavior of plastic concrete in triaxial compression, Cem. Concr. Res. 35 (2005) 412–419. [36] CLUSTER 2 Production and Application of Blended Cements, European Construction in Service of Society. [37] Z. Li, T.-A. Ohkubo, Y. Tanigawa, Yield model of high fluidity concrete in fresh state, J. Mater. Civ. Eng. 16 (2004) 195–201. [38] M.R. L’Hermite, M.G. Tournon, La vibration du béton frais, Annales de l’Institut Technique du Batiment et des travaux Publics, 1948. [39] Z. Li, Investigation of shear flow of self-compacting concrete, Proceedings of ‘‘The Second International Symposium” of Self – Compacting – Concrete, COMS Engineering Corporation, Fukui, Japan, 2001. [40] H. Yim, J. Kim, S. Kwon, Effect of admixtures on the yield stresses of cement pastes under high hydrostatic pressures, Materials 9 (2016) 147. [41] J.H. Kim, S.H. Kwon, S. Kawashima, H.J. Yim, Rheology of cement paste under high pressure, Cem. Concr. Compos. 77 (2017) 60–67.
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[42] L.K. Mettler, F.K. Wittel, R.J. Flatt, H.J. Herrmann, Evolution of strength and failure of SCC during early hydration, Cem. Concr. Res. 89 (2016) 288–296. [43] C.-A. Graubner, T. Proske, Materialversuche zur Reibung und zum Verformungsverhalten von frischem Beton sowie Entwicklung eines analytischen Modells zur Bestimmung der Schalungsbelastung, Schalungsbelastung durch Hochleistungsbetone mit fließfähiger Konsistenz, 2009. [44] W. Brameshuber, H. Beitzel, M. Beitzel, C. Bohnemann, E. Boska, F. Dehn, C.-A. Graubner, A. König, C. Motzko, H.S. Müller, K. Pistol, T. Proske, C. Stettner, K. Zilch, Formwork pressure induced by highly flowable concretes – material investigations and large-scale tests, Struct. Concr. 12 (2011) 270–280. [45] C.-A. Graubner, T. Proske, Frischbetondruck bei Verwendung von Selbstverdichtendem Beton, Beton- Stahlbetonbau 104 (2009) 88–96. [46] F. Mahaut, S. Mokéddem, X. Chateau, N. Roussel, G. Ovarlez, Effect of coarse particle volume fraction on the yield stress and thixotropy of cementitious materials, Cem. Concr. Res. 38 (2008) 1276–1285. [47] S.H. Kwon, J.H. Kim, S.P. Shah, Development and applications of the intrinsic model for formwork pressure of self-consolidating concrete, Int. J. Concr. Struct. Mater. 6 (2012) 31–40. [48] Germann Instruments A/S, ICAR Rheometer – Manual, Copenhagen, Denmark, 2015. [49] E.P. Koehler, D.W. Fowler, Development of a Portable Rheometer for Fresh Portland Cement Concrete – Research Report ICAR – 105-3F, 2004. [50] X. Chateau, Particle packing and the rheology of concrete, in: Understanding the Rheology of Concrete, Elsevier, 2012, pp. 117–143. [51] F. de Larrard, Concrete Mixture Proportioning: A Scientific Approach, 9, 1999.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
A new test method to characterize the pressure-dependent shear behavior of fresh concrete Tilo Proske, Moien Rezvani ⇑, Carl-Alexander Graubner Institute of Concrete and Masonry Structures, Technische Universität Darmstadt, Franziska-Braun-Str. 3, 64287 Darmstadt, Germany
h i g h l i g h t s The shear resistance of concrete is significantly influenced by the actual pressure level. The influence of pressure on the yield stress increases as the flowability of concrete decreases. Significant influence of pressure level on yield stress was observed at early state. The volume fraction of aggregate is the main governing parameter on the pressure-dependent shear behavior.
