1. INTRODUCTION
⌅To
ensure the safety, long-term performance, and durability of concrete
structures in an accelerated construction schedule, an understanding of
concrete behavior and an evaluation of its mechanical properties at
early ages plays an important role in the construction quality control
and planning process. Concrete’s compressive strength and dynamic
modulus at an early age dramatically impact its long-term efficiency,
durability, and properties. Early age of concrete is typically defined
as the first few hours or days after casting concrete, which is marked
by the setting and hardening processes as hydration occurs. During this
time, the fluid phase of fresh concrete transitions into the hardened
state, resulting in the development of mechanical properties, heat
release, and deformations due to the success of the hydration reactions.
The mechanical properties of early-age concrete develop at different
rates, depending on mixture proportions, including the fiber content,
water-to-cement (w/c) ratio, age, and curing conditions (11.
Nehdi, M.L.; Soliman, A.M. (2011) Early-age properties of concrete:
Overview of fundamental concepts and state-of-the art research. Constr. Mater. 164 [2], 57-77. https://doi.org/10.1680/coma.900040.
, 22. Pane, I.; Hansen, W. (2002) Early-age creep and stress relaxation of concrete containing blended cements. Mater. Struc. 35, 92. https://doi.org/10.1007/BF02482107.
).
For example, while the gain in concrete compressive strength is rapid
at an early age, with approximately 65-75 percent of the compressive
strength reached in the first 7 days, the gain in concrete elastic
modulus is extremely rapid at an early age, with approximately 90
percent of the elastic modulus reached in the first 24 hours. This
rapid, early gain in strength and dynamic modulus is directly linked to
the increase of the gel/space ratio of calcium silicate hydrate, and it
can be affected by the change in mix proportions, such as the addition
of fiber reinforcement (33. Neville, A.M. (2004) Properties of Concrete, 4th edition. Wiley Harlow, New York, USA, (2004).
).
Concrete
is a widely used construction material due to its high compressive
strength, although it has a relatively low tensile strength when
compared to other construction materials. Therefore, concrete is often
reinforced with structural fibers to enhance its mechanical and physical
properties. Fiber-reinforced concrete (FRC) is classified into steel
fiber-reinforced concrete (SFRC), glass fiber-reinforced concrete
(GFRC), synthetic fiber-reinforced concrete, and natural
fiber-reinforced concrete (44.
American Society for Testing Materials (2015) Standard specification for
fiber-reinforced concrete. ASTM C1116. 100 Barr Harbor Drive, PO Box
C700, West Conshohocken, PA 19428-2959, USA, (2015). https://doi.org/10.1520/C1116_C1116M-10AR15.
).
Traditionally, to ensure the quality, durability, and safety of
concrete, the compressive strength of cylindrical concrete specimens is
determined experimentally using a destructive test (55.
American Society for Testing Materials (2012) Standard test method for
compressive strength of cylindrical concrete specimens. ASTM C39. 100
Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, USA,
(2012). https://doi.org/10.1520/C0039_C0039M-12.
), and the static modulus of elasticity of concrete is calculated using a similar destructive test (66.
American Society for Testing Materials (2010) Standard test method for
static modulus of elasticity and Poisson’s ratio of concrete in
compression. ASTM C469. 100 Barr Harbor Drive, PO Box C700, West
Conshohocken, PA 19428-2959, USA, (2010). https://doi.org/10.1520/C0469_C0469M-10.
).
However, these destructive test methods are outdated for three reasons.
First, they are time-consuming because strength requirements are
typically calculated after 28 days when most of the concrete strength is
achieved. Second, they require several samples since the test method is
destructive and compliance with the standards demands at least the
average of a few samples measured at the same age. Third, the workload
is high and expensive for sampling, curing, and transporting, and for
the testing equipment. In addition, the use of these test methods for
in-service evaluation is inefficient because cores need to be extracted
from the structure, and extracting too many cores will harm the
integrity of the structure and the data obtained are confined to the
cores’ locations.
As an alternative to destructive testing,
several studies exist in the literature containing empirical equations
based on nondestructive test methods, such as the ultrasonic pulse
velocity (UPV), to predict the compressive strength and dynamic modulus
of concrete (7-97. Lin, Y.; Kuo, S-F.; Hsiao, C.; Lai, C-P. (2007) Investigation of pulse velocity-strength relationship of hardened concrete. ACI Mater. J. 104 [4], 344-350. https://doi.org/10.14359/18823.
8.
Mahure, N.; Vijh, G.; Sharma, P.; Sivakumar, N.; Ratnam, M. (2011)
Correlation between pulse velocity and compressive strength of concrete.
Inter. J. Ear. Sci. Eng. 4 [6], 871-874.
9. Khademi, F.;
Akbari, M.; Jamal, S.M. (2016) Prediction of concrete compressive
strength using ultrasonic pulse velocity test and artificial neural
network modeling. Roma. J. Mater. 46 [3], 343-350.
).
Most of these studies focus on predicting plain concrete’s compressive
strength and dynamic modulus based on UPV after 28 days, while
considering mixture parameters such as cement type, cement content,
aggregate size, aggregate content, and w/c ratio. Studies including the
effects of a wider mixture of parameters, such as the addition of
different types of fibers and different fiber volume fractions on UPV,
are limited and cannot be utilized for all types of FRC and at different
ages. On the other hand, it has been proven that the fibers influence
the compressive strength, the duration for the peak load, and the energy
absorption under uniaxial compressive loading at the early ages (1010. Ding, Y.; Kusterle, W. (2000) Compressive stress-strain relationship of steel fibre-reinforced concrete at early age. Cem. Concr. Res. 30 [10], 1573-1579. https://doi.org/10.1016/s0008-8846(00)00348-3.
).
