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Publication Number: FHWA-HRT-06-117
Date: December 2006

Chapter 3: Effectof Air Content and W/C on Freeze-Thaw Resistance

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In order to evaluate the relation between concrete microstructure and its freeze-thaw resistance, an experiment was designed for concretes with fresh air contents (total air) ranging from 2.5 to 4.5 percent and w/c ranging from 0.4 to 0.5; freeze-thaw testing was performed using ASTM C 666,(21) Procedure A. Specimens were monitored for changes in resonant frequency (ASTM C 215(22)) and mass at regular intervals. ASTM C457(18) air void system evaluations (both modified point count and linear traverse) were conducted on hardened specimens from each mix.

Experimental Investigation

The experiment was designed as a central composite design in the two variables, with a total of 13 mixes (4 factorial points, 4 axial points, and 5 center points). Central composite design, (which is enhanced factorial design), is widely used for fitting a second-order response surface. It allows estimation of a full quadratic model for each response. Response surface methodology consists of a set of statistical methods normally applied in situations where several factors (for instance, the proportions of individual materials in concrete)—in this case w/c ratio and air entrained—influence one or more performance characteristics, or responses (freeze-thaw resistance, for example).

The experiment design of this study consists of 2k factorial points, 2*k axial points, and 5 center points, where k is the independent variable (in this case 2). The design is shown in table 1. Shaded mixes represent the control mix (center point), which was repeated several times.

Table 1. Experiment design for mixes 115–118.
Run # Mix ID W/C Total fresh air (%) Point type
1 115-1 0.45 3.5 Center
2 115-2 0.45 3.5 Center
3 115-3 0.40 4.5 Factorial
4 116-1 0.45 3.5 Center
5 116-2 0.40 2.5 Factorial
6 116-3 0.45 2.5 Axial
7 117-1 0.50 3.5 Axial
8 117-2 0.50 2.5 Factorial
9 117-3 0.45 4.5 Axial
10 117-4 0.45 3.5 Center
11 118-1 0.50 4.5 Factorial
12 118-2 0.45 3.5 Center
13 118-3 0.40 3.5 Axial

Materials included Type I Portland cement (ASTM C 150(36)), #57 crushed limestone coarse aggregate, ASTM C33(37) natural sand (quartz), and tap water (material properties can be found in appendix A). The air-entraining admixture was a Vinsol resin-based admixture meeting ASTM C 260(38) (AASHTO M154(39)). Concrete was mixed in a 0.25 m3 drum mixer according to ASTM C192.(40) The batch size was 0.07 m3. Mix proportions actually used are shown in table 2.

Fresh concrete tests included slump (ASTM C 143(41)), fresh air content (ASTM C231(14)), and unit weight (ASTM C 138(41)). Five 75- by 100- by 400-mm prisms (for freeze-thaw testing) and eight 100- by 200-mm cylinders (for strength testing and ASTM C457 evaluations(18)) were cast for each mix. Admixture dose is given in liters (L) per 100 kg of cement.

Table 2. Mixture proportions for mixes 115–118.
Mix ID W/C Coarse agg (SSD)
kilogram
(kg)/m3
Fine agg (SSD)
kg/m3
Cement
kg/m3
Water
kg/m3
AEA
L/100 kg
WRA†
L/100 kg
115-1 0.45* 976 866 355 163 0.033 0.260
115-2 0.45 978 870 355 160 0.033 0.260
115-3 0.40 978 891 355 142 0.072 0.260
116-1 0.45 970 889 352 159 0.007 0.319
116-2 0.40 979 944 356 142 0.003 0.260
116-3 0.45 980 898 356 160 0.008 0.260
117-1 0.50 979 825 356 178 0.023 0.260
117-2 0.50 978 850 355 178 0.002 0.260
117-3 0.45 975 854 355 160 0.046 0.260
117-4 0.45 976 869 355 160 0.028 0.260
118-1 0.50 978 797 355 178 0.043 0.260
118-2 0.45 978 870 355 160 0.023 0.260
118-3 0.40 981 920 357 143 0.036 0.260

* Actual as-batched w/c was 0.46 for this mix.
† WRA—water reducing admixture

Results

The fresh concrete properties for each mix are shown in table 3.

