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The effects of pumping on the air content and void structure of air-entrained, wet mix fibre reinforced shotcrete
Journal Pre-proof The Effects of Pumping on the Air Content and Void Structure of Air-Entrained, Wet Mix Fibre Reinforced Shotcrete S. Talukdar, R. Heere
PII:
S2214-5095(19)30390-0
DOI:
https://doi.org/10.1016/j.cscm.2019.e00288
Reference:
CSCM 288
To appear in:
Case Studies in Construction Materials
Received Date:
18 July 2019
Revised Date:
3 October 2019
Accepted Date:
8 October 2019
Please cite this article as: Talukdar S, Heere R, The Effects of Pumping on the Air Content and Void Structure of Air-Entrained, Wet Mix Fibre Reinforced Shotcrete, Case Studies in Construction Materials (2019), doi: https://doi.org/10.1016/j.cscm.2019.e00288
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
The Effects of Pumping on the Air Content and Void Structure of Air-Entrained, Wet Mix Fibre Reinforced Shotcrete
S. Talukdar1 and R. Heere2
1. PhD, PEng, Instructor, British Columbia Institute of Technology, Burnaby, BC, Canada
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of
2. MASc, PEng, Senior Engineer, Metro Testing Laboratories, Burnaby, BC, Canada
Abstract
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Keywords
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The air content and void structure of hardened shotcrete is essential for its durability and resistance against freeze thaw damage. However, producing a mix which is pumpable, shootable, and also meets durability requirements is not always easy. The situation is further complicated when adding fibres to the mix. This paper presents some observations concerning air content and air void systems of fresh and hardened fibre reinforced shotcrete mixes used for a ground support project in Western Canada. The most critical findings were that altering the time the mix spends in the line and presumably while under pressure may affect the overall air content and the air void structure of the hardened material, and that the pumping process itself has an adverse effect on the total air content and spacing factor in the hardened material.
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Shotcrete, Air Voids, Air Content, Spacing Factor, Slump, Pumping, Durability, Freeze Thaw Introduction and Background
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The (wet-mix) shotcreting process consists of pumping a plastic or flowable concrete through a conveying hose or tube, and then adding pressurized air at the nozzle to spray the concrete towards the substrate at a high velocity to achieve compaction. It is used for a variety of structural and repair applications including shoring walls, lining tunnels, or renovating existing structures (Choi et al, 2016).
Incorporating an adequate number of properly sized and distributed air voids in shotcrete is key to avoiding freeze-thaw induced cracking and scaling (Choi et al, 2016). Air is typically entrained by the addition of an air entraining admixtures before or during mixing. Such admixtures are often used in combination with water reducing and viscosity modifying admixtures to produce a mix which meets specifications for plastic air content while remaining pumpable and shootable. Proper use of airentraining admixtures ensures the development of the correct spacing, size, and amount of air voids. A well-entrained air-void system provides empty chambers within the hydrated matrices to relieve the internal hydrostatic pressure driven by the expansion of water on freezing. 1
Once concrete has set, the casts of the original air bubbles remain in the hardened concrete as voids. They are commonly referred to as the “air-void system” in hardened concrete. The major parameters of the air-void system are the total air content, average spacing factor between adjacent air voids, and specific surface (PCA, 1998).
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The common method of measuring air content in fresh concrete is via the pressure method using an air meter (ASTM C231, CSA A23.2-4C). The difficulty of obtaining the bubble size distribution in fresh concrete generally makes it more practical to measure the total air content and use it as a quality control measure on its own. However, overall freeze-thaw performance of a mix is affected by the size, spacing and distribution of air voids within a mix, rather than just by overall air content. All of the above characteristics of air-void systems are typically evaluated only when called for by a specification (which is rare) or as a result of substandard field performance. These analyses of air-void size, spacing and distribution are almost always performed on hardened concrete specimens. Furthermore, in the event that air content in fresh concrete is compared to air content in hardened concrete, differences commonly exist (PCA, 1998).
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Spacing factor and specific surface are typically determined by analyzing hardened concrete specimens using the linear traverse method (ASTM C457). The nominal distance between adjacent voids is termed the spacing factor (Powers, 1949). The spacing factor is commonly assumed to be roughly the distance water would have to travel before entering an air void, to relieve the pressure. If this distance is less than the critical maximum distance at which excessive stresses develop, the concrete would be adequately protected. Therefore, smaller spacing is better. According to CSA A23.1-14 Clause 4.3.3.4, hardened concrete is considered to have a satisfactory air-void system when the average spacing factor from all tests does not exceed 230 m, with no single result greater than 260 m, and the air content is ≥3.0%. For concrete with a water-to-cementing materials ratio of 0.36 or less, the average spacing factor shall not exceed 250 m, with no single value greater than 300 m. Specific Surface is a measure of voids surface area per unit volume of voids. Specific surface is therefore a good indicator of average void size. As average void size increases, specific surface decreases (Elkey et al, 1994).
