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

Chapter 4: Useof Synthetic Air-Entraining Admixture

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The scarcity of Vinsol resin admixture is responsible for the increasing use of synthetic admixtures. The freeze-thaw performance of marginal air mixes containing synthetic admixtures was investigated in a comparison with Vinsol resin. This experiment was made up of concretes with a wide range of air contents, batched with two different air-entraining admixtures (AEA), Vinsol and a synthetic. 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

Two sets of tests were performed—one for each of the two air-entraining admixtures: set 1 containing Vinsol resin air-entraining admixture and set 2 containing synthetic air-entraining admixture. The mix proportion of the two sets was the same. Each set consisted of five concrete mixtures proportioned with w/c ratios of 0.45 and target fresh air contents of 2.5 to 4.5 percent, in increments of 0.5 percent. In set 1, an additional non-air-entrained concrete mixture was also proportioned. The materials used are shown in table 8.

Table 8. Materials used for set 1 (mixes 223–302—Vinsol resin (VR) AEA) and set 2 (346–350—(synthetic) SYN AEA).
Component Set 1 (223–302—VR AEA)) Set 2 (346–350—SYN AEA))
Water Municipal tap water Municipal tap water
Cement Type I Type I
Fine aggregate Natural sand Natural sand
Coarse aggregate #57 Crushed limestone #57 Crushed limestone
AEA type Vinsol resin Synthetic
WRA ASTM C 494 Type A*(42) ASTM C 494 Type A*(42)

* The water reducer admixture used in both sets was the same and meet the requirements of ASTM C 494(42) Type A, water-reducing, Type B, retarding, and Type D, water-reducing and retarding, admixtures.

The mixture proportions used for sets 1 and 2 are shown in tables 9 and 10.

Table 9. Mixture proportions for set 1 (223–302)—w/c=0.45.
Mix ID Target air
(%)
Coarse agg (SSD)
kg/m3
Fine agg (SSD)
kg/m3
Cement
kg/m3
Water
kg/m3
Vinsol AEA
L/100kg
WRA
L/100kg
223 4.0 1015 836 356 160 0.033 0.210
224 3.5 1015 836 356 160 0.026 0.210
225 3.0 1015 836 356 160 0.035 0.210
226 4.5 1015 836 356 160 0.048 0.210
227 2.5 1015 836 356 160 0.013 0.210
302 1015 876 356 160 0.000 0.415
Table 10. Mixture proportions for set 2 (mixes 346–350)—w/c=0.45
Mix ID Target air
(%)
Coarse agg (SSD)
kg/m3
Fine agg (SSD)
kg/m3
Cement
kg/m3
Water
kg/m3
Synthetic AEA
L/100kg
WRA
L/100kg
348 2.5 1015 857 356 160 0.028 0.266
346 3.0 1015 861 356 160 0.039 0.266
347 3.5 1015 857 356 160 0.016 0.266
349 4.0 1015 805 356 160 0.079 0.266
350 4.5 1015 805 356 160 0.138 0.266

The concrete was mixed in batches of 0.042 m3  in a drum mixer with 0.125 m3 capacity. From each mix in set 1 (223–302), three 100- by 200-mm cylinders for compressive strength and five 75- by 100- by 400-mm beams for freeze-thaw testing were cast. Two cylinders 150- by 300- mm were cast for air void system analysis (ASTM C 457(18)). From each mix in set 2, the same number and type of cylinders were cast along with four beams (same size as set 1).

For set 1, using the Vinsol resin AEA, the mix sequence was as follows:

  1. The coarse and fine aggregates were added to the mixer and mixed for 30 seconds.
  2. The AEA was added to part of the water, the AEA and water were added to the mixer, and the materials were mixed for 30 seconds.
  3. The WRA was added to the remaining water, the WRA and water were added to the mixer, the cement was added to the mixer, and the materials were mixed for 4 minutes.
  4. The mixer was stopped for a 2-minute rest period.
  5. The materials were mixed for 2 additional minutes.

For set 2, using the synthetic AEA, various trial mix sequences were carried out until the target air content was achieved. The final mix sequence used was quite different from the regular mix procedures used, including the use of warm water and the addition of the two admixtures at the same time:

  1. The coarse and fine aggregates were added to mixer and mixed for 30 seconds.
  2. The AEA and WRA were added to the entire amount of water, the AEA, WRA, and water were added to the mixer, and the materials were mixed for 30 seconds.
  3. The cement was added to the mixer and the materials were mixed for 3 minutes.
  4. The mixer was stopped for a 3-minute rest period.
  5. The materials were mixed for 2 additional minutes.