Subsequently, there is a need for equations specifically proposed to
predict the early-age compressive strength and dynamic modulus of
different types of FRC based on UPV.
A number of research studies
have investigated the relationship between UPV and the compressive
strength and dynamic elastic modulus of plain concrete. However, as
discussed, better estimation of these properties in terms of the
ultrasonic pulse velocity measurement depends on numerous mixture
parameters, such as fiber type and volume fraction, w/c ratio,
temperature, coarse aggregate, shape, and cement type (77. Lin, Y.; Kuo, S-F.; Hsiao, C.; Lai, C-P. (2007) Investigation of pulse velocity-strength relationship of hardened concrete. ACI Mater. J. 104 [4], 344-350. https://doi.org/10.14359/18823.
, 88.
Mahure, N.; Vijh, G.; Sharma, P.; Sivakumar, N.; Ratnam, M. (2011)
Correlation between pulse velocity and compressive strength of concrete.
Inter. J. Ear. Sci. Eng. 4 [6], 871-874.
, 1111. Elvery, R.; Ibrahim, L. (1976) Ultrasonic assessment of concrete strength at early ages. Mag. Concr. Res. 28 [97], 181-190. https://doi.org/10.1680/macr.1976.28.97.181.
). While plain concrete was the focus of many studies (7-97. Lin, Y.; Kuo, S-F.; Hsiao, C.; Lai, C-P. (2007) Investigation of pulse velocity-strength relationship of hardened concrete. ACI Mater. J. 104 [4], 344-350. https://doi.org/10.14359/18823.
8.
Mahure, N.; Vijh, G.; Sharma, P.; Sivakumar, N.; Ratnam, M. (2011)
Correlation between pulse velocity and compressive strength of concrete.
Inter. J. Ear. Sci. Eng. 4 [6], 871-874.
9. Khademi, F.;
Akbari, M.; Jamal, S.M. (2016) Prediction of concrete compressive
strength using ultrasonic pulse velocity test and artificial neural
network modeling. Roma. J. Mater. 46 [3], 343-350.
, 12-1412.
Naik, T.; Malhotra, V.; Popovics, J. (2003) The ultrasonic pulse
velocity method. In: Handbook on nondestructive testing of concrete,
Second Edition, 8-1 to 8-19, CRC Press, (2003).
13. Nash’t, I.;
A’bour, S.; Sadoon, A. (2005) Finding an unified relationship between
crushing strength of concrete and non-destructive tests. Mid. East Nond. Test. Conf. Exhi. 27-30. Nov., Bahrain, Manama, (2005).
14. Kheder, G. (1999) A two stage procedure for assessment of in situ concrete strength using combined non-destructive testing. Mater. Struct. 32, 410. https://doi.org/10.1007/BF02482712.
),
the addition of structural fibers was either ignored or limited to a
specific type of fiber. Additionally, the relationship between the steel
fiber-reinforced concrete’s UPV and its compressive strength was
studied extensively, but fewer studies can be found for polypropylene,
nylon, and glass fiber-reinforced concrete (15-1815.
Gebretsadik, B. (2013) Ultrasonic pulse velocity investigation of steel
fiber reinforced self-compacted concrete. UNLV Theses, Dissertations,
Professional Papers, and Capstones, 1828, University of Nevada, Las
Vegas, USA. https://doi.org/10.34917/4478246.
16. Nitin; Verma, S.K. (2016) Effect on mechanical properties of concrete using nylon fibers. Inter. Res. J. Eng. Tech. 3 [7], 1751-1755.
17. Raouf, Z.; Ali, Z. (1983) Assessment of concrete characteristics at an early age by ultrasonic pulse velocity. J. Build. Res. 2 [1], 31-44.
18. Suksawang, N.; Wtaife, S.; Alsabbagh, A. (2018) Evaluation of elastic modulus of fiber-reinforced concrete. ACI Mater. J. 115 [2], 239-249. https://doi.org/10.14359/51701920 .
).
One investigation of the relationship between the compressive stress
and the strain of steel fiber-reinforced concrete at early age showed
that the fiber content and age of concrete affect the energy absorption
of FRC (1010. Ding, Y.; Kusterle, W. (2000) Compressive stress-strain relationship of steel fibre-reinforced concrete at early age. Cem. Concr. Res. 30 [10], 1573-1579. https://doi.org/10.1016/s0008-8846(00)00348-3.
).
Therefore, the effect of different types of structural fibers on the
early-age compressive strength and the modulus of elasticity of FRC at
various fiber volume fractions needs to be investigated further.
In the present study, FRC cylindrical specimens were cast, cured, and tested for compressive strength, dynamic modulus, and ultrasonic pulse velocity. The cement type and aggregate size remained constant, while the effect of change in fiber type, fiber volume fraction, water-to-cement ratio, and test age on the prediction of FRC compressive strength and dynamic modulus were investigated. Two sets of new empirical equations were proposed to predict the compressive strength and dynamic modulus of FRC based on UPV. The first set of equations predicted the compressive strength of early-age steel, polypropylene, nylon, and glass FRC. The second set of equations predicted the dynamic modulus of early-age steel, polypropylene, nylon, and glass FRC. The accuracy of these new equations was tested by measuring the coefficient of variation between the measured values and the predicted values from the proposed equations.
2. EXPERIMENTAL PROGRAM
⌅An experimental program was designed and conducted to establish a correlation between FRC’s early-age UPV and its early-age compressive strength and dynamic modulus. This program involved 189 specimens of 100 mm × 200 mm FRC cylinders with different mixture proportions. The UPV, dynamic modulus, and compressive strength were measured using an ultrasonic concrete tester, a resonance test gauge, and a compression test machine, respectively. The experimental program outline is shown in Table 1.
