The aim of this research is to provide a quantitative method for evaluating concrete segregation. Because of various conditions of concrete materials, mix proportions, and delivery, concrete can be segregated. The acquisition inspection executed in construction field for supplied ready-mixed concrete is an important quality control process for concrete. Among the inspections conducted at the project site, segregation of concrete mixture should be evaluated before placing the concrete mixture, currently a qualitative inspection on concrete segregation was conducted. For a normal concrete mixture with slumping behavior, shear slump or collapse slump often occur as an indication of segregation. The suggested evaluation index of segregation for normal concrete (EISN) was induced from the shape of the concrete slumping: relation between the maximum distance of flow and the minimum distance of flow. To evaluate the feasibility of EISN, two different concrete mixture conditions were tested. The recommended EISN parameter of segregation is 1.09 using the three grades of concrete quality. This new quantitative method of evaluating segregation of the concrete mixture is expected to contribute to a more efficient quality control in concrete construction.
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1 Introduction
Because of changing conditions of concrete materials, such as aggregate quality or other supplementary cementitious materials, determining the appropriate concrete mix proportion needed to assure a good mixture consistency is difficult. The purpose of a concrete mix design is to secure the desirable performance at fresh state and hardened state concrete such as workability, strength, durability, and uniform appearance with acceptable economic advantage. Since concrete is a heterogeneous material with a wide range of particle sizes and weight, sustaining uniformity is important to secure a desirable performance during fresh state. Hence, the meaning of ‘workability’ should not only contain fluidity with good mobility, but also uniformity with segregation resistance (Tattersall & Banfill, 1983). A well-designed mixture should have a segregation resistance during the working processes of delivering and placing; thus, unit water content and sand-to-aggregate ratio (S/a) are the representative factors dominating segregation (or uniformity) of the concrete mixture. Unit water content is the amount of water for unit volume (one cubic meter) of concrete mixture. For mixture proportioning, unit water content is determined by the target slump (ACI Committee211, 2002; de Larrard, 1999). Hence, the unit water content can be considered as a determining factor for workability of the concrete mixture. On the other hand, with a consideration that concrete mixture is a suspension of aggregates within the cement paste (Erdem et al., 2010; Toutou et al., 2007), if the water-to-cement ratio remains the same, the unit water content is a portion of the cement paste. Therefore, an increase or decrease of the unit water content means an increase or decrease of cement paste to carry the aggregates. S/a is the ratio of a fine aggregate (sand) volume to the entire aggregate volume within the unit volume (one cubic meter) of the concrete mixture (Aïssoun et al., 2016; Bassuoni & Nehdi, 2009; Khayat et al., 1996; Popovics, 1973). To determine the appropriate S/a, the S/a should be as high as possible unless segregation has not occurred. Namely, the S/a is directly related to the segregation of the concrete, i.e., it is related through cohesion of the concrete mixture because the sand particles (fine aggregate) are relatively fine particles in the concrete mixture (Mindess et al. 2002).
Most construction sites use ready-mixed concrete manufactured in certified plants. Nevertheless, there is a possibility of segregation of concrete mixture due not only to the concrete mix proportion and materials, but also due to other factors such as a long transfer time; thus, the supplied concrete should be inspected at the project site. According to the construction specifications or guides from many countries (ACI Committee311, 2004; Japan, 2009; Korea Standards Association, 2011); however, generally, the segregation of the concrete mixture is inspected by observing the conditions of the concrete mixture after the slump test, and this evaluation can be debatable because it is a qualitative method. Therefore, in this research, regarding a normal concrete mixture with a relatively low slump range (generally less than 150 mm) with slumping behavior, a quantitative method of evaluating segregation of concrete is introduced using the data obtained from the slump test result. The results of this research are expected to contribute on providing a new quantitative approach to segregation evaluation for normal concrete mixtures using simple slump test results.