a r t i c l e
i n f o
Article history: Received 29 April 2019 Received in revised form 4 October 2019 Accepted 12 October 2019
Keywords: Yield stress Plastic viscosity Shear behavior Variable pressure Rheometer
a b s t r a c t The development of formwork pressure is essentially affected by the rheological behavior of fresh concrete, while the shear behavior itself might be influenced by the magnitude of normal pressure. A test set-up was developed in which the fresh mortar and concrete can be sheared in a pressure cell by a vane as a part of a concrete rheometer (ICAR). The concrete is directly loaded in the pressure cell by a hydraulic jack. Preliminary shear tests were conducted on concretes with moderate and high flowability at constant water/binder-ratio of 0.4 with the focus on the static yield stress. The additional pressure was applied up to 240 kPa. The consistency class of the concrete F4, F5 and F6 was varied by choosing the paste volume to 300, 315 and 330 l/m3, respectively. The results show the significant influence of applied pressure on the measured yield torque or yield stress in early concrete age especially for concretes with moderate flowability (F4) or with relatively high volume fraction of aggregates respectively. However, a significant increase of yield resistance by applied pressure was identified for all concretes after 90 min concrete age. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The rheology as one of the fundamental characteristics of the fresh concrete can influence the production, processing, formwork pressure, surface quality as well as the mechanical and durability properties of concrete significantly [1–6]. A detailed knowledge of the rheological behavior of fresh concrete at variable pressure levels is essential for an accurate calculation and prediction of formwork pressure and can help to improve current practice in formwork design [7]. Further on, the high-pressure processing (e.g. pumping, extrusion and printing) of fresh concrete can be improved by a better understanding of the pressure-dependent rheological behavior, in addition to other effects as shown e.g. in Refs. [2,8]. Fig. 1(c and d) illustrates the development of yield stress s0 of fresh concrete and concrete pressure rh depending on the ⇑ Corresponding author. E-mail address: [email protected] (M. Rezvani). https://doi.org/10.1016/j.conbuildmat.2019.117255 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
casting time t and vertical pressure rv(h) (due to concrete weight). The surface friction sw is neglected in the shown example. As plotted, an exponential evolution of yield stress versus time can be assumed for fresh concrete [9,10]. It is shown in Fig. 1(c) that the yield stress under vertical pressure (s0(rv), red line) can be higher than the yield stress under atmospheric pressure (s0(ratm), blue line). In this case an increased yield stress leads to a smaller but more realistic formwork pressure (Fig. 1(d) red line, rh(rv) < rh(ratm)). Several efforts were carried out in the literature to predict the lateral pressure of concrete on the formwork by considering the time-depending rheology, especially based on the yield stress of concrete [4,9,11–19]. Among them, several models [3,15,16,19] are based on the pressure-dependent rheology of concrete. They address that the increased pressure due to the concrete weight can influence the shear resistance of the fresh concrete. Extensive investigations have already been conducted on the rheological characteristics of concrete under atmospheric pressure
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T. Proske et al. / Construction and Building Materials 233 (2020) 117255
Placing hose Yield stress
Concrete level
Pressure on formwork 0
Fresh concrete
Surface friction w= 0
Without surface friction
Example: casting rate x setting time formwork height
0
b
Aged concrete (setting)
0
(
atm)
c
h( atm)
h(t)
d
=
hydro
Influence of pressure
h(t)
a
v(h)
( v)
h( v)
Fig. 1. Influence of the pressure level on the yield stress of fresh concrete s0 and the effects on formwork pressure rh in tall formwork.
by many researchers [1,20–26]. In some cases the pumpability of concrete was tested in special pipe rheometers under slightly increased pressure [8,27]. Especially in tall formwork or in concrete pipes during pumping the impact of the pressure level on the rheology can be significant. Astonishingly, systematic investigations to analyze the influence of the pressure on the concrete rheology are missing so far. Therefore, the existing model approaches to describe the flow behavior of fresh concrete are primarily applicable for atmospheric state [25,28,29], but not necessarily for higher pressure. It is essential that further investigations on the influence of the pressure level on concrete rheology should focus on the early as well as the aging state of concrete up to the setting. Aim of the present study was to develop a new test set-up in which the concrete could be sheared directly by means of a vane rheometer under pressurized condition. In first measurements, the influence of variable pressure levels on the shear behavior of fresh concrete as a function of cement paste volume and consistency class was analysed. Knowledge from future direct measurement on concrete conducted by the developed test device can provide a more precise prediction of the rheological behavior and formwork pressure.