Slumps were quite low at w/c of 0.40 and 0.45 (13 mm or less) but increased to 50 mm or more at w/c=0.50. Slump also increased slightly with air content at w/c=0.50.

The mean 28-day strengths (21-day strengths for mixes 115-1, 115-2, and 115-3) and standard deviations are shown in table 4.

Table 3. Fresh concrete properties for mixes 115–118.
Mix ID W/C Slump (mm) Total air content (%) Unit weight (kg/m3)
115-1 0.46 0 3.6 2361
115-2 0.45 5 3.6 2368
115-3 0.40 0 4.6 2374
116-1 0.45 15 3.4 2379
116-2 0.40 0 2.5 2401
116-3 0.45 15 2.4 2416
117-1 0.50 65 3.5 2390
117-2 0.50 50 2.7 2366
117-3 0.45 0 4.4 2371
117-4 0.45 15 3.8 2358
118-1 0.50 70 4.6 2352
118-2 0.45 5 3.6 2377
118-3 0.40 0 3.3 2387
Table 4. 28-Day strength results for mixes 115–118.
Mix ID W/C Total fresh
air content
(%)
Mean 28-day
strength*
(megapascals (MPa))
Std. dev.
(MPa)
115–1* 0.46 3.6 39.9 0.3
115–2* 0.45 3.6 43.1 1.6
115–3* 0.40 4.6 43.1 1.7
116–1 0.45 3.4 50.3 1.4
116–2 0.40 2.5 53.2 0.2
116–3 0.45 2.4 50.3 0.8
117–1 0.50 3.5 38.6 5.8
117–2 0.50 2.7 40.8 1.0
117–3 0.45 4.4 44.3 1.5
117–4 0.45 3.8 46.3 4.8
118–1 0.50 4.6 41.0 1.3
118–2 0.45 3.6 49.0 0.8
118–3 0.40 3.3 49.3 0.0

* 21-day strengths are reported for 115-1, 115-2, and 115-3. All results based on 2 tests of 100- by 200-mm cylinders.

The results of freeze-thaw testing are summarized in table 5. DF ranged from 3.3 to 94.8 percent. With one exception (mix 116-1), mixes with greater than 3.0 percent fresh air content performed well (DF > 80) through more than 300 cycles of freezing and thawing. All specimens suffered some mass change (loss) during testing. Mass losses ranged from 0.61 to 3.66 percent. The mass loss can be attributed to surface scaling, which occurred on all beams. Any mass gain resulting from water entering the concrete through cracks was obscured by the losses due to scaling.

Table 5. Summary of freeze-thaw test results for mixes 115–118.
Mix ID W/C Fresh air (%) Cycles Final RDM (%) DF (%) Mass change (%)
115-1 0.46 3.6 300 84.2 84.2 –3.66
115-2 0.45 3.6 300 80.3 80.3 –3.28
115-3 0.40 4.6 300 85.7 85.7 –2.63
116-1† 0.45 3.4 103 51.5 17.7 –0.66
116-2 0.40 2.5 132 48.0 21.1 –0.61
116-3 0.45 2.4 191 57.7 36.7 –2.60
117-1 0.50 3.5 303* 89.6 90.5 –3.53
117-2 0.50 2.7 38 26.4 3.3 0.13
117-3 0.45 4.4 303* 90.9 91.8 –3.39
117-4 0.45 3.8 303* 90.3 91.2 –1.88
118-1 0.50 4.6 300 94.8 94.8 –1.73
118-2 0.45 3.6 300 92.7 92.7 –1.18
118-3 0.40 3.3 300 92.4 92.4 –0.92

* The values of DF are corrected to 300 cycles. The actual relative dynamic modulus is shown in final RDM column.
† Not included when averaging center mixes.

Tables 6 and 7 summarize the results of ASTM C457(18) modified point count (MPC) and linear traverse (LT) evaluations on polished surfaces cut axially from 100- by 200-mm cylinders.

The significantly different freeze-thaw resistance of mix 116-1, when compared to the other center mixes, and its low durability factor can be explained by its air void system. It seems that the fresh air content of mix 116-1 was not properly determined, so although the percentage of fresh air showed to be within the target range, both modified point count and linear traverse results show a different scenario. Not only was the air content much lower than the other center mixes, but also the specific surface was much lower and the spacing factor was much higher. As a result, when averaging center mixes, mix 116-1 is disregarded.