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2 Quantitative Evaluation Method for Concrete Segregation
2.1 Current Methods
For normal concrete mixtures, slump test is a well-known test method for their ability to assess fresh state mixture conditions, that is, workability and uniformity (Mindess et al., 2002; Neville, 2012). Hence, on the project site, the segregation or cohesiveness of the concrete mixture is evaluated during the slump test. According to Neville (Neville, 2012), shear and collapse slump can be a harsh mixture with insufficient cohesiveness (see Fig. 1). Additionally, from Maruya et al.’s technical report (Maruya et al., 2013), a new method of investigating concrete segregation was suggested. The method is observing the remaining circle shape on top of the slumped concrete after hitting the slump plate with a rubber hammer and causing the concrete to fly up to 47 cm (see Fig. 2). Although this suggested that the method can evaluate the segregation or segregation resistance of concrete mixture, this method is still a qualitative method and it cannot be clear for “remaining circle”.
Fig. 1
Possible shapes of a slump.
Fig. 2
Schematic process of segregation evaluation for concrete mixture suggested by Maruya et al. (2013).
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×
As a quantitative evaluation method for segregation of concrete, several test methods were suggested such as column test (ASTM International, 2014), and sieve segregation test (British Standard Institution, 2010). Nevertheless, these test methods are suitable for not a normal concrete mixture but a high workability concrete mixtures or self-consolidating concrete. From the rheological aspect, concrete flow behavior can be analyzed using a Bingham model where the fluidity is measured based on yield stress and plastic viscosity (Papanastasiou, 1987). The cohesiveness of the mixture is related to the plastic viscosity of the mixture, and the segregation of the concrete mixture is induced by the insufficient plastic viscosity of the mortar mixture and cement paste for segregation of coarse aggregates (honeycomb) and water (bleeding), respectively (Han, 2014; Tregger et al., 2012). Therefore, measuring rheological properties of a concrete mixture can be a good method of evaluating the segregation resistance of a concrete mixture quantitatively. However, for normal concrete mixtures within the slump test range, it is hard to measure the rheological properties with currently available rheometers (Koehler & Fowler, 2004) because of excessively high torque on the motor of the rheometer. Therefore, even a fluidity evaluation using rheometer and rheology can be applied for high fluidity concrete mixture. Moreover, rheometers are expensive, and their raw data are not only difficult to use, but also the rheological test is hard to apply at all construction project sites.
2.2 Suggested Method
Considering the shape of the normal concrete after the slump test, the segregation of concrete can be determined based on the shear slump due to insufficient cohesion (see Fig. 3). As shown in Fig. 3, measuring slump flow is not a valid. However, the concrete mixture with shear slump will collapse on one side of the concrete, and the spread distance will differ depending on the condition of the sides. Thus, the segregation possibility of a relatively low slump can be evaluated by calculating the ratio of the maximum distance of slump flow to the minimum distance of slump flow, which can be expressed as:
where EISN means evaluation index for segregation of normal concrete.
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3 Experiment
3.1 Experimental Plan
In this research, to evaluate the applicability of EISN for normal concrete mixtures, two different concrete mixture types were prepared: normal concrete mixture with relatively high water-to-cement ratio and high-strength concrete mixture with normal slump range. Although EISN is designed for normal concrete mixture with slumping behaviors (generally less than 150 mm), it can be also used to evaluate the concrete mixture of slumping range but designed as a high-strength concrete. As shown in Table 1, the water-to-cement ratios of each case was fixed to 0.50 and 0.30 for normal concrete mixture and high- strength concrete with normal slump range, respectively. Specifically, for high-strength concrete mixture, to control the fluidity to a similar range of the normal concrete mixture, superplasticizer was added 0.25% of the cement mass. Regarding two different concrete mixtures, S/a and unit water content were changed to induce segregation from 0.35 to 0.55, and from 165 to 205 kg/m3. To evaluate the workability and segregation of the mixtures, slump test was conducted, and slump and slump flow values were measured. The segregation of concrete mixtures was determined based on observation, and the observation results were compared with EISN from slump and slump flow data. Additionally, air content and unit volume weight were checked, and compressive strength was also tested at 3, 7, and 28 days.
Table 1
Experimental plan.
Mixture conditions
w/c*
0.50 (for normal concrete mixture)
0.30 (for high-strength concrete mixture
Target air content (%)
4.5 ± 1.5
S/a**
0.35, 0.40, 0.45, 0.50, 0.55
W (kg/m3)***
165, 175, 185, 195, 205
Tests
Fresh state
Slump, slump flow, air content, unit volume weight
Hardened state
Compressive strength (@ 3, 7, and 28 days)
*w/c water-to-cement ratio.
