Development of Steel Fiber-Reinforced Expanded-Shale Lightweight Concrete with High Freeze-Thaw Resistance

International Joint Research Lab for Eco-Building Materials and Engineering of Henan, North China University of Water Resources and Electric Power, Huayuan Campus, No. 36 Beihuan Road, Zhengzhou 450045, China Henan Province Collaborative Innovation Center for High-Efficient Utilization and Support Engineering of Water Resources, North China University of Water Resources and Electric Power, Longzihu Campus, No. 136 Jinshui East Road, Zhengzhou 450046, China School of Civil Engineering and Communication, North China University of Water Resources and Electric Power, Huayuan Campus, No. 36 Beihuan Road, Zhengzhou 450045, China


Introduction
On the purpose of utilizing the local raw materials, steel fiberreinforced expanded-shale lightweight concrete (SFRELC) was developed, which applied the expanded shale as coarse aggregates and the lightweight sand of expanded-shale's byproduct or the manufactured sand as fine aggregates.Based on the systematically experimental studies, SFRELC has reliable basic mechanical properties [1][2][3][4], reasonable strength development [5], enhanced carbonization resistance, and reduced shrinkage [5,6].To verify the possibility of applying SFRELC in the wet environments at cold and severe cold areas of China [7], the experimental study was carried out on the freeze-thaw resistance of SFRELC in this paper.
After searching the literature, although few investigations were found to research the freeze-thaw resistance of steel fiber-reinforced lightweight aggregate concrete (SFRLAC), the results exhibited a good prospect for the development of SFRELC with high freeze-thaw resistance.Ishida et al. [8] reported that the freeze-thaw resistance of SFRLAC could be improved by the increase in the bond force between slenderer steel fibers with a large bond area and a high-strength matrix.Huo et al. [9,10] discovered that the freeze-thaw resistance of pumice lightweight concrete could be enhanced by adding hybrid fibers (steel fiber and polypropylene fiber) due to the decrease of strength loss, although the bond between steel fibers and the matrix became weaker with the increase of freeze-thaw cycles.Li et al. [11] concluded that steel fibers can improve the freeze-thaw resistance of SFRLAC characterized by the mass loss rate and the relative dynamic modulus of elasticity, as the matrix spalling from frost heaving was restrained by steel fibers.
In view of the benefit of air-entraining to freeze-thaw resistance of concrete [12][13][14][15][16], the content of the airentraining agent was also considered as a main factor in this paper.e freeze-thaw resistance of SFRELC was experimentally studied and comprehensively evaluated by the indexes of the mass loss rate, relative dynamic modulus of elasticity, and relative flexural strength.e compound effect of the air-entraining agent and steel fibers and the beneficial effect of manufactured sand on freeze-thaw resistance of SFRELC are analyzed.e mechanisms are explored with the aid of test results of water penetration of SFRELC.Suggestions are given out for the optimal mix design of SFRELC with high freeze-thaw resistance.

Raw Materials.
e ordinary silicate cement was grade 42.5, the water requirement of normal consistency was 26.4%, and the initial and final setting times were 150 min and 248 min.e compressive strength was 22.8 MPa at 3 days and 50.8 MPa at 28 days, and the tensile strength was 4.1 MPa at 3 days and 8.0 MPa at 28 days.e sintering expanded shale with a maximum size of 20 mm was used as coarse aggregates sieved in continuous gradation based on the maximum density principle [17,18].e bulk and particle densities were 800 kg/m 3 and 1274 kg/m 3 , the cylinder compressive strength was 5.0 MPa, the 1-hour water absorption was 6.1%, and the mud content was 0.2%.
Two kinds of fine aggregates were used, respectively.One was the lightweight sand made of the byproduct of sintering expanded shale in continuous gradation with a size of 1.6-5 mm [17,18] and another was the manufactured sand [19,20].Table 1 lists their physical properties.
e steel fiber was of milling type with length l f � 30 mm and equivalent diameter d f � 0.8 mm. e aspect ratio l f /d f � 37.5.
Figure 1 shows the morphology features of coarse expanded shale, lightweight sand, manufactured sand, and steel fibers.