Table 6. Modified point count (MPC) results for mixes 115–118.
Mix ID Fresh air
(%)
Air
(%)
Paste
(%)
Voids
counted
MCL
(mm)
Voids per m Specific
surface
(mm-1)
Spacing
factor
(mm)
115-1 3.6 4.0 28.0 313 0.302 138 13.6 0.406
115-2 3.6 3.7 25.5 219 0.385 96 10.6 0.521
115-3 4.6 4.1 27.0 321 0.286 142 14.5 0.381
116-1 3.4 2.5 28.5 109 0.525 49 7.6 0.876
116-2 2.5 3.7 28.4 127 0.660 55 6.1 0.940
116-3 2.4 2.4 28.1 132 0.404 59 10.0 0.686
117-1 3.5 6.2 25.4 400 0.353 175 11.3 0.368
117-2 2.7 3.8 26.8 105 0.822 47 4.9 1.118
117-3 4.4 4.6 30.7 394 0.264 173 15.2 0.343
117-4 3.8 5.0 29.5 300 0.386 132 10.7 0.483
118-1 4.6 7.2 26.2 658 0.249 289 16.2 0.229
118-2 3.6 3.9 27.1 264 0.330 116 12.5 0.445
118-3 3.3 3.9 26.6 243 0.364 106 10.5 0.483

Notes: All results are averages of two tests on two different polished surfaces.
MCL=Mean chord length

Table 7. Linear traverse (LT) results for mixes 115–118.
Mix ID Fresh air
(%)
Air
(%)
Voids
counted
MCL
(mm)
Voids per m Specific
surface
(mm-1)
Spacing
factor
(mm)
115-1 3.6 4.8 386 0.282 169 14.1 0.363
115-2 3.6 6.1 300 0.465 130 8.6 0.533
115-3 4.6 5.3 463 0.262 201 15.3 0.320
116-1 3.4 3.8 140 0.612 63 6.5 0.879
116-2 2.5 4.9 192 0.587 83 6.8 0.744
116-3 2.4 3.6 178 0.460 79 8.7 0.673
117-1 3.5 5.4 393 0.315 173 12.7 0.381
117-2 2.7 4.1 127 0.732 55 5.5 1.011
117-3 4.4 5.3 473 0.257 209 15.6 0.315
117-4 3.8 4.9 327 0.345 142 11.6 0.437
118-1 4.6 5.6 581 0.221 256 18.1 0.264
118-2 3.6 4.2 276 0.345 122 11.5 0.472
118-3 3.3 3.9 221 0.399 98 10.0 0.564

Notes: All results are based on one test.
Spacing factors were calculated using paste content from MPC.
MCL=Mean chord length

Discussion and Analysis

Figures 16–18 show the influence of the air content (based on fresh air content) on durability. It can be observed that the mixes with fresh air content in the levels of 3.5 percent and 4.5 percent present similar freeze-thaw resistance. They last at least 300 cycles and their durability factors are higher than 80 percent, except for mix 116-1. On the other hand, the mixes with air content around 2.5 percent present much lower freeze-thaw resistance. A correlation of 0.78 was obtained between fresh air content and durability factor. The legends in the figures indicate the mix ID for the plotted points, and text boxes in the figure provide a summary of air content, and DF for each mix. The center mixes are represented by their average RDM (mix 116-1 was not included).

The graph shows the relative dynamic modulus, R D M, versus cycles. The horizontal axis represents the number of cycles, ranging from 0 to 350. The vertical axis is the relative dynamic modulus, R D M, in percentage, ranging from 0 to 120. This graph uses the air content as a variable, the water-cement ratio being kept the same at 0.40. The first, a curve with solid rhombs, corresponds to the mixes with air content 2.5 percent. The second curve, with triangles, corresponds to the mixes with air content 3.3 percent. The third curve, with solid squares, corresponds to the mixes with air content 4.6 percent. The plot shows that the mixes with fresh air content in the levels of 3.3 percent and 4.5 percent present similar freeze-thaw resistance. They last at least 300 cycles and their durability factors are higher than 80 percent. On the other hand, the mixes with air content 2.5 percent present much lower freeze-thaw resistance. The legends in the figures indicate the mix I D for the plotted points, and text boxes in the figure provide a summary of air content, and D F for each mix, such as D F equals 21.1 for air content of 2.5 percent, D F equals 85.7 for air content of 4.6 percent, and D F equals 92.4 for air content of 3.3 percent.