**S/a sand-to-aggregate ratio.
***W unit water content.
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3.2 Materials and Test Methods
Ordinary Portland cement from South Korea was the cement used for this research. The properties of this cement are similar to the Type I ordinary Portland cement from ASTM C150 (ASTM International, 2012a). According to the manufacturer, the specific gravity is 3.15 and fineness is 3,390 cm2/g. For mixing water, tap water was used. Fine aggregate was natural river sand (specific gravity: 2.59, fineness modulus 2.21, and 0.08 mm sieve pass rate: 2.87) was used while coarse aggregate was crushed rock. To satisfy gradation of coarse aggregate, the nominal maximum size of 25 and 10 mm aggregates were mixed with ratio of 8 to 2. For coarse aggregate, specific gravity was 2.67, and fineness modulus was 6.59. As a chemical admixture, air entrainer and a superplasticizer (from Dongnam corp.) was used. The air entrainer was used for both normal and high-strength concrete mixtures, while the superplasticizer was used only for high-strength concrete mixture. All chemical admixtures used were general products from a South Korean vendor and the superplasticizer used was a naphthalene-based high-range water reducer.
Concrete mix proportions were designed based on the testing factors of unit water content and S/a as shown in Tables 2 and 3 for normal concrete and high-strength concrete, and an air entrainer was added as 0.01% of the cement mass to satisfy the target air content for the control mixture of S/a at 0.45 and the unit water content at 185 kg/m3. Concrete mixing was conducted with a pan-type mixer with a 60-L capacity. The mixing protocol followed ASTM C192 (ASTM International, 2013a). All fresh state tests were executed immediately after the mixing, and a concrete cylindrical mold was cured as per ASTM C192 with diameter 100 mm and height 200 mm and was used until the designated ages were determined for the compressive strength test.
Table 2
Mix proportions of normal concrete mixtures.
w/c*
Proportion**
Unit volume weight (kg/m3) ***
SP/AE**** (kg/m3)
S/a
W
OPC
Fine aggregate
Coarse aggregate
0.50
0.35
165
330
624
1163
0.99/0.03
175
350
609
1135
1.05/0.04
185
370
594
1079
1.11/0.04
195
390
579
1052
1.17/0.04
205
410
564
1107
1.23/0.04
0.40
165
330
713
1073
0.99/0.03
175
350
696
1047
1.05/0.04
185
370
679
1022
1.11/0.04
195
390
662
996
1.17/0.04
205
410
645
971
1.23/0.04
0.45
165
330
802
984
0.99/0.03
175
350
783
960
1.05/0.04
185
370
763
937
1.11/0.04
195
390
744
913
1.17/0.04
205
410
725
890
1.23/0.04
0.50
165
330
891
894
0.99/0.03
175
350
870
873
1.05/0.04
185
370
848
852
1.11/0.04
195
390
827
830
1.17/0.04
205
410
806
809
1.23/0.04
0.50
165
330
980
805
0.99/0.03
175
350
957
786
1.05/0.04
185
370
933
766
1.11/0.04
195
390
910
747
1.17/0.04
205
410
886
728
1.23/0.04
*w/c water-to-cement ratio.
**S/a sand-to-aggregate ratio, W unit water content.
***OPC ordinary Portland cement.
****SP superplasticizer, AE air entrainer.
Table 3
Mix proportions of high-strength concrete mixtures.