Jo
It is generally accepted that the wet-mix shotcreting process as well as pumping concrete over longer distances correlates to a substantial loss of initial air contents (Vosahik, 2018), and a detrimental modification of the air voids system (Pleau et al, 1995) (Elkey et al, 1994). The three mechanisms that are believed to account for the diminished air content individually or, in combination with one another are suction, dissolution, and mechanical rupture when the mix impacts a surface.
The suction mechanism occurs when the concrete is subjected to lower than atmospheric pressures. In a piston-actuated pump, a pump cylinder fills up with concrete not only due to gravity alone but also by suction from the retracting piston. This suction causes the air to expand to larger bubbles and (later) escape from the concrete (Scholer and Grossman, 1998).
2
The dissolution mechanism is explained by Dyer (1991). His hypothesis is that while the concrete is pressurized, the smaller air bubbles dissolve in the surrounding water. When the concrete depressurizes upon exiting the hose, air is expelled from the water and can form bubbles again. However, such secondary air bubbles tend to be larger, and have less advantageous spacing factors.
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Yingling et al. (1992) report that the air loss mechanism in pumped concrete may largely be due to the impact of rapidly moving concrete contacting stationary objects (such as shotcrete impacting the substrate at high velocity) thereby breaking the internal air voids through mechanical action. Pleau et al (1995) concluded that impact destroys mainly larger air voids, whereas the number of smaller air voids (<300 μm in diameter) is less affected.
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Consequently, the mix designer should take into account the effects which pumping and placement will have on the air voids system of the mix after it has been discharged from the nozzle. The mix must be fluid enough to be easily pumped through a hose, but stiff enough so that it does not sag or slough after placement. A good compromise was proposed by Jolin and Beaupré (2000) who developed the ‘Temporary High Initial Air Content’ concept. This concept is a clever and simple system by which the workability of the fresh concrete is increased to meet the pumpability requirement by introducing a large amount of entrained air bubbles into the mix, recognizing that a significant amount of air will be lost between discharge and placement. Therefore, it is common to batch the mix with a high initial (asbatched) air content (typically between 8 and 20%) recognizing that this high air content will reduce to 3 to 6% in the in-place shotcrete. Hence, the high initial air content will not detrimentally affect the compressive strength. More details on this concept can be found in Jolin and Beaupre (2000) and Zhang (2012).
Jo
The Temporary High Initial Air Concept works well to meet requirements for pumpability and shootability of the mix. In addition, the measured ‘as-shot’ air content typically meets specifications when measured using a regular air meter. In fact, it is common practice for practitioners to test the air content of a shotcrete mix at the truck once, and then after discharge from the nozzle to obtain a value for the as-shot air content. The difference is then used to correlate and establish an approximate value for air loss in conveying and placement process, such that subsequent batches need only be tested at the truck (Zhang, 2012).
This remainder of this paper presents some observations concerning air content and air void systems of fresh and hardened fibre reinforced shotcrete mixes used for a ground support project in Western Canada. In particular, the effects of pumping air-entrained wet mix shotcrete on its air content and air void system are discussed, along with the possible ramifications of using different pump rates.
3
4.0
Equipment, Materials, and Procedure
The fibre reinforced wet-mix shotcrete was similar to materials used for some of the ground support work in Western Canada. Two different placement machines were used, here referred to as Sprayer 1 and Sprayer 2. Technical specifications for each sprayer are presented in Table 1.
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The pump lines’ internal diameters were 75 mm on both machines. The shotcrete mix design is provided in Table 2:
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To facilitate pumping and spraying with accelerator addition at the nozzle, the typical as-batched slump was approximately 200 mm. In order to evaluate how the air contents varied during placement, they were measured in four steps: A. Air content was first measured using an air meter immediately upon discharge from the truck.
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B. Material directly after discharge from the truck was used to cast cylinders for further air voids analysis and compressive strength testing in the laboratory.
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C. A shotcrete sample was sprayed directly into the pot of the air meter and tested (as-shot).
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D. A sample of the material which had just been sprayed onto the substrate was scraped off and consolidated into cylinder moulds for further air voids analysis (as per ASTM C457) and compressive strength testing (as per CSA A23.2-14C / ASTM C39) in the laboratory.
This sampling procedure was conducted at three different discharge rates of 9, 11 and 17 m3/h. Note that no accelerator was added to the mix for any of the tests.
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In addition, one batch of fresh shotcrete, which was placed at 17 m3/h, was repeatedly tested as above at 3 ages. The last 2 tests were performed on the material after it was retempered with high range water reducing admixture to study the effect of retempering on slump, air content and the air void structure.
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Results and Discussion:
4
5.1
Pump Rates
The measured air contents, spacing factors, specific surfaces and compressive strengths at different pump rates are presented in Table 3:
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We clearly see a decrease in air content from the as-batched to as-shot state. The decrease was measured both in the air meter readings on site, and in the air void analysis conducted in the laboratory. Furthermore, as discussed in PCA (1998), when the air content of fresh concrete is compared to the air content in hardened concrete, it is common to find a discrepancy. Therefore, these results are quite plausible.