Mix Proportion of SFRELC.
e mix proportion of SFRELC was designed in accordance with the specifications of Chinese standards [17,21], where the absolute volume method was adopted.e volume fraction of steel fibers (ρ f ),  2 Advances in Materials Science and Engineering the content of the air-entraining agent (m ae ), and the type of fine aggregates were considered as the test parameters.Based on previous studies of fundamental properties of SFRELC with good workability [1][2][3][4][5][6], the water-cement ratio was fixed as w/c � 0.30, while the sand ratio was 42% and the dosage of the water reducer was 4.0% cement mass.Table 2 lists their combinations for the test of 9 trials, where the letter in mix no. is the identifier of fine aggregates and the following digits represent m ae and ρ f .Based on previous experimental studies, prewetting the lightweight aggregates had beneficial effects on the mechanical properties especially on the shrinkage reduction of SFRELC [1][2][3][4][5][6]. is may also be beneficial to decrease the water penetration and to increase the freeze-thaw resistance of SFRELC.Zhao et al. [22] reported that the water penetration was reduced for SFRLAC with saturated lightweight aggregates, and Ali et al. [23] reported that the freeze-thaw resistance of lightweight aggregate concrete was improved at early ages by increasing the saturation level of aggregates.Although there were some contract conclusions [24,25] or no relationship [26,27] reported, the differences may be resulted from the different pores' structure (open or closed) and water absorption of lightweight aggregates [15,28,29].
erefore, the expanded shale and lightweight sand of this experiment were prewetted as the saturated dry surface by using the additional water counted with the 1-hour water absorption.All mixes of this study had good workability with slump of 120 mm-150 mm.

Preparation of Specimens.
Specimens for the freeze-thaw test were 100 mm × 100 mm × 400 mm prisms; 189 specimens for 9 trials were prepared, and each trial had 21 specimens.Specimens for the water penetration test were circular truncated cones with a bottom diameter of 185 mm, tip diameter of 175 mm, and height of 150 mm; 54 specimens for 9 trials were prepared, and each trial had 6 specimens.
e single horizontal shaft forced mixer was used.e expanded shale and lightweight sand (except for M0.08/0.8)were firstly prewetted in the mixer for 1 hour, and then, the manufactured sand (only for M0.08/0.8), the cement, and half dosage of the mix water were added and mixed for 30 s.During the mixing, the water reducer and air-entraining agent as well as residual mix water were added.After that, the steel fiber was sprinkled into the mixer and mixed for 3 min.e specimens formed by steel moulds on the vibration platform.After being cast for 24 hours, they moved from moulds and cured in the standard curing room for 28 days before testing.

Test Methods.
Test methods of this experiment were in accordance with the specifications of the Chinese standard GB/T 50082 [30] and ASTM standard C666 [31].e freezethaw resistance was measured by using the test method for rapid freezing and thawing in water, and the main test apparatuses were the rapid freeze-thaw test machine, the tester of the dynamic modulus of elasticity, the hydraulic universal test machine, and the balance.
e freeze-thaw resistance was presented by the mass loss rate (Δm n ) and the relative dynamic modulus of elasticity (P n ) calculated as follows: where m 0 and m n are the mass of the specimen at the beginning and after n cycles of the freeze-thaw test, respectively, and f 0 and f n are the transversal base frequency of the specimen at the beginning and after n cycles of the freeze-thaw test, respectively.e durability factor (DF) was used to evaluate the freeze-thaw resistance [7]: where f 300 is the transversal base frequency of the specimen after 300 cycles of the freeze-thaw test.Before 300 cycles of freezing and thawing, if the relative dynamic modulus of elasticity (P n ) and the mass loss rate (Δm n ) reached 60% and 5%, respectively, at n cycles, then the DF was computed as follows: Based on previous studies [9][10][11], the strength loss of SFRLAC is better to reflect the freeze-thaw resistance.
erefore, the flexural strength of SFRELC was tested in accordance with the specification of the Chinese standard Advances in Materials Science and Engineering GB/T 50081 [32].e concentrated loads were exerted on the three dividing points, and the relative exural strength (f r ) was de ned as follows: where f 0 and f n are the exural strength of the specimen at the beginning and after n cycles of the freeze-thaw test, respectively.
To explain the freeze-thaw mechanism in aspect of the water transport property of SFRELC, the depth of water penetration was measured [30].e main test apparatuses were the testing machine for water penetration of concrete, the hydraulic universal test machine, and the steel ruler.After the specimens in a group were xed in the testing machine, the hydraulic pressure was exerted within 1.2 ± 0.05 MPa for 24 hours.en, the specimens were split on the hydraulic universal test machine.
e depth of the water stain was measured by the steel ruler at 10 points in equal space divided along the bottom splitting line.e depth of water penetration of each specimen was counted as the mean value of these 10 points and that of one group (h p ) was the mean value of six specimens.