Portland Cement | Type I/II |
Coarse Aggregate Maximum Size | 4.7625 mm (0.1875″) |
Fiber Types | Nylon, polypropylene, steel, and glass |
Fiber Volume Fraction (%) | 0.5, 1.0, and 1.5 |
Water-to-Cement Ratio | 0.40, 0.45, and 0.50 |
Specimen Geometry | Cylinder 100 mm × 200 mm (4″ × 8″) |
Curing Time (days) | 1, 3, 7, 28 |
Destructive Test | Compression test machine |
Nondestructive Tests | Ultrasonic concrete tester, resonance test gauge |
Mechanical Properties Evaluated | Compressive strength, dynamic modulus |
2.1. Materials
⌅Portland cement type I/II was combined with gravel; sand; water; and nylon, polypropylene, steel, and glass fibers to produce four different types of FRC. The fiber properties are presented in Table 2.
).
Stainless Steel | AR Glass | Virgin Nylon | Polypropylene | |
---|---|---|---|---|
Filament Diameter, d (mm) | 1.18 | 0.014 | 0.038 | 1.52 |
Fiber Length, l (mm) | 25.4 | 13 | 19 | 19 |
Density, ρ (kg/m3) | 7800 | 2700 | 1150 | 910 |
Tensile Strength, τ (MPa) | 1030 | 2000 | 300 | 410 |
Flexural Strength, σ (GPa) | 203 | 77 | 2.8 | 5.6 |
Melting Point (°C) | 1516 | 1121 | 225 | 160 |
Water Absorption | Nil | < 1% | 3% by Weight | Nil |
Alkali Resistance | High | High | High | Excellent |
Corrosion Resistance | High | High | High | High |
2.2. Mixture proportions
⌅Reinforcing fibers are added to concrete at different dosages depending on the intended application and fiber type. Table 3 shows the fiber addition rates that manufacturers recommend, and Table 4 lists studies that examined the performance of various types of fiber-reinforced concrete at different fiber volume fractions. It can be observed from Table 3 and Table 4 that the fiber volume fraction range of 0% vol. to 1.0% vol. is applicable for all types of fibers considered in this investigation. In addition, having the same fiber volume fraction range allows for easy comparison between fibers. Therefore, the research team designed 21 mixes comprising nylon, polypropylene, steel, and glass fibers at fiber volume fractions (Vf) of 0.5% vol., 0.75% vol., and 1.0% vol., and w/c ratios of 0.40, 0.45, and 0.50. Table 5 shows the different concrete mix proportions; Vf is fiber volume fraction, W/C is water-to-cement ratio, C is cement, CA is coarse aggregate, FA is fine aggregate, and W is water.
).
Fiber Addition Rates | Nylon | Polypropylene | Steel | Glass |
---|---|---|---|---|
Plastic Shrinkage Cracking | 0.6 kg/m3 | 0.9 kg/m3 | 10-15 kg/m3 | 0.3-0.6 kg/m3 |
Structural Performance | - | - | 15-80 kg/m3 | 5-15 kg/m3 |
No. | Reference | Fiber Volume Fraction (Vf) Ranges | |||
---|---|---|---|---|---|
Steel | Glass | Nylon | Polypropylene | ||
1 | (1515.
Gebretsadik, B. (2013) Ultrasonic pulse velocity investigation of steel
fiber reinforced self-compacted concrete. UNLV Theses, Dissertations,
Professional Papers, and Capstones, 1828, University of Nevada, Las
Vegas, USA. https://doi.org/10.34917/4478246. ) | 0-2% | - | - | - |
2 | (2020. Bobde, S.P.; Gandhe, G.R.; Tupe, D.H. (2018) Performance of glass fiber reinforced concrete. Inter. J. Advan. Res. Ideas Inno. Tech. 4 [3], 984-988. ) | - | 0-5% | - | - |
3 | (2121.
Zheng, Y.; Wu, X.; He, G.; Shang, Q.; Xu, J.; Sun, Y. (2018) Mechanical
properties of steel fiber-reinforced concrete by vibratory mixing
technology. Adv. Civil Eng. 2018, 1-11. https://doi.org/10.1155/2018/9025715. ) | 0-2% | - | - | - |
4 | (2222. Ramli, M.; Hoe, K.W. (2010) Influences of short discrete fibers in high strength concrete with very coarse sand. Amer. J. Appl. Scie. 7 [12], 1572-1578. https://doi.org/10.3844/ajassp.2010.1572.1578
) | - | 0-2.4% | - | - |
5 | (1616. Nitin; Verma, S.K. (2016) Effect on mechanical properties of concrete using nylon fibers. Inter. Res. J. Eng. Tech. 3 [7], 1751-1755. ) | - | - | 0-1.5% | - |
6 | (2323. Pawade, P.; Nagarnaik, P.; Pande, A. (2011) Performance of steel fiber on standard strength concrete in compression. Inter. J. Civil Struc. Eng. 2 [2], 483-492. ) | 0-1.5% | - | - | - |
7 | (2424. Mohod, M.V. (2015) Performance of polypropylene fiber reinforced concrete. IOSR J. Mech. Civil Eng. 12 [1], 28-36. ) | - | - | - | 0-2% |
8 | (1818. Suksawang, N.; Wtaife, S.; Alsabbagh, A. (2018) Evaluation of elastic modulus of fiber-reinforced concrete. ACI Mater. J. 115 [2], 239-249. https://doi.org/10.14359/51701920 . ) | 0-2% | - | - | 0-2% |
Fiber Type | ID | Vf (%) | W/C | C (kg/m3) | CA (kg/m3) | FA (kg/m3) | W (kg/m3) | Fiber (kg/m3) |
---|---|---|---|---|---|---|---|---|
Plain Concrete | Mix 1 | 0.00 | 0.40 | 503.3 | 709.7 | 986.5 | 201.3 | 0.0 |
