Superplasticizers
2.6 The effects of superplasticizers on the properties of hardened concrete
Page 93
Fig. 2.18 Slump loss at 21°C of superplasticized concretes with OPC and CAE or SNF ploymer-based admixtures. The figures on the slump-loss curves indicate the percentage of the superplasticizer active ingredient by mass of cement.
2.5.3 Setting time
The effect that superplasticizers have on the setting times of concrete depends on a number of factors including the type of superplasticizer, cement composition, and particularly whether there is a simple addition of the admixture to the concrete or if a reduction in water–cement ratio is made. In general it can be stated that:
1. With a direct addition of superplasticizer to obtain highly workable concrete, initial and final setting times are invariably increased in the order SMF < SNF < polyacrylates. At normal dosages this increase rarely exceeds two hours for materials that are not intentionally formulated to retard.
2. When the water–cement ratio is reduced to give a similar slump to a control mix, the setting time is normally very similar to the control; perhaps a small decrease in the case of SMF and SNF [64] and a slight increase in the case of polyacrylate-based materials [57], normally no greater than one hour either way.
2.6 The effects of superplasticizers on the properties of
Page 94 ability to protect reinforcing steel. It is also important that these properties are maintained during the service life and that any addition, such as superplasticizers, have no adverse effects on these attributes.
2.6.1 Compressive strength
In general, superplasticizers will not have an adverse effect on strength, or strength development of concrete when added to produce highly workable concrete without reducing the water content. In fact several studies have indicated that an increase in strength may occur, and in some cases this can
superplasticizer compared to concrete made with 400 kg of normal Portland cement per m3 in the stiff to low workability ranges (25–100 mm slump).
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Page 95 be substantial [65], as shown in Fig. 2.19 for an SMF-based material. Table 2.4 (column 3) shows data for a NSF superplasticizer where strengths are essentially the same as a control.
When superplasticizers are used to reduce water content of the concrete mix, the increase in strength is normally considered to follow the Abram’s
Table 2.4 Freeze-thaw resistance of superplasticized concrete (Rixom)
Mix no. 1 2 3
Description Control Addition of admixture normal workability
Addition of admixture self- compacting concrete Mix design:
10 mm gravel (rounded
irregular) (kg) 8 8 8
Zone 3 sand (kg) 4 4 4
OPC (kg) 2 2 2
Water (litre) 1.1 0.9 1.0
Admixture (% by weight of
cement) 0 2.5 2.5
Properties of plastic concrete:
Slump (mm) 60 60 Collapse
Air content (%) 1.8 1.7 2.2
Compacting factor 0.87 0.90 —
Density (kg m−3) 2394 2430 2394
Properties of hardened concrete (N mm−2):
1 day 6.3 12.9 6.0
7 days 31.2 40.2 32.4
28 days 41.2 64.3 42.0
Adsorption (% of dry cube weight)
10 min 1.3 0.8 1.1
30 min 1.9 1.0 2.0
60 min 2.9 1.8 2.8
24 h 5.2 2.6 5.0
ISAT method
Initial surface adsorption test BS 1881, Part 5, 1970
30 min 0.42 0.08 0.31
60 min 0.26 0.03 0.20
Freeze-thaw dilation test
(dilation at 50 cycles, %) 0.10 0.030 0.075
DHO test (number of cycles to cause 0.5 mg mm−2 scaling
6 32 12
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Page 96
Fig. 2.20 The relationship between water–cement ratio and compressive strength of concretes containing superplasticizers.
Fig. 2.21 Shrinkage strains in each finished model wall compared with prisms at 91 days.
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Page 97
Table 2.5 Mixture proportions and properties of fresh and hardened concrete (water reduced)
Mix
Series Type of
concrete Mixture proportions and
properties of fresh concrete Properties of hardened concrete Modulus of
elasticity (GPa) Water
content (kg m
−3)
W/c
Ratio Entrained
air (%) Slump
(mm) Density (kg m
−3)
Compressive strength of 150 × 300 mm
cylinders (MPa)
Flextural strength of 90 × 100 × 40 mm prisms (MPa)
7 days 28 days 91 days
28 days 1 Reference
(Type I cement)
147 0.49 5.2 75 2348 26.8 32.8 37.8 6.1 32
2 Type I cement and SP-M
120 0.40 5.6 80 2362 37.3 44.0 48.5 7.0 37
3 Type I cement and SP-N
120 0.40 6.0 70 2350 35.5 39.3 47.6 7.0 37
4 Type I cement and SP-L
120 0.40 5.6 80 2360 36.3 42.6 49.9 6.6 36
5 Reference (Type II cement)
147 0.49 4.9 85 2354 25.6 36.6 42.4 6.0 32
6 Type II cement and SP-M
120 0.40 5.6 90 2362 36.3 47.6 55.0 6.9 37
7 Type II cement and SP-N
121 0.40 5.3 75 2377 36.9 47.6 55.8 7.2 37
8 Type II cement and SP-L
121 0.40 4.8 75 2385 35.0 47.6 55.8 7.3 36
9 Reference (Type V cement)
144 0.48 5.4 90 2352 19.1 32.2 38.0 5.0 32
11 Type V cement and SP-N
118 0.38 5.3 80 2381 33.0 42.0 48.5 5.7 35
12 Type V cement and SP-L
118 0.38 5.2 85 2379 32.8 42.4 50.3 6.2 35
*Coarse aggregate was crushed limestone with a maximum size of 19 mm. Fine aggregate was natural sand. Air-entraining admixture was a sulfonated hydrocarbon.
