Effects of different chemical admixtures on fresh and hardened properties of prolonged mixed concrete and their cost-effectiveness were investigated. Influence of sand to aggregate volume ratio, cement content, and use of chilled mixing water on the properties of prolonged mixed concrete was studied as well. Different concrete mixtures were prepared using five different types of chemical admixture (one water reducer based on lignosulfonate and four superplasticizers based on sulfonated naphthalene polymer, polycarboxylic ether, second-generation polycarboxylic ether polymer, and organic polymer), varying s/a ratio (0.40 and 0.45) and cement content (340 kg/m3 and 380 kg/m3) and using chilled mixing water. Slump tests were performed at 15-minute intervals to assess the fresh performance of each prolonged mixed concrete mixture. 100 mm by 200 mm cylindrical concrete specimens were prepared and tested for compressive strength, Young’s modulus, splitting tensile strength, and ultrasonic pulse velocity. Results indicate that concretes with sulfonated naphthalene polymer-based superplasticizer and second-generation polycarboxylic ether-based superplasticizer show best performances in both fresh and hardened states. Concrete with lignosulfonate-based water reducer exhibits poor performance in comparison with the concretes with superplasticizers. The cost per unit compressive strength of concrete with sulfonated naphthalene polymer-based superplasticizer is lower compared with the concretes with other types of chemical admixture.
In recent years, the demand for ready-mix concrete (RMC) has increased rapidly in Dhaka city. The primary reasons behind this are convenience of using RMC in high-rise structures, shortage of space at construction site, saving of time related to the preparation of concrete on-site, and better quality of RMC compared to the conventional concrete. Nevertheless, because of heavy traffic congestions throughout Dhaka, especially during weekdays, the time required to haul RMC from plant to construction site is very high. Therefore, keeping concrete workable for such a long time without compromising the required strength has become one of the most challenging tasks in the RMC industry. Moreover, high ambient temperature in summer makes the situation worse, since high temperature adversely affects the workability of fresh concrete [
A good number of researches were carried out to understand the effects of different types of chemical admixture on properties of concrete. For instance, Topçu and Ateşin [
Although numerous studies were conducted with different types of chemical admixture, very few of them addressed the influence of chemical admixtures on properties of prolonged mixed concrete [
With the viewpoint of the above discussion, a detailed experimental investigation has been carried out with some chemical admixtures which are used extensively in the ready-mix concrete industry of Bangladesh. The effects of these admixtures and their dosages on fresh properties (workability) and hardened properties (compressive strength, Young’s modulus, splitting tensile strength, and ultrasonic pulse velocity (UPV)) of concrete have been studied so as to identify the best type of chemical admixture. The cost-effectiveness of using the chemical admixtures in concrete has also been analyzed. The effects of sand to aggregate volume (s/a) ratio, cement content (C), and use of chilled mixing water on fresh and hardened properties of prolonged mixed concrete have been investigated as well.
In this study, five different types of chemical admixture were used. One of the admixtures was water reducer (WR) and the rest four were superplasticizers (SP1, SP2, SP3, and SP4). All the types of chemical admixture comply with ASTM C494. The chemical and physical properties of the chemical admixtures, and their dosage ranges as recommended by the manufacturers for the best performance, are mentioned in Table
Properties and recommended ranges of dosage of chemical admixtures.
Chemical admixture | Composition | Appearance | Specific gravity at 25°C | Recommended dosage range (% weight of cement) |
---|---|---|---|---|
WR | Lignosulfonate based | Dark brown liquid | 1.17 | 0.23–0.47 |
SP1 | Sulfonated naphthalene polymer based | Dark brown liquid | 1.24 | 0.87–2.23 |
SP2 | Polycarboxylic ether based | Light brown liquid | 1.05 | 0.42–1.26 |
SP3 | Second-generation polycarboxylic ether polymer based | Light brown liquid | 1.10 | 0.55–1.32 |
SP4 | Organic polymer based | Dark brown liquid | 1.19 | 0.71–1.31 |
Gradations of (a) coarse aggregate (crushed stone) and (b) fine aggregate (river sand).
