The influences of fines content, methylene blue (MB) value, and lithology of crushed sand (CS) on frost durability and strength of concrete were investigated, and the frost durability and strength of crushed sand concrete (CSC) and river sand concrete (RSC) were compared. The results show that inclusion of fines improves CSC compressive strength and reduces frost durability of C30 CSC when fines content reaches 10%, whereas it has little negative influence on frost durability of C60 CSC. Increasing MB value does not negatively affect compressive strength of C30 CSC but decreases compressive strength of C60 CSC and frost durability of CSC, and the reduction is more pronounced when MB value exceeds 1.0. Lithology has no prominent influence on frost durability and compressive strength of CSC within the lithologies (dolomite, limestone, granite, basalt, and quartz) studied. Though compressive strength of CSC is a little higher than RSC under equal water to cement ratio, frost durability of CSC is no better than RSC especially for C30 CSC, and air-entraining agent is suggested for enhancing frost durability of C30 CSC exposed to freezing environment.
Fine aggregate is a necessary raw material for making concrete, and its characteristics can impact concrete properties from workability to strength and durability. In developing countries where infrastructure constructions are massively needed like China, growing depletion and expansive cost of river sand (RS) have resulted in extensive application of CS. CS refers to fine particles finer than 4.75 mm produced by rock crushing. Generally speaking, CS particles are more angular and much tougher when compared with RS particles and particularly contain 10–20% of fines which are finer than 75
A number of researches have been done on properties of CSC so far. It has been established that qualified concrete with satisfactory strength and durability can successfully be prepared using CS [
Properties of CS are greatly affected by parent rock and crushing technology [
Therefore, this research reports an investigation on the effects of CS characteristics on frost durability of low- and high-strength CSC. In the study, frost durability of CSC was examined using the rapid freezing-thawing (FT) method and the test results were compared with those of RSC. The compressive strength of CSC was also investigated. Experimental results of this research can verify crucial factors affecting frost durability of CSC and provide technical guidance for application of CSC in cold regions.
The cement used was 42.5-grade ordinary Portland cement and its physical properties were displayed in Table
Physical properties of used cement.
Density (g/cm3) | Standard consistency (%) | Setting time (min) | Flexural strength (MPa) | Compressive strength (MPa) | |||
---|---|---|---|---|---|---|---|
Initial | Final | 3 d | 28 d | 3 d | 28 d | ||
3.13 | 27.0 | 125 | 190 | 6.8 | 9.3 | 30.3 | 51.2 |
Properties of used fine aggregates.
Lithology | Apparent density (kg/m3) | Crushing value (%) | Void ratio (%) | Fineness modulus |
Water absorption (%) | Main mineral composition |
---|---|---|---|---|---|---|
RS | 2630 | 12.4 | 36.0 | 2.81 | 0.53 | Quartz |
DL | 2837 | 23.2 | 44.5 | 3.15 | 0.78 | Dolomite |
LS | 2667 | 24.2 | 40.6 | 3.17 | 0.80 | Limestone |
GR | 2650 | 18.7 | 42.7 | 3.17 | 0.87 | Quartz, sanidine, mica |
BA | 2895 | 19.7 | 45.3 | 3.24 | 0.96 | Augite, anorthite |
QZ | 2657 | 21.3 | 42.3 | 3.08 | 1.07 | Quartz, albite |
Crushing value of aggregates was determined according to JTG E42-2005 [
The relative dynamic modulus of elasticity
Fines were incorporated in concrete as replacement of sand at variable contents, and the mix proportions of the influence of fines content on concrete frost durability and strengths are shown in Table
Concrete mix proportions and strengths of resultant concrete.
