Chapter 2: Concrete Mix Design
2.1 Introduction
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Design Concrete mix design is important in the safety, durability and service life of reinforced concrete structures. Through proper resource allocation, engineers can balance out the materials they require to provide the needed strength and still have some workability and resistance to environmental factors. In this project, the mixture is customised to the requirements of the location of Zone Z on the 5th floor, where slabs and beams are exposed to diverse loads, and among them, a special water storage pressure. The design is related to compliance with AS3600 durability provisions. To define the mix proportions, the American Volumetric Method is followed to provide a systematic way of attaining dependable performance.
2.2 Strength Requirements
In this project, the concrete grade will be that of N32, which is equivalent to a compressive strength (f’c), of 32 MPa at 28 days. This grade is typical in the structural activity where beams and slab need to perform considerable imposed loads supported as well as being able to be serviced (Fan et al. 2021). Given the salient structural members in the Zone Z of the 5th floor are a continuous T-beam and a two-way slab each of which experienced variable live loads and special water loads caused by the Jacuzzi, a dependable strength margin is a prerequisite.
There is a basis of varying materials and workmanship to consider and provide the target mean strength at 40.5 MPa; it is a safety margin. This margin overcomes the few test samples which are usually available at the time construction is in progress, and this is standard practice to minimise the risk of underperformance. N32 had been chosen to favour both economical choice and at the same time, high-grade materials like N40 can boost material costs, not required, thus low grades accentuate structural safety (Zhang et al. 2022).
In general, the selected strength class allows keeping the beam and slab in Zone Z within the limit state requirements of ultimate and serviceability limits, which gives an assurance of the performance and durability in the long term of the structure.
2.3 Water–Cement Ratio
Another most important parameter concerning concrete mix design is the water-cement ratio (w /c ratio) as it directly determines strength, durability, and workability. In this project, one has considered a value of 0.41, which is appropriate to non-air-entrained concrete subjected to a mild interior environment. This choice follows the durability demands in AS3600 as well, as with the exposure category of A1 that comprises interior concrete that is completely shielded against external forces of the weather or destructive elements.
An effective ratio of w/c is 0.41, which offers a good compromise point between strength and workability (Beckmann et al. 2021). At this ratio, it is anticipated that the mean strength of the concrete will meet the target point of 40.5MPa, whereby the designed members are capable of bearing both dead and live load whilst remaining serviceable through analysing the design life of the building (Zhang & Zhang, 2021). A lesser ratio would enhance strength but may decrease the workability; also, placement and compaction may be a challenge in slab/ beam areas. On the other hand, an increased ratio would enhance workability but at a greater cost of strength and permeability, preventing durability at long-term orientation.
In this way, the chosen w/c ratio displays sufficient strength, workability, and durability against deterioration of the concrete mix that would be used in Zone Z beams and slabs.
2.4 Selection of Materials
The definitiveness of the concrete productions depends on the materials used in order to realise the desired performance. In this project, the chosen material of coarse aggregate is a 25 mm sub-angular crushed stone, which can be favoured by being highly strong and able to lock with each other and relatively less in water demand than rounded aggregates (Raphael et al. 2023). It enhances bonding with cement paste because of its angularity, wherein the cement mix gives the required compressive strength.
These include a 2.60 fine aggregate modulus of fineness, which implies a medium level of grading to workability and long-life. Bulk density was used to ensure that there is an appropriate proportioning between sand and coarse aggregate. The sand adds enough sand to allow the concrete to be kept workable and at the same time, not excessively shrink.
Cement is predetermined fixed at 441kg/m3, well in excess of the 310kg/m3 minimum limit of structural concrete laid out in AS3600. This increased cement-based material helps to sustain the intended performance in terms of mean strength, as well as enhance durability (Alabbasi et al. 2023). The proportion of water is fixed at 181 kg/m3 to represent the sub-angular form of the aggregates, conforming to providing sufficient workability.
It has an assumed air content of 1.5-2% of air, which is common in the mild conditions in non-air-entrained concrete. A defined slump of 75 mm is taken, a figure chosen to ensure that it is easy to incorporate into slabs and beams and at the same time to maintain sufficient cohesion to avoid excessive segregation during manipulation.