w/c*
Proportion**
Unit volume weight (kg/m3)***
SP/AE**** (kg/m3)
S/a
W
OPC
Fine aggregate
Coarse aggregate
0.30
0.35
165
550
547
1056
1.38/0.06
175
583
529
1021
1.46/0.06
185
617
511
986
1.54/0.06
195
650
492
950
1.63/0.07
205
683
474
915
1.71/0.07
0.40
165
550
625
974
1.38/0.06
175
583
604
742
1.46/0.06
185
617
583
909
1.54/0.06
195
650
563
877
1.63/0.07
205
683
541
844
1.71/0.07
0.45
165
550
703
893
1.38/0.06
175
583
680
864
1.46/0.06
185
617
656
834
1.54/0.06
195
650
633
804
1.63/0.07
205
683
609
774
1.71/0.07
0.50
165
550
781
812
1.38/0.06
175
583
755
785
1.46/0.06
185
617
729
758
1.54/0.06
195
650
703
731
1.63/0.07
205
683
677
704
1.71/0.07
0.50
165
550
860
731
1.38/0.06
175
583
831
707
1.46/0.06
185
617
802
682
1.54/0.06
195
650
773
658
1.63/0.07
205
683
745
633
1.71/0.07
*w/c water-to-cement ratio.
**S/a sand-to-aggregate ratio, W unit water content.
***OPC ordinary Portland cement.
****SP superplasticizer, AE air entrainer.
To evaluate air content and unit volume weight, ASTM C231 (ASTM International, 2010a), and C138 (ASTM International, 2013b) methods were executed. The slump test was conducted as per ASTM C143 (ASTM International, 2012b) and the EISN was calculated as shown in Eq. (1). Concrete flow distance was measured at both maximum and minimum distances as per ASTM C1611 (ASTM International, 2010b) without the inverted cone. The compressive strength was measured as per ASTM C39 (ASTM International, 2012c) at designated ages.
4 Results and Discussion
4.1 Normal Concrete Mixture
4.1.1 Basic Fresh State Properties
The basic properties of fresh state concrete were measured. These include slump, air content, and unit volume weight. Regarding slump test results, Fig. 4a, b shows the effects of S/a and unit water content on slump. Generally, slump varies from zero to 275 mm depending on test factors. For S/a, although S/a is known as controlling the cohesion of the mixture, increasing S/a decreased the slump of the mixtures. However, comparing the influence of S/a and unit water content on slump, unit water content showed higher influence on slump than S/a as widely known. Specifically, when unit water content was 165 kg/m3 (the lowest level under this research’s scope), slump was not influenced by S/a. Regarding the influence of unit water content on the slump of the mixture as shown in Fig. 4b, based on 185 kg/m3 of the unit water content, as the unit water content increased, the slope of the increasing slump became relatively gentle as the changing slope threshold of the slump started at approximately 175 mm (between 150 and 200 mm) of slump. It can be considered that the mixture intends to flow rather than slump over the 185 kg/m3 of unit water content. In other words, measuring slump cannot be measured efficiently after it reaches the slump threshold of approximately 175 mm.
Fig. 4
Effects of S/a and unit water content on slump of normal concrete mixtures.
×
Regarding the air content and unit volume weight of the concrete, as shown in Fig. 5a, b, although all cases satisfied the target air content range, air content decreased with increased S/a, and increased with increased unit water content. Naturally, the unit volume weight of concrete showed an inverse proportional relationship. The unit volume weight’s influence on S/a affects air content, which means that increasing fine aggregate increased the removal of the air void. On the other hand, increasing the unit water content increased the portion of the cement paste to the aggregate. The increasing portion of cement paste means increasing a portion of the volume where the air void is located; thus, the amount of air entrainer increases with the weight of the cement even though the dosage is fixed.
Fig. 5
Effects of S/a and unit water content on air content of normal concrete mixtures.
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4.1.2 Segregation Evaluation (EISN)
To evaluate the segregation of concrete mixtures after the slump tests, the shape of the concrete slump is shown in Fig. 6. The segregation of concrete mixtures was evaluated by observation, and maximum and minimum distances were measured to calculate the EISN. The results of observation for segregation and EISN are summarized in Table 4 where the S/a decreases, and the unit water content increases causing segregation to occur. From EISN values calculated from maximum and minimum distances of slump flow, all values were distributed between 1.00 and 1.16. Specifically, based on similar results occurring for segregation, the EISN values were decreased with increased S/a and increased with increased unit water content as shown in Fig. 7a, b, respectively. Namely, with an increasing EISN value (low S/a, and high unit water content), a possible segregation of the concrete mixtures exists. As shown in Eq. (1), 1.00 of EISN means same distance between the maximum and the minimum distance of slump flow, and thus it is highly possible that the true slump will appear without segregation or collapse slump. Therefore, Fig. 8 shows 1.09 of EISN as the maximum border when evaluating a non-segregating concrete mixture. Additionally, based on the EISN and concrete segregation resistance, the concrete mixtures could be categorized into three quality grades. Quantitatively, it can be suggested to categorize EISN for grade I is between 1.00 and 1.03; grade II is between 1.03 and 1.06, and grade III is between 1.06 and 1.09.