Strength grade | Fines content | Unit weight (kg/m3) | Compressive strength (MPa) | ||||||
---|---|---|---|---|---|---|---|---|---|
Cement | Water | CA | CS (DL) | Fines | WR | 3 d | 28 d | ||
C30 | Control | 327 | 180 | 1079 | 814 | 0 | 0.59 | 21.3 | 38.1 |
5% | 327 | 180 | 1079 | 773.3 | 40.7 | 0.65 | 23.8 | 39.0 | |
10% | 327 | 180 | 1079 | 732.6 | 81.4 | 0.65 | 25.6 | 41.3 | |
15% | 327 | 180 | 1079 | 691.9 | 122.1 | 0.65 | 27.4 | 43.3 | |
20% | 327 | 180 | 1079 | 651.2 | 162.8 | 0.65 | 27.7 | 44.9 | |
|
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C60 | Control | 531 | 170 | 1061 | 738 | 0 | 1.86 | 52.1 | 71.1 |
3% | 531 | 170 | 1061 | 715.9 | 22.1 | 2.06 | 52.7 | 76.1 | |
7% | 531 | 170 | 1061 | 686.3 | 51.7 | 2.06 | 57.8 | 78.9 | |
10% | 531 | 170 | 1061 | 664.2 | 73.8 | 2.06 | 58.8 | 77.7 | |
15% | 531 | 170 | 1061 | 627.3 | 110.7 | 2.06 | 59.4 | 72.9 |
The data in Table
The influences of fines content on frost durability of C30 and C60 CSC are shown in Figures
Effect of fines content on frost durability of C30 CSC: (a)
Effect of fines content on frost durability of C60 CSC: (a)
In Figure
Fines belong to a kind of nonpozzolanic filler. The increase of fines content decreases the relative amount of cement in powdery materials of concrete, so that there is not enough cement paste to coat and bind the all aggregates [
Frost durability of C30 CSC is inherently poor owning to high W/C. Additionally, cement content of C30 CSC is relatively low. Thus inclusion of fines significantly decreases the relative amount of cement and thereby reduces frost durability. However, C60 CSC has high frost durability due to low W/C and high cement content, which cause frost durability of C60 CSC to be much less sensitive to variation in fines content as compared to that of C30 CSC.
MB value of CS was adjusted by incorporating clay as partial replacements of fines at fixed fines content of 10%. Concrete mix proportions and compressive strengths are shown in Table
Concrete mix proportions and strengths of resultant concrete.
Strength grade | MB value |
Unit weight (kg/m3) | Compressive strength (MPa) | WA (%) | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Cement | Water | CA | CS (DL) | Fines | Clay | WR | 3 d | 28 d | |||
C30 | Control | 327 | 180 | 1079 | 814 | 0 | 0 | 0.59 | 21.3 | 38.1 | 0.96 |
0.3 | 327 | 180 | 1079 | 773.3 | 81.4 | 0 | 0.65 | 25.6 | 41.3 | 0.94 | |
0.5 | 327 | 180 | 1079 | 773.3 | 79.9 | 1.5 | 0.65 | 25.9 | 43.7 | 0.93 | |
1.0 | 327 | 180 | 1079 | 773.3 | 76.3 | 5.1 | 0.65 | 26.3 | 44.6 | 0.97 | |
1.4 | 327 | 180 | 1079 | 773.3 | 73.6 | 7.8 | 0.65 | 26.2 | 44.3 | 1.01 | |
2.0 | 327 | 180 | 1079 | 773.3 | 69.9 | 11.5 | 0.65 | 26.0 | 43.9 | 1.05 | |
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C60 | Control | 531 | 170 | 1061 | 738 | 0 | 0 | 1.86 | 52.1 | 71.1 | 0.63 |
0.3 | 531 | 170 | 1061 | 664.2 | 73.8 | 0 | 2.06 | 58.8 | 77.7 | 0.60 | |
0.5 | 531 | 170 | 1061 | 664.2 | 72.4 | 1.4 | 2.06 | 58.0 | 72.4 | 0.61 | |
1.0 | 531 | 170 | 1061 | 664.2 | 69.2 | 4.7 | 2.06 | 53.1 | 71.7 | 0.67 | |
1.4 | 531 | 170 | 1061 | 664.2 | 66.7 | 7.1 | 2.06 | 52.0 | 65.9 | 0.80 | |
2.0 | 531 | 170 | 1061 | 664.2 | 63.4 | 10.4 | 2.06 | 48.5 | 64.0 | 0.97 |
As can be seen in Table
However, in the case of C60 CSC, compressive strengths decrease with increase of MB value from 0.3 to 2.0, and they become lower than the control when MB value exceeds 1.0, which varies from the influence of MB value on compressive strength of C30 CSC. The minimum compressive strengths of 3 d and 28 d are observed at MB value of 2.0, which are about 7% and 10% lower than the control.