Nylon | Mix 2 | 0.50 | 0.40 | 500.8 | 706.1 | 981.5 | 200.3 | 5.7 |
Mix 3 | 0.75 | 0.40 | 499.5 | 704.3 | 979.1 | 199.8 | 8.5 | |
Mix 4 | 1.00 | 0.40 | 498.3 | 702.6 | 976.6 | 199.3 | 11.4 | |
Mix 5 | 0.75 | 0.45 | 487.3 | 687.1 | 955.0 | 219.3 | 8.5 | |
Mix 6 | 0.75 | 0.50 | 475.6 | 670.6 | 932.1 | 237.8 | 8.5 | |
Polypropylene | Mix 7 | 0.50 | 0.40 | 500.8 | 706.1 | 981.5 | 200.3 | 4.5 |
Mix 8 | 0.75 | 0.40 | 499.5 | 704.3 | 979.1 | 199.8 | 6.8 | |
Mix 9 | 1.00 | 0.40 | 498.3 | 702.6 | 976.6 | 199.3 | 9.1 | |
Mix 10 | 0.75 | 0.45 | 487.3 | 687.1 | 955.0 | 219.3 | 6.8 | |
Mix 11 | 0.75 | 0.50 | 475.6 | 670.6 | 932.1 | 237.8 | 6.8 | |
Steel | Mix 12 | 0.50 | 0.40 | 500.8 | 706.1 | 981.5 | 200.3 | 39.0 |
Mix 13 | 0.75 | 0.40 | 499.5 | 704.3 | 979.1 | 199.8 | 58.5 | |
Mix 14 | 1.00 | 0.40 | 498.3 | 702.6 | 976.6 | 199.3 | 78.0 | |
Mix 15 | 0.75 | 0.45 | 487.3 | 687.1 | 955.0 | 219.3 | 58.5 | |
Mix 16 | 0.75 | 0.50 | 475.6 | 670.6 | 932.1 | 237.8 | 58.5 | |
Glass | Mix 17 | 0.50 | 0.40 | 500.8 | 706.1 | 981.5 | 200.3 | 13.5 |
Mix 18 | 0.75 | 0.40 | 499.5 | 704.3 | 979.1 | 199.8 | 20.2 | |
Mix 19 | 1.00 | 0.40 | 498.3 | 702.6 | 976.6 | 199.3 | 27.0 | |
Mix 20 | 0.75 | 0.45 | 487.3 | 687.1 | 955.0 | 219.3 | 20.2 | |
Mix 21 | 0.75 | 0.50 | 475.6 | 670.6 | 932.1 | 237.8 | 20.2 |
2.3. Specimen preparation
⌅Twenty-one
separate mixes (batches) were prepared and nine specimens per mix were
produced, yielding a total of 189 specimens. These 189 specimens were
categorized into five groups: the first group (Mix 1) had specimens with
no fibers, the second group (Mixes 2-6) had specimens with nylon
fibers, the third group (Mixes 7-11) had specimens with polypropylene
fibers, the fourth group (Mixes 12-16) had specimens with steel fibers,
and the fifth group (Mixes 17-21) had specimens with glass fibers. The
concrete was mixed, placed, consolidated, and cured in accordance to
ASTM C192 (2525.
American Society for Testing Materials. (2019) Standard practice for
making and curing concrete test specimens in the laboratory. ASTM C192.
100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,
USA, (2019). https://doi.org/10.1520/C0192_C0192M-19.
).
2.4. Ultrasonic pulse velocity measurement
⌅The ultrasonic pulse velocity of each specimen was determined according to ASTM C597 (2626.
American Society for Testing Materials. (2016) Standard test method for
pulse velocity through concrete. ASTM C597. 100 Barr Harbor Drive, PO
Box C700, West Conshohocken, PA 19428-2959, USA, (2016). https://doi.org/10.1520/C0597-16.
)
after the specimens had cured in water for 1, 3, 7, and 28 days. The
UPV was determined by the direct transmission configuration, where the
transmitter and receiver transducers are placed directly opposite each
other on parallel surfaces. The pulse velocity (V) was calculated by
dividing the length (L) of the specimen by the transit time (T). The UPV
of each specimen was calculated in kilometers per second (km/s); an
average of four transit time measurements was used. Additionally, the
test results of at least three specimens per mix determined the UPV of
each mix at each age.
2.5. Dynamic modulus measurement
⌅The dynamic modulus of each specimen was determined according to ASTM C215 (2727.
American Society for Testing Materials. (2019) Standard test method for
fundamental transverse, longitudinal, and torsional resonant
frequencies of concrete specimens. ASTM C215. 100 Barr Harbor Drive, PO
Box C700, West Conshohocken, PA 19428-2959, USA, (2019). https://doi.org/10.1520/C0215-19.
)
using the resonance test gauge after the specimens had cured in water
for 3, 7, and 28 days. The dynamic modulus was determined by the impact
resonance method using the longitudinal configuration, where the
accelerometer and the hammer strike are directly opposite each other on
parallel surfaces. Dynamic modulus of each specimen was then calculated
based on the longitudinal frequency, mass, geometry, and dimension of
the specimen. When defining the dynamic modulus of each specimen in GPa,
an average of three longitudinal frequency measurements was used.
Additionally, the test results of at least three specimens per mix
determined the dynamic modulus of each mix at each age.
2.6. Compressive strength measurement
⌅The compressive strength of each of the 189 cylinders was tested according to ASTM C39 (55.
American Society for Testing Materials (2012) Standard test method for
compressive strength of cylindrical concrete specimens. ASTM C39. 100
Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, USA,
(2012). https://doi.org/10.1520/C0039_C0039M-12.
