Understanding and Assessing Concrete Bleeding

Concrete, a fundamental construction material, requires careful evaluation to ensure its quality. One crucial aspect is the assessment of bleeding, which gauges the amount of water that separates from a freshly mixed concrete batch. This test helps in understanding the concrete’s workability and consistency.

Apparatus and Equipment

To conduct the Test for Bleeding of Concrete, specific apparatus and accessories are essential:

1. Cylindrical Container

The cylindrical container, made of steel, possesses a volume of 0.01 m³, with an inside diameter of 250 mm and an inside height of 280 mm.

2. Weighing Scale

A digital weighing scale with an accuracy of up to 0.005 is required for precise measurement of specimen samples.

3. Pipette

Utilized to draw off the bleed water from the specimen.

4. Tamping Bar

A rounded steel bar, 16 mm in diameter and 610 mm in length, used for compacting the concrete layers.

5. Graduated Cylinder

A 100 cm³ graduated cylinder is necessary for measuring the water quantity.

Test Procedure

The procedure for the Test for Bleeding of Concrete involves several steps:

Concrete Preparation:

Take a sufficient quantity of freshly mixed concrete with a known water content.

Filling the Container:

Fill the cylindrical container with the concrete mix in five layers, each 50 mm thick.
Compact each layer with a tamping bar, providing a minimum of 60 strokes after filling.

Surface Preparation:

After compacting the final layer, level and smooth the top surface using a trowel.

Initial Measurements:

Immediately after troweling, note the time and weigh the container.
Place the container on a leveled surface, free from vibrations, and cover the top surface to prevent evaporation.

Bleeding Water Collection:

At a temperature between 25°C to 29°C, use a pipette to collect water every 10 minutes for the first 40 minutes.
Subsequently, collect water every 30 minutes until bleeding ceases.
If water collection becomes difficult on the flat top surface, slightly tilt the container.

Final Measurements:

Note the weight of water collected in the graduated jar.

Calculations

The percentage of bleeding water can be calculated using the following equations:

Bleeding Water % (Equation 1):

Bleeding water % = (B/W) × 100
Where B = Total quantity of bleeding water, W = Total quantity of water in the concrete mix.

Bleeding Water % (Equation 2):

Bleeding water percentage of net mixing water contained within the test specimen is given by:
- (D / (S – w)) × 100
- Where D = Total mass of bleeding water, w = Net mass of water (total water – water absorbed by aggregates), S = Total mass of concrete mix, W = Total mass of water in the mix.

Equation 2 is considered more accurate since it accounts for water absorbed by aggregates.

Results

The bleeding water percentage for the given concrete mix is calculated using the appropriate equation.