Fig. 6
Shapes of the slumping concrete after slump test for normal concrete mixtures.
Table 4
Slump and slump flow test results and EISN calculation data of normal concrete mixtures.
Mixture*
Slump (mm)
Slump flow (mm)
Segregation
S/a
W
Average
Max. (a)
Min. (b)
EISN (a/b)
Evaluation**
0.35
165
35
208
210
205
1.02
O
175
150
293
310
275
1.13
X
185
185
335
360
310
1.16
X
195
230
435
460
410
1.12
X
205
275
550
580
520
1.12
X
0.40
165
20
205
205
205
1.00
O
175
130
253
260
245
1.06
O
185
205
373
390
355
1.10
X
195
220
445
470
420
1.12
X
205
270
540
570
510
1.12
X
0.45
165
30
208
210
205
1.02
O
175
120
235
240
230
1.04
O
185
195
333
345
320
1.08
O
195
205
350
370
330
1.12
X
205
235
433
455
410
1.11
X
0.50
165
45
213
215
210
1.02
O
175
105
218
220
215
1.02
O
185
175
323
330
315
1.05
O
195
200
423
435
410
1.06
X
205
225
430
450
410
1.10
X
0.55
165
10
203
205
200
1.03
O
175
80
210
210
210
1.00
O
185
160
308
315
300
1.05
O
195
185
400
410
390
1.06
O
205
210
415
425
405
1.05
O
*S/a sand-to-aggregate ratio, W unit water content.
**Segregation of the concrete mixtures was evaluated by observation (O: non-segregation, X: segregation).
Fig. 7
Effects of S/a and unit water content on EISN of normal concrete mixtures.
Fig. 8
Categorizing grades of concrete mixtures depending on EISN for normal concrete mixture.
×
×
×
4.1.3 Compressive Strength
Compressive strength of the concrete mixture was evaluated based on S/a, and unit water content. Theoretically, changing S/a or unit water content should not affect the compressive strength of the concrete mixture since the water-to-cement ratio is same. However, Fig. 9a, b shows data with a scattered trend toward error for averaged compressive strength values from five different unit water contents with S/a. Compressive strength gradually increased with increased S/a as shown in Fig. 10a. Similar values usually appeared regardless of unit water content change (Fig. 10b). Compressive strength gradually increased with an increase in S/a. This was a result of improved packing with small particles of fine aggregate. Based on this result, it can be considered that segregation or poor quality of fresh state properties of concrete mixture can affect the compressive strength of hardened state concrete.
Fig. 9
Effects of S/a and unit water content on compressive strength of normal concrete mixture depending on age. Each data point represents an averaged value from five-unit water content cases and S/a cases for the graph of S/a and the graph of unit water content, respectively.
Fig. 10
Effects of S/a and unit water content on slump of high-strength concrete mixtures.
×
×
4.2 High-Strength Concrete Mixture
4.2.1 Basic Fresh State Properties
Unlike the normal concrete mixtures, although high-strength concrete mixtures were designed to have a similar range of slump values, high-strength concrete mixtures contain high volume of cement which can cause high viscosity and superplasticizer with dispersing action. For the slump, as designed, all mixture slump values were no higher than 215 mm. Depending on the S/a, as shown in Fig. 10a, the slump decreased with increased S/a. The effect of unit water content is shown in Fig. 10b, where slump increased with increased unit water content. Although both S/a and unit water content affected the slump of the concrete mixtures, unit water content had a greater effect than S/a on the slump of the concrete mixtures as well as the normal concrete mixtures. Regarding air content and unit volume weight, air content decreased as the S/a increased as shown in Fig. 11a and increased as the unit water content was increased as shown in Fig. 11b. The unit volume weight had an inverse proportional relationship with air content. Comparing S/a and unit water content, unit water content had more of an effect on air content than S/a.
