Air-entraining agents
3.4 The effects of air-entraining agents on the properties of plastic concrete
Page 120 Air-entrained pastes possess a higher viscosity than pastes with little or no air content, mainly because of the bridging effect of cement particles by air bubbles increasing the structure of the system.
There is no evidence to suggest that the presence of air-entraining agents of the type normally available commercially alter, in any way, the eventual hydration products of the cement.
3.4 The effects of air-entraining agents on the properties of
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Fig. 3.16 Air content as a function of admixture addition level (Johnson).
Fig. 3.17 Air content as a function of admixture addition level for cements of different fineness (Mayfield).
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Page 122 (b) Mixing techniques
The change in air content on prolonged mixing has been studied under laboratory conditions for two cements with results shown in Table 3.8 [19]. The data indicate that the maximum level of air entrainment is rapidly achieved in the case of the lower-cement-content mix and is progressively lost as mixing continues. In the case of the higher-cement-content mix, some 5–10 min are required to reach the maximum air content which then
diminishes on continued mixing. Field trials in a full-size mixer were carried out on the same admixtures using the 340 kg m−3 mix and the results are given in Table 3.9.
The two studies suggest that it is unlikely that more than about 1% of entrained air will be lost by mixing for up to 30 minutes.
The mix capacity was also varied using the full-size mixer with the same concrete mixes and admixtures. Results are given in Table 3.10 where it will be seen that the effect of batch size is only slight and, in the higher cement content mixes, shows a trend towards higher air content as the capacity of the mixes is approached.
(c) Cement characteristics
The characteristics and quantity of cement used in the production of air-entrained concrete can have a pronounced effect on the air content and/or
Table 3.8 The effect of mixing time on the level of air entrainment in laboratory mixes
Mixing time (min.) Air content (%)
No addition Admixture
A B C
256 kg m−1 cement
2 1.8 5.3 6.0 3.8
5 1.1 5.2 5.5 3.4
10 1.0 4.2 4.9 2.1
15 1.0 3.3 4.2 1.9
30 1.0 2.5 3.4 1.1
60 1.0 2.0 1.5 1.0
340 kg m−3 cement
2 1.3 4.5 4.4 3.1
5 1.0 5.2 5.9 2.1
10 0.9 5.8 5.9 1.4
15 0.8 5.7 5.8 1.3
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Table 3.9 The effect of mixing time on the level of air entrainment in a full-size mixer
Mixing time (min.) No addition Admixture
A B C
15 1.0 3.8 — 3.3
30 — 3.2 6.0 3.2
45 — 3.0 5.5 3.0
60 — 2.7 4.9 2.5
75 — 2.7 4.1 2.6
90 — 2.4 3.3 2.4
Table 3.10 Slightly higher air contents are obtained as the volume of mix approaches that of the mix capacity, particularly for the higher cement content mix
Percentage of mixes capacity Air content (%)
No addition Admixture
A B C
256 kg m−3 cement
20 1.5 5.1 5.1 2.7
40 1.6 6.1 5.1 4.2
60 1.5 6.1 6.0 3.8
80 1.5 5.8 6.0 3.5
100 1.5 6.0 6.2 3.9
340 kg m−3 cement
20 0.7 3.0 3.0 1.9
40 0.8 4.4 3.9 2.2
60 1.0 4.5 4.1 2.4
80 1.0 4.3 4.1 2.9
100 1.1 4.6 4.5 3.0
the dosage of admixture required to obtain the necessary air content. This is illustrated by the evaluation [19] of 12 different cements in an identical mix using a standard dosage of admixture. Results are given in Table 3.11.
Large variations in air content due to a change in cement source are, therefore, quite possible and, although it is not possible to quantify the effect of all cement variables, the following data are relevant.
FINENESS
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The fineness of cement is a major factor in determining the quantity of air-entraining admixture required to incorporate a given amount of entrained
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Table 3.11 Cement type and source can influence the volume of air obtained in both plain and air-entrained mixes
Cement code Air entrained (%)
No addition Admixture
1 2 3
A 3.1 5.2 5.3 3.6
B 1.2 5.3 4.4 3.2
C 1.2 5.4 5.5 3.0
D 1.1 5.1 5.4 2.0
E 1.4 3.9 3.5 2.8
F 2.3 6.2 5.4 4.5
G 2.4 7.9 6.3 4.3
H 1.0 5.4 4.5 3.6
I 1.3 6.2 5.2 3.0
J 1.8 7.8 8.1 4.6
K 1.0 6.9 5.9 3.6
L 1.7 7.2 5.8 4.3
air. Table 3.12 summarizes the results for three cements differing only in their specific surface area [18].