In set 1, the admixtures were added to the mix separately (AEA first, then WRA); the water was at room temperature. In set 2, however, both admixtures were added at the same time (in accordance with the manufacturer’s recommendations), and the mixing water was warm (around 38 °C), in order to reduce the amount of air entrained. This procedure was necessary because, even when very small amounts of AEA were used, the air content produced exceeded the target values. In both sets of mixes, the coarse aggregates were batched dry, while the sand was batched moist. For each mix, the mixing water contents were adjusted accordingly.

All test specimens were cured in saturated limewater at 23±2 ºC. Compressive strength cylinders were cured for 28 days and freeze-thaw specimens for 14 days. The cylinder ends were ground in a concrete end grinder prior to strength testing per ASTM C 39.(43)

The specimens were tested in accordance with ASTM C666, Procedure A.(21) The specimens were monitored for changes in resonant frequency in accordance with ASTM C 215(22) and for mass changes (to the nearest 1 g) 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.

Results

The fresh concrete properties for sets 1 and 2 are shown in tables 11 and 12, respectively. A tolerance of 0.2 percent (deviation from target value) for fresh air content was considered acceptable.

Table 11. Fresh concrete properties for set 1 (VR AEA).
Mix ID Slump (mm) Air content (%) Unit weight (kg/m3)
223 44 4.0 2368
224 51 3.6 2379
225 44 3.1 2397
226 25 4.7 2349
227 44 2.7 2393
302 25 2.0 2400
Table 12. Fresh concrete properties for set 2 (SYN AEA).
Mix ID Slump (mm) Air content (%) Unit weight (kg/m3)
346 25 3.2 2363
347 19 3.5 2360
348 19 2.3 2390
349 25 4.0 2345
350 25 4.3 2339

The strength test results are shown in tables 13 and 14.

Table 13. 28-Day strengths for set 1 (VR AEA).
Mix ID W/C Fresh air content (%) Mean 28-day strength (MPa) Std. dev. (MPa)
223 0.45 4.0 36.4 2.0
224 0.45 3.2 39.4 1.6
225 0.45 3.1 38.5 1.8
226 0.45 4.7 38.1 1.3
227 0.45 2.7 43.2 0.1
302 0.45 2.0 (Non-air-entrained) 49.5 4.1

All results based on 3 tests of 100- by 200-mm cylinders.

Table 14. 28-Day strengths for set 2 (SYN AEA).
Mix ID W/C Fresh air content (%) Mean 28-day strength (MPa) Std. dev. (MPa)
346 0.45 3.2 44.8 1.0
347 0.45 3.5 35.0 1.2
348 0.45 2.3 42.5 1.0
349 0.45 4.0 38.1 0.4
350 0.45 4.3 32.6 1.8

All results based on 3 tests of 100- by 200-mm cylinders

Table 15 presents the air void system for set 1 (223–227). The air void parameters were determined according to ASTM C 457(18) (linear traverse) and represent the average of two measurements. The combined linear traverse and point count results can be found in appendix B. The air system of mix 302 (set 1) (non-entrained-air concrete) was not determined.

Table 15. Air void system of set 1 (VR AEA) measured by linear traverse.
Mix Fresh air (ASTM C 231) Air (%) ASTM C 457 Chord length
(mm)
Voids counted Mean chord length (mm) Voids per m Specific surface (mm2/mm3) Spacing factor (mm)
223 4 2.4 55 276 0.22 120 19.9 0.38
224 3.6 2.8 64 215 0.30 93 13.5 0.49
225 3.1 4.2 94 288 0.33 126 12.2 0.45
226 4.7 4.7 106 495 0.21 215 18.7 0.28
227 2.7 3.3 74 212 0.35 93 11.5 0.54

All mixes of set 1 presented marginal air void contents. The spacing factors were higher than the maximum value (0.2 mm) normally required for a good freeze-thaw resistance (most of them were above 0.36 mm) and the specific surface areas were lower than the normally desired (24 mm-1) for the total air volume in the range of the mixes for this study. Some of the mixes had specific surface area half of that, for example mix 227.

One could expect that the freeze-thaw performance of those mixes would not be adequate. Nevertheless, table 16 shows that DFs were above 80 percent, excepting for the non-air-entrained mix 302, which could be considered a satisfactory performance. All the air-entrained mixes withstood at least 300 cycles, excluding beam 224-A5 that suffered some damage during the handling of the specimen not related to testing. The tables and the graphs of the RDM  over cycles can be found in appendix B.