)
after the specimens had cured in water for 3, 7, and 28 days. To
determine the compressive strength of each mix at each age, an average
of three cylinder compression tests per mix was used.
3. ANALYSIS AND DISCUSSION OF RESULTS
⌅3.1. Ultrasonic pulse velocity
⌅Some
researchers have studied the development of ultrasonic pulse velocity
of plain concrete over time and concluded that the rate of gain of UPV
is high during early ages and then slows at later ages (2828.
Yoon, H.; Kim, Y.J.; Kim, H.S.; Kang, J.W.; Koh, H-M. (2017) Evaluation
of early-age concrete compressive strength with ultrasonic sensors. Sensors. 17 [8], 1817. https://doi.org/10.3390/s17081817.
).
Few investigations have been conducted on the development of the UPV of
fiber-reinforced concrete over time and/or compared the development of
the UPV of different fiber types over time. The development of the UPV
over time of plain concrete, M1 (Mix 1 in Table 5),
nylon fiber-reinforced concrete (NFRC), polypropylene fiber-reinforced
concrete (PFRC), steel fiber-reinforced concrete, and glass
fiber-reinforced concrete with different mix proportions is shown in Figure 1.
Figure 1 shows that, for these FRC mixes, the gain in UPV was rapid up to 7 days after pouring and slowed afterward, which was a result of the hydration process of concrete. The hydration process was faster at early ages because there are many un-hydrated compounds and empty spaces in the cement paste that can be filled with gel. Therefore, as time passes, the empty spaces were filled with calcium silicate hydrate, thus increasing the UPV. Materials with high density, good quality, and continuity had high velocities, while materials with low density, several cracks, and voids had slow velocities. The UPV of plain concrete (M1) was faster than that of NFRC (M2-M6) and PFRC (M7-M11) because both NFRC and PFRC have lower densities than concrete, and nylon and polypropylene fibers reduce the workability of concrete, thus forming voids. The UPV of plain concrete (M1) was closer to or lower than SFRC, such as Mix 12 (M12), which had a higher UPV compared to plain concrete, because steel fibers have a higher density than concrete, and the fiber volume fraction of 0.5% vol. and water-to-cement ratio of 0.4 do not dramatically affect the workability or voids in the concrete. On the other hand, the UPV of plain concrete (M1) was higher than SFRC (M13 and M14) because fiber volume fractions higher than 0.5% reduce the workability of concrete, thus forming voids. In addition, the UPV of plain concrete (M1) was higher than SFRC (M15 and M16) because at w/c ratios higher than 0.4, the voids in the concrete increased, and having a high w/c ratio means less cement and aggregate content, resulting in slower velocities. The UPV of plain concrete (M1) was higher than GFRC, despite glass having a higher density than concrete, because glass fibers scatter unevenly and significantly impact the workability of concrete, thus forming voids. It can be concluded from Figure 1 that different types of structural fibers affected the UPV of FRC in different ways, depending on fiber type and volume fraction and water-to-cement ratio, and therefore a unique equation that predicts the compressive strength or dynamic modulus of all types of FRC based on UPV would not be the most accurate one. The measurements at early ages and 28 days and the observed increased rate pattern in UPV showed that a relationship exists between the early-age measurements and the mechanical properties of concrete.
3.2. Workability
⌅In
the current study it was observed that Polypropylene fibers increase
entrapped air voids at 1.0% or higher fiber volume fractions, that
results in decreasing of the concrete workability and creating
difficulties when compacting the mixes, which is in agreement with the
observations in similar studies (2929. Madhavi, T.C.; Raju, L.S.; Mathur, D. (2014). Polypropylene fiber reinforced concrete - a review. Inter. J. Emer. Tech. Advan. Engin. 4 [4], 114-119.
). Nylon fibers are hydrophilic, therefore they can absorb a small amount of water during mixing (3030. Zollo, R.F. (1997) Fiber-reinforced concrete: an overview after 30 years of development. Cem. Concr. Comp. 19 [2], 107-122. https://doi.org/10.1016/S0958-9465(96)00046-7.
).
This characteristic can be favorable for the distribution of nylon
fibers during mixing, but excess absorption at higher fiber volume
fractions may adversely affect the workability of the mixtures. Similar
observations were reported elsewhere (3131. Song, P.S.; Hwang, S.; Sheu, B.C. (2015). Strength properties of nylon and polypropylene fiber reinforced concretes. Cem. Concr. Res. 35 [8], 1546-1550. https://doi.org/10.1016/j.cemconres.2004.06.033.
).
It was also emphasized that the structural fibers can reduce the
workability of concrete mixtures and cause fiber ball production at
mid-to-high fiber volume fractions, resulting in a lack of homogeneity (44.
American Society for Testing Materials (2015) Standard specification for
fiber-reinforced concrete. ASTM C1116. 100 Barr Harbor Drive, PO Box
C700, West Conshohocken, PA 19428-2959, USA, (2015). https://doi.org/10.1520/C1116_C1116M-10AR15.
). In the following sections the effect of reduction in workability on mechanical properties of specimens will be discussed.
3.3. Compressive strength
⌅The
gain in concrete compressive strength is rapid at an early age. This
rapid early gain in strength is directly linked to the increase of the
gel/space ratio of calcium silicate hydrate (33. Neville, A.M. (2004) Properties of Concrete, 4th edition. Wiley Harlow, New York, USA, (2004).
).
The development of compressive strengths over time of plain concrete
(M1), NFRC, PFRC, SFRC, and GFRC with different mix proportions is shown
in Figure 2.