2. Pressure dependency of concrete yield stress 2.1. Mechanical background and assumptions The shear resistance of a material at a very small shear rate under atmospheric pressure is called cohesion, which is a result of inter-particle interactions, namely friction and adhesion. The shear resistance can increase under an additional normal pressure. Such growth in the yield stress can be attributed to the increase of friction between solid particles s due to higher effective stress r0n arising from the applied normal stress rn (see Fig. 2). The intensity of the pressure-dependent friction is often connected to the dilatation of the particles system during shearing. Recent studies on fresh mortar and concrete show that the dilatancy of solid particles can result in an increase of the yield stress as the concrete suspension has a quasi-granular behavior [30]. It was observed, that two phenomena control the pressure-dependency during shearing; 1) the dilatancy of the interconnected particle network which will increase the shear resistance and 2) the collapse of cement colloidal network which decreases the friction [30]. However, for high flowable concretes like SCC, the dilatancy will increase the internal friction especially in aging state due to the
hydration and formation of the particle network. In this case they will behave as ‘‘hard” suspensions as defined in [31]. The stress-dependent yield stress of the fresh concrete s0(rn) can be characterized by specific material models which are applied in the soil mechanics. One option could be the Mohr-Coulomb failure criterion which is already used by several researchers (see e.g. [3,32–37]) to describe the pressure-dependent shear behavior of fresh concrete, with the material parameters cohesion c (stressindependent term) and friction angle u (stress-dependent term). The yield condition is represented by the Mohr’s failure envelope. Hereby the shear stress s0 is a function of cohesion c, normal stress rn and friction angle u:
s0 ¼ c þ rn tanu
ð1Þ
It must be kept in mind that the yield stress s0 can only be considered for the design of formwork if the concrete is deformed, at least with a shear angle at which the maximum shear stress is reached. Otherwise the formwork pressure would be underestimated. However, in the design of conventional formwork, a certain formwork deformation is allowed for the limit state (for both serviceability and ultimate limit state). In this cases, the concrete should be sheared sufficiently. It is further to notice that the internal friction (adherence) represents the maximum achievable value of the surface friction sw between formwork surface and fresh concrete. 2.2. Measurements regarding the pressure-dependency of concrete shear resistance The friction angle of vibrated concrete was determined experimentally in several studies [33,38]. Ritchie [33] conducted triaxial tests on the fresh concrete, where aggregate and cement content, w/c-ratio as well as slump value of fresh concrete (according to DIN EN 12350-2) were varied. l’Hermite&Tournon [38] investigated the influence of vibration on the friction angle and cohesion. They concluded a significant reduction of shear stress during vibration especially at low normal pressure levels. Only a few studies are conducted concerning the influence of pressure on the flow behavior of highly flowable cement paste and concrete (SCC) in fresh state. The yield stress of fresh concretes was investigated by Li et al. [37,39]. The results showed that yield stress of all used materials increases linearly by growth of normal stress. However, the influence of normal stress on concrete was more significant compared to mortar and cement paste mixtures due to a larger friction angle of concrete with u 3:0 . Fig. 3 (left) shows the principle effect of the normal stress rn on the shear
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Cement particles
n
=
Aggregates
n'
+u ( n) Transfer of normal load by granular skeleton (effective stresses n') and shear component (
Water and Air
Residual load transfer by the liquid phase (pore water pressure u) More solid-solid contact by: - Hindered expansions (dilatation) of the particle system - Compression of the liquid/gaseous phase - Loss of water (e. g. hydration)
( n
n)
=
n'
Increased friction between solid particles (e.g. aggregates and hydration products)
+u
Fig. 2. Contribution of normal pressure on the shear resistance of fresh concrete.
t = constant
0
n2
0,atm
n1
n2 n1
0
0
n2
n1 0
Shear rate [1/s]
c
c F1 cF 6 cSCC 0
n
tan n
tan
F1
Friction angle F6
Consistency class according to EN 206-1 and DIN EN 12350-5
0
SCC Normal pressure
n
Fig. 3. Influence of normal pressure rn and shear rate c_ on shear resistance of SCC [38] (left); influence of normal pressure rn on yield stress s0 and internal friction angle u respective of concretes with different consistency class [3] (right).