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In all cases, the Air Void Analysis indicated that the Spacing Factor for the hardened as–shot samples exceeded the acceptable maximum limit of 230 m. The larger spacing factors indicate that although the air content meets prescribed proportions, the distance between adjacent bubbles is greater than specified and adequate protection against freeze-thaw damage may not be provided (Pigeon et al, 1995). This finding is also supported by a recent study by Yun et al, 2019. They investigated the effects of spraying on the air content of wet mix shotcrete, and also concluded that the pumping and shooting processes substantially affected the air-void characteristics such as air content, spacing factor and specific surface area of air entrained into wet mix shotcrete. Furthermore they indicated that it would be difficult for the wet-mix shotcrete mixures they studied to meet the freeze-thaw criteria for conventional cast-in-place concrete.
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Reviewing overall changes in air content indicates that as the pump rate increases, loss of air content seems to reduce. In other words, higher pump rates appear to result in less reduction of air content than lower pump rates. This trend was noticed both for fresh concrete and for hardened specimens (Figure 1).
Jo
Overall, analysis of the results lends credence to a combination of suction, dissolution and impact affecting the air content and voids structure of the shotcrete. In particular, the dissolution mechanism would explain the change in the air void structure and the spacing factor. Boulet et al (1997) reported that the dissolution mechanism affects the smaller entrained air bubbles rather than the larger ones. Their paper stated that when concrete was exposed to a 5.5 MPa pressure, the number of voids decreased by two orders of magnitude, and all of the voids less than 100 m disappeared. In their study, Pleau et al (1995) also observed that the loss of air due to dissolution correlates with a very significant decrease of the number of small air voids (< 100 μm in diameter). Vosahik et al (2018) have also hypothesized that the dissolution mechanism may be more pronounced in ‘higher flow’ mixes as they are being pumped. Due to the lower viscosity of such mixes, they will shear more readily, causing 5
greater disturbance to air voids. In this research project, the initial slump of the trial batches was around 200 mm, meaning they were ‘higher flow’ mixes.
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Pleau et al (1995) state that the detrimental influence of pumping due to the dissolution of small air voids is also a function of the length of time during which this pressure is applied. At higher pump rates, for a constant line geometry, shotcrete would be in the line for a shorter amount of time. Therefore, it is suggested that at higher pumping rates, for probably only slightly increased pressure, the dissolution of air content (and loss of small air voids) in the shotcrete may be reduced, as it is subjected to such pressure for a shorter time. Thereafter, if the fraction of small air voids in the mix exiting the nozzle increases, the amount of total air loss would decrease, as the impact mechanism has less effect on small air voids then on larger ones. This theory logically describes the trend noticed in Figure 1.
Retempering
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After shotcrete exits the nozzle, larger air bubbles may be able to consolidate and reform. However, at that time the air void structure is already damaged due to the dissolution of the smaller air bubbles, increasing the spacing factor. Elkey et al (1994) also concluded that the duration and magnitude of the static pressure on fresh concrete contributed to the shift in the air void distribution and worsening of the associated parameters. Therefore, regardless of the pump rate the air void structure and resistance to freeze thaw damage may still be adversely affected.
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For the samples of shotcrete that were pumped at 17 m3/h, the effects of changing the slump by retempering the mix with high range water reducing admixture are presented in Table 4 and Figure 2:
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Overall, in this study, it appears that retempering the mix does not significantly affect the overall plastic air contents of the mix when measured as discharged from the mixer truck, and as-shot. Retempering also does not appear to significantly affect the air contents in hardened concrete samples. As expected, the overall air content of the concrete decreased when it was pumped and shot. Although samples were not cast for the as-shot material, it was confirmed that the spacing factor for the as-batched samples met maximum allowable limits.
Zhang (2012), however sounds a note of caution. Certain brands of high range water reducing admixtures can increase the air content. Therefore, while there appeared to be a minimal effect on the overall air content of the mix using a particular high-range water reducing admixture, the findings may not be the same for other types of such admixtures. Other studies have also found a relation between air content and slump. Du and Folliard (2005) noted an increase in slump from 75 to 150 mm (with all other mixture parameters remaining the same) will increase the air content; however, above a slump of 150 mm, the large air bubbles become less stable due to buoyancy forces and the air content drops. 6
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Conclusions Although the ‘temporary high initial air’ concept works well to pump and place a mix which meets specifications for air content, the process of pumping the material appears to adversely affect the air void structure, and increase the spacing factor to a potentially unacceptably high level which may affect its durability and resistance against freeze thaw damage.
2.
It is postulated that while pumping the material will adversely affect the air void structure regardless of the pump rate, the overall change in the structure may be decreased by reducing the retention time of fresh concrete in a pressurized system.
3.
The common practice of testing the air content of a shotcrete mix at the mixer truck once (as batched), and again at the nozzle (as-shot) to establish an approximate ratio for air loss in the mix due to pumping and shooting appears to be acceptable. However, it should only be done if the pump rate of the placing machine, line length, line geometry and all other relevant variables remain relatively constant. Altering any of these variables may affect the overall air content and the air void structure of the as-shot material.