Figure 2
shows that approximately 75 percent of concrete’s compressive strength
was achieved in the first 7 days for all mixes, due to the hydration
process, which had a faster rate at early ages. Fibers provided internal
reinforcement due to the fiber-bridging effect. The fiber-bridging
constitutive law describes the relationship between the bridging stress
transferred across a crack and the opening of this crack. However, if
the workability of concrete is not high enough, the fiber-bridging
effect and the performance of the FRC is reduced. Incorporating an
admixture could influence the performance of FRC; for instance, the
workability of FRC can be improved by using high-range water-reducing
admixtures to address this problem (3232.
Thirumurugan, S.; Sivakumar, A. (2013) Compressive strength index of
crimped polypropylene fibers in high strength cementitious matrix. World Appli. Scien. J. 24 [6], 698-702.
, 3333. Yang, E-H.; Wang, S.; Yang, Y.; Li, V.C. (2008) Fiber-bridging constitutive law of engineered cementitious composites. J. Advan. Concr. Tech. 6 [1], 181-193. https://doi.org/10.3151/jact.6.181.
).
However, that was not the focus of the current study. While the
ductility and failure patterns in all FRC samples compared to plain
concrete were observed to be improved during the testing process, in the
fiber volume fraction range discussed in the present work, the
compressive strength of plain concrete (M1) was depicted as higher than
the compressive strengths of NFRC (M2-M6), PFRC (M7-M11), and GFRC
(M17-M21) because the properties of nylon, polypropylene, and glass
fibers were not high enough to compensate for the reduction in concrete
workability. The compressive strength of SFRC (M12-M16) was
significantly higher than the compressive strengths of NFRC (M2-M6),
PFRC (M7-M11), and GFRC (M17-M21) due to its superior fiber properties,
but it was only slightly higher than the compressive strength of plain
concrete (M1) due to the fiber volume fraction range considered in this
study, reduced workability, and, subsequently, the lack of complete
fiber-bridging effect.
3.4. Dynamic modulus
⌅The
gain in concrete elastic modulus is extremely rapid at an early age.
This rapid early gain in strength is directly linked to the increase of
the gel/space ratio of calcium silicate hydrate (33. Neville, A.M. (2004) Properties of Concrete, 4th edition. Wiley Harlow, New York, USA, (2004).
).
The development of the dynamic modulus over time of plain concrete,
NFRC, PFRC, SFRC, and GFRC with different mix proportions is shown in Figure 3.
Figure 3 shows that the development of the dynamic modulus of concrete was extremely rapid; approximately 80 percent of FRC’s dynamic modulus was achieved in the first 3 days for all mixes. This occurred because the mixture’s stiffness increases as the concrete hardens and due to the progress in the hydration process. It can also be observed that the effects of cement and aggregate content on the dynamic modulus were more impactful than the fiber volume fraction. The dynamic modulus of plain concrete was greater than that of NFRC, PFRC, and GFRC, as the elastic properties of nylon, polypropylene, and glass fibers affect the elastic properties of the composite material. The dynamic modulus of SFRC was significantly greater than that of NFRC, PFRC, and GFRC because steel fibers have superior elastic modulus, but again it can be affected due to the reduced workability.
3.5. Prediction of early-age compressive strength of FRC
⌅The
relationship between UPV and compressive strength of concrete at the
age of 28 days has been investigated extensively in previous works,
while only a few studies have discussed the relationship between UPV and
compressive strength of concrete at early ages. It has been observed
that the relationship between concrete compressive strength and
ultrasonic pulse velocity is better estimated by utilizing the
exponential equation forms as shown in Table 6. However, some linear relationships have also been found in the literature (77. Lin, Y.; Kuo, S-F.; Hsiao, C.; Lai, C-P. (2007) Investigation of pulse velocity-strength relationship of hardened concrete. ACI Mater. J. 104 [4], 344-350. https://doi.org/10.14359/18823.
, 88.
Mahure, N.; Vijh, G.; Sharma, P.; Sivakumar, N.; Ratnam, M. (2011)
Correlation between pulse velocity and compressive strength of concrete.
Inter. J. Ear. Sci. Eng. 4 [6], 871-874.
, 2828.
Yoon, H.; Kim, Y.J.; Kim, H.S.; Kang, J.W.; Koh, H-M. (2017) Evaluation
of early-age concrete compressive strength with ultrasonic sensors. Sensors. 17 [8], 1817. https://doi.org/10.3390/s17081817.
, 3434. Popovics, S.; Rose, J.; Popovics, J. (1990) The behaviour of ultrasonic pulses in concrete. Cem. Concr. Res. 20 [2], 259-270. https://doi.org/10.1016/0008-8846(90)90079-D.
).
A new empirical equation capable of predicting the early-age
compressive strength of different types of fiber-reinforced concrete at
different fiber volume fractions and water-to-cement ratios is presented
in this section.
Reference | Equation | Limitation |
---|---|---|
(3535. Nematzadeh, M.; Poorhosein, R. (2017) Estimating properties of reactive powder concrete containing hybrid fibers using UPV. Comp. Concr. 20 [4], 491-502. https://doi.org/10.12989/cac.2017.20.4.491. ) | ƒc = 0.013e1.959Vp | Containing silica fume, superplasticizer, and steel fiber at 1, 2, and 3%. Age - 28 days |
(3535. Nematzadeh, M.; Poorhosein, R. (2017) Estimating properties of reactive powder concrete containing hybrid fibers using UPV. Comp. Concr. 20 [4], 491-502. https://doi.org/10.12989/cac.2017.20.4.491. ) | ƒc = 0.016e2.411Vp | Containing silica fume, superplasticizer, and PVA fiber at 0.25, 0.5, and 0.75%. Age - 28 days |
(3636.
