This study aims to estimate the pumpability and shootability of wet-mix shotcrete (WMS) made with crushed aggregates and various admixtures such as silica fume, fly ash, ground granulated blast furnace slag (GGBFS), metakaolin, and steel fiber based on rheological properties. The IBB rheometer was employed as an apparatus to measure the rheological properties of freshly mixed shotcrete such as flow resistance and torque viscosity. Results have shown that the use of silica fume and metakaolin led to satisfactory pumpability, whereas mixtures with fly ash and steel fiber failed to meet the pumping criteria at normal pump pressure. The build-up thickness, an indicator to represent shotcrete shootability, was predicted to vary between 68 and 218 mm, demonstrating that the use of admixtures resulted in a wide spectrum of shootability. In particular, the use of metakaolin was found to substantially increase the predicted build-up thickness only with a small replacement. The findings of this study are expected to be used as an easy-to-use guideline for estimating pumpability and shootability of WMS when no compliance testing data is available.
Pumpability and shootability are important functional parameters considered in the design procedure for wet-mix shotcrete (WMS) mixtures [
One feasible approach to address this limitation is to indirectly estimate the pumpability through rheological properties of fresh materials, namely, yield stress and plastic viscosity of the Bingham model (or equivalent). Several studies have been conducted to estimate the pumpability of normal- and high-performance shotcrete/concrete based on the rheology and other flow characteristics. Browne and Bamforth [
Even though many theoretical and practical research attempts to quantify the pumpability of WMS have been made based on prevailing theories of rheology, research studies on shootability predictions have been relatively limited. This is because the shootability is generally subjected to a number of construction uncertainties (such as workmanship of a nozzle operator and field conditions including climates as well as equipment/method used), which made the research approach towards shootability estimation quite challenging [
A commercially available Type I portland cement complying with KS L 5201 was employed. The specific gravity and fineness of the portland cement used were 3.15 and 3,289 cm2/g, respectively. The chemical compositions were as follows: 20.8% SiO2, 6.3% Al2O3, 61.2% CaO, 3.3% MgO, 2.3% SO3, and 0.61% loss-on-ignition (LOI).
Crushed granite obtained from the surrounding rock of tunnel walls was used as coarse aggregate after being sieved and washed. The coarse aggregate used had a maximum aggregate size of 10 mm, a specific gravity of 2.65, and a fineness modulus of 5.70. As fine aggregate, washed crushed sand with a specific gravity of 2.62 and a fineness modulus of 2.77 was employed. The coarse and fine aggregates used in this study conformed to the gradation specifications of ASTM C33.
A Type I fly ash as per the classification of the Korean Industrial Standards (KS) was used. The chemical properties of the fly ash used were 41.5% SiO2, 16.8% C, 22.9% Al2O3, 4.91% CaO, and 1.84% Fe2O3, all of which met the requirements of KS L 5405. The density of the fly ash used was 2.25 g/cm3, which complied with the specified lower limit of 1.95 g/cm3.
Silica fume with a density of 3.0–3.5 g/cm3, a specific surface area of 200,470 cm2/g, a 0–45
Ground granulated blast furnace slag (GGBFS) with a density of 2.90 g/cm3, a specific surface area of 4,306 cm2/g, an activity index at 91 days of 117, and a flow of 102% was used. The chemical properties of the GGBFS used were as follows: 5.61% MgO, 1.0% SO3, 0.43% LOI, and 0.007% chloride ion.
Metakaolin with a specific gravity of 2.63 and a specific area of 12,000 cm2/g was adopted as a mineral admixture. The chemical compositions of the metakaolin used were 56% SiO2, 37% Al2O3, 2.4% Fe2O3, 0.2% CaO, 0.3% MgO, 0.8% K2O, and 0.2% Na2O.
A bundle-type steel fiber with a density of 7.80 g/cm3, a tensile strength of 1,195.5 MPa, and an aspect ratio of 60 (
22 mixtures with a constant water-to-cementitious material ratio (w/cm) of 0.41 and a fine aggregate-to-total aggregate fraction (S/a) of 0.71 were prepared while varying replacement/addition levels as seen in Table
Mixture proportions of WMS.
