3.1 Fresh properties
The results of unit weight, air content, slump flow, V- funnel, U-box, and L-box tests are presented in Table 5. The aim is to compare the passing capability, filling capability, and segregation resistance of different SCC mixtures based on EFNARC [26].
Table 5. Properties of fresh concrete mixtures.
Mixtures
|
Unit weight (kg/m3)
|
Air content (%)
|
Slump flow (mm)
|
V-funnel (second)
|
U-box (mm)
|
L-box (mm/mm)
|
Reference (R)
|
2401
|
2.0
|
700
|
12
|
60
|
0.9
|
S
|
2350
|
2.1
|
650
|
13
|
50
|
0.86
|
HA
|
2396
|
2.5
|
710
|
11
|
40
|
0.91
|
AE
|
2345
|
4.0
|
660
|
14
|
50
|
0.85
|
HR
|
2345
|
2.3
|
640
|
13
|
50
|
0.86
|
S+HA
|
2380
|
2.4
|
640
|
12
|
70
|
0.91
|
S+AE
|
2365
|
4.1
|
720
|
10
|
40
|
0.92
|
S+HR
|
2375
|
2.0
|
690
|
11
|
50
|
0.86
|
HA+AE
|
2350
|
3.6
|
700
|
11
|
50
|
0.92
|
HA+HR
|
2370
|
2.5
|
685
|
11
|
40
|
0.93
|
HR+AE
|
2370
|
4.3
|
720
|
11
|
50
|
0.93
|
S+HA+HR
|
2379
|
2.4
|
690
|
11
|
40
|
0.88
|
S+AE+HR
|
2360
|
3.9
|
720
|
14
|
70
|
0.90
|
S+AE+HA
|
2360
|
4.0
|
725
|
13
|
60
|
0.93
|
HA+AE+HR
|
2364
|
4.2
|
720
|
14
|
70
|
0.92
|
S+AE+HA+HR
|
2365
|
3.7
|
715
|
13
|
60
|
0.91
|
3.1.1 Unit weight
The unit weight values are also illustrated in Fig. 2 for the sake of comparison. The effect of chemical admixtures on the settlement of SCC can be clearly grasped from this Fig 2. Accordingly, the addition of solely AE or HR to the mixture resulted in a decrease of 2.33% in unit weight as compared to the base (reference) mixture. The unit weight of the SCC mixture with only shrinkage reducer (S) was 2.12% lower than the reference value. Among the mixes with only one type of admixture, specimen HA had the highest unit weight value, which was almost equal to the respective value of the plain specimen.
If it is compared the mixes with more than one type of additive, the simultaneous use of HA and AE (HA+AE) resulted in the greatest decrease (2.12%) in unit weight. SCC with S+HR, S+HA+HR, and S+HA had closer unit weights to the reference mixture, which were only 1.08, 0.92, and 0.88% smaller than the reference value, respectively. The unit weights of specimens S+AE+HA and S+AE+HR were about 1.71% smaller than the reference value, while the specimen HA+AE+HR had a 1.54% lower unit weight compared to the plain mixture. The use of S+AEor S+AE+HA+HR decreased the unit weight by 1.50%, yet the use of HA+HR or HR+AE by 1.29% compared to the base specimen.
In general, the mixes with HA had higher unit weight values than the ones without HA. In other words, the hardening-accelerating additive has a slight or no influence on the unit weight. The use of HR and AE yields a slight decrease in the unit weight of SCC when used alone or with other chemical additives. This is because air-entraining (AE) admixture typically plays a critical role in reducing the surface tension at the water-air interface and protects concrete against freezing and thawing damage by inducing an air content of approximately 6% in the mixture [27-32]. Increasing the air content generally reduces the unit weight and the concrete strength [32].
3.1.2Air content
The air content values of the mixtures are shown in Fig. 3 for comparison. These values ranged between 2 and 4.3%. The air contents of the mixtures with air-entraining (AE) additive ranged from 3.6 to 4.3%, implying the major contribution of this additive to the air content as expected. The air content of fresh SCC with no additive was 2%, while this content increased to 4% in the sole presence of AE. The air content increased further to 4.1 and 4.3% when AE was used together with S and HR, respectively. The air content of HA+AE remained below the respective value of mixture AE. To be more specific, the inclusion of shrinkage-reducing (S) and heat-reducing (HR) additive in addition to AE contributed to the air content, while the inclusion of hydration-accelerating (HA)admixture had a reverse effect. All of the mixtures with two or three more additives in addition to AE had air contents in the vicinity of the mixture with only AE (4%). The low unit weights of the mixtures with AE can be attributed to the high air contents of these mixtures.