Nematzadeh, M.; Dashti, J.; Ganjavi, B. (2018) Optimizing compressive
behavior of concrete containing fine recycled refractory brick aggregate
together with calcium aluminate cement and polyvinyl alcohol fibers
exposed to acidic environment. Constr. Build. Mater. 164, 837-849. https://doi.org/10.1016/j.conbuildmat.2017.12.230. ) | ƒc = 0.15e1.40V |
Containing
recycled factory brick aggregate, calcium aluminate cement, silica
fume, superplasticizer, and polyvinyl alcohol fibers at 0.5% fiber
volume fraction.
Age - 7 to 63 days |
(3737.
Nematzadeh, M.; Fallah-Valukolaee, F. (2017) Erosion resistance of
high-strength concrete containing forta-ferro fibers against sulfuric
acid attack with an optimum design. Constr. Build. Mater. 154, 675-686. https://doi.org/10.1016/j.conbuildmat.2017.07.180. ) | ƒc = 33.27e0.582V |
High-strength concrete containing 0, 0.2, 0.4, and 0.6% of twisted bundle non-fibrillated, monofilament, and fibrillated polypropylene network plus silica fume. Age - 7 to 63 days |
(77. Lin, Y.; Kuo, S-F.; Hsiao, C.; Lai, C-P. (2007) Investigation of pulse velocity-strength relationship of hardened concrete. ACI Mater. J. 104 [4], 344-350. https://doi.org/10.14359/18823. ) | ƒc = 0.00055e2.5V |
Age - 28 days CA = 1100 kg/m3 |
(1111. Elvery, R.; Ibrahim, L. (1976) Ultrasonic assessment of concrete strength at early ages. Mag. Concr. Res. 28 [97], 181-190. https://doi.org/10.1680/macr.1976.28.97.181. ) | ƒc = 0.0012e2.27V |
Age of 3 hr and over Temperature 0° to 60°C |
(1313.
Nash’t, I.; A’bour, S.; Sadoon, A. (2005) Finding an unified
relationship between crushing strength of concrete and non-destructive
tests. Mid. East Nond. Test. Conf. Exhi. 27-30. Nov., Bahrain, Manama, (2005). ) | ƒc = 1.19e0.715V |
Age - 7 to 138 days Cubes |
(3838. Jones, R. (1962) Non-destructive Testing of Concrete. Cambridge University Press, London, (1962). ) | ƒc = 2.8e0.53V | Concrete slabs |
(1717. Raouf, Z.; Ali, Z. (1983) Assessment of concrete characteristics at an early age by ultrasonic pulse velocity. J. Build. Res. 2 [1], 31-44. ) | ƒc = 2.016e0.61V | Concrete cubes |
(1212.
Naik, T.; Malhotra, V.; Popovics, J. (2003) The ultrasonic pulse
velocity method. In: Handbook on nondestructive testing of concrete,
Second Edition, 8-1 to 8-19, CRC Press, (2003). ) | ƒc = (-109.6 + 33V) | Concrete cylinders |
(88.
Mahure, N.; Vijh, G.; Sharma, P.; Sivakumar, N.; Ratnam, M. (2011)
Correlation between pulse velocity and compressive strength of concrete.
Inter. J. Ear. Sci. Eng. 4 [6], 871-874. ) | ƒc = 9.502V - 18.89 |
Age - 7 and 28 days
Cubes M15 grade |
(88.
Mahure, N.; Vijh, G.; Sharma, P.; Sivakumar, N.; Ratnam, M. (2011)
Correlation between pulse velocity and compressive strength of concrete.
Inter. J. Ear. Sci. Eng. 4 [6], 871-874. ) | ƒc = 2.701V - 17.15 |
Age - 7 and 28 days
Cubes M20 grade |
(88.
Mahure, N.; Vijh, G.; Sharma, P.; Sivakumar, N.; Ratnam, M. (2011)
Correlation between pulse velocity and compressive strength of concrete.
Inter. J. Ear. Sci. Eng. 4 [6], 871-874. ) | ƒc = 4.104V - 19.23 |
Age - 7 and 28 days
Cubes M35 grade |
(1414. Kheder, G. (1999) A two stage procedure for assessment of in situ concrete strength using combined non-destructive testing. Mater. Struct. 32, 410. https://doi.org/10.1007/BF02482712. ) | ƒc = 8.4 * 10-9(V * 103)2.5921 | Age - 7 to 90 days |
Where ƒc is compressive strength in MPa and V is ultrasonic pulse velocity in km/s.
The proposed Equation [1a] and Equation [1b]
predict the compressive strength of 3-day and 7-day SFRC, GFRC, PFRC,
and NFRC with fiber volume fractions ranging from 0.5% vol. to 1% vol.
and water-to-cement ratios ranging from 0.40 to 0.50. The development
trend with hydration time is the same for both the early-age compressive
strength and early-age ultrasonic pulse velocity, where compressive
strength and ultrasonic pulse velocity increase exponentially over time (99.
Khademi, F.; Akbari, M.; Jamal, S.M. (2016) Prediction of concrete
compressive strength using ultrasonic pulse velocity test and artificial
neural network modeling. Roma. J. Mater. 46 [3], 343-350.
).
Therefore, the prediction of FRC’s compressive strength at early ages
based on UPV was expressed using an exponential relationship. A
coefficient of variation (COV) was used to test the accuracy of the
proposed equation by comparing the measured and predicted results. The
proposed equations showed good agreement with the measured values at the
age of 3 days, as shown in Figure 4, and at the age of 7 days, as shown in Figure 5.
For G, P, and NFRC
For SFRC
Where ƒc is compressive strength (MPa), V is ultrasonic pulse velocity (km/s), and α is calculated using Equation [2] and the fiber properties in Table 2. GFRC, PFRC, and NFRC were grouped together (Equation [2a]) because they had a low density, while SFRC remained alone (Equation [2b]) because it had a higher density.