Table 16. Durability factor—results for set 1 (VR AEA).
Results are sorted by percent fresh air content.
Mix Fresh air (%) Durability factor
A1 A2 A3 A4 A5 Proc A avg Proc A std dev
302 2.0 non A/E 14.4 14.5 16.7 12.1 15.4 14.4 1.9
227 2.7 86.7 88.1 86.8 89.9 76.8 87.9 1.5
225 3.1 89.4 90.5 90.0 90.8 88.2 90.2 0.6
224 3.6 85.7 87.2 85.4 84.4 00.0* 85.7 1.2
223 4.0 89.6 88.5 89.7 84.4 92.0 88.9 2.8
226 4.7 92.1 94.0 93.0 95.0 95.3 93.5 1.3

* The DF for 224-A5 was not included when calculating averages and standard deviations

Figure 28 shows the RDM versus cycles for one of the air entrained concretes (mix 225), which is representative of the mixes of set 1, excepting mix 302 (figure 29).

The graph shows the relative dynamic modulus, R D M, versus cycles. Five individual specimens are represented. 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. Five specimens A 1, A 2, A 3, A 4, and A 5 were tested according to the procedure A. The graph indicates that the five specimens display almost similar freeze-thaw resistance. The five curves are overlapping. The relative dynamic modulus, R D M, corresponding to 300 cycles is about 90 percent.

Figure 28. Graph. Relative dynamic modulus versus cycles for mix 225
(VR AEA—3.1 percent fresh air content). Individual specimens are shown.
“A” stands for specimens tested according to Procedure A.

The graph shows the relative dynamic modulus, R D M, versus cycles. Five individual specimens are represented. 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. Five specimens A 1, A 2, A 3, A 4, and A 5 were tested according to the procedure A. The graph indicates that the five specimens display almost similar freeze-thaw resistance. The five curves are overlapping. For a non-air-entrained mix, such as mix 302, the relative dynamic modulus, R D M, corresponding to about 80 cycles is around 60 percent.

Figure 29. Graph. Relative dynamic modulus versus cycles for mix 302 (non-air-entrained). Individual specimens are shown. “A” stands for specimens tested according to Procedure A.

Set 2 presented a much better air system, with respect to spacing factor and specific surface area, but most of the mixes remained in the range of marginal air void parameters (table 17). The air void parameters were determined according to ASTM C 457(18) (linear traverse) and represented the average of two measurements. Figures 30 and 31 show the differences in the air void chord size distributions from linear traverse results for the two different admixture types, sets 1 and 2, respectively. It is important to mention that the air was well distributed and no clustering was observed.

Table 17. Air void system of mixes 346–350 (set 2—SYN AEA) measured by linear traverse.
Mix Fresh air (ASTM C 231) Air (%) ASTM C 457 Accum. chord length (mm) Accum. voids counted Mean chord length (mm) Voids per m Specific surface (mm2/mm3) Spacing factor (mm)
346 3.2 4.4 101 632 0.16 280 25.2 0.21
347 3.5 4.6 104 642 0.16 280 25.0 0.22
348 2.3 4.2 95 352 0.27 154 15.0 0.37
349 4 4.5 101 887 0.11 388 35.3 0.15
350 4.3 5.0 114 966 0.12 423 33.8 0.15

The graph shows the bubble size distribution, determined by C 457 (linear traverse) for five mixes (223, 224, 225, 226, and 227). The horizontal axis represents the bubble size, in microns, ranging 0 to 25 to 1000 to 3000. The vertical axis represents the number of bubbles, ranging from 0 to 400. The distribution graph indicates that most bubbles, for the 223, 224, 225, 226, and 227mixes, fall into the following groups: 50 to 75, 100 to 125, and 150 to 175 microns. The number of bubbles per size distribution is up to 100 for the 50 to 75 and 100 to 125 groups (in microns).

Figure 30. Graph. Bubble size distribution by C 457 (linear traverse) of set 1 with Vinsol resin admixture.

The graph shows the bubble size distribution, determined by C 457 (linear traverse) for five mixes (346, 347, 348, 349, and 350). The horizontal axis represents the bubble size, in microns, ranging from 0 to 25 to 1000 to 3000. The vertical axis represents the number of bubbles, ranging from 0 to 400. The distribution graph indicates that most bubbles, for the 346, 347, 348, 349, and 350 mixes, fall into the following groups: 0 to 25, 50 to 75, and 100 to 125 microns. The number of bubbles per size distribution is up to 380 for the 50 to 75group (in microns).