Mass Loss Rate.
e mass change of the SFRELC matrix comes mainly from two parts: one is the increment due to the water absorbed in the pores and capillaries of concrete during freezing and thawing and another is the decrement due to the surface peeling of set cement and aggregates.When the latter is greater than the former, the mass loss rate computed by formula (1) is positive, which always means the better internal structure the matrix has, and the freeze-thaw damage takes place from the surface successively.When the latter is lower than the former, the mass loss rate is negative, which always means the poor internal structure the matrix has, and the freeze-thaw damage takes place because of the internal expansion of absorbed water inside the pores and capillaries [12,33].
Figure 2 presents the changes of mass loss rate of SFRELC with the freeze-thaw cycles.e negative mass loss rate of L0.00/0.0increased by adding steel bers in L0.00/0.8.
is condition was improved successively by the addition of the air-entraining agent from 0.4‰ to 1.2‰.When the content of the air-entraining agent was not less than 0.8‰, the mass loss rate of SFRELC became positive normally.Under the condition of SFRELC with 0.8‰ air-entraining agent, the mass loss rate changed from negative to positive with ρ f 0.4% and 0.8% before 175 freeze-thaw cycles, and then, it remained positive and increased with the increasing freeze-thaw cycles.When ρ f 1.2%, the mass loss rate remained positive all the time.When N 300 and Δm n 0.42% and 0.51% for SFRELC with ρ f 0.8% and 1.2%, the mass loss rate was the same for L0.08/0.0without steel bers.Compared to L0.08/0.8 with lightweight sand, the mass loss rate of M0.08/0.8 with manufactured sand changed slightly until N 300.
e changes of mass loss rate were identical to the water penetration properties as shown in Figure 3. Compared to L0.00/0.0,L0.08/0.0 had a higher resistance to water penetration with 7.27% reduction of h p and L0.00/0.8 had a lower resistance to water penetration with 25% increment of h p . is exhibited the di erent roles of the air-entraining agent and steel bers a ecting the microstructure of SFRELC.e air-entraining agent imported even dispersed bubbles with minuteness and closed and mutually uncorrelated characteristics [13][14][15], which led to the higher density of SFRELC matrix with improved uniformity of zigzag capillaries.e steel bers increased the connectivity of internal pores and capillaries due to the defects of interfaces along steel bers in the matrix [9,10,22].erefore, the results of water penetration of SFRELC re ected the compound function of the air-entraining agent and steel bers.
With the increase of m ae 0.04‰-0.Combined with h p of L0.12/0.8,more air-entraining was needed for L0.08/1.2 to get the reduced h p .In a word, the balance between volume fraction of steel bers and airentraining controlled the water permeability of SFRELC.When replacing lightweight sand by manufactured sand, the resistance of SFRELC to water penetration increased with the reduction of h p from 10.2 mm to 8.1 mm. is is due to the bene cial e ects of stone powder in manufactured sand [19,20]: the microaggregate lling e ect on density, the activity e ect and crystal nuclei e ect on cement hydration degree, and the enhancement e ect on hardness and bond property of the interfaces among the composites.eoretically, these e ects should also comprehensively improve the interfaces among steel bers and set cement.
Given above, the mass loss rate re ected some information about the freeze-thaw resistance of SFRELC, where the negative and positive values gave the relative degree of water absorption and surface peeling o during the cycles of freezing and thawing.However, because of the integrity maintained and undetected surface scaling of SFRELC specimens, the mass loss rate was less changed with the increasing freeze-thaw cycles, and it was not a sensitive parameter to represent the freeze-thaw resistance of SFRELC.