resistance s of SCC in a shear test. It indicates that the shear stress increases as the normal stress grows. Yim et al. [40] investigated the effects of different supplementary cementitious materials (SCM) and admixtures on the rheological properties of different cement pastes at two pressure levels. It was concluded that the influence of hydrostatic pressure on plastic viscosity of cement paste is negligible while the yield stress may be affected by the amount and type of used SCM. The results of Kim et al. [41] have shown only limited influence of the pressure on the rheology of cement paste. Triaxial compression tests on mortar mixtures with different binder types and water-cementitious material ratios (w/cm) were presented by Khayat et al. [32]. The results show friction angles between 25 and 31° for low flowable mortars with flow diameter of approx. 190 mm following 25 drops according to ASTM C 1437. The reason to use such mortars with relatively low workability compared to self-compacting properties was to ensure sufficient cohesion to prevent the specimens collapse during its mounting in a vertical position in the flexible membrane of the triaxial cell. Therefore, the application of the triaxial test is not suitable to investigate the pressure-dependent rheology of highly-flowable concrete like SCC. Studies on the influence of pressure on the flow behavior of SCC in aging state are very limited in the literature. Mettler et al. [42] investigated the strength and failure development of SCC and calculated the cohesion and friction angle based on compressive and
tensile strength (e.g. for the Mohr-Coulomb and Drucker-Prager model). It is shown that the friction angle rises from 0 to 48° during the transition of concrete from a non-Newtonian fluid to a cohesive material. Indirect measurements on pressure-dependent shear behavior of concrete were also conducted by the current authors using a self-developed test apparatus described in Refs. [3,43–45]. The concretes were placed in a hard-plastic cubic cell (25 25 25 cm3) in which the concrete sample was loaded directly in vertical direction. During the application of vertically pressure the resulting horizontal stresses was measured. These tests were conducted on highly workable concretes (SCC, F6, F5) as well as on low and moderate workable concretes (consistency class F1 to F4), just after mixing up to the final setting time of the concrete (maximum 10 h), early to aging state, respectively. The shear behavior of the fresh concrete was evaluated indirectly using appropriate mechanic models (e.g. Mohr’s model). The material parameters friction angle u and cohesion c (or yield stress s0) were calculated based on the ratio of horizontal pressure rh to vertical pressure rv (lateral pressure factor k) for an active soil state. The experiments were performed with both rigid and deformable formwork. The results showed that the friction angle u of SCC is about zero at early concrete age but increases significantly at aging state [3,44]. Fig. 3 (right) reveals further that the consistency class of concrete might be an indicator for the pressure-dependent yield stress. The friction angle u increased
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with the decrease of workability even at early state. The effect of time (aging state) on the pressure-dependent shear stress s and the friction angle u was also remarkable for all consistencies. 2.3. Conclusion based on the existing measurements For fresh cement pastes at early state which are watersaturated, the friction angle can be assumed with u = 0 [20]. On concrete level, it is assumed that in the early state, the friction angle of a very flowable concrete (e.g. SCC) tends to about zero as all solid particles are highly dispersed by water and superplasticizers [3]. This behavior should be independent of the volume of the suspended solids if the paste volume is high enough to prevent a dilatation effect of the coarser particles. However, in the aging state the solid particles will be increasingly connected by agglomeration and loss of water due to hydration. This will lead to an increase of the friction angle. Stiffer concrete (e.g. consistency classes F1–F4) exhibits larger friction angles even in early state, due to a higher content of solid particles and/or lower dispersion. In aging state the increase of yield stress may be more significant compared to SCC because of the higher volume fraction of coarse aggregates (see also Ref. [46]). 3. Preliminary observations of aggregate displacement during shearing To understand the aggregate displacement during shearing which may control the pressure dependent shear behavior of concrete (see Fig. 4), exemplary analysis were conducted by the authors on model concrete samples with X-ray computed tomography technique. For this purpose, a versatile high-resolution 3D computed tomography General Electric v|tome|x s 240 d (micro CT and nano CT) and 3D metrology was used. To figure out the relative particle displacement Dla and rotation of the particle da in the system by a shear process, the scans were conducted on samples before and after applying a shear displacement. In this observation, the maximum shear angle c was about 30° and applied in a special mold with acrylic glass walls. The two smaller opposite walls were rotatable, whereas the upper and bottom surfaces were profiled. To achieve a sufficient resolution and minimize the wall-effect, the dimension of concrete sample and the maximum aggregate size
Before shearing
a
Δla
After shearing
Fig. 4. Computer tomography images of a concrete sample with w/c = 0.40 at early age before and after shearing.