Differences in cement fineness that are normally experienced could lead to a doubling or halving of admixture requirements, but it is worth noting that in the study from which the above data are derived, an addition level of 10.0 ml per batch would have produced concrete conforming to a 3–6% requirement for all three cements.
CEMENT CONTENT .
The amount of entrained air decreases with increasing cement content [20] and typically an increase in cement content of 90 kg m−3 will reduce the volume of entrained air in concrete by about 1% of the volume of concrete.
Table 3.12 The amount of air-entraining agent required to obtain 4% air is increased for higher surface area cements
Cement Specific surface area Quantity of admixture required (ml 0.0368 m−3 batch) for 4% air
A 2750 6.5
B 3750 10.0
C 4750 14.0
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Page 125 ALKALI CONTENT
An investigation [21] of mortars containing cements differing only in their alkali content indicated that when the alkali (as Na2O) in the water in contact with the cement reaches about 0.8% by weight of water, the amount of air- entraining agent required to achieve a given air content is minimized. This is shown in Fig. 3.18.
This effect also applies to alkali added as part of the admixture formulation, it is particularly important in cements with low alkali analysis and is illustrated in Table 3.13 [22].
(d) Workability
If the addition of the air-entraining agent is maintained at a constant level, a more workable mix will entrain more air than a less workable one. However, for very workable concrete of slump greater than 180 mm, the air will be more rapidly lost before placing.
Fig. 3.18 The effect of alkali content of cements on the air-entraining capacities of neutralized wood resins in concrete (Greening).
Table 3.13 The alkalinity of the admixture influences the air content obtained, particularly with cements of low alkali analysis
Air-entraining admixture pH Na2O (%) Air content (%) of cement
1 9.4 0.35 5.5
2 10.6 0.38 7.4
Page 126 (e) Temperature
The temperature of the concrete has a significant influence on the amount of air entrained in concrete by a
standard addition level of admixture; the higher the temperature, the lower the air content. A typical set of results is shown in Fig. 3.19 [23]. The effect is more marked at higher slump values.
(f) Aggregate type and content
COARSE AGGREGATE
There is no evidence to suggest that coarse aggregate shape or geological origin affect the amount of air entrainment obtained. The only exception is where crushed rock aggregates contain an appreciable quantity of dust which could influence the fine aggregate gradings considered below.
FINE AGGREGATE
The amount of air that is entrained in concrete increases with an increase in the sand content and this is shown for a variety of sand gradings in two
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Fig. 3.19 Relationship between temperature and air content of concrete.
Page 127 concrete mixes in Fig. 3.20 [24]. As a guide, an increase in the sand content of 5% will lead to an increase in air content of 1–1.5%.
A relationship exists between the size of bubbles that can be accommodated in a system containing fine particles of 300–600 µm diameter varies from about 30 to 100 µm. Since a large proportion of the bubbles in air-entrained concrete are less than 100 µm diameter, it is clear that this particle size is of considerable importance in
determining the amount of air entrained. Therefore, even at constant sand content, an increase in the properties of particles of this size will lead to an increase in air content. This effect is shown for a number of sand contents in Fig. 3.21 [24].
(g) Fine fillers and pozzolans
When fine particle material of less than 20 µm diameter is included in the mix design, the amount of air- entraining agent must be increased to obtain the required air content [25]. The effect is largely independent of coarse aggregate type. Some results are shown in Fig. 3.22, where it is apparent that the effect is considerable in the case of fly ash and pumice [26]. Although these figures are relevant to a very high replacement of
cementitious materials, even at normal replacement levels of 20–30% fly ash, the concrete will require three or four times the quantity of air-entraining agent in comparison to a concrete containing no fly ash to entrain the same volume
Fig. 3.20 The effect of sand content on the air entrainment of concrete at two addition levels of air- entraining agent, the fineness modulus (FM) of the sand also being varied (Craven).
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Fig. 3.21 The effect of fine aggregate of 3–600 µm diameter on the air entrainment of concrete (Craven).
of air. The effect of fly ash on air entrainment is also dependent on its carbon content (LOI – loss of ignition).
(h) The presence of water-reducing admixtures
If the concrete to be air entrained contains a water-reducing agent of the lignosulfonate or hydroxycarboxylic acid type, a reduction of 50–60% in the quantity of air-entraining agent can be made in comparison to a mix not containing the water-reducing agent [27, 28].
3.4.2 The stability of the entrained air
Following the removal of the concrete from the mixer, subsequent transport, handling and placing techniques can cause reductions in the air content and in the air void characteristics. However, it must be borne in mind that measurement of air content is usually by the pressure meter method, which includes a compaction stage where the large voids that could be lost in handling operations are removed prior to measurement. In view of this, losses during handling and transport are usually less than 0.5% of the concrete volume, as shown in Fig. 3.23 [29].