The air contents of mixtures without AE were close to the respective reference (base) values, which are2.1, 2.3, 2.5, and 2% for mixtures with only S, only HR, only HA, and both S and HA, respectively.
3.1.3 Slump flow
The slump flow diameters of the mixtures in Table 5 are also depicted in Fig. 4 for comparison. While the flow diameter of the base SCC specimen was 700 mm, the flow diameters of the S+AE+HA, HA+AE+HR, and S+AE mixtures were measured as 725, 720, and 720 mm, respectively. The minimum flow diameter was measured as 640 mm in HR and S+HA series. Although the AE mixture had a smaller flow diameter (660 mm) than the reference, the flow diameters of all mixtures containing other chemicals in addition to AE exceeded the reference value.
For instance, the flow diameter of S+AE+HA exceeded the base value by 3.57%, while the flow diameters of S+AE, HR+AE, HA+AE+HR, and S+AE+HR were 2.85% greater than the reference value. Similarly, the respective value of the S+AE+HA+HR mixture was 2.14 % greater than the flow diameter of the reference mixture. In other words, the addition of one, two, or three admixtures to SCC in addition to AE has a positive influence on the flowability. Parallel to this finding, the previous studies reported that the slump increases by approximately 100 mm per 1% air [33].
The flow diameters ofHA, S+HR, S+HA+HR were 1.42% above the reference value. The flow diameters of HR+AE, AE, S, HR, and S+HA series were 2.14, 5.72, 7.14%, 8.57, 8.57%, respectively, below the base value.
3.1.4 V-funnel
The V-funnel flow times of the mixtures, given in Table 5, are depicted in Fig. 5.
Minimum flow time among the mixtures was recorded as 10 seconds in S+AE, while SCC with HA, HA+AE, HA+HR, HR+AE, S+HR, and S+HA+HR series had flow times of 11 s, which is 8.33% smaller than the respective value of the base specimen. To be more specific, the addition of only HA or binary combinations of the admixtures decreased the flow time, i.e., increased the flowability, of SCC. Among the binary combinations of admixtures, only the S+HA mixture had an identical flow time (12 seconds) to the base mixture. Furthermore, among the mixtures with a single additive, only the mixture with HA was more flowable than the plain mixture.
The addition of only AE, HA+AE+HR, and S+AE+HR increased the flow time to 14 seconds, i.e., by 16.67%, while the addition of only S, only HR, S+AE+HA, and S+AE+HA+HR increased this time to 13 seconds, i.e., by 8.33%, compared to the base specimen. Except for the mixture S+HA+HR, the combinations of admixtures composed of three or four chemicals had higher flow times, i.e., less flowability, as compared to the reference mixture. When all results are examined, the flow time values are close to each other, implying that the combination of chemical additives does not exhibit negative effects on the flow time of SCC.
3.1.5 U-box
Next, the U-box passing values of the fresh mixtures, given in Table 5, are illustrated in Fig. 6 for comparison. U-box test is based on the measurement of filling a second chamber by passing through concrete reinforcement. The passing values indicate the difference between the levels of two compartments of the U-box apparatus and the flowability increases as this value approaches zero, meaning that the second compartment is filled to the same level as the first one.
In the presence of a single admixture and binary combinations of admixtures, the passing values remained below the base value (60 mm). The fresh properties of SCC mixtures with only HA, HA+HR, S+AE, and S+HA+HR had 40 mm passing values. The fresh properties of SCC mixtures with only AE, only S, only HR, HA+AE, HR+AE, and S+HR had 50 mm and the mixtures with HA+AE+HR and S+AE+HR had 70 mm passing values. Similar to the findings from the V-funnel test, the addition of HA, the binary combinations including HA (except for S+HA), and the ternary S+HA+HR combination increased the flowability of concrete mixture, i.e., decreased the passing value. The hydration-accelerating admixture contributes to the flowability of SCC when used alone in the mixture or combination with the other admixtures.