were selected 40 40 80 mm3 and 8 mm, respectively. The model concrete had w/c = 0.40 with a paste volume of 330 l/m3 (consistency class of F4). Test was conducted at quasi-early state (scan after approx. 15 min) as well as at aging state. Images captured from concrete sample before and after shearing is shown in Fig. 4. As Fig. 4 reveals, a significant relative particle displacement induced by shearing, including a rotation of several aggregates. Applying a shear to the specimen results in a rearrangement of the solid skeleton. In regions where solid-solid contacts exist, it is expected that the presence of a normal pressure will increase the required shear stress for the given deformation of c. It is to keep in mind that a direct contact of the aggregates is not necessary in all cases as the other finer solid particles are also included in the transfer of stresses in the overall skeleton. These mechanisms will be discussed specifically in Section 5.2. Due to the small particle size of the sand, these particles are not well to distinguish. However, it should be mentioned that the target of this analysis was to verify the displacement and rotation of the coarse aggregates during shear.
4. Development of a special testing device for rheological measurements on freh concrete under variable pressure A special test set-up was developed to enable a direct measurement of mortar and concrete yield stress under variable pressure level (see Fig. 5). The test apparatus is based on an existing test machine which is described in [3,43,44]. The fresh concrete is placed in a hard-plastic cubic cell (25 25 25 cm3). To prevent significant errors in the experiments, e. g. due to the ‘‘wall effect”, the maximum aggregate diameter of the concrete should not exceed 8 mm. The cell is confined in the horizontal direction by a rigid steel frame. The fresh concrete sample can be loaded vertically by a hydraulic jack. Two pressure sensors in the vertical walls measure the resulting horizontal pressure. The 4-blade vane of a rheometer (ICAR) is positioned in the testing cell. The ICARRheometer is fixed on the bottom of the steel frame. A sealed ball bearing at the entering point to the cell ensures a low friction during rotation. The drainage of paste and water is hindered additionally by special sealing grease with sufficient viscosity. The complete-sealing and suitability of the developed test set-up were verified by using wallpaper adhesive and fine mortar, respectively. Compared to test set-ups in which the pressure is applied with air or liquid (for example, [40,41,47]), the developed method may have the advantage to transfer the load to concrete in a more realistic way, especially if the processing of concrete in tall formwork systems is simulated by application of external pressure. In the developed test set-up, both the liquid phase and the granular phase are loaded directly. This probably leads to higher effective stresses in the granular system and therefore a higher shear resistance compared to a loading with air or liquid. Furthermore, during shearing the granular particles have no room for a dilatation at the surface as it might happen in set-ups where the pressure is applied with air or liquid. Compared to triaxial tests, the developed test set-up has the advantage of a directly and precise measurements especially for concretes with low friction angles. A further advantage is a fast preparation and execution of tests. Furthermore, the triaxial testing is not suitable to investigate the shear resistance of highlyflowable concrete because of the impossible installation of the workable concrete in the cell [30,32]. Based on the obtained maximum torque, the corresponding yield stress can be calculated by a suitable approach [48,49]. Assuming a uniform distribution of the shear stress in the shearing zone, calculation of yield stress using Eq. (2) is possible based on the measured maximum torque Tmax, the vane height H and the
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Side view
Section A-A
Vertical pressure Rigid frame
4 60
127
60
Pressure sensor
60
250
A
60
250
Vane
250
250
20
A
127
Fresh concrete
Ball bearing with seal Rigid subframe
ICAR
Dimensions in mm
ICARRheometer
Fig. 5. Developed test set-up to measure the pressure-dependent shear behavior of fresh concrete.