Figure 16. Graph. Relative dynamic modulus versus cycles for mixes with water-cement ratio=0.40.

The graph shows the relative dynamic modulus, R D M, versus cycles. The horizontal axis represents the number of cycles, ranging from 0 to 350. The vertical axis is the relative dynamic modulus, R D M, in percentage, ranging from 0 to 120. This complex graph uses the air content as a variable, the water-cement ratio being kept the same at 0.45. The first, a curve with solid rhombs, corresponds to the mixes with air content 2.4 percent. The second curve, with solid squares, corresponds to the mixes with air content 3.7 percent. The third curve, with triangles, corresponds to the mixes with air content 4.4 percent. The plot shows that the mixes with fresh air content in the levels of 3.7 percent and 4.4 percent present similar freeze-thaw resistance. They last at least 300 cycles, and their durability factors are higher than 85 percent. On the other hand, the mixes with air content around 2.4 percent present much lower freeze-thaw resistance. The legends in the figures indicate the mix I D for the plotted points, and text boxes in the figure provide a summary of air content, and D F for each mix, such as D F equals 36.7 for air content of 2.4 percent, D F equals 87.1for air content of 3.7 percent, and D F equals 91.8 for air content of 4.4 percent.

Figure 17. Graph. Relative dynamic modulus versus cycles for mixes with water-cement ratio=0.45.

The graph shows the relative dynamic modulus, R D M, versus cycles. The horizontal axis represents the number of cycles, ranging from 0 to 350. The vertical axis is the relative dynamic modulus, R D M, in percentage, ranging from 0 to 120. This graph uses the air content as a variable, the water-cement ratio being kept the same at 0.50. The first, a curve with solid rhombs, corresponds to the mixes with air content 2.7 percent. The second curve, with solid squares, corresponds to the mixes with air content 3.5 percent. The third curve, with triangles, corresponds to the mixes with air content 4.6 percent. The plot shows that the mixes with fresh air content in the levels of 3.5 percent and 4.6 percent present similar freeze-thaw resistance. They last at least 300 cycles and their durability factors are higher than 90 percent. On the other hand, the mixes with air content around 2.7 percent present much lower freeze-thaw resistance. The legends in the figures indicate the mix I D for the plotted points, and text boxes in the figure provide a summary of air content, and D F for each mix, such as D F equals 30.3 for air content of 2.7 percent, D F equals 90.5 for air content of 3.5 percent, and D F equals 94.8 for air content of 4.6 percent.

Figure 18. Graph. Relative dynamic modulus versus cycles for mixes with water-cement ratio=0.50.

The water-cement ratio (within the range tested) does not appear to play a significant role on the freeze-thaw resistance (figures 19 and 20). The correlation between water-cement ratio and durability factor was 0.04. Only for mixes with designed air content of 2.5 percent (figure 21), the mix with w/c=0.5 (117-2) shows a much lower freeze-thaw resistance. Nevertheless, this difference in performance seems to be much more related to the air void system (low specific surface and high spacing factor) than to the w/c ratio. It is confirmed if mixes 116-2 and 116-3 are compared, where the latter presents higher specific surface, lower spacing factor, and as a result, better freeze-thaw resistance.

The graph shows the relative dynamic modulus, R D M, versus cycles. The horizontal axis represents the number of cycles, ranging from 0 to 350. The vertical axis is the relative dynamic modulus, R D M, in percentage, ranging from 0 to 120. This graph uses the water-cement ratio as a variable, the air content being kept the same at 3.5 percent. The first, a curve with solid rhombs, corresponds to the mixes with water-cement ratio of 0.40. The second curve, with solid squares, corresponds to the mixes with water-cement ratio of 0.45. The third curve, with triangles, corresponds to the mixes with water-cement ratio of 0.50. The plot shows that the water-cement ratio does not affect the freeze-thaw resistance, and the difference in performance is related to the air content much more than to the water-cement ratio. They last at least 300 cycles, and their durability factors are higher than 80 percent. The legends in the figures indicate the mix I D for the plotted points, and text boxes in the figure provide a summary of air content and D F for each mix, such as D F equals 92.4 for water-cement ratio of 0.40, D F equals 84.9 for water-cement ratio of 0.45, and D F equals 87.1 for water-cement ratio of 0.50.