3.1.6 U-box
The highest L-box passing ratio values were found to belong to SCC specimens with HA+HR, HR+AE, and S+AE+HA as 93%. The passing ratios of SCC specimens, with HA+AE, S+AE, and HA+AE+HE, were measured as 92%, while the respective values of specimens with HA, S+HA, and S+AE+HA+HR were 91%. The passing ratio of SCC without additive (base) and with S+AE+HR was measured as 90% and the related values of S+HA+HR, S, HR, and S+HR specimens as 88, 86, 86, and 86%, respectively.
Accordingly, the SCC specimens with HA+HR, HR+AE, and S+AE+HA have a more homogeneous structure compared to other SCC mixtures and the flow was observed to be continuous throughout the horizontal chamber in these mixtures. When all results are examined, all values are close to each other. The combination of chemical additives did not show a major negative effect on the flow time of SCC. Similar to the results from the U-box test, the L-box test indicated that the combinations with air-entraining and hydration-accelerating admixtures create more flowable mixtures of SCC.
In general, the use of chemical additives, especially the hardening-accelerating, air-entraining, and hydration heat-reducing admixtures, and their binary combinations improve the flow properties of the SCC. Similar to the positive contribution of hydration accelerators on workability, SCC mixtures with air-entraining admixture show superior passing ability. When the air-entraining admixture is not used, large, entrapped pores are irregularly distributed in the SCC and this irregular distribution reduces the workability. But the pore size decreases, and the distribution of pores is enhanced with the inclusion of air-entraining admixture, and thus increasing workability [34]. Similar results were attained in the L-box test (Fig. 7). This test is applied to measure the flowability of SCC through reinforcing bars. In the test, the criterion of the passing ratio to exceed 0.8 is sought [26].
3.2 Hardened properties
The 2-day and 28-day compressive strength and 28-day splitting tensile strength test results were also explored in the present study and the values are given in Table 6.
Table 6. Properties of hardened properties of SCC.
Mixtures
|
2-Day Compressive Strength(MPa)
|
28-Day Compressive Strength(MPa)
|
28-Day Splitting Tensile Strength(MPa)
|
Reference (R)
|
44.5
|
63.5
|
4.2
|
S
|
42.0
|
60.5
|
4.0
|
HA
|
45.5
|
62.0
|
3.9
|
AE
|
41.0
|
57.0
|
3.6
|
HR
|
34.0
|
59.5
|
4.1
|
S+HA
|
43.0
|
62.0
|
3.9
|
S+AE
|
40.5
|
58.5
|
3.8
|
S+HR
|
35.5
|
59.5
|
3.9
|
HA+AE
|
41.5
|
59.0
|
3.8
|
HA+HR
|
36.5
|
59.0
|
3.9
|
HR+AE
|
27.0
|
57.5
|
3.7
|
S+HA+HR
|
38.0
|
59.5
|
4.1
|
S+AE+HR
|
37.0
|
59.0
|
3.9
|
S+AE+HA
|
41.5
|
60.0
|
3.8
|
HA+AE+HR
|
37.5
|
57.5
|
3.8
|
S+AE+HA+HR
|
39.0
|
60.0
|
4.0
|
3.2.1 Compressive strength
The compressive strength test results of different SCC mixtures are shown in Fig. 8.
The 2-day compressive strength values of the specimens ranged between 27.0and 45.5 MPa. The 28-day compressive strength, on the other hand, varied between 57.0 and 63.5 MPa. In plain words, the variation in compressive strength decreased significantly at 28 days. As expected, the hydration-accelerating (HA) additive had a positive influence on the early-age compressive strength. To be more specific, the strength of the mixture HA exceeded the related strength of the base mixture by 2.25%, while the 28-day strength of HA remained below the base value. This situation has shown that the hardening accelerator contributes to strength at an early age as shown in the literature [27,28]. The other specimens with a single additive had 2- and 28-day compressive strength values below the plain mixture. The lowest 2-day compressive strength value among the mixtures with a single additive was measured in mixture HR, whose2-day strength was 23.59 % smaller than the base value. The lowest 28-day strength value, on the other hand, was measured in mixture AE.