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Declaration of interests
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☒ 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.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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References Boulet, D., “Influence du pompage sur les caractéristiques du réseau de bulles d'air du béton”. Masters Thesis, Laval University, 1997 (In French) Choi, P., Yeon, J.H., Yun, K., “Air-void structure, strength, and permeability of wet-mix shotcrete before and after shotcreting operation: The influences of silica fume and air-entraining agent”. Cement and Concrete Composites (July):69-77, 2016. Du, L. Folliard, K.J., “Mechanisms of Air Entrainment in concrete”. Cement and Concrete Research, 35:1463-1471, 2005.
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Dyer, R.M., An investigation of Concrete Pumping Pressure and the Effects on the Air Void system of Concrete. Masters Thesis, Department of Civil Engineering, University of Washington, 1991.
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Elkey, W., Janssen, D.J., Hover, K.C., “Concrete Pumping Effects on Entrained Air Voids”. Research Project T9233, Washington State Department of Transportation (1994).
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Jolin, M., and Beaupré, D., “Temporary High Initial Air Content Wet Process Shotcrete,” Shotcrete, 2(1): 22-23, 2000. Pigeon M., Marchand J., Pleau R. “Frost resistant concrete” Construction and Building Materials 10(5):339–348 (1995).
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Pleau R., Pigeon M., Lamontagne A., Lessard M., “Influence of pumping on characteristics of air-void system of high-performance concrete”. Transportation Research Record, 1478:30–36, 1995.
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Portland Cement Association, “Control of Air Content in Concrete”. Concrete Technology Today, 19(1):13, 1998.
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Powers, T.C., with discussion by Willis, T.F., “The Air Requirement of Frost Resistant Concrete,” Proceedings, Highway Research Board, 29(184-211), 1949. Scholer, J.G, Grossman, C., “Controlling air content in concrete that is being pumped, a synthesis study”. Indiana Department of Transportation, August, 1998. Vosahik, J., Riding, K.A., Feys, D., Lindquist, W., Keller, L., Van Zetten, S., Schulz, B., “Concrete Pumping and its effect on the air void system”. Materials and Structures, 2018 51:94. Yingling, J., Mullings, G.M., Gaynor, R.D., “Loss of Air Content in Pumped Concrete”, 14(10):57-61, 1992.
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Yun, K.K., Choi, P., Yeon, J.H., “Microscopic investigations on the air-void characteristics of wet-mix shotcrete”. Journal of Materials Research and Technology. In Press (2019). Zhang, L., “Air Content in Shotcrete: As-Shot Versus As-Batched”. Shotcrete, (Winter):50-54, 2012.
8
Change in Air Content vs Pump Rate Pump Rate (m3/h) 0
5
10
15
20
-2 -4 -6
Air Meter
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Lab Analysis
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Change in Air Content (%)
0
-14
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-16
Air Content vs Slump
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25 20
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15 10 5 0 140
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Air Content (%)
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Figure 1: Change in Air Content vs Pump Rate
160
180
200
220
Slump (mm)
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Figure 2: Air Content vs Slump
9
Air Meter (Truck) Air Meter (As Shot) Air Void Analysis (Truck)
Table 1: Equipment Details
Max. Vertical Reach (m)
Max Lateral Reach (m)
Max Pump Output (m3/h)
Sprayer 1 Sprayer 2
17 10
15 8
30 20
Max Pump Pressure on Concrete (MPa) 7.5 6.5
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Equipment
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Table 2: Shotcrete Mix Design (/m3)
0.3 0.8 2.7
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Air Entraining Admixture (Surfactant) Retarding Admixture (Hydration Controlling) High Range Water Reducing Admixture (Polycarboxylate)
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Mass (kg) 425 490 1115 23 166 5
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Material Cement Coarse Aggregate Fine Aggregate Silica Fume Water Macrosynthetic Fibre
Table 3: Measured Air Contents and Spacing Factors
Hardened Air Void Analysis as batched (%) 20.3
Hardened Air Void Analysis As Shot (%) -
Spacing Specific Compressive Factor Surface Strength (mm) (mm-1) (MPa)
19.0
Air Meter Reading As Shot (%) -
0.15
8
37.9
-
4.4
-
6.9
0.25
16
56.9
-
4.0
-
6.4
0.32
14
60.7
Pump Rate
Air Meter Reading as batched (%) 18.0
Hardened Air Void Analysis as batched (%) 19.4
Hardened Air Void Analysis As Shot (%) -
Spacing Specific Compressive Factor Surface Strength (mm) (mm-1) (MPa)
Before pumping 17 m3/h
Air Meter Reading As Shot (%) -
-
4.1
-
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Sprayer 1 Pump Rate
Air Meter Reading as batched (%)
Before pumping 11 m3/h 3
9 m /h
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8.1
12
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Sprayer 2
0.08
0.26
19
48.8
14
57.1
Table 4: Air Content and Spacing Factors of Retempered Mix
Air Meter Reading, as batched (%)
150 190 215
17.0 18.0 19.0
Air Meter Reading as shot (%) 5.4 4.1 5.4
Hardened Air Void Analysis as batched (%) 18.2 19.4 18.7
Spacing Factor as batched (mm) 0.1 0.08 0.09
Specific Compressive Surface as Strength as batched (mm-1) batched (MPa) 15 45.3 19 48.8 15 45.9
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Slump as batched (mm)
13
PII:
S2214-5095(19)30390-0
DOI:
https://doi.org/10.1016/j.cscm.2019.e00288
Reference:
CSCM 288
To appear in:
Case Studies in Construction Materials