2.5 Mix Proportioning Process
The mix of the concrete was prepared in compliance with the American Volumetric Method, i.e. all the components are measured in terms of volume and transformed into weight to be batched conveniently (Song et al. 2024). The calculation of the required water level of 181 kg/m3 based on a 25 mm aggregate dimension and even a slump grade of 75mm initiates this spacious method.
Based on the desired means of 0.41, the chosen water-cement ratio, 441 kg/m3 of cement can be obtained, which also meets the minimum of AS3600.
The second step is to ascertain the amount of coarse aggregate. In the case of 25 mm sub-angular crushed stone, the bulk density assumed is 1600 kg/m3, and the volumetric proportions give the mixed aggregate of about 1202 kg/m3. The remaining part of the volume is occupied by the aggregate fine, which will be computed, 643 kg/m3 based on a 2.60 fineness modulus (Pan & Zhang, 2023). These proportions are physically sufficient to maintain good workability and good strength at the same time, keeping the segregation and bleeding at a minimum.
This results in the final mix composition of each cubic metre is: Water = 181 kg, Cement = 441 kg, Fine Aggregate = 643 kg and Coarse Aggregate = 1,202 kg with the air content of 1.5- 2% (Olsson et al. 2023).
2.6 Mix Design Process
Step 1. Strength Requirements
1.Specified Concrete Grade
For this project, concrete of Grade N32 is selected, with a nominal design compressive strength f’c=32MPa, which falls within the typical range of 21–35 MPa as specified in the standards.
2.Limited Number of Test Samples
Since the number of test samples is less than 15, statistical methods (such as standard deviation correction) cannot be reliably used to determine the target mean strength. Therefore, the additive method is adopted to ensure an adequate safety margin.
3.Target Mean Strength Determination
To account for potential underestimation due to statistical uncertainty, a fixed safety margin of 8.5 MPa is added to the nominal design strength:
Step 2. Determine Water‐Cement Ratio
According to the ACI recommended tables:
If air-entrainment is used →w/c ratio≈0.32
If no air-entrainment is used → w/c ratio = 0.41
5.Environmental Exposure Conditions
The project is located in Canberra, Australia, which is classified as a low exposure environment (A2). No freeze-thaw cycles or chemical attack is expected. Therefore, air-entrainment is not required, and the selected water-cement ratio is 0.41.
Step 3. Coarse Aggregate Requirements
According to the structural design drawings, the plate thickness is 200mm and the minimum cross-section of the beam is also 200mm.
Thus The minimum size of concrete slab is 200 mm.
Minimum concrete cover = 40 mm
Minimum clear spacing between reinforcement bars = 200 mm
The maximum aggregate size is designed 25 mm
According to the structural design:
Minimum member size/dimension (slab thickness or minimum beam cross-section) = 200 mm → Limit = 200 ÷ 5 = 40 mm
Minimum clear spacing between reinforcement bars = 200 mm → Limit = (3/4) × 200 = 150 mm
Minimum concrete cover = 40 mm → Limit = (3/4) × 40 = 30 mm·
By comparison, the three limits are 40 mm, 150 mm, and 30 mm.
The governing (smallest) value = 30 mm.
Therefore, the selected 25 mm coarse aggregate size sub-angular, rough-textured crushed stone satisfies all the requirements and is acceptable for the design.
Step 4. Air Entrainment Requirements
In this project, since the indoor slabs and beams are not subjected to prolonged moisture or freeze-thaw conditions, the use of an air-entraining agent is unnecessary. In other words, non-air-entrained concrete is the most appropriate choice for the design.
Referring to Table 7.6, when the nominal maximum coarse aggregate size is 25 mm, the target air content for non-air-entrained concrete should be 1.5%.
Step 5. Workability Requirements
Slump is an indicator of concrete workability. A low slump makes placement difficult, while a high slump may cause segregation and bleeding. In this design, a slump of 75 mm was chosen to ensure adequate workability for beams and slabs without quality issues.
Step 6. Water Content
In this design, the selected 25 mm sub-angular crushed stone with a target slump of 75 mm requires 193 kg/m³ of mixing water (from Table 7.8 for angular aggregate). After applying the correction for sub-angular shape (−12 kg/m³), the final water content is 181 kg/m³.