vane diameter D. The used vane has H = 127 mm and D = 127 mm. Therefore, Eq. (2) can be simplified as Eq. (3).
s0 ¼ s0 ¼
2 T max
D p 3
H D
ð2Þ
þ 13
2 T max 0:1273 p 1 þ 13
¼ 233 T max
ð3Þ
Tmax in Nm s0 in Pa After the maximum yield torque Tmax is reached during a test on concrete and mortar, a plug zone may be formed directly at the surrounding (cylindrical shape) of the vane. Outside this cylinder the concrete will be quasi at rest. Therefore, the cubic container geometry is suitable for the measurement of yield stress but less for the measurement of plastic viscosity.
5. First studies on static yield stress of fresh concrete under pressure 5.1. Materials and methods The tested concretes had a w/b-ratio of 0.4 and different consistency classes (F4, F5 and F6 according to EN 206-1 and DIN EN 12350-5). The binder consists of 70% Portland cement CEM I 52,5 R and 30% limestone powder (LS). The paste volume was approx. 300 l/m3 for concrete with consistency class F4, 310 l/m3 for class F5 and 330 l/m3 for class F6. The water to binder ratio (CEM I + LS) was 0.40. Aggregates with a maximum diameter of 8 mm were used. To achieve the aimed flow value according to DIN EN 12350–5 a superplasticizer (PCE) was applied. The concrete temperature was approx. 22 °C. Table 1 shows the mix proportion and the target workability. A concrete batch of 35 l for each test was mixed in a single shaft mixer. Firstly, the air dried aggregates were mixed together with
Table 1 Mix proportion and the consistency of the tested concretes. Concrete characteristics
Concrete F4
Concrete F5
Concrete F6
284 130 159 894 894 2.5 30.5 2 67.5 81.4 F4 70 30 0.4 51 43 45 38
294 132 164 883 883 2.1 31.3 2 66.6 80.9 F5 70 30
314 135 176 859 859 1.4 33.1 2 64.9 79.9 F6 70 30
58 44 42 39
69 55 46 44
Mix design CEM I 52,5 R Limestone powder Water (excluding sorption water by aggregates) Aggregates 0–2 mm, naturally rounded Aggregates 2–8 mm, naturally rounded Superplasticizer Paste volume (binder, water and SP) Air content Volume of aggregates Volume of solids Consistency class (DIN EN 206-1) Binder CEM I 52,5 R Limestone powder w/b-ratio Consistency f (t = 0 min) f (t = 30 min) f (t = 60 min) f (t = 90 min)
kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 wt% of CEM I Vol% Vol% Vol% Vol% – wt% wt% – cm
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T. Proske et al. / Construction and Building Materials 233 (2020) 117255
cement and limestone powder for 1 min. Afterwards water and superplasticizer were added and mixed for 5 min with 30 rpm. The flow values measured with the flow table test according to DIN EN 12350-5 are presented in Table 1. The flow value f was measured for all concretes at different times after mixing. Fresh concrete which was at rest was filled in the cone and compacted slightly with a rod for each table test. The cone was lifted and the concrete was dropped 15 times according to the standard test method. In this studies, the concrete samples were tested at applied pressure levels of 0 kPa (only concretes self-weight), 24, 80 and 240 kPa. The vertical pressure was raised stepwise from 0 to 240 kPa followed by unloading to 0 kPa. The concrete samples were sheared at each target pressure level under a slow rotational speed of 0.001 rps for approx. 30 s. The applied shear profile is plotted in Fig. 6. For each measurement the torque evolution was recorded and the maximum torque Tmax (static yield value s0) was determined. The load cycle and measurement procedure was conducted at 10 min after concrete placement and repeated at 30, 60 and 90 min. At each concrete age, all pressure levels were applied within a very short time (approx. 5 min in total). This short time ensures that the consistency of concrete does not change significantly. 5.2. Experimental results The results of the maximum yield torque and yield stress calculated with Eq. (3) respectively are shown exemplarily for concrete F4 in Fig. 7. Evident is a significant influence of the