Received Date:
18 July 2019
Revised Date:
3 October 2019
Accepted Date:
8 October 2019
Please cite this article as: Talukdar S, Heere R, The Effects of Pumping on the Air Content and Void Structure of Air-Entrained, Wet Mix Fibre Reinforced Shotcrete, Case Studies in Construction Materials (2019), doi: https://doi.org/10.1016/j.cscm.2019.e00288
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
The Effects of Pumping on the Air Content and Void Structure of Air-Entrained, Wet Mix Fibre Reinforced Shotcrete
S. Talukdar1 and R. Heere2
1. PhD, PEng, Instructor, British Columbia Institute of Technology, Burnaby, BC, Canada
1.0
of
2. MASc, PEng, Senior Engineer, Metro Testing Laboratories, Burnaby, BC, Canada
Abstract
2.0
Keywords
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The air content and void structure of hardened shotcrete is essential for its durability and resistance against freeze thaw damage. However, producing a mix which is pumpable, shootable, and also meets durability requirements is not always easy. The situation is further complicated when adding fibres to the mix. This paper presents some observations concerning air content and air void systems of fresh and hardened fibre reinforced shotcrete mixes used for a ground support project in Western Canada. The most critical findings were that altering the time the mix spends in the line and presumably while under pressure may affect the overall air content and the air void structure of the hardened material, and that the pumping process itself has an adverse effect on the total air content and spacing factor in the hardened material.
3.0
ur na
Shotcrete, Air Voids, Air Content, Spacing Factor, Slump, Pumping, Durability, Freeze Thaw Introduction and Background
Jo
The (wet-mix) shotcreting process consists of pumping a plastic or flowable concrete through a conveying hose or tube, and then adding pressurized air at the nozzle to spray the concrete towards the substrate at a high velocity to achieve compaction. It is used for a variety of structural and repair applications including shoring walls, lining tunnels, or renovating existing structures (Choi et al, 2016).
Incorporating an adequate number of properly sized and distributed air voids in shotcrete is key to avoiding freeze-thaw induced cracking and scaling (Choi et al, 2016). Air is typically entrained by the addition of an air entraining admixtures before or during mixing. Such admixtures are often used in combination with water reducing and viscosity modifying admixtures to produce a mix which meets specifications for plastic air content while remaining pumpable and shootable. Proper use of airentraining admixtures ensures the development of the correct spacing, size, and amount of air voids. A well-entrained air-void system provides empty chambers within the hydrated matrices to relieve the internal hydrostatic pressure driven by the expansion of water on freezing. 1
Once concrete has set, the casts of the original air bubbles remain in the hardened concrete as voids. They are commonly referred to as the “air-void system” in hardened concrete. The major parameters of the air-void system are the total air content, average spacing factor between adjacent air voids, and specific surface (PCA, 1998).
-p
ro
of
The common method of measuring air content in fresh concrete is via the pressure method using an air meter (ASTM C231, CSA A23.2-4C). The difficulty of obtaining the bubble size distribution in fresh concrete generally makes it more practical to measure the total air content and use it as a quality control measure on its own. However, overall freeze-thaw performance of a mix is affected by the size, spacing and distribution of air voids within a mix, rather than just by overall air content. All of the above characteristics of air-void systems are typically evaluated only when called for by a specification (which is rare) or as a result of substandard field performance. These analyses of air-void size, spacing and distribution are almost always performed on hardened concrete specimens. Furthermore, in the event that air content in fresh concrete is compared to air content in hardened concrete, differences commonly exist (PCA, 1998).
ur na
lP
re
Spacing factor and specific surface are typically determined by analyzing hardened concrete specimens using the linear traverse method (ASTM C457). The nominal distance between adjacent voids is termed the spacing factor (Powers, 1949). The spacing factor is commonly assumed to be roughly the distance water would have to travel before entering an air void, to relieve the pressure. If this distance is less than the critical maximum distance at which excessive stresses develop, the concrete would be adequately protected. Therefore, smaller spacing is better. According to CSA A23.1-14 Clause 4.3.3.4, hardened concrete is considered to have a satisfactory air-void system when the average spacing factor from all tests does not exceed 230 m, with no single result greater than 260 m, and the air content is ≥3.0%. For concrete with a water-to-cementing materials ratio of 0.36 or less, the average spacing factor shall not exceed 250 m, with no single value greater than 300 m. Specific Surface is a measure of voids surface area per unit volume of voids. Specific surface is therefore a good indicator of average void size. As average void size increases, specific surface decreases (Elkey et al, 1994).
Jo
It is generally accepted that the wet-mix shotcreting process as well as pumping concrete over longer distances correlates to a substantial loss of initial air contents (Vosahik, 2018), and a detrimental modification of the air voids system (Pleau et al, 1995) (Elkey et al, 1994). The three mechanisms that are believed to account for the diminished air content individually or, in combination with one another are suction, dissolution, and mechanical rupture when the mix impacts a surface.