Step 7. Cement Content
The cement content was determined using the water–cement ratio relationship：
For this design, the selected parameters are:
Mixing water content
W_Water=181Kg/m^3
Water–cement ratio=0.41
According to the specifications, the minimum cement requirement for a 25 mm nominal maximum aggregate size is 310 kg/m³. The calculated cement content of 441 kg/m³ is higher than this minimum requirement, ensuring adequate strength and durability for the mix.
The mass of concrete about 25mm aggregate size is 2376kg/m^3
Step 8. Fine Aggregate Requirements
In this design, the bulk density of coarse aggregate was taken as 1600 kg/m³，
The fineness modulus (FM) of the fine aggregate was selected as 2.60.
Concrete Strength Design – Summary
Design strength: Concrete grade N32 is selected, with a target compressive strength of 32 MPa.
Target mean strength: Due to limited test samples, a safety margin of 8.5 MPa is added, resulting in a target strength of 40.5 MPa.
Water-cement ratio: Given Canberra’s low exposure environment, non-air-entrained concrete is used with a w/c ratio of 0.41.
Coarse aggregate: Selected 25 mm sub-angular crushed stone meets all spacing and section size limits.
Air content: As per design and environment, air content is 2%.
Slump (workability): Target slump is 75 mm for proper placement without segregation.
Water content: Adjusted to 181 kg/m³ after considering aggregate shape.
Cement content: Calculated as 441 kg/m³, which satisfies the minimum requirement of 310 kg/m³.
Fine aggregate: Computed as 643 kg/m³ based on remaining percentage of total concrete volume.
2.7 Final Mix Design and Verification
The result of the final mix design, which was formulated by using the American Volumetric Method, is the following mix: 181 kg of water, 441 kg of cement, 643 kg of fine aggregate, 1,202 kg of coarse aggregate, and 1.5-2.0% of air that defines the mix. The resulting proportions yield cement content significantly greater than the necessary minimum 310 kg/m 3, and a water-cement ratio of 0.41, since the ratio provides durability properties that align with UnderAS3600 area Exposure, Class A1 moisture and independent of temperature ( interior, mild environment) durability properties.
Checking of the mix shows that it achieved the two premises of the project of strength and serviceability. The average strength at 40.5 Mpa is higher than that of the provided 32 MPa grade, and it provides a suitable tolerance on variation of materials (Friese et al. 2022). The slump of 75 mm was full in order to have a working mix to allow the beam and slab to be homogeneous and at the same time imparts cohesiveness to the mix to ensure that the mix does not separate.
The combination of materials and cement content incorporated can be readily mixed, compacted and finished without excessive Shrinkage or chances of crack propagation. It is noteworthy that the mix may also be used in the Zone Z, where the slabs and beams are required to hold standard occupancy loads and other hydrostatic pressure imposed by a Jacuzzi (Kazemi et al. 2023). Design Final, thus, validates successful functions as far as strength, durability and serviceability use is concerned.
| Material | Quantity (kg/m³) | Notes |
| Water | 181 | Ensures workability (Slump: 75 mm) |
| Cement | 441 | > AS3600 minimum of 310 kg/m³ |
| Fine Aggregate | 643 | Based on 2.60 fineness modulus |
| Coarse Aggregate | 1,202 | 25 mm sub-angular crushed stone |
| Air Content | 1.5 – 2.0 % | Non-air-entrained; for mild exposure |
| Water-Cement Ratio | 0.41 | Meets strength and durability balance |
| Target Mean Strength | 40.5 MPa | Exceeds characteristic f’c = 32 MPa |
Table 1: Final Concrete Mix Proportions
2.8 Summary
The study established a concrete mix design of structural elements in Area Z in the 5th floor in Zone Z, which was developed through the application of the American Volumetric Method. The ultimate ratio of mixes is 181kg of water, 441kg cement, 643kg fine and 1202kg gross per cubic metre, resulting in a water/cement ratio of 0.41. The mix fulfils all the AS3600 requirements of longevity during the Exposure Class A1 and has sufficient workability and strength. The design has a target mean strength of 40.5 MPa and is hence in compliance with the structural requirement and serviceability requirements.
Reference List
Journals
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