Concrete is a common building material that is used in a variety of construction projects, from sidewalks to skyscrapers. While it is known for its strength and durability, concrete is not immune to a phenomenon known as bleeding. Bleeding of concrete refers to the migration of water to the surface of freshly poured concrete, causing a layer of water to form on the top. It may seem like a minor issue, but bleeding can have serious consequences for the structural integrity and aesthetic appearance of concrete. In this article, we will explore the causes, effects, and prevention of bleeding in concrete to help you understand this phenomenon in more detail.
Table of Contents
Test for Bleeding of Concrete
Test for Bleeding of Concrete is an important measurement in assessing the quality and strength of concrete. Bleeding is the process by which water in the freshly mixed concrete rises to the surface due to the settlement of solid particles, causing a layer of cement paste to form on the surface. This can have a negative impact on the durability and strength of the concrete, leading to cracking and surface defects. Hence, it is necessary to conduct a test for bleeding to ensure the overall quality of the concrete.
The most common method used for testing bleeding of concrete is the ASTM C 232, which is a standard test method for Bleeding of Concrete. This test determines the amount of bleeding in freshly mixed concrete by measuring the quantity of water that separates from the concrete mixture and accumulates on its surface.
The test procedure involves preparing a 200 mm diameter mold, which is lined with filter paper at the bottom. A known quantity of concrete is then placed in the mold and compacted using a tamping rod. The mold is kept on a glass plate for support and immersed in a water bath maintained at a temperature of 23°C. A weight is placed on top of the mold to simulate the vertical pressure exerted by the weight of additional concrete layers. The test is carried out for a period of 5 minutes, and the amount of water accumulated on the surface is measured.
The results of the test are reported as the percentage of the gross volume of the concrete mixture. The accepted value for this test is less than 3% for concrete containing fine aggregates and less than 1.5% for concrete containing both fine and coarse aggregates.
There are several factors that can affect the results of the bleeding test, such as the characteristics of the cement, types and amounts of admixtures, water-cement ratio, temperature, and humidity. Thus, it is essential to carefully control these factors to obtain accurate and consistent results.
In conclusion, the test for bleeding of concrete is a crucial evaluation procedure that helps civil engineers to determine the quality and strength of the concrete. It is essential to conduct this test to ensure that the concrete is free from excessive water and meets the required standards for construction projects.
Apparatus of Bleeding of Concrete
Bleeding of concrete is a common phenomenon that occurs during the mixing and placement of concrete. It refers to the process by which water rises to the surface of freshly placed concrete, leaving behind a layer of cement paste at the surface. This can result in a weaker and less durable concrete structure, making it important to control bleeding during construction. The apparatus used for bleeding of concrete consists of the following components:
1. Funnel: The funnel is used to create a vertical column of concrete in order to measure the rate of bleeding. It is made of metal or glass and has a diameter of 150mm and a height of 300mm.
2. Graduated Glass Cylinder: This is used to measure the total amount of water that has bled out of the concrete. It has a capacity of 1000ml and is marked in 10ml intervals.
3. Perspex Plate: The Perspex plate is placed on top of the funnel to prevent the evaporation of water from the surface of the concrete. It also helps in maintaining a constant temperature and humidity during the test.
4. Stop Watch: A stopwatch is used to measure the time taken for the water to bleed out of the concrete. This is an important factor in determining the bleed rate.
5. Balance: A balance is used to weigh the concrete sample before and after the bleeding test. This helps in calculating the amount of water that has bled out of the concrete.
6. Thermometer: A thermometer is used to measure the temperature of the concrete sample during the test. This is important as temperature can affect the rate of bleeding.
7. Measuring Cylinder: A measuring cylinder is used to measure the initial quantity of water added to the concrete mix, as well as the amount of water that has bled out of the concrete.
8. Mixer: A mixer is used to prepare the concrete mix before the bleeding test. It ensures a uniform and consistent mix of all the ingredients.
9. Trowel: A trowel is used to transfer the concrete mix from the mixer to the funnel for testing.
All these apparatus are essential for conducting the bleeding test and obtaining accurate results. They should be calibrated and maintained properly to ensure reliable results. The bleeding test helps in understanding the workability and stability of concrete and is an important factor in determining the quality of the final concrete structure.
Test Procedure of Bleeding of Concrete
Bleeding of concrete is a common phenomenon that occurs during the early stages of concrete placement and can lead to several defects in the finished product. It is the process of separation of water from the freshly placed concrete, resulting in the accumulation of water on the surface. This can cause various problems such as an uneven surface, reduced strength, and durability of the concrete. Therefore, it is essential to perform the bleeding test to identify any issues and take necessary steps to rectify them. The following is the procedure for testing bleeding of concrete.
1. Preparation of Test Samples:
Firstly, prepare the concrete sample of adequate quantity according to the project requirements. Ensure that the mixture is homogeneous and free from any air voids. The sample should be prepared using the same materials and proportions as the actual concrete used in the construction.
2. Molding of Test Specimens:
Next, mold the test specimens using a compaction method that is similar to the construction site. It is recommended to use a vibrator to remove any entrapped air and to make sure the concrete is properly compacted. Once the specimens are molded, label them with a unique identification number for tracking purposes.