The suction mechanism occurs when the concrete is subjected to lower than atmospheric pressures. In a piston-actuated pump, a pump cylinder fills up with concrete not only due to gravity alone but also by suction from the retracting piston. This suction causes the air to expand to larger bubbles and (later) escape from the concrete (Scholer and Grossman, 1998).
2
The dissolution mechanism is explained by Dyer (1991). His hypothesis is that while the concrete is pressurized, the smaller air bubbles dissolve in the surrounding water. When the concrete depressurizes upon exiting the hose, air is expelled from the water and can form bubbles again. However, such secondary air bubbles tend to be larger, and have less advantageous spacing factors.
ro
of
Yingling et al. (1992) report that the air loss mechanism in pumped concrete may largely be due to the impact of rapidly moving concrete contacting stationary objects (such as shotcrete impacting the substrate at high velocity) thereby breaking the internal air voids through mechanical action. Pleau et al (1995) concluded that impact destroys mainly larger air voids, whereas the number of smaller air voids (<300 μm in diameter) is less affected.
ur na
lP
re
-p
Consequently, the mix designer should take into account the effects which pumping and placement will have on the air voids system of the mix after it has been discharged from the nozzle. The mix must be fluid enough to be easily pumped through a hose, but stiff enough so that it does not sag or slough after placement. A good compromise was proposed by Jolin and Beaupré (2000) who developed the ‘Temporary High Initial Air Content’ concept. This concept is a clever and simple system by which the workability of the fresh concrete is increased to meet the pumpability requirement by introducing a large amount of entrained air bubbles into the mix, recognizing that a significant amount of air will be lost between discharge and placement. Therefore, it is common to batch the mix with a high initial (asbatched) air content (typically between 8 and 20%) recognizing that this high air content will reduce to 3 to 6% in the in-place shotcrete. Hence, the high initial air content will not detrimentally affect the compressive strength. More details on this concept can be found in Jolin and Beaupre (2000) and Zhang (2012).
Jo
The Temporary High Initial Air Concept works well to meet requirements for pumpability and shootability of the mix. In addition, the measured ‘as-shot’ air content typically meets specifications when measured using a regular air meter. In fact, it is common practice for practitioners to test the air content of a shotcrete mix at the truck once, and then after discharge from the nozzle to obtain a value for the as-shot air content. The difference is then used to correlate and establish an approximate value for air loss in conveying and placement process, such that subsequent batches need only be tested at the truck (Zhang, 2012).
This remainder of this paper presents some observations concerning air content and air void systems of fresh and hardened fibre reinforced shotcrete mixes used for a ground support project in Western Canada. In particular, the effects of pumping air-entrained wet mix shotcrete on its air content and air void system are discussed, along with the possible ramifications of using different pump rates.
3
4.0
Equipment, Materials, and Procedure
The fibre reinforced wet-mix shotcrete was similar to materials used for some of the ground support work in Western Canada. Two different placement machines were used, here referred to as Sprayer 1 and Sprayer 2. Technical specifications for each sprayer are presented in Table 1.
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The pump lines’ internal diameters were 75 mm on both machines. The shotcrete mix design is provided in Table 2:
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To facilitate pumping and spraying with accelerator addition at the nozzle, the typical as-batched slump was approximately 200 mm. In order to evaluate how the air contents varied during placement, they were measured in four steps: A. Air content was first measured using an air meter immediately upon discharge from the truck.
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B. Material directly after discharge from the truck was used to cast cylinders for further air voids analysis and compressive strength testing in the laboratory.
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C. A shotcrete sample was sprayed directly into the pot of the air meter and tested (as-shot).
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D. A sample of the material which had just been sprayed onto the substrate was scraped off and consolidated into cylinder moulds for further air voids analysis (as per ASTM C457) and compressive strength testing (as per CSA A23.2-14C / ASTM C39) in the laboratory.
This sampling procedure was conducted at three different discharge rates of 9, 11 and 17 m3/h. Note that no accelerator was added to the mix for any of the tests.
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In addition, one batch of fresh shotcrete, which was placed at 17 m3/h, was repeatedly tested as above at 3 ages. The last 2 tests were performed on the material after it was retempered with high range water reducing admixture to study the effect of retempering on slump, air content and the air void structure.
5.0
Results and Discussion:
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5.1
Pump Rates
The measured air contents, spacing factors, specific surfaces and compressive strengths at different pump rates are presented in Table 3:
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We clearly see a decrease in air content from the as-batched to as-shot state. The decrease was measured both in the air meter readings on site, and in the air void analysis conducted in the laboratory. Furthermore, as discussed in PCA (1998), when the air content of fresh concrete is compared to the air content in hardened concrete, it is common to find a discrepancy. Therefore, these results are quite plausible.