applied pressure on the measured yield stress. For an applied pressure of approx. 30 kPa the yield increased up to about 100%. However, the increase is lower at higher pressure levels but still significant. Results for the concrete F5 are plotted in Fig. 8. As it reveals, three effects can be detected to influence the yield stress under variable pressure state: 1) the pressure effect, 2) the effect of pre-shearing and 3) the aging effect. The influence of the applied pressure can be identified as the difference between the measurements at 240 kPa and unloaded state at one cycle. In this case the influence of pre-sharing and aging is almost negligible (pure pressure effect). The hysteresis within the shearing in one cycle shows the pre-shearing effect. On the other hand, the difference between cycles at different concrete ages shows the aging effect. Both effects are influenced by thixotropy and structural build-up of the concrete which depend on the degree of disturbance during the test. Probably a certain plug zone is formed at the surrounding of the vane, but limited by the very small applied shear angle. The increase of shear resistance by the growing pressure level is clearly visible in Fig. 8. However, this effect is not very significant compared to concrete F4. The difference between the measured yield stresses at unloaded samples represents the impact of preshearing. It is visible that the pre-shearing history affects the magnitude of the yield stress notably. Meanwhile, it can be seen that a rise in the yield values is influenced by the aging effect. It should be mentioned that the magnitude of these effects depends on the mix proportion of concrete. Fig. 9 illustrates results for concrete F6. A relatively low influence of pressure on the yield stress at the early age of 30 min is visible. A high impact of applied pressure on the yield stress was observed at the age of 90 min. The increase of static yield stress at atmospheric pressure s0,atm for the concrete F6 – also expressed as thixotropy Athix – shows a value of approx. Athix = 0.3 Pa/s which is in the range of conventional SCC. The influence of the consistency class on the pressure dependent yield stress is shown in Fig. 10. Interestingly, the concretes F6 and F5 show no significant increase of yield stress with increasing pressure in early age. However, a high impact of pressure on the yield stress was measured during the test with concrete F4. Due to the comparable low internal friction angles and the high stiffness of the container, the horizontal pressure was measured in the same magnitude as the vertical pressure at early stages for all concrete (hydrostatic pressure plus surcharge load).
Fig. 6. Shear profile applied for the measurements.
Fig. 7. Measured yield torque and stress at different times and pressure levels for consistency class F4.
Fig. 8. Measured yield torque and stress at different times and pressure levels for consistency class F5.
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T. Proske et al. / Construction and Building Materials 233 (2020) 117255
5.3. Discussion
Fig. 9. Measured yield torque and stress at different times and pressure levels for consistency class F6.
It can be figured out that at early state the influence of pressure on yield stress is only considerable for concrete with low or moderate workability. In this case, the cement particles are not well dispersed by superplasticizers (see [32]) or the paste volume is so low that the solid particles stay in contact (Fig. 11(a)) and are able to transfer effective stresses in the solid system (Fig. 11 left). An increase of paste volumes in concretes with higher workability (F5 and F6) reduces the pressure-dependency at early age significantly. The distance between the aggregates is increasing and the direct contact and therefore the friction resistance is very limited (Fig. 11(c)). Also the better dispersion of the smaller particles reduces the contacts between the solid particles. This reduces the load transfer and friction resistance of the solids accordingly. The internal friction angle u of the concrete was calculated based on Eqs. (1) and (4). The cohesion c is assumed equal to the yield stress at atmospheric pressure s0;atm . This gives:
u ¼ tan1
Fig. 10. Yield torque for consistency class F4, F5 and F6 at concrete age of 10 min versus applied pressure.