Properties of coarse aggregate.
Aggregate type | Specific gravity | Absorption capacity (%) | Abrasion (%) (as per ASTM C131) | SSD unit weight (kg/m3) | Fineness modulus |
---|---|---|---|---|---|
Crushed stone | 2.56 | 2.39 | 38.30 | 1549 | Controlled as per ASTM C33 |
Properties of fine aggregate.
Aggregate type | Specific gravity | Absorption capacity (%) | SSD unit weight (kg/m3) | Fineness modulus |
---|---|---|---|---|
River sand | 2.45 | 3.30 | 1520 | 2.52 |
Seven different types of concrete mixture were prepared for this study, namely, M1, M2, M3, M4, M5, M6, and RM. The ambient temperature was 25°C ± 2°C when the mixtures were prepared. All the mixtures were prepared using a laboratory mixer having a capacity of 75 l. Each mixture was initially mixed for 5 minutes at a speed of 20 rpm to ensure its homogeneity. After that, the mixing was carried out at a speed of 6 rpm. This low rotational speed was chosen to simulate the mixing process of ready-mix concrete in truck mixer. The mixer was stopped briefly at 15-minute intervals in order to conduct slump tests. The mixing process of each concrete mixture was continued until the final slump became less than or equal to 2 cm.
Each M1 concrete mixture was prepared using the average of the maximum and minimum dosages of every chemical admixture recommended by the respective manufacturer. On the other hand, M2, M3, M4, M5, and M6 mixtures were prepared using the maximum recommended dosages of admixtures. During the preparation of M1 and M2 mixtures, the entire dosage of admixture was applied with water at the beginning of mixing. However, in case of M3, M4, M5, and M6 mixtures, the total dosage of admixture was applied in two stages: at first, 2/3 of the dosage of admixture was applied with water, and the rest 1/3 was applied when the slump value of prolonged mixed concrete became less than or equal to 3 cm.
In M1, M2, and M3 mixtures, the water to cement (W/C) ratio, s/a ratio, and cement content (C) were, respectively, 0.40, 0.40, and 340 kg/m3. In M4 mixture, the s/a ratio was increased to 0.45, but the W/C ratio and cement content were kept similar to M1, M2, and M3 mixtures. In M5 and M6 mixtures, the cement content was raised to 380 kg/m3 of concrete, but W/C and s/a ratios were kept, respectively, to 0.40 and 0.40. In the RM (reference mix), no chemical admixture was used, and the W/C ratio, s/a ratio, and cement content were, respectively, 0.40, 0.40, and 340 kg/m3. Prior to the preparation of each mixture, both the coarse and fine aggregates were brought to saturated surface dry (SSD) condition, so that the W/C ratio of the mix would remain unaffected.
In case of M6 mixture, instead of using plain water, chilled water was used (half of the total required water (by weight) as per mix design was plain water and the rest half was ice) to keep the temperature of the concrete mix low. The initial temperature of the chilled water was 0°C ± 1°C. It should be noted that, in M4, M5, and M6 mixtures, only the two types of admixture that gave best performances in M1, M2, and M3 mixtures were used. The details of different types of mixture are presented in Table
Details of different types of concrete mixture.
Mix type | Details |
---|---|
M1 | W/C = 0.40; s/a = 0.40; |
M2 | W/C = 0.40; s/a = 0.40; |
M3 | W/C = 0.40; s/a = 0.40; |
M4 | W/C = 0.40; s/a = 0.45; |
M5 | W/C = 0.40; s/a = 0.40; |
M6 | W/C = 0.40; s/a = 0.40; |
RM (reference mix) | W/C = 0.40; s/a = 0.40; |
Cylindrical concrete specimens of 100 mm diameter and 200 mm height were made for assessing the hardened properties of concrete. A total of 22 independent cases and 242 specimens were investigated. The mixture proportions of all 22 cases are summarized in Table
Mixture proportion of concrete.