Admixture | Replacement1 (%) | Weight per unit volume (kg/m3) | |||||||
---|---|---|---|---|---|---|---|---|---|
SF | FA | GGBFS | MK | C | W | F/A | C/A | ||
SF | 0 | 0 | — | — | — | 440 | 180 | 1159 | 480 |
5 | 22 | — | — | — | 418 | 180 | 1153 | 478 | |
9.1 | 40 | — | — | — | 400 | 180 | 1149 | 476 | |
15 | 66 | — | — | — | 374 | 180 | 1142 | 473 | |
|
|||||||||
FA | 0 | 40 | 0 | — | — | 400 | 180 | 1149 | 476 |
10 | 40 | 44 | — | — | 356 | 180 | 1137 | 471 | |
20 | 40 | 88 | — | — | 312 | 180 | 1125 | 466 | |
30 | 40 | 132 | — | — | 268 | 180 | 1113 | 461 | |
|
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GGBFS | 0 | 40 | — | 0 | — | 400 | 180 | 1149 | 476 |
10 | 40 | — | 44 | — | 356 | 180 | 1146 | 475 | |
20 | 40 | — | 88 | — | 312 | 180 | 1144 | 474 | |
30 | 40 | — | 132 | — | 268 | 180 | 1142 | 473 | |
40 | 40 | — | 176 | — | 224 | 180 | 1140 | 472 | |
|
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MK | 0 | 40 | — | — | 0 | 400 | 180 | 1149 | 476 |
5 | 40 | — | — | 22 | 378 | 180 | 1146 | 475 | |
10 | 40 | — | — | 44 | 356 | 180 | 1144 | 474 | |
15 | 40 | — | — | 66 | 334 | 180 | 1141 | 473 | |
|
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Steel fiber | 02 | 40 | — | — | — | 400 | 180 | 1149 | 476 |
302 | 40 | — | — | — | 400 | 180 | 1149 | 476 | |
402 | 40 | — | — | — | 400 | 180 | 1149 | 476 | |
502 | 40 | — | — | — | 400 | 180 | 1149 | 476 | |
602 | 40 | — | — | — | 400 | 180 | 1149 | 476 |
The IBB rheometer, of which the original and revisited designs were devised at The University of British Columbia and IBB Rheology Inc., respectively, was employed to monitor the rheological properties of freshly mixed shotcrete (see Figure Calibrate the torque resolution (±0.5 N·m) and set the built-in strain gage to a null position. Pour freshly mixed shotcrete into a 21-liter mixing bowl up to a level of 200 mm. Lift the mixing bowl until the H-shaped impeller is fully buried in the mixture. Begin operation with the lowest rotational speed, and gradually increase the speed as required.
IBB rheometer: (a) front and rear view and (b) schematic diagram.
During the operation, the torque applied to the rotary impeller as it stirs the mixture with a specified revolving speed was continuously measured by means of a load cell. The testing was conducted under a constant temperature condition of 23°C so as to eliminate the unexpected effect of temperature. Figure
Typical data obtained from the IBB rheometer.
The torque viscosity and flow resistance have comparable physical meanings to yield stress and plastic viscosity, respectively, by definition of the Bingham model as follows [
The pumpability of WMS was estimated based on previously published data by Beaupre [
Rheological properties for different mixtures and their borderline values [
The shootability of WMS is often characterized by a build-up thickness. In this study, flow resistance was chosen as an indicator to predict the build-up thickness of WMS because the build-up thickness had a fairly proportional relationship to flow resistance (see Figure
Relationships between rheological parameters and build-up thickness: (a) flow resistance versus build-up thickness; (b) torque viscosity versus build-up thickness [
Two different relationships presented in former studies—one based on the two-point test [
The results of rheometer tests for various silica fume mixtures are shown in Figure
Exerted torque versus rotational speed relationship for silica fume mixtures.
Figure
Exerted torque versus rotational speed relationship for fly ash mixtures with 9.1% silica fume.
The experimental data and their regression lines for GGBFS mixtures are presented in Figure
Exerted torque versus rotational speed relationship for GGBFS mixtures with 9.1% silica fume.
Figure
Exerted torque versus rotational speed relationship for metakaolin mixtures with 9.1% silica fume.
The rheometer test results for steel fiber mixtures with a 9.1% silica fume replacement are given in Figure
Exerted torque versus rotational speed relationship steel fiber mixtures with 9.1% silica.
Figures
Variations in rheological properties for various WMS mixtures: (a) variations in flow resistance and (b) variations in torque viscosity.
Figure
Pumpability predictions for various WMS mixtures.
Furthermore, the results of pumpability prediction for fly ash mixtures are presented in Figure
When GGBFS was used, two mixtures (i.e., 0 and 40%) successfully obtained the pumpable condition, while other mixtures failed to meet the pumping requirements. However, it appears that the GGBFS itself had a nonsignificant effect on the pumpability because the variations in torque viscosity and flow resistance per the replacement level were quite small compared with other mixture groups. This, in turn, implies that the high fineness of the GGBFS used (4,310 cm2/g) had a minimal effect on the rheological properties of WMS.
The results given in Figure
All mixtures with steel fibers were found to fall on the “nonpumpable” zone. This is because the steel fiber additions enormously increased the torque viscosity as high as 9.18–12.48 N·m·s—about 3.17–4.3 times greater than the border line value for torque viscosity—while keeping a similar level of initial slump (i.e., 215–230 mm). Accordingly, when steel fibers are to be used for a shotcrete project, compliance testing may need to be conducted prior to field practice to check the actual pumpability.