Relative Dynamic Modulus of Elasticity.
e dynamic modulus of elasticity is closely linked with the constitutes and microstructures of concrete, which is a ected sensitively by the interior pores and unsubstantial interface [12,33].Figure 4 exhibits the changes of relative dynamic modulus of elasticity with freeze-thaw cycles a ected by the content of the air-entraining agent.Compared to L0.00/0.0,P n of L0.08/0.0decreased slowly with the increasing freeze-thaw cycles.is is identical to the previous studies for the freezethaw resistance of lightweight aggregate concrete [9,[13][14][15], which bene ts from the air-entraining e ectiveness for the improvement of the microstructure with proper amount of closed and uniformly distributed microbubbles, resulting in the cutting-o of pores and capillaries inside concrete.At the same time, adding steel bers in L0.00/0.8without the airentraining agent led to the drop down of P n . is is consistent with the increased depth of water penetration as shown in Figure 3.With the increasing content of the airentraining agent, the reduction of P n became slow, while the changes of P n for L0.08/0.8 and L0.08/0.0 were almost the same.When m ae 0.12‰, the P n of L0.12/0.8decreased slightly until N 300.erefore, the P n of SFRELC was sensitive to the addition of steel bers and the air-entraining agent.For the design of freeze-thaw resistance of SFRELC, the balance between the bene cial e ect of the air-entraining agent and the harmful e ect of steel bers should be comprehensively considered.
Figure 5 exhibits the changes of relative dynamic modulus of SFRELC with freeze-thaw cycles a ected by the volume fraction of steel bers.With m ae 0.08‰, the P n reduced slowly with the increasing volume fraction of steel bers.When ρ f 1.2%, the P n reduced slightly.is exhibited that, with proper content of the air-entraining agent, SFRELC reached the higher freeze-thaw resistance with the assistance of a lager amount of steel bers.Despite the increase of water penetration to some extent as shown in Figure 3, the bene cial e ects of steel bers on the connement of internal shortages and the bridging on microcracks appeared successively [6,11], and the integrity of SFRELC was maintained by overcoming the ice expansion in pores with the increasing freeze-thaw cycles.erefore, the compound e ect of the air-entraining agent and steel bers led to the high freeze-thaw resistance of SFRELC.
Meanwhile, Figure 5 also exhibits the changes of relative dynamic modulus of SFRELC with freeze-thaw cycles affected by the ne aggregate.With m ae 0.08‰ and ρ f 0.8%, the SFRELC with manufactured sand had higher freeze-thaw resistance than that using lightweight sand. is is identical with the experimental results of water penetration shown in Figure 3. Table 3 lists the test results of the DF of SFRELC.Table 4 presents the minimum value of the DF specified in the Chinese standard for the durability design of concrete structures [7].It can be seen that air-entraining is quite necessary for the SFRELC with certain freeze-thaw resistance even in the partial freezing area.With proper contents of the air-entraining agent and steel fibers, SFRELC with high freeze-thaw resistance can be applied in every freezing area, even in the highly saturated environmental condition of severe cold areas.5 lists the test results of the relative flexural strength of SFRELC with the increase of freeze-thaw cycles.e changes of relative flexural strength were similar to those of relative dynamic modulus of elasticity, which appeared more sensitive to the freeze-thaw cycles.Without adding the air-entraining agent, SFRELC lost the flexural strength rapidly after 75 freezethaw cycles.When ρ f � 0.8%, the relative flexural strength of SFRELC obviously improved with the increase of m ae from 0.04‰ to 0.08‰.When m ae ≥ 0.08‰, SFRELC with ρ f ≥ 0.8% had higher relative flexural strength.Replacing lightweight sand with manufactured sand also gave the SFRELC with high relative flexural strength.

Relative Flexural Strength of SFRLAC. Table
ese demonstrated that the flexural strength of SFRELC decreased with the increase of freeze-thaw cycles due to the weakened bond of steel fibers with the matrix.

Conclusion
In this paper, the effects of air-entraining, steel fibers, and fine aggregate type on the freeze-thaw resistance of SFRELC were experimentally studied and explained on the mechanisms in consistent with the test results of water penetration.
e conclusions can be drawn as follows: (1) Due to the water importing and exporting peculiarity of SFRELC, the mass loss rate could not reach the limit of 5%.It was not a good index to evaluate the damage of SFRELC due to freezing and thawing.
Comparatively, the relative dynamic modulus of elasticity of SFRELC had a clear trend to reflect the freeze-thaw resistance, and the relative flexural strength of SFRELC was more sensitive to the freezethaw cycles.
(2) Air-entraining was quite necessary for the development of SFRELC with high freeze-thaw resistance.Steel fibers were harmful to the freeze-thaw resistance of SFRELC without air-entraining.However, the harmful effect of steel fibers could be overcome with the assistance of air-entraining.With proper air-entraining, the freeze-thaw resistance of SFRELC increased with the increasing volume fraction of steel fibers.is exhibited the compound effect of the air-entraining agent and steel fibers on the freeze-thaw resistance of SFRELC.
(3) e replacement of lightweight sand with manufactured sand in SFRELC could enhance the freezethaw resistance.e reduction of relative dynamic modulus of elasticity became slow with the increasing freeze-thaw cycles, while the reduction of relative flexural strength became smaller.
(4) e proper mix proportion design should pay attention to the compound effect of the air-entraining agent and steel fibers, which controlled the degree of freeze-thaw resistance of SFRELC.In this experimental study, several instances of SFRELC with high freeze-thaw resistance were developed to meet the requirement of applying in freezing areas even in severe cold areas under highly saturated environmental condition; however, the quantitative relations for the design should be further studied.

8 Figure 4 : 8 Figure 5 :
Figure 4: Changes of the relative dynamic modulus of elasticity a ected by content of the air-entraining agent.

Table 1 :
Physical properties of lightweight sand and manufactured sand.

Table 2 :
Mix proportion of concrete.

Table 3 :
Test results of the DF of SFRELC.

Table 4 :
Specified minimum value of the DF.

Table 5 :
Test results of the relative flexural strength of SFRELC.