Concrete mix type | W/C | s/a | Admixture type | Unit content (kg/m3) | Admixture dosage (% weight of cement) | |||
---|---|---|---|---|---|---|---|---|
Cement | Sand | Aggregate | Water | |||||
M1 | 0.40 | 0.40 | WR | 340 | 721 | 1130 | 136 | 0.35 |
SP1 | 340 | 718 | 1125 | 136 | 1.55 | |||
SP2 | 340 | 719 | 1127 | 136 | 0.84 | |||
SP3 | 340 | 719 | 1127 | 136 | 0.94 | |||
SP4 | 340 | 719 | 1127 | 136 | 1.01 | |||
M2 | 0.40 | WR | 340 | 721 | 1130 | 136 | 0.47 | |
SP1 | 340 | 716 | 1122 | 136 | 2.23 | |||
SP2 | 340 | 718 | 1125 | 136 | 1.26 | |||
SP3 | 340 | 718 | 1125 | 136 | 1.32 | |||
SP4 | 340 | 718 | 1126 | 136 | 1.31 | |||
M3 | 0.40 | WR | 340 | 721 | 1130 | 136 | 0.47 | |
SP1 | 340 | 716 | 1122 | 136 | 2.23 | |||
SP2 | 340 | 718 | 1125 | 136 | 1.26 | |||
SP3 | 340 | 718 | 1125 | 136 | 1.32 | |||
SP4 | 340 | 718 | 1126 | 136 | 1.31 | |||
M4 | 0.45 | SP1 | 340 | 806 | 1029 | 136 | 2.23 | |
SP3 | 340 | 808 | 1032 | 136 | 1.32 | |||
M5 | 0.40 | SP1 | 380 | 686 | 1075 | 152 | 2.23 | |
SP3 | 380 | 688 | 1079 | 152 | 1.32 | |||
M6 | 0.40 | SP1 | 380 | 686 | 1075 | 152 | 2.23 | |
SP3 | 380 | 688 | 1079 | 152 | 1.32 | |||
RM (reference mix) | 0.40 | — | 340 | 722 | 1132 | 136 | — | |
|
||||||||
Total number of cases = 22 | ||||||||
Cylinders per case = 3 × 3 (compressive strengths at 7 days, 28 days, and 90 days) + 2 (splitting tensile strengths at 28 days) = 11 | ||||||||
Total number of cylinders = 11 × 22 = 242 |
All the concrete mixtures with chemical admixtures (namely, M1, M2, M3, M4, M5, and M6) were subjected to prolonged mixing. Slump tests were done for concrete mixtures subjected to prolonged mixing at 15-minute intervals as per ASTM C143. When the slump of each mixture became less than or equal to 2 cm, the mixing process was stopped, and the mixture was poured into cylindrical molds to prepare 100 mm by 200 mm cylindrical concrete specimens. After casting of concrete specimens, they were cured initially for 24 hours by covering the cylindrical molds with wet clothes and polythene to prevent moisture loss. The specimens were demolded after 24 hours of casting and then cured under water till the age of testing as per ASTM C31.
The cylindrical specimens were tested for compressive strength and Young’s modulus at 7 days, 28 days, and 90 days and for splitting tensile strength at 28 days according to ASTM specifications. Prior to compressive strength test, the UPV test was conducted for unloaded water-saturated concrete specimens according to ASTM C597.
According to the JSCE 2007 guidelines for concrete [
The slump test results of RM, M1, M2, and M3 mixtures at 15-minute intervals are presented in Figure
Slump test results of RM, M1, M2, and M3 mixtures (W/C = 0.40, s/a = 0.40, and
Results presented in Figure
Figure
The fresh performances of M3, M4, M5, and M6 concrete mixtures with sulfonated naphthalene polymer-based SP1 and second-generation polycarboxylic ether-based SP3 are presented in Figure
Slump test results of M3, M4, M5, and M6 mixtures: (a) sulfonated naphthalene polymer-based SP1 (maximum dosage of admixture in two stages) and (b) second-generation polycarboxylic ether-based SP3 (maximum dosage of admixture in two stages).