Table
Recommended replacement/addition ranges for each admixture.
Mixture group | Pumpable (%) | Nonpumpable (%) |
---|---|---|
Silica fume only | 5–15 | <5 |
Fly ash | — | All |
GGBFS | 30–40 | <30 |
9.1% silica fume | ||
Metakaolin | 0–10 | >10 |
Steel fiber | — | All |
The build-up thickness of various WMS mixtures, as a measure of shootability, was predicted using the regression equation derived in Figure
Shootability (build-up thickness) predictions for various WMS mixtures.
The maximum build-up thickness for fly ash mixtures was obtained when 10% fly ash was used. The build-up thickness then decreased as the fly ash replacement increased above 10%. The inconsistent trend of build-up thickness changes according to the replacement level may be due to the effect of different air contents among the mixtures—the air contents for 0, 10, 20, and 30% fly ash mixtures were 18, 15, 11, and 10%, respectively. A further investigation needs to be conducted to clearly identify the sole influence of flow resistance on the build-up thickness.
The build-up thickness of GGBFS mixtures was predicted as 65–114 mm, which was relatively smaller than that of other mixtures. This is most likely because, as previously described, GGBFS gave rise to the ball-bearing effect, thereby increasing the workability of mixtures. Given the relatively smaller build-up thicknesses and little effect of GGBFS replacement level on the build-up thickness, it appears that GGBFS is not a suitable admixture for shootability improvements.
Metakaolin mixtures led to an overall increase in predicted build-up thickness with an increased metakaolin replacement. Specifically, a substantial increase in build-up thickness was observed when the replacement level became greater than 5%. This implies that caution should be taken to control the build-up thickness when proportioning shotcrete mixtures with more than 5% metakaolin, while up to 5% metakaolin can be used without essential modifications to mixture proportions.
The predicted build-up thickness for steel fiber mixtures was found to increase until the addition level reached up to 50 kg/m3, whereas a noticeable reduction was observed at a higher addition level beyond 50 kg/m3. Even though the build-up thicknesses predicted were overall sufficient for the use in actual practice, however, steel fiber may not be a suitable additive given the poor pumpability of WMS, as previously discussed.
Table
Predicted build-up thickness ranges for WMS mixtures meeting pumping requirements.
Mixture group | Pumpable range (%) | Predicted build-up thickness (mm) |
---|---|---|
Silica fume only | 5–15 | 97–173 |
Fly ash | — | — |
GGBFS | 30–40 | 68–84 |
9.1% silica fume | ||
Metakaolin | 0–10 | 114–218 |
Steel fiber | — | — |
Overall range | 5–15 | 68–218 |
In this paper, the pumpability and shootability of wet-mix shotcrete (WMS) with crushed aggregates and various admixtures were estimated based on a series of rheometer tests. Optimum replacement/addition ranges for each admixture were also proposed as a guideline for actual practice, along with expected build-up thicknesses upon shooting. Based on the findings of this study, the following conclusions can be made: Silica fume had positive effects on both pumpability and shootability by decreasing torque viscosity and increasing flow resistance simultaneously. Fly ash replacements increased the torque viscosity of WMS, which may lead to poor pumpability. Ground granulated blasted furnace slag (GGBFS) replacements tended to reduce the shootability of WMS, while it had a nonsignificant effect on the pumpability. Metakaolin replacements significantly increased the shootability of WMS, while keeping a similar level of pumpability across the mixtures. Adding steel fibers resulted in a substantial increase in torque viscosity, which may not be a good strategy to enhance the pumping performance. While all fly ash and steel fiber mixtures failed to meet the pumping requirements, several silica fume, GGBFS, and metakaolin mixtures complied with the pumping criteria at normal pump pressure. In particular, silica fume and metakaolin mixtures were found to have good pumping performance. A wide range of build-up thicknesses between 68 and 218 mm was obtained for WMS mixtures with various type and dosage of admixtures. Particularly, the use of metakaolin dramatically increased the build-up thickness only with a 10% replacement.
The results outlined in this study would be used as a simple estimate of the pumpability and shootability predictions, as far as no compliance testing is conducted for a given project. A little more conservative approach is recommended taking into account the errors that may be caused by measurements and material variability. Future research will be performed to understand how the use of those admixtures plays a role in actual pumpability and shootability.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research was supported by both grant A (13RDRP-B066780) and grant B (15TLRP-B079261-02), funded by Ministry of Land, Infrastructure and Transport of Korean government, and was partly supported by Kangwon National University (Grant no. 120131837), Chuncheon, Republic of Korea.