3. Curing of Test Specimens:
After molding, the test specimens should be cured in a water tank or moist environment for 28 days. This is crucial for achieving accurate test results.
4. Removal from Molds:
After 28 days, remove the specimens from the molds and allow them to air dry for 24 hours.
5. Preparing the Test Setup:
The test setup consists of a flat plate and a 100mm diameter cylinder. On the flat plate, place the cylinder and fill it with freshly mixed concrete up to about three-quarters of its height. Make sure there are no air pockets between the concrete and the cylinder walls.
6. Measuring the Initial Height of Concrete:
Measure the initial height (H1) of concrete from the bottom of the cylinder to the surface of the concrete.
7. Observation and Recording of Surface Water:
Allow the concrete to settle and flow for 15 minutes. After this time, observe and record the amount of free water on the surface of the concrete. This is known as the bleeding water and is calculated as the difference between the initial height (H1) and the final height (H2) of the concrete.
Bleeding water amount = (H1 – H2)
8. Calculation and Reporting of Results:
Once the bleeding water has been measured and recorded, calculate the bleeding rate. This is done by dividing the bleeding water amount by the surface area of the concrete sample. The results should be reported in terms of milliliters per square centimeter per minute (ml/cm2/min).
9. Repeat the Test:
To ensure accurate results, it is recommended to perform the bleeding test on at least three specimens and take the average value.
10. Interpretation of Results:
According to IS: 1199-1959, the acceptable rate of bleeding should not exceed 5 ml/cm2/min. Any values above this indicate excessive bleeding, which can lead to defects in the concrete. In such cases, necessary precautions should be taken to reduce the bleeding rate.
In conclusion, the bleeding test is a simple yet essential test to determine the quality of concrete. It helps to identify any potential issues with the concrete mixture and allows for necessary measures to be taken to prevent them. As a civil engineer, it is crucial to follow the proper procedure for testing
Calculations for Bleeding of Concrete
Bleeding is a common phenomenon that occurs during the process of concrete placement and consolidation. It refers to the upward movement of water within the newly placed concrete, causing it to rise to the surface and accumulate. This leads to a reduction in the quality and strength of concrete as well as aesthetic issues such as surface cracking and discoloration.
Calculations for bleeding of concrete are essential to understand the extent of this phenomenon and take appropriate measures to control it. The following are the key calculations involved in assessing and managing concrete bleeding:
1. Water-Cement (W/C) Ratio:
The water-cement ratio is a crucial parameter in determining the amount of bleeding in concrete. It is the ratio of the weight of water to the weight of cement in a concrete mix. A higher W/C ratio results in excessive bleeding, while a lower ratio can cause inadequate workability and compaction. The ideal W/C ratio should be determined based on the specific requirements of the project, including the strength and durability of the concrete.
2. Slump Test:
The slump test is a standard method used to assess the consistency and workability of fresh concrete. It involves filling a cone-shaped mold with concrete and measuring the settlement or slump of the concrete after the mold is removed. A higher slump indicates a higher amount of bleeding, and adjustments must be made to the mix design to reduce the W/C ratio and control the bleeding.
3. Air Content:
Air entrainment is an essential aspect of concrete mix design, especially in harsh climatic conditions. The presence of air voids in concrete can reduce the amount of bleeding by providing an escape route for the water to rise to the surface. The air content is typically measured using the pressure method, where the volume of air is determined by subjecting a known amount of concrete to a specific pressure.
4. Aggregate Properties:
The properties of aggregates used in concrete, such as shape, size, and texture, can significantly impact the amount of bleeding. A mix with poorly graded aggregates is prone to bleeding, as the voids between particles allow water to rise easily. On the other hand, a well-graded mix with a suitable proportion of coarse and fine aggregate can reduce the chances of bleeding due to its uniformity and compactness.
5. Temperature and Humidity:
The ambient temperature and relative humidity during concrete placement can also affect the level of bleeding. Higher temperatures and low humidity can accelerate the rate of evaporation of water, resulting in more bleeding. Hence, it is essential to consider the weather conditions and plan the concrete placement accordingly to minimize the effects of bleeding.
In addition to these calculations, it is crucial to monitor and record the actual bleeding of concrete during placement and consolidation. This can be done by visually observing the surface of concrete for signs of bleeding, such as the presence of water on the surface. If the bleeding is excessive, appropriate measures such as using admixtures or retarding agents or adjusting the mix design must be taken to control it.
In conclusion, understanding and accurately calculating the factors that contribute to concrete bleeding is crucial for producing durable and aesthetically pleasing concrete structures. Proper mix design, monitoring, and control can help minimize the adverse effects of bleeding and ensure the desired quality and performance of concrete.
Conclusion
In conclusion, bleeding of concrete is a normal and inevitable process in the construction of various structures. It occurs due to the settling and segregation of the components of concrete, and can have both positive and negative effects on the final strength and durability of the structure. It is important for construction professionals to understand the causes and mechanisms of bleeding, as well as the proper methods for controlling and minimizing it. With proper knowledge and techniques, bleeding can be managed effectively, ensuring the overall quality and longevity of the concrete structure. It is crucial to address any issues related to bleeding during construction to avoid potential problems and ensure a successful project. By understanding all aspects of bleeding of concrete, constructors can achieve optimal results and deliver high-quality structures that can withstand the test of time