*SP-M, SP-N, and SP-L refer to melamine, naphthalene and lignosulfonate type superplasticizers (SP) respectively.
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Page 98
Table 2.6 Data on creep measurements for control and water-reduced superplasticized concrete
Admixtures Mixture proportions Properties of fresh
concrete Creep measurements on 150 × 300 mm cylinders
Superplasticizer
type Superplasticizer
AEA* W/c
ratio by weight
Type 1 cement (kg m
−3)
FA CA Entrained
air (%) Slump
(mm) Density (kg m
−3)
f 'c at 28 days
f 'c at 63 days
Stress
applied Stress–
strain ratio
Test creep strain (in./
in. × 10p
−6)† (ml kg−1 of
cement)
(kg m−3) (MPa)
Control — 0.31 0.49 298 817 1082 5.3 80 2344 34.3 37.4 15.2 0.44 1101‡
Melamine 23.6 1.18 0.40 304 835 1106 5.4 75 2365 45.2 50.8 19.6 0.43 1085‡
Naphthalene 9.1 3.15 0.40 303 832 1102 6.0 80 2357 47.4 51.1 20.3 0.43 1107‡
Lignosulfonate 25.6 0.34 0.40 305 839 1111 3.4 40 2377 46.0 48.7 19.8 0.43 1157‡
*Air-entraining agent.
†Total creep strain is obtained by structuring shrinkage and elastic strain at loading from the strain readings.
‡At 334 ± 10 days.
Page 99 law curve, although consideration of the compilation of data in Fig. 2.20 does indicate a trend to higher values for the superplasticized concrete. The increases in strength by using superplasticizers to reduce the water–cement ratio appear to be consistent over several cement and superplasticizer types and continues beyond the 28-day result, as shown in Table 2.5, which also indicates how flexural strength and modulus are similarly affected [56].
2.6.2 Shrinkage and creep
Most of the studies in this area have indicated that superplasticized concrete has shrinkage and creep
characteristics similar to plain concrete. Figure 2.21 illustrates the shrinkage results of plain and superplasticized concrete [66], while Table 2.6 shows similar results for creep [67].
Fig. 2.22 The effect of admixture on the change in weight of water- and sulfate-stored concrete specimens.
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Page 100
Table 2.7 Air-void determinations on hardened concrete
Mix
no. Properties of fresh concrete Air-Void determinations on hardened concrete*
Water–
cement ratio (by wt)
Nature of mix Air content (%)
Paste content (%)
Voids in concrete (%)
Specific surface area (mm)
Void spacing factor (mm)
1 0.70 Plain 2.4 24.6 2.4 10 0.62
3 0.70 Air entrained 6.2 21.2 9.0 14 0.23
3 0.70 Air entrained and
superplasticized† 6.7 20.9 9.4 12 0.26
4 0.50 Plain 2.2 27.0 2.0 12 0.59
5 0.50 Air entrained 6.8 23.2 7.7 28 0.13
6 0.50 Air entrained and superplasticized†
6.5 23.2 7.1 21 0.18
7 0.35 Plain 2.0 34.1 1.4 13 0.73
8 0.35 Air entrained 5.7 31.2 6.0 32 0.14
9 0.35 Air entrained and
superplasticized† 5.0 30.7 4.8 21 0.23
*Data supplied by Ontario Hydro.
†A napthalene-based superplasticizer was used at a dosage rate of 0.75% by weight of cement.
Page 101 2.6.3 Freeze–thaw durability .
In the early days of the use of superplasticized concrete, some concerns were aired regarding the resistance of air- entrained concrete containing superplasticizers to freeze–thaw cycling. However, more recent research has indicated the following:
1. In air-entrained superplasticized concrete, the commonly accepted minimum value of the spacing factor of the air void system (0.2 mm) to provide adequate freeze–thaw protection is usually exceeded [68–71].
2. Despite the fact that the minimum spacing factor is exceeded, the concrete freeze–thaw resistance of the concrete does not appear to be adversely affected [71]. Table 2.7 clearly shows how the presence of an SNF superplasticizer increases the spacing factor of the air void system at each of the three water–cement levels evaluated [71].
2.6.4 Sulfate resistance
Research into the susceptibility of superplasticized concrete to sulfate attack has concluded that there is no significant difference between plain concrete and the admixture-containing concrete [71, 72]. Figure 2.22 presents some data for concretes containing an SNF superplasticizer exposed to a 3% magnesium sulfate solution.
References
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