For G, P, and NFRC
For SFRC
Where σ is fiber flexural strength (GPa), ɭ is fiber length, d is fiber diameter, t is age (days), τ is fiber tensile strength (MPa), and ρ is fiber density (kg/m3).
For Figure 4 and Figure 5, the 45-degree line represents a perfect correlation between the predicted compressive strength and the measured compressive strength. Data points above this line represent unconservative deviations of the compressive strength equation 1, while data points below this line represent conservative deviations. The coefficient of variation represents the variability between the predicted and measured results. A low COV indicates good agreement between the predicted and measured values. Therefore, the proposed equations showed good agreement with the measured values.
3.6. Prediction of the early-age dynamic modulus of FRC
⌅The
development trend of compressive strength and dynamic modulus and
ultrasonic pulse velocity with hydration time was observed to be
exponential at the early ages. Therefore, the prediction of FRC’s
compressive strength and dynamic modulus at early ages based on
ultrasonic pulse velocity is expressed using an exponential relationship
which is well aligned with similar studies in the literature (99.
Khademi, F.; Akbari, M.; Jamal, S.M. (2016) Prediction of concrete
compressive strength using ultrasonic pulse velocity test and artificial
neural network modeling. Roma. J. Mater. 46 [3], 343-350.
).
In addition, the proposed equations can predict the compressive
strength and dynamic modulus of multiple fiber reinforced concrete
types, due to the incorporation of different fiber properties as
variables in the equations such as: fiber flexural strength, fiber
length, fiber diameter, fiber tensile strength, and fiber density. The
proposed Equation [3a] and Equation [3b]
predict the dynamic modulus of 3-day and 7-day SFRC, GFRC, PFRC, and
NFRC with fiber volume fractions ranging from 0.5% vol. to 1% vol. and
water-to-cement ratios ranging from 0.4 to 0.5. The development trend of
dynamic modulus and ultrasonic pulse velocity with hydration time is
exponential at early ages (3939.
Haque, M.A.; Rasel-Ul-Alam, Md. (2018) Non-linear models for the
prediction of specified design strengths of concretes development
profile. HBRC J. 14 [2], 123-136. https://doi.org/10.1016/j.hbrcj.2016.04.004.
).
Therefore, the prediction of FRC’s early-age dynamic modulus was
calculated using an exponential equation. A COV was used to test the
accuracy of the proposed equation by comparing the measured and
predicted results. The proposed equations showed good agreement with the
measured values at the age of 3 days, as shown in Figure 6, and at the age of 7 days, as shown in Figure 7.
For G, P, and NFRC
For SFRC
Where Ed is the dynamic modulus (GPa), V is the ultrasonic pulse velocity (km/s), and α is calculated using Equation [4] and the fiber properties listed in Table 2. GFRC, PFRC, and NFRC were grouped together because they each had a low density, while SFRC remained alone because it had a high density.
For G, P, and NFRC
For SFRC
Where σ is fiber flexural strength (GPa), ɭ is fiber length, d is fiber diameter, t is age (3 or 7 days), τ is fiber tensile strength (MPa), and ρ is fiber density (kg/m3).
Figure 6 and Figure 7, the 45-degree line represents a perfect correlation between the predicted dynamic modulus and measured dynamic modulus. Data points above this line represent unconservative deviations of the dynamic modulus Equation [3], while data points below this line represent conservative deviations. The coefficient of variation represents the variability between the predicted and measured results. A low COV indicates good agreement between the predicted and measured values; therefore, the proposed equations showed good agreement with the measured values.
4. CONCLUSIONS
⌅This paper investigates the correlation between the early-age ultrasonic pulse velocity and the early-age compressive strength and dynamic modulus of nylon, polypropylene, steel, and glass fiber-reinforced concrete. The study was needed because the preliminary study conducted prior to this research revealed that the existing equations did not provide a good prediction of fiber-reinforced concrete’s early-age compressive strength and/or dynamic modulus based on UPV. The mixture parameters investigated included fiber volume fractions of 0.5% vol., 0.75% vol., and 1.00% vol., and water-to-cement ratios of 0.40, 0.45, and 0.50. The ultrasonic pulse velocity, compressive strength, and dynamic modulus were measured for the specimens using an ultrasonic concrete tester, compression test machine, and resonance test gauge, respectively. Two sets of equations were proposed to predict the early-age compressive strength and dynamic modulus of FRC based on ultrasonic pulse velocity. The first set of equations predicted the 3-day and 7-day compressive strength of nylon, polypropylene, steel, and glass fiber-reinforced concrete. The second set of equations predicted the 3-day and 7-day dynamic modulus of nylon, polypropylene, steel, and glass fiber-reinforced concrete. The proposed equations can predict the early-age compressive strength and dynamic modulus of multiple fiber-reinforced concrete types, due to the incorporation of different fiber properties as variables in the equations. No other equations have been found in the literature capable of accurately predicting the early-age compressive strength and dynamic modulus of multiple types of fiber-reinforced concrete with different mixture parameters.
The accuracy of these new equations was tested by measuring the coefficient of variation between the measured values and the predicted values from the proposed equations. The coefficients of variation between the measured and predicted compressive strengths showed reasonable agreement with the measured values, and ranged from 6.4 to 14.6 percent. The coefficients of variation between the measured and predicted dynamic moduli also showed reasonable agreement with the measured dynamic moduli, and ranged from 3.3 to 9.2 percent. Based on these results, it appears that the proposed Equation [1a] and Equation [1b] can accurately predict the 3-day and 7-day compressive strength of nylon, polypropylene, steel, and glass fiber-reinforced concrete. Similarly, based on the results, it appears that the proposed Equation [3a] and Equation [3b] can accurately predict the 3-day and 7-day dynamic modulus of nylon, polypropylene, steel, and glass fiber-reinforced concrete.