The 2-day and 28-day compressive strength values of all mixtures with two, three, and four additives were smaller than the respective values of the plain mixture. For instance, the 2-day compressive strengths of HR+AE, S+HR, and S+AE+HR series were 39.33, 20.22, and 17.98% lower than the base value. Among the mixtures with more than one additive, the S+HA mixture had the highest 2- and 28-day strength values, which were much closer to the related values of the reference mixture. For brevity, chemical additives had negative effects on the compressive strength of SCC, and this negative influence was more pronounced in the early-age strength, particularly in the mixtures with no HA. The negative influence of additives on compressive strength decreased in time. Strikingly, the addition of air-entraining and/or heat-reducing chemicals reduced the early-age compressive strength up to 39.33%, while this reduction became more insignificant in the long term. The 28-day compressive strength values of SCC specimens with AE, HR+AE, HA+AE+HR, and S+AE were 10.24, 9.45, 9.45, and 7.87% smaller than that of the base specimen. The reason for AE to reduce the concrete strength stems from the fact that AE increases the entrapped air content in the mixture. With the increase in the air content, the compactness of SCC decreases, and its porosity increases, so the strength decreases. These results are consistent with the ones in the literature [32].
Recall that, with the addition of shrinkage reducing and hydration accelerating admixture SCC had the highest 2- and 28-day strength values. As can be seen in Fig. 9, this results in considerably depending on dense Calcium-Silicate-Hydrate (C-S-H) gel-forming. The forming C-S-H gel decreases the number of pores in the cementitious composites due to the filling effect of pores [38-43]. The morphological structure of C-S-H gel resembles a web-like structure from weak crystalline fibers. During the hydration process, Calcium-Silicate-Hydrate gels concentrate around the hydrated cement particles and cover all particles.
3.2.2 Splitting tensile strength
The 28-day splitting tensile strength test results explored in the present study and the results are shown in Fig. 10.
The lowest and highest 28-day splitting tensile strength results were obtained as 3.6 and 4.2 MPa, respectively. Similar to the compressive strengths, all of the mixtures had tensile strength values lower than the related value of the reference mixture. As in 28-day compressive strength, SCC specimens with AE and HR+AE had the lowest tensile strength values, which were 14.28 and 11.90%, respectively, smaller than the related strength of the plain mixture. In summary, the utilization of different combinations of chemical additives did not contribute to the compressive and tensile strength.
3.3 Effect of shrinkage reducing admixture on length changes of SCC
3.3.1 Negative shrinkage
Negative shrinkage developments of SCC with 1% SRA and control SCC specimens were shown in Fig. 11.
As shown in Fig. 11, at any age of water curing, specimens of SCC with 1%SRA exhibited higher negative shrinkage than the control specimen. It is thought that SRA addition forms an adverse pressure acting against the cohesion of the gel particles, that SRA affects the water absorbed, C-S-H gels and pores, and that it supports particles lift from each other possessing thickened the absorbed water layer because of the water absorbed by CSH gels [35].
3.3.2 Drying shrinkage
Drying shrinkage developments of SCC with 1% SRA and control SCC specimens were shown in Fig. 12.
Drying shrinkage increased until the end of 180 days of total curing for both SCC with 1%SRA and SCC control specimens. The control specimens showed higher drying shrinkage than that of SCC with 1%SRA specimens at the end of curing age. Research determines the effects of SRA on shrinkage as a complex mechanism [36]. it is usually believed that SRA helps decrease drying shrinkage by decreasing the surface tension [37]. SRA addition plays a key role in drying shrinkage and is particularly striking in decreasing it at the early ages.
3.3.3 Negative shrinkage due to W/D cycles in NaSO4 solution
Negative shrinkage developments of SCC with 1% SRA and control SCC specimens after subjected to W/D cycles Na2SO4 solution were shown in Fig. 13.
According to Fig. 13, both SCC with 1% SRA and control SCC specimens illustrated increases in negative shrinkage that they expanded upon exposure to W/D cycles in sulfate suspension and the trend of expansion was continuous until the end of 180 days which was awaited because of the blended ingress of both water and sulfate ions (that lead to ettringite) causing expansion as inferred previously [15].