Factors Affecting The Workability Of Concrete

In this post, we will discuss the factors that affect the workability of concrete. Based on our understanding of practicality and its importance from the previous article, we will take an in-depth look at these essential factors. Let’s see how these elements affect the ease and efficiency of working with concrete in construction projects.

Water-Cement Ratio
Aggregates cement Ratio
Mix Proportion Aggregate and cement Ratio
Types of Cement
Temperature
Humidity and Environmental Conditions
Use of Admixtures

1. Water-Cement Ratio

The water-cement ratio significantly affects the workability of concrete. A higher amount of water per cubic meter increases the fluidity of concrete, which is an important factor affecting workability.

According to Abran’s law, the strength of concrete is directly related to the water-cement ratio. Specifically, the amount of cement in concrete depends entirely on the water-cement ratio. In fresh concrete, water content increases relative to strength as voids decrease. As a result, lower water-cement ratio results in reduction of void volume and solid volume ratio, thereby yielding stronger hardened concrete.

The water-cement ratio essentially measures voids directly related to the amount of empty pore space in hardened concrete. Similarly, the amount of cement in fresh concrete is directly related to the amount of solid in hardened concrete. In the hardened state, the strength of concrete is inversely proportional to the water-cement ratio, as shown in the figure. In particular, the compressive strength of concrete does not strictly follow a hyperbolic curve at low water-cement ratios because while water is available for cement hydration, some amount of the cement paste remains unhydrated.

The strength of concrete depends on the following factors like,

Water Cement Ratio
Cement to aggregate ratio
Maximum overall size
Physical properties of aggregates

The strength of concrete is significantly affected by the presence of air voids within the mix. The higher the percentage of air voids, the lower the strength of the resulting concrete. These air voids are formed due to evaporation of water and entrapment of trapped air during the concrete making process.
The strength of cement decreases with age. This reduction in strength is particularly noticeable when the direction of load is perpendicular to the axis of the concrete cube, at which point the load carrying capacity is considered maximum.
Optimizing the cement-to-aggregate ratio plays an important role in increasing the strength of concrete. Increasing this ratio increases ultimate strength, albeit to a certain extent.
In terms of the type of aggregate used, crushed aggregate provides maximum strength due to its ability to reduce voids within the concrete structure.

Furthermore, when comparing different aggregate shapes, round spherical or cuboidal aggregates, when compacted, contain less voids than irregular and flaky aggregates of the same nominal size. As a result, they contribute to higher concrete strength.

Here are the maximum permissible water-cement ratios for various exposure conditions

For mild exposure conditions (e.g., interior walls, above ground) Maximum W/C Ratio: 0.55
For moderate exposure conditions (for example, general outdoor construction): Max W/C Ratio: 0.5
For severe risk situations (e.g., coastal areas, exposure to aggressive chemicals): Maximum W/C Ratio: 0.45

2. Aggregates cement Ratio

Aggregate cement ratio (A – C ratio) is the ratio of the weight of aggregate to the weight of cement in the concrete mix. It is an important factor that affects the workability, strength and durability of concrete.

A higher A/C ratio results in a more workable concrete, but it also reduces the strength and durability of the concrete. This is because cement paste is responsible for binding the aggregates together and filling the spaces between them. If there is not enough cement paste, the concrete will be weak and more likely to crack.

The ideal A/C ratio for a given application will depend on several factors, including the required strength and durability of the concrete, exposure conditions, and workability requirements.

For structural concrete, the A/C ratio should be 6:1 or less.
For exposed concrete, the A/C ratio should be 5:1 or less.
For high-performance concrete, the A/C ratio should be 4:1 or less.

3. Aggregate size

The size and shape of aggregates play an important role in determining the workability of concrete. Larger aggregates have less surface area, requiring less water for wetting and less paste to smooth the surface, reducing internal friction. As a result, for a given amount of water and paste, larger aggregate sizes increase workability.

3. Aggregate Shape

Furthermore, the Shape of the aggregate has a profound effect on workability. Angular, elongated or flaky aggregates result in harder concrete than rounded or cubic aggregates. Round aggregates have less surface area and fewer voids, and because of their size, abrasion resistance is significantly reduced.

Due to these factors or the above reasons, river and gravel aggregates provide higher workability to concrete than crushed aggregates and sand.

4. Mix Proportion Aggregate and cement Ratio

Proper proportion of cement, water, aggregates and admixture is necessary to obtain the desired workable consistency in concrete.
A well-balanced mix ensures that the concrete is easily workable without being too hard or too liquid.
Adjustments to the mix proportions allow tailoring of concrete to meet specific project requirements and environmental conditions.
Achieving the correct mix ratio is a critical step in optimizing functionality while maintaining structural integrity and durability.

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5. Types of Cement

Different types of cement have different chemical and physical properties that affect the behaviour and workability of concrete. Engineers select the appropriate cement type based on project specifications and desired workability for efficient construction.
Understanding the properties of different types of cement helps in achieving the desired workability and overall quality of concrete.

6. Temperature

Temperature affects the rate of hydration, setting time and workability of the concrete mix.
Higher temperatures may cause faster setting and may require adjustment to maintain desired functionality during hot weather conditions.
Cold temperatures can reduce workability, requiring measures to ensure proper mixing and placement of concrete.
Managing the temperature of concrete is important to maintain workability and achieve the required strength and durability of the structure.

7. Humidity and Environmental Conditions

The level of humidity in the environment affects the water demand and workability of the concrete mix.
In dry conditions, low humidity can accelerate water evaporation, affecting functionality and requiring timely adjustment.
Understanding how different environmental conditions affect workability is important for successful concrete placement and finishing.
Adapting the mix to suit different environmental conditions helps maintain optimum functionality during the manufacturing process.

Use of Admixtures

Admixtures are additives that are used to enhance specific properties of concrete, including workability.
Plasticizers and superplasticizers are common admixtures that improve workability by reducing water content while maintaining the desired consistency.
The use of admixtures allows precise control over workability, enabling the concrete to meet specific project requirements.
Proper selection and dosage of admixture is important to achieve the desired workability and overall performance of the concrete mix.

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