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In all cases, the Air Void Analysis indicated that the Spacing Factor for the hardened as–shot samples exceeded the acceptable maximum limit of 230 m. The larger spacing factors indicate that although the air content meets prescribed proportions, the distance between adjacent bubbles is greater than specified and adequate protection against freeze-thaw damage may not be provided (Pigeon et al, 1995). This finding is also supported by a recent study by Yun et al, 2019. They investigated the effects of spraying on the air content of wet mix shotcrete, and also concluded that the pumping and shooting processes substantially affected the air-void characteristics such as air content, spacing factor and specific surface area of air entrained into wet mix shotcrete. Furthermore they indicated that it would be difficult for the wet-mix shotcrete mixures they studied to meet the freeze-thaw criteria for conventional cast-in-place concrete.
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Reviewing overall changes in air content indicates that as the pump rate increases, loss of air content seems to reduce. In other words, higher pump rates appear to result in less reduction of air content than lower pump rates. This trend was noticed both for fresh concrete and for hardened specimens (Figure 1).
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Overall, analysis of the results lends credence to a combination of suction, dissolution and impact affecting the air content and voids structure of the shotcrete. In particular, the dissolution mechanism would explain the change in the air void structure and the spacing factor. Boulet et al (1997) reported that the dissolution mechanism affects the smaller entrained air bubbles rather than the larger ones. Their paper stated that when concrete was exposed to a 5.5 MPa pressure, the number of voids decreased by two orders of magnitude, and all of the voids less than 100 m disappeared. In their study, Pleau et al (1995) also observed that the loss of air due to dissolution correlates with a very significant decrease of the number of small air voids (< 100 μm in diameter). Vosahik et al (2018) have also hypothesized that the dissolution mechanism may be more pronounced in ‘higher flow’ mixes as they are being pumped. Due to the lower viscosity of such mixes, they will shear more readily, causing 5
greater disturbance to air voids. In this research project, the initial slump of the trial batches was around 200 mm, meaning they were ‘higher flow’ mixes.
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Pleau et al (1995) state that the detrimental influence of pumping due to the dissolution of small air voids is also a function of the length of time during which this pressure is applied. At higher pump rates, for a constant line geometry, shotcrete would be in the line for a shorter amount of time. Therefore, it is suggested that at higher pumping rates, for probably only slightly increased pressure, the dissolution of air content (and loss of small air voids) in the shotcrete may be reduced, as it is subjected to such pressure for a shorter time. Thereafter, if the fraction of small air voids in the mix exiting the nozzle increases, the amount of total air loss would decrease, as the impact mechanism has less effect on small air voids then on larger ones. This theory logically describes the trend noticed in Figure 1.
Retempering
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5.2
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After shotcrete exits the nozzle, larger air bubbles may be able to consolidate and reform. However, at that time the air void structure is already damaged due to the dissolution of the smaller air bubbles, increasing the spacing factor. Elkey et al (1994) also concluded that the duration and magnitude of the static pressure on fresh concrete contributed to the shift in the air void distribution and worsening of the associated parameters. Therefore, regardless of the pump rate the air void structure and resistance to freeze thaw damage may still be adversely affected.
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For the samples of shotcrete that were pumped at 17 m3/h, the effects of changing the slump by retempering the mix with high range water reducing admixture are presented in Table 4 and Figure 2:
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Overall, in this study, it appears that retempering the mix does not significantly affect the overall plastic air contents of the mix when measured as discharged from the mixer truck, and as-shot. Retempering also does not appear to significantly affect the air contents in hardened concrete samples. As expected, the overall air content of the concrete decreased when it was pumped and shot. Although samples were not cast for the as-shot material, it was confirmed that the spacing factor for the as-batched samples met maximum allowable limits.
Zhang (2012), however sounds a note of caution. Certain brands of high range water reducing admixtures can increase the air content. Therefore, while there appeared to be a minimal effect on the overall air content of the mix using a particular high-range water reducing admixture, the findings may not be the same for other types of such admixtures. Other studies have also found a relation between air content and slump. Du and Folliard (2005) noted an increase in slump from 75 to 150 mm (with all other mixture parameters remaining the same) will increase the air content; however, above a slump of 150 mm, the large air bubbles become less stable due to buoyancy forces and the air content drops. 6
6.0
Conclusions Although the ‘temporary high initial air’ concept works well to pump and place a mix which meets specifications for air content, the process of pumping the material appears to adversely affect the air void structure, and increase the spacing factor to a potentially unacceptably high level which may affect its durability and resistance against freeze thaw damage.
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It is postulated that while pumping the material will adversely affect the air void structure regardless of the pump rate, the overall change in the structure may be decreased by reducing the retention time of fresh concrete in a pressurized system.
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The common practice of testing the air content of a shotcrete mix at the mixer truck once (as batched), and again at the nozzle (as-shot) to establish an approximate ratio for air loss in the mix due to pumping and shooting appears to be acceptable. However, it should only be done if the pump rate of the placing machine, line length, line geometry and all other relevant variables remain relatively constant. Altering any of these variables may affect the overall air content and the air void structure of the as-shot material.