Fig. 11
Effects of S/a and unit water content on air content of high-strength concrete mixtures.
×
4.2.2 Segregation Evaluation (EISN)
Figure 12 shows the segregation evaluation measures of the concrete mixtures based on the shapes of the concrete after the slump test. Because of the relatively high cohesiveness of the mixtures due to the low water-to-cement ratio, these mixtures were hard to evaluate as a segregation or collapsed slump for high-strength concrete mixtures. This also relates to the relatively low slump values of high-strength concrete mixtures compared to the slump value of normal concrete mixtures (comparing Fig. 5 with Fig. 13). Additionally, to compare to the other evaluation methods, the remaining circle method by Maruya et al. (Maruya et al. 2013) was used (see Fig. 13). As shown in the figure, the Maruya et al.’s method determined the mixtures with less than 0.40 of S/a as a segregation although the mixtures were not segregated. To calculate EISN, the maximum and minimum distances were measured (see Table 5). As shown in Fig. 14a, b, there was no clear relationship between EISN and S/a and unit water content. The relation between slump and EISN is expressed in Fig. 15 in which all cases show less than 1.09 of EISN values without any segregation, while the Maruya et al.’s method evaluated as a segregation in some cases. Namely, in the case of high-strength concrete mixtures with a high solid volume fraction of 0.30 water-to-cement ratio due to high cohesiveness or plastic viscosity, shear slump or collapse slump did not occur, and based on the EISN method, there was no evaluation as a segregation. On the other hand, according to Maruya et al.’s evaluation method, because there was no remaining ring at S/a 0.35 and 0.40, the concrete mixtures were evaluated as segregated mixtures, although low unit water content mixtures did not show the segregation (see Fig. 15). Therefore, it can be stated that Maruya et al.’s method is not completely suitable for a high viscous concrete mixture such as high-strength concrete mixtures with low water-to-cement ratio (high solid volume fraction). Furthermore, additional study on the feasibility of the EISN method is needed for slumping concrete mixtures with high viscosity.
Fig. 12
Shapes of the slumping concrete after slump test for high-strength concrete mixtures.
Fig. 13
Evaluation of segregation of high-strength concrete mixtures by Maruya et al. method. The segregation evaluation was also conducted by the same method, and the circles on the concrete show the consistently appearing circle shape.
Table 5
Slump and slump flow test results and EISN calculation data of high-strength concrete mixtures.
Mixture*
Slump (mm)
Slump flow (mm)
Segregation
S/a
W
Average
Max. (a)
Min. (b)
EISN (a/b)
Evaluation**
0.35
165
55
203
205
200
1.03
X
175
105
210
215
205
1.05
X
185
163
275
280
270
1.04
X
195
195
283
285
280
1.02
X
205
215
233
335
330
1.02
X
0.40
165
43
203
205
200
1.03
X
175
80
205
210
200
1.05
X
185
135
245
250
240
1.04
X
195
175
268
270
265
1.02
X
205
200
295
295
295
1.00
O
0.45
165
25
200
200
200
1.00
O
175
55
205
205
205
1.00
O
185
115
240
245
234
1.05
O
195
160
253
260
245
1.06
O
205
170
273
275
270
1.02
O
0.50
165
20
200
200
200
1.00
O
175
40
203
205
200
1.03
O
185
100
233
235
230
1.02
O
195
135
243
245
240
1.02
O
205
155
265
275
255
1.08
O
0.55
165
0
200
200
200
1.00
O
175
35
203
205
200
1.03
O
185
70
205
205
205
1.00
O
195
90
215
220
210
1.05
O
205
130
257
260
255
1.02
O
*S/a sand-to-aggregate ratio, W unit water content.
**Segregation of the concrete mixtures was evaluated by Maruya et al. suggested method (O: non-segregation; X: segregation).
Fig. 14
Effects of S/a and unit water content on EISN of high-strength concrete mixtures.
Fig. 15
Categorizing grades of concrete mixtures depending on EISN for normal concrete mixtures. The evaluation of segregation or non-segregation is the result of Maruya et al.’s method, and this method is recognized as limited under these conditions.