Figure 19. Graph. Relative dynamic modulus versus cycles for mixes with designed air void content of 3.5 percent.

The graph shows the relative dynamic modulus, R D M, versus cycles. The horizontal axis represents the number of cycles, ranging from 0 to 350. The vertical axis is the relative dynamic modulus, R D M, in percentage, ranging from 0 to 120. This graph uses the water-cement ratio as a variable, the air content being kept the same at 4.5 percent. The first curve, with solid rhombs, corresponds to the mixes with water-cement ratio of 0.40. The second curve, with solid squares, corresponds to the mixes with water-cement ratio of 0.45. The third curve, with triangles, corresponds to the mixes with water-cement ratio of 0.50. The plot shows that the water-cement ratio does not affect the freeze-thaw resistance and the difference in performance is related to the air content much more than to the water-cement ratio. They last at least 300 cycles, and their durability factors are higher than 85 percent. The legends in the figures indicate the mix I D for the plotted points, and text boxes in the figure provide a summary of air content and D F for each mix, such as D F equals 85.7 for water-cement ratio of 0.40, D F equals 91.8 for water-cement ratio of 0.45, and D F equals 94.8 for water-cement ratio of 0.50.

Figure 20. Graph. Relative dynamic modulus versus cycles for mixes with designed air void content of 4.5 percent.

The graph shows the relative dynamic modulus, R D M, versus cycles. The horizontal axis represents the number of cycles, ranging from 0 to 250. The vertical axis is the relative dynamic modulus, R D M, in percentage, ranging from 0 to 120. This graph uses the water-cement ratio as a variable, the air content being kept the same at 2.5 percent. The first curve, with triangles, corresponds to the mixes with water-cement ratio of 0.50. The second curve, with solid rhombs, corresponds to the mixes with water-cement ratio of 0.40. The third curve, with solid squares, corresponds to the mixes with water-cement ratio of 0.45. The plot shows that the water-cement ratio plays a significant role on the freeze-thaw resistance only when the air content is low (2.5 percent in this test). The legends in the figures indicate the mix I D for the plotted points, and text boxes in the figure provide a summary of air content and D F for each mix, such as D F equals 3.3 for water-cement ratio of 0.50, D F equals 21.1 for water-cement ratio of 0.40, and D F equals 36.7 for water-cement ratio of 0.45.

Figure 21. Graph. Relative dynamic modulus versus cycles for mixes with designed air void content of 2.5 percent.

The mass change over cycles can be associated with concrete deterioration. A mass gain can be an indication of cracking formation and water absorption through the cracks. On the other hand, the mass loss can also be related to concrete deterioration in the case where concrete specimens scale significantly during testing. This set of mixes (115–118) did not show any mass gain. Also, the mass loss did not present any trend in relation to air void system parameters (figures 22–24).

The graph shows the mass change, versus cycles. The horizontal axis represents the number of cycles, ranging from 0 to 350. The vertical axis is the mass change, in percentage, ranging from negative 4.00 to positive 1.00. This graph uses the air content as a variable, the water-cement ratio being kept at 0.40. The first curve, with solid rhombs, corresponds to the mixes with air content 2.5 percent. The second curve, with solid squares, corresponds to the mixes with air content 3.3 percent. The third curve, with triangles, corresponds to the mixes with air content 4.6 percent. The plot shows that the mass loss does not indicate any trend in relation to air void system parameters. The mixes with an air content of 2.5 percent show a mass change of about negative 0.5 percent after 130 cycles. The mixes with an air content of 3.3 percent show a mass change of about negative 0.9 percent after 300 cycles. The mixes with an air content of 4.6 percent show a mass change of about negative 2.8 percent after 300 cycles.

Figure 22. Graph. Mass change versus cycles for mixes with water-cement ratio=0.40.