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Declaration of interests
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☒ 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.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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References Boulet, D., “Influence du pompage sur les caractéristiques du réseau de bulles d'air du béton”. Masters Thesis, Laval University, 1997 (In French) Choi, P., Yeon, J.H., Yun, K., “Air-void structure, strength, and permeability of wet-mix shotcrete before and after shotcreting operation: The influences of silica fume and air-entraining agent”. Cement and Concrete Composites (July):69-77, 2016. Du, L. Folliard, K.J., “Mechanisms of Air Entrainment in concrete”. Cement and Concrete Research, 35:1463-1471, 2005.
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Dyer, R.M., An investigation of Concrete Pumping Pressure and the Effects on the Air Void system of Concrete. Masters Thesis, Department of Civil Engineering, University of Washington, 1991.
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Elkey, W., Janssen, D.J., Hover, K.C., “Concrete Pumping Effects on Entrained Air Voids”. Research Project T9233, Washington State Department of Transportation (1994).
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Jolin, M., and Beaupré, D., “Temporary High Initial Air Content Wet Process Shotcrete,” Shotcrete, 2(1): 22-23, 2000. Pigeon M., Marchand J., Pleau R. “Frost resistant concrete” Construction and Building Materials 10(5):339–348 (1995).
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Pleau R., Pigeon M., Lamontagne A., Lessard M., “Influence of pumping on characteristics of air-void system of high-performance concrete”. Transportation Research Record, 1478:30–36, 1995.
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Portland Cement Association, “Control of Air Content in Concrete”. Concrete Technology Today, 19(1):13, 1998.
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Powers, T.C., with discussion by Willis, T.F., “The Air Requirement of Frost Resistant Concrete,” Proceedings, Highway Research Board, 29(184-211), 1949. Scholer, J.G, Grossman, C., “Controlling air content in concrete that is being pumped, a synthesis study”. Indiana Department of Transportation, August, 1998. Vosahik, J., Riding, K.A., Feys, D., Lindquist, W., Keller, L., Van Zetten, S., Schulz, B., “Concrete Pumping and its effect on the air void system”. Materials and Structures, 2018 51:94. Yingling, J., Mullings, G.M., Gaynor, R.D., “Loss of Air Content in Pumped Concrete”, 14(10):57-61, 1992.
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Yun, K.K., Choi, P., Yeon, J.H., “Microscopic investigations on the air-void characteristics of wet-mix shotcrete”. Journal of Materials Research and Technology. In Press (2019). Zhang, L., “Air Content in Shotcrete: As-Shot Versus As-Batched”. Shotcrete, (Winter):50-54, 2012.
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Change in Air Content vs Pump Rate Pump Rate (m3/h) 0
5
10
15
20
-2 -4 -6
Air Meter
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Lab Analysis
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Change in Air Content (%)
0
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Air Content vs Slump
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15 10 5 0 140
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Air Content (%)
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Figure 1: Change in Air Content vs Pump Rate
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180
200
220
Slump (mm)
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Figure 2: Air Content vs Slump
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Air Meter (Truck) Air Meter (As Shot) Air Void Analysis (Truck)
Table 1: Equipment Details
Max. Vertical Reach (m)
Max Lateral Reach (m)
Max Pump Output (m3/h)
Sprayer 1 Sprayer 2
17 10
15 8
30 20
Max Pump Pressure on Concrete (MPa) 7.5 6.5
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Equipment
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Table 2: Shotcrete Mix Design (/m3)
0.3 0.8 2.7
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Air Entraining Admixture (Surfactant) Retarding Admixture (Hydration Controlling) High Range Water Reducing Admixture (Polycarboxylate)
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Mass (kg) 425 490 1115 23 166 5
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Material Cement Coarse Aggregate Fine Aggregate Silica Fume Water Macrosynthetic Fibre
Table 3: Measured Air Contents and Spacing Factors
Hardened Air Void Analysis as batched (%) 20.3
Hardened Air Void Analysis As Shot (%) -
Spacing Specific Compressive Factor Surface Strength (mm) (mm-1) (MPa)
19.0
Air Meter Reading As Shot (%) -
0.15
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37.9
-
4.4
-
6.9
0.25
16
56.9
-
4.0
-
6.4
0.32
14
60.7
Pump Rate
Air Meter Reading as batched (%) 18.0
Hardened Air Void Analysis as batched (%) 19.4
Hardened Air Void Analysis As Shot (%) -
Spacing Specific Compressive Factor Surface Strength (mm) (mm-1) (MPa)
Before pumping 17 m3/h
Air Meter Reading As Shot (%) -
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4.1
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Sprayer 1 Pump Rate
Air Meter Reading as batched (%)
Before pumping 11 m3/h 3
9 m /h
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Sprayer 2
0.08
0.26
19
48.8
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57.1
Table 4: Air Content and Spacing Factors of Retempered Mix
Air Meter Reading, as batched (%)
150 190 215
17.0 18.0 19.0
Air Meter Reading as shot (%) 5.4 4.1 5.4
Hardened Air Void Analysis as batched (%) 18.2 19.4 18.7
Spacing Factor as batched (mm) 0.1 0.08 0.09
Specific Compressive Surface as Strength as batched (mm-1) batched (MPa) 15 45.3 19 48.8 15 45.9
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Slump as batched (mm)
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