Figure 31. Graph. Bubble size distribution by C 457 (linear traverse) of set 2 with synthetic air-entraining admixture.

However, the freeze-thaw performance of set 2 was worse than that of set 1 (table18 and figures 32–35). Only mix 350 (the highest air volume, lowest spacing factor, and highest specific surface) had a DF above 80 percent. The tables of the RDM over cycles and the combined linear traverse and point count results can be found in appendix B.

Table 18. Durability factor—results for set 2 (SYN AEA).
Results are sorted by percentage of fresh air.
Mix Fresh air (%) A1 A2 A3 A4 Proc A avg Proc A std dev
348 2.3 38.3 22.2 29.4 24.9 28.7 7.1
346 3.2 66.2 46.0 56.9 53.4 55.6 8.4
347 3.5 68.0 78.3 77.1 78.8 75.6 5.1
349 4.0 82.4 62.5 50.6 66.9 65.6 13.2
350 4.3 76.6 86.2 83.1 83.5 82.3 4.1

The graph shows the durability factor, D F, as a function of designed fresh air for two sets of mixes. Set 1 was prepared with Vinsol resin air-entrained admixture and Set 2 with synthetic air-entrained admixture. The horizontal axis represents the designed fresh air, in percent, for Set 1 and Set 2, ranging from 2.5 to 4.5. The vertical axis is the durability factor, in percent, ranging from 0 to 120. Set 1 is represented by dotted columns and Set 2 by hatched columns. The graph indicates that Set 1 presents better freeze-thaw performance than Set 2. The durability factor for Set 1 is above 80 percent, while durability factor for Set 2 is from 30 to 80 percent, with the lowest corresponding to a designed air content of 2.5 percent.

Figure 32. Graph. Comparison between mixes prepared with Vinsol resin air-entrained admixture (set 1) and synthetic air-entrained admixture (set 2).

The graph shows the durability factor, D F, as a function of the spacing factor for two sets of mixes. Set 1 was prepared with Vinsol resin air-entrained admixture and Set 2 with synthetic air-entrained admixture. The horizontal axis represents the spacing factor, in millimeters, for Set 1 and Set 2, ranging from 0.00 to 0.60. The vertical axis is the durability factor, in percent, ranging from 0 to 120. Set 1 is represented by solid rhombs and Set 2 by squares. The graph indicates that the is a clear change in behavior around a Spacing factor of 0.25 millimeters, as follow: the durability factor for Set 1 is above 80 percent for a spacing factor between 0.25 and 0.55 millimeters, while durability factor for Set 2 is from 20 to 85 percent, with the lowest corresponding to a spacing factor of 0.35 millimeters.

Figure 33. Graph. Relation between durability factor and spacing factor of mixes with Vinsol resin admixture (set 1) or synthetic admixture (set 2).

The graph shows the durability factor, DF, as a function of the specific surface for two sets of mixes. Set 1 was prepared with Vinsol resin air-entrained admixture and Set 2 with synthetic air-entrained admixture. The horizontal axis represents the specific surface, in millimeters to the power of negative one, for Set 1 and Set 2, ranging from 0 to 40. The vertical axis is the durability factor, in percent, ranging from 0 to 120. Set 1 is represented by solid rhombs and Set 2 by squares. The graph indicates that there is a clear change in behavior around a spacing surface of 25 millimeters to the power of negative one, as follows: the durability factor for Set 1 is above 80 percent for a specific surface between 10 and 20 millimeters to the power of negative one, while durability factor for Set 2 is from 30 to 85 percent, the lowest corresponding to a specific surface of 15 millimeters to the power of negative one.

Figure 34. Graph. Relation between durability factor and specific surface of mixes with Vinsol resin admixture (set 1) or synthetic admixture (set 2).

The graph shows the durability factor, D F, as a function of the air content measured according to A S T M C 457 for two sets of mixes. Set 1 was prepared with Vinsol resin air-entrained admixture and Set 2 with synthetic air-entrained admixture. The horizontal axis represents the air content measured according to A S T M C 457, in percent, for Set 1 and Set 2, ranging from 0 to 6. The vertical axis is the durability factor, in percent, ranging from 0 to 120. Set 1 is represented by solid rhombs and Set 2 by squares. The graph indicates that Set 1 presents better freeze-thaw performance than Set 2. The durability factor for Set 1 is above 80 percent for an air content between 2.5 and 5.0 percent, while durability factor for Set 2 is from 30 to 85 percent, with the lowest corresponding to an air content of 4.5 percent.