Figure
From Figure
Figure
Hardened properties of RM, M1, M2, and M3 concretes (W/C = 0.40, s/a = 0.40, and
The Bangladesh National Building Code (BNBC) 2006 [
It can be seen from Figure
The present study focused on assessing the fresh and hardened performances of concrete for admixture dosages not exceeding the maximum limits suggested by respective manufacturers. Therefore, the study did not address the influence of overdosing of admixture on concrete properties. Nevertheless, the study conducted by Gagné et al. [
Figure
Figure
Hardened properties of M3, M4, M5, and M6 concretes at the age of 28 days: (a) compressive strength, (b) Young's modulus, (c) splitting tensile strength, and (d) ultrasonic pulse velocity.
It should be noted that, with the increase of s/a ratio from 0.40 to 0.45, the workability of prolonged mixed concrete decreased, but the compressive strength of concrete increased. Therefore, when the workability of ready-mix concrete is of primary concern, it should be kept into consideration that the s/a ratio is not too high. On the other hand, if the strength of concrete is of primary concern, the s/a ratio should not be too low.
Results presented in Figure
Figure
Results presented in Figure
Figure
Compressive strength gains of different types of concrete made with sulfonated naphthalene polymer-based SP1 with respect to age.
Costs of 1 m3 of concretes made with maximum recommended dosages of different types of chemical admixture and cost of 1 m3 of concrete made without admixture are shown in Figure
Costs of 1 m3 of concretes (W/C = 0.40, s/a = 0.40, and
The 28-day strengths of concretes prepared with different types of chemical admixture are different. Therefore, the costs per unit 28-day compressive strength (1 MPa) of 1 m3 concretes prepared with different types of chemical admixture are presented in Figure
Unit strength costs of 1 m3 of concretes (W/C = 0.40, s/a = 0.40, and
From the scope of this investigation, the following conclusions can be drawn: Sulfonated naphthalene polymer-based superplasticizer shows best performance in improving workability of fresh concrete in comparison with other chemical admixtures. Second-generation polycarboxylic ether-based superplasticizer can be categorized as the second best chemical admixture in improving workability of fresh concrete. Concretes made with sulfonated naphthalene polymer-based superplasticizer and second-generation polycarboxylic ether-based superplasticizer exhibit higher compressive strengths, splitting tensile strengths, Young’s moduli, and UPVs than concretes prepared with other admixtures. Superplasticizers show better performances in improving fresh and hardened properties of concrete compared to water reducers. The compressive strength of concrete increases with increase of admixture dosage, when the dosage of admixture is within the range recommended by the manufacturer. Applying dosage of chemical admixture in two stages helps to keep concrete workable for longer duration compared to applying the same dosage of admixture at the beginning of mixing process. Workability of fresh concrete decreases with the increase of s/a ratio from 0.40 to 0.45. However, the compressive strength of concrete increases when the s/a ratio is increased from 0.40 to 0.45. If chilled water is used in concrete mix, it remains workable for longer period in comparison with the concrete mix with plain water. Use of chilled water in fresh concrete causes reduction in early concrete strength but improves the long-term strength property of concrete. Unit strength cost of concrete made with sulfonated naphthalene polymer-based superplasticizer is lower compared to other chemical admixtures.
The authors declare that they have no conflicts of interest.
The authors express their sincere gratitude and appreciation to Islamic University of Technology (IUT), Gazipur, Bangladesh, and the Structural Engineers Limited (SEL), Bangladesh, for financing the research project. The cement used in this study was provided by Seven Circle Bangladesh Limited. The authors wish to express their gratitude to Seven Circle Bangladesh Limited as well.