Aging state t1 < tset
Unhydrated cement particle dispersed by Fine aggregates superplasticizers Water
(a)
s0 c s0 s0;atm ¼ tan1 Dr n Dr n
Cement agglomeration
Partially hydrated cement
(b)
Low/moderate workable concrete
>> 0
>0 (c)
(d)
Highly workable concrete (SCC)
0
ð4Þ
At early age, the concrete F4 shows already a considerable friction angle of approx. u10 min = 1.5° (for Drn = 80 kPa). During time evolution the friction angle increases for F4 to approx. u30 min = 3.5° (for Drn = 24 kPa). The friction angle of the concretes F5 and F6 in early age is negligible. It is outlined that in highly dispersed systems, the volume fraction of aggregate is the main governing parameter on the pressuredependent shear behavior. However, it is ascertained that the volume fraction of aggregates at which the pressure has a significant influence on the yield stress depends on a number of parameters, like the maximum aggregate diameter, the dilatation behavior of the aggregates (angle of dilatation), the packing density of aggregates or the maximum particle volume fraction as described in Ref. [50]. Also the properties of the cement paste will be significant as there exists an interaction of all solid particles regarding the packing density including the wall and loosening effect (see [51]). In addition to the above described factors, a low dispersion of the colloid particles or a relatively low content of superplasticizer
Early state t0 Coarse aggregate
>0
Fig. 11. Illustration of the shear resistance depending on the solid particles in fresh concrete at early and aging state.
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T. Proske et al. / Construction and Building Materials 233 (2020) 117255
will provoke higher internal friction angles (as shown in Ref. [32]). This can be attributed to the formation of colloidal agglomerates. It is further to mention that the air content of the concrete may be relevant as compression reduces the air volume and therefore increases the solid volume ratio in the concrete at higher pressure levels. This will be investigated in the future studies specifically. At aging state the influence of pressure on the yield stress increases continuously as the contact between the solid particles increases do to colloidal build-up and hydration (Fig. 11(b) + (d)). The solid-solid load transfer increases accordingly and at the same time the friction resistance of the solid skeleton. This is more significant for concretes with low or moderate workability. At aging state, the friction angle increased faster for concrete F5 with approx. u30 min = 0.3° whereas concrete F6 showed this friction angle later at 90 min. It is to mention that the pre-shearing effect presumably reduced the friction angle significantly. Future measurements should be conducted on undisturbed samples for every pressure level and concrete age. It is to mention that the present studies are only first experiments on selected concrete mixes with specific additives as the cement and superplasticizer which can have a significant influence on the workability loss. To give general explanations and to improve the existing rheological models, more investigations on the pressure-dependency are necessary.
6. Conclusion and outlook The shear resistance of concrete may be significantly influenced by the actual pressure level. A testing device was developed in which the fresh concrete is sheared in a cubic pressure cell by an inserted vane as a part of a concrete rheometer (ICAR). The concrete is directly loaded in the pressure cell by a hydraulic jack. Preliminary tests on concretes with different consistency classes have revealed a notable influence of the applied pressure on the yield torque or yield stress. The results obtained for concrete with moderate flowability (consistency class F4) showed that the yield torque is significantly affected by the pressure, while this effect was insignificant for concretes with a larger table flow and a higher paste volume. However, a significant increase in yield resistance was identified for all concretes under higher pressure at aging state. Considerable friction angles of approx. u = 1.5° and 3.5° were observed for concrete with consistency class of F4 at early and aging states, respectively. Whereas the friction angles of the concretes F5 and F6 in early age were negligible. At aging state, the friction angles increased for concrete F5 with approx. u = 0.3° after 30 min while concrete F6 showed a similar friction angle later at 90 min. It is outlined that in highly dispersed systems, the volume fraction of aggregate is the main governing parameter on the pressuredependent shear behavior. At early state, a significant influence of pressure level on yield stress was observed for an aggregate volume / above its critical value /c,pressure = 0.67. Furthermore, it was figured out that this critical aggregate volume is about 85% of the packing density of highly compacted aggregates /m. Furthermore, the present study and the literature show that the interlocking effect of all solids (aggregates and finesses) in combination with the degree of dispersion (by superplasticizers) plays an important role regarding the pressure-dependent behavior. It should be mentioned that further systematic measurements to identify the influence of pressure level on the yield stress should be conducted on samples which are not sheared previously by the vane (without pre-shearing) which do not show a disturbed shear and plug zone. An improved testing device with a cylindrical pressure cell will allow a more accurate study on both yield stress and viscosity. In the future, such measurements on moderate and
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