Figure 35. Graph. Relation between durability factor and hardened air content of mixes with Vinsol resin admixture (set 1) or synthetic admixture (set 2).

Figure 36 shows the DF versus fresh air for sets 1 and 2. For set 1, it can be observed that the marginal air void concretes had similar freeze-thaw resistance, but if no air entrainment is provided, the freeze-thaw resistance of the concrete is much poorer.For set 2, the higher the fresh air content (ASTM C 231(14)), the higher the DF.

The graph shows the average durability factor, D F, as a function of the fresh air content for two sets of mixes. Set 1 was prepared with Vinsol resin air-entrained admixture and Set 2 with synthetic air-entrained admixture. The horizontal axis represents the fresh air content, in percent, for Set 1 and Set 2, ranging from 1.0 to 5.0. The vertical axis is the average durability factor, in percent, ranging from 0 to 120. Set 1 is represented by solid rhombs, and Set 2 by squares. The graph indicates that both mixes have almost similar freeze-thaw resistance for low fresh air content, but Set 1 presents better freeze-thaw resistance than Set 2 when air entrainment is provided. The durability factor for Set 1 is above 80 percent for a fresh air content higher than 2.5 percent, while durability factor for Set 2 is from 60 to 80 percent, for a fresh air content higher than 3.0 percent.

Figure 36. Graph. Relation between durability factor and fresh air content of mixes with Vinsol resin admixture (set 1) or synthetic admixture (set 2).

All specimens suffered some mass change (loss) during testing. The mass loss is a good indication of the scaling of the specimen (figure 37). Mass losses ranged from 0.32 to 4.03 percent (figure 38). Any mass gain due to water entering the concrete through cracks was obscured by the losses due to scaling. No correlation was observed between mass loss and the freeze-thaw performance of sets 1 and 2. The mass change versus cycles can be found in appendix B.

The picture shows the scaling of a concrete prism after testing. The aggregate is exposed in the surface of the prism.

Figure 37. Photo. Scaling of typical specimen. The specimens tended to scale toward the center region of the beam specimens corresponding to the area where the metal containers bulged due to ice formation between the concrete and the container.

The graph shows the mass change, as a function of designed fresh air for two sets of mixes. Set 1 was prepared with Vinsol resin air-entrained admixture and Set 2 with synthetic air-entrained admixture. The horizontal axis represents the designed fresh air, in percent, for Set 1 and Set 2, ranging from 2.5 to 4.5. The vertical axis is the mass change, in percent, ranging from negative 5 to 0. Set 1 is represented by dotted columns, and Set 2 is represented by hatched columns. The graph indicates that no correlation was observed between the mass loss and the designed fresh air. The highest mass loss was observed for Set 2 for air content of 2.5 percent and the lowest for Set 2 for air content of 3.0 percent.

Figure 38. Graph. Mass change of mixes with Vinsol resin admixture (set 1) or synthetic admixture (set 2).

It must be pointed out that set 1 and set 2 differ only in the type of air-entraining admixture—set 1 has Vinsol resin and set 2, synthetic. For the mixes prepared in this study and for the specific admixtures used, the Vinsol resin mixes exhibited better freeze-thaw resistance although they had a worse air void system.

The reasons for this unexpected observation are not known. It is possible that the water reducer or the cement used had an influence in the efficiency of the air void system. Another possibility is that the air-entraining admixture contains nonionic surfactants, which could result in a lack of a hydrophobic “tail” oriented towards the interior of the air bubbles, preventing water intrusion as pressure develops during freezing.(26) A previous study(44) showed that the cement-alkali level may have a negative impact on the air void system, and as a consequence for the freeze-thaw performance, on concretes with synthetic air-entraining admixture.

There are well-established thresholds for the air void parameters, which date from the time when only Vinsol resin admixtures were available. Experience shows that these limits (> 6 ± 1 percent air, specific surface ≥ 24 mm2/mm3, and spacing factor ≤ 0.20 mm) would be expected to give good concrete freeze-thaw resistance. The test data presented in this chapter suggest these limits may not be adequate to assure durability for some air entrained concrete containing synthetic admixtures.

There is insufficient data in this study to generalize this finding for all Vinsol resin and synthetic admixtures and all levels of air content. More research is needed in order to confirm this finding.

 

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