×
×
×
×
4.2.3 Compressive Strength
For compressive strength of the concrete mixture, all values were within 49.4–57.4 MPa. As shown in Fig. 16a, b, the influence of S/a and unit water content was not significant for compressive strength. Specifically, in the case of S/a, the compressive strength was increased by increasing S/a until S/a 0.45, and the compressive strength either remained constant or decreased after reaching S/a 0.45. Notably, when increasing a portion of fine aggregate, it can fill the void of the concrete mixture while excessively high portions of fine aggregate does not help to increase compressive strength. For unit water content, compressive strength generally decreases when unit water content increases within a small or limited range. This study examined the compressive strength results pertaining to S/a and unit water content used during the mix proportioning process. Study results indicate that even though the factor dominating compressive strength is not the S/a and unit water content, an inappropriate range of S/a and unit water content can be harmful to the compressive strength of concrete mixtures.
Fig. 16
Effects of S/a and unit water content on compressive strength of high-strength concrete mixtures.
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4.3 Valid Range for Slump or Slump Flow
In this research, for the normal concrete mixture with slumping behavior, slump test was conducted and slump, and slump flow were measured to calculate the EISN. An EISN formula was determined to obtain segregation of the concrete mixture with slumping for both normal and high-strength concrete mixtures. A valid range of EISN was determined by measuring the slump and slump flow of concrete mixtures; thus, “the slumping concrete mixture” was defined. Generally, a slump test is conducted for the concrete mixture with slumping and normal workability, while slump flow tests are conducted for the concrete mixture with flowing and high workability. In a typical slump flow test, the flowing concrete mixture has no meaningful height for slump. In rheological aspect, the flowing concrete mixture has very low or no yield stress. Based on the definition of yield stress pertaining to the minimum stress that must be overcome for starting the mixture flow, the flowing concrete mixture begins to flow after lifting the slump cone. Because of the low or no yield stress of the concrete mixture, the concrete mixture can flow and spread widely whether the mixture suffers segregation or not; thus, no stacking occurs in the mixture to be measured as a slump. Furthermore, if there is a stacking of the mixture for a flowing concrete mixture, it is a stack of coarse aggregates due to the separation of the mortar or cement paste (segregation). Figure 17 shows the relation between slump flow and slump in normal concrete mixture, the figure shows the changing slope relationship. From the trend line, the slump value for changing the slope of the trend line (the point of the tangent line slope is one) can be calculated as 115.50 mm. Therefore, based on this result, a valid range of slump test and slump flow test results can be determined as an approximately 120 mm of slump. However, with the slump result of the normal concrete mixture in Fig. 4, it is applicable for practical conditions that 150 mm of slump as a boarder of the valid range of slump. Additionally, for EISN, it can be state that the concrete mixture with less than 120 mm slump is valid for EISN.
Fig. 17
Relationship between slump and slump flow for normal concrete mixtures.
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5 Conclusions
In this research, a quantitative evaluation method for segregation of normal concrete is introduced, which uses observation after the slump test, currently. Based on the collapsed slump or shear slump shape, the shape of the segregated concrete mixture was defined after the slump test, and the quantitative evaluation method was defined. By measuring both maximum and minimum flow distances of the concrete mixture after the slump test, the EISN value can be calculated and based on 1.09 of EISN value, thereby allowing the segregation of concrete mixture to be assessed. This method is also applicable for high-strength concrete mixtures including high volume of powders and superplasticizers as long as the concrete mixture is slumping. Additionally, according to the relation between slump and slump flow, a valid range of slump test and slump flow test was determined, and a valid range of EISN was also defined. Currently, determining segregation of concrete relies on the observation of engineers, and the evaluation method is qualitatively conducted. By using the method introduced in this paper, quality control of ready-mixed concrete can be improved in the normal strength concrete range.
Acknowledgements
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2018R1C1B6005814)
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Authors information
In-Deok Han, Ph. D., Associate Professor, Department of Architectural Engineering, Jeju National University.
Dongyeop Han, Ph. D., Associate Professor, Department of Architectural Engineering, and Engineering Research Institute, Gyeongsang National University.
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