The graph shows the mass change, versus cycles. The horizontal axis represents the number of cycles, ranging from 0 to 350. The vertical axis is the mass change, in percentage, ranging from negative 4.00 to positive 1.00. This graph uses the air content as a variable, the water-cement ratio being kept at 0.45. The first curve, with solid rhombs, corresponds to the mixes with air content 2.4 percent. The second curve, with solid squares, corresponds to the mixes with air content 3.7 percent. The third curve, with triangles, corresponds to the mixes with air content 4.4 percent. The plot shows that the mass loss does not indicate any trend in relation to air void system parameters. The mixes with an air content of 2.4 percent shows a mass change of about negative 3.4 percent after 220 cycles. The mixes with an air content of 3.7 percent show a mass change of about negative 2.9 percent after 300 cycles. The mixes with an air content of 4.4 percent show a mass change of about negative 3.3 percent after 300 cycles.

Figure 23. Graph. Mass change versus cycles for mixes with water-cement ratio=0.45.

The graph shows the mass change, versus cycles. The horizontal axis represents the number of cycles, ranging from 0 to 350. The vertical axis is the mass change, in percentage, ranging from negative 4.00 to 1.00. This graph uses the air content as a variable, the water-cement ratio being kept at 0.50. The first curve, with solid rhombs, corresponds to the mixes with air content 2.7 percent. The second curve, with solid squares, corresponds to the mixes with air content 3.5 percent. The third curve, with triangles, corresponds to the mixes with air content 4.6 percent. The plot shows that the mass loss does not indicate any trend in relation to air void system parameters. The mixes with an air content of 2.7 percent show a mass change of about negative 0.2 percent after 50 cycles. The mixes with an air content of 3.5 percent show a mass change of about negative 3.6 percent after 300 cycles. The mixes with an air content of 4.6 percent show a mass change of about negative 1.8 percent after 300 cycles.

Figure 24. Graph. Mass change versus cycles for mixes with water-cement ratio=0.50.

In terms of air measurement, the fresh air void content, when measured according to ASTM C 231,(14) was always lower than the linear traverse air volume (measured according to ASTM C 457(18)) and, in most of the cases, lower than the modified point count, as well (figure 25).

The graph shows the comparison among modified point count test, linear traverse test, and fresh air content measured according to A S T M C 231. The horizontal axis represents the mix I D (13 mixes). For each mix, a modified point count test, linear traverse test, and fresh air content are represented by a graphed column: a solid hatched column represents the modified point count (M P C); a hatched column represents linear traverse test (L T) and a clear column represents the fresh air content measured according to A S T M C 231. The vertical axis is the air content, in percentage, ranging from 0.0 to 8.0 percent. The graph shows that the fresh air void content, measured according to A S T M C 231, was always lower than the linear traverse air volume (A S T M C 457), and in most of the cases, lower than the modified point count. No trend was observed between linear traverse and modified point count. According to the graph, M P C ranges from 2.2 to 7.1 percent, L T ranges from 3.5 to 7.1 percent, and air content measured according to A S T M C 231 ranges from 2.3 to 4.6 percent.

Figure 25. Graph. Comparison among modified point count test, linear traverse test, and fresh air content.

The spacing factor versus the relative dynamic modulus (figure 26) shows a clear trend (with correlation of 0.91): the higher the spacing factor, the lower the RDM. The specific surface shows the same trend (figure 27) but with a lower correlation (0.77).

The graph shows the relative dynamic modulus, R D M, versus spacing factor. The horizontal axis represents the spacing factor, ranging from 0 to 1.5 millimeters. The vertical axis is the relative dynamic modulus, R D M, in percentage, ranging from 0 to 100. The graph indicates that, the higher the spacing factor, the lower the R D M. A spacing factor of less than 0.5 confers an R D M of more than 80 percent. A spacing factor of 1.0 confers an R D M of less than 5 percent.

Figure 26. Graph. Relation between spacing factor and relative dynamic modulus.

The graph shows the relative dynamic modulus, R D M, versus specific surface. The horizontal axis represents the specific surface, ranging from 0 to 20. The vertical axis is the relative dynamic modulus, R D M, in percentage, ranging from 0 to 100 millimeters to the power of negative one. The graph indicates that the higher the specific surface, the lower the R D M. The graph shows that there is a noticeable change in behavior around 8 percent air content, such as: below 8 millimeters to the power of negative one , the R D M is more than 80 percent, and above 8 millimeters to the power of negative one , the R D M is less than 40 percent.

Figure 27. Graph. Relation between specific surface and relative dynamic modulus.

 

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