C30 concrete is vulnerable to segregation and bleeding for its high W/C and low cement content. Inclusion of fines containing clay enhances cohesiveness of fresh concrete and fills in the voids in hardened concrete, which may create a denser matrix and thus improve strength (Figure
SEM images of ITZ in C30 CSC: (a) the control; (b) MB value 1.0; (c) MB value 2.0.
SEM images of ITZ in C60 CSC: (a) the control; (b) MB value 1.0; (c) MB value 2.0.
Effect of MB value on frost durability of C30 CSC: (a)
Effect of MB value on frost durability of C60 CSC: (a)
Influences of MB value on
MB value of CS reflects the relative amount of clay in fines, and an increase of MB value indicates increasing amount of clay in concrete [
One RS and five CSs of different lithologies were selected to prepare concrete according to mix proportions in Table
Concrete mix proportions and strengths of resultant concrete.
Strength grade | Lithology | Unit weight (kg/m3) | Compressive strength (MPa) | ||||||
---|---|---|---|---|---|---|---|---|---|
Cement | Water | CA | Sand | Fines | WR | 3 d | 28 d | ||
C30 | RS | 327 | 180 | 1079 | 814 | 0 | 0.59 | 20.1 | 37.4 |
DL | 327 | 180 | 1079 | 814 | 0 | 0.59 | 21.3 | 38.1 | |
LS | 327 | 180 | 1079 | 814 | 0 | 0.59 | 21.9 | 39.2 | |
GR | 327 | 180 | 1079 | 814 | 0 | 0.59 | 21.6 | 40.3 | |
BA | 327 | 180 | 1079 | 814 | 0 | 0.59 | 22.6 | 40.1 | |
QZ | 327 | 180 | 1079 | 814 | 0 | 0.59 | 20.3 | 38.9 | |
|
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C60 | RS | 531 | 170 | 1061 | 738 | 0 | 1.86 | 50.5 | 69.9 |
DL | 531 | 170 | 1061 | 738 | 0 | 1.86 | 52.1 | 71.1 | |
LS | 531 | 170 | 1061 | 738 | 0 | 1.86 | 53.5 | 72.9 | |
GR | 531 | 170 | 1061 | 738 | 0 | 1.86 | 55.7 | 75.8 | |
BA | 531 | 170 | 1061 | 738 | 0 | 1.86 | 54.9 | 74.8 | |
QZ | 531 | 170 | 1061 | 738 | 0 | 1.86 | 53.9 | 72.8 |
As shown in Table
The influences of lithology on
Effect of lithology on frost durability of C30 concrete: (a)
Effect of lithology on frost durability of C60 concrete: (a)
Figures
For C30 concrete with low strength and impermeability, CS particles with high crushing value (low mechanical strength) and water absorption (high permeability), together with the interface between sand particle and cement paste, might be damaged by freezing of water penetrated into concrete, which leads to frost durability of C30 CSC being reduced and poorer than that of C30 RSC. However, as for C60 concrete having high strength and impermeability, the sand particles are so well bonded by cement paste that neither the sand particle nor interface could be frozen to damage. Thus, frost durability of C60 CSC is rarely affected and nearly equal to that of C60 RSC.
Crushing values and water absorptions of the used CS are almost around 20% and 0.9%, respectively, which may account for inconspicuous frost durability difference of concrete prepared with CS of various lithologies. Parent rock lithology can affect water absorption and crushing value of CS, and some researcher believed they were related to frost durability of concrete [
Based on the experimental results of this research, the following conclusions can be drawn: Increasing fines content of dolomite CS improves concrete strength and reduces frost durability of C30 CSC when fines content reaches 10%. However, it has little negative effect on frost durability of C60 CSC. Increasing MB value of dolomite CS does not negatively affect strength of C30 CSC but decreases strength of C60 CSC and frost durability of CSC, and the reduction is more pronounced when MB value exceeds 1.0. CS lithology has no prominent influence on frost durability and strength of CSC within the lithologies studied. Compressive strength of CSC is a little higher than RSC under equal W/C, yet frost durability of CSC is no better than RSC especially for C30 CSC. To ensure frost durability of CSC, fines content and MB value of CS should be limited to be less than 10% and 1.0. Air-entraining agent is suggested for enhancing frost durability of C30 CSC exposed to freezing environment.
The authors declare that there are no competing interests regarding the publication of this paper.