Self-compacting concrete (SCC) is a unique type of concrete that possesses exceptional flowability, homogeneity, and the ability to fill every corner of the formwork without segregation or the need for vibration. To classify a concrete mix as SCC, it must fulfill the requirements for several key characteristics, including filling ability, passing ability, segregation resistance, and viscosity.
To determine whether a concrete mix meets these requirements, the European Federation of National Associations Representing for Concrete (EFNARC) recommends a set of testing procedures. These procedures are designed to measure the flowability of the concrete, its ability to pass through obstructions without segregation, its resistance to segregation, and its viscosity.
By conducting these tests, engineers and contractors can ensure that the SCC they are using meets the required specifications for each of these critical characteristics. Ultimately, these procedures help to ensure that self-compacting concrete performs as expected, delivering the exceptional properties that make it such a valuable material in construction projects.
Test Methods For Workability of Self Compacting Concrete
1. Slump Flow & T50cm Test
The purpose of the slump flow test is to evaluate the ability of concrete to flow horizontally in the absence of obstructions. This test is widely used and provides a reliable assessment of the concrete’s filling ability. It can be performed on-site and is also useful in determining the concrete’s resistance to segregation. During the test, the slump cone is lifted, and a stopwatch is started to measure the time it takes for the concrete to reach a 500 mm mark. This time is known as the T50 time, which indicates the rate of spread of the concrete.
Equipment
To conduct a slump test, the usual slump cone is used. This cone has a base diameter of 200 mm, top diameter of 100 mm, and height of 300 mm. Additionally, a stiff base plate is needed, which should be at least 700 mm on each side. Before beginning the test, concentric circles are marked around the center point where the slump cone will be placed. A firm circle is drawn at a diameter of 500 mm.
In addition to the slump cone and base plate, several other tools are required for the test. These include a trowel and scoop for measuring and manipulating the concrete, a measuring tape for taking measurements, and a stopwatch for timing. All of these tools are essential for accurately conducting the slump test and obtaining reliable results.
Procedure
To conduct a test that requires about 6 liters of concrete, the first step is to place the baseplate on level ground. The next step is to position the slump cone centrally on the baseplate. Then, using a scoop, fill the cone with concrete without tamping it down. Level off the concrete with a trowel and remove any excess that may be lying on the baseplate. Once this is done, lift the cone up vertically and let the concrete flow freely. Finally, measure the final diameter of the concrete in two perpendicular directions and calculate the average of the two diameters to obtain the slump flow in mm. It is important to note that no water or cement paste or mortar should be visible at the edge of the spread concrete, as this can affect the accuracy of the test.
Interpretation
The ability of concrete to fill formwork under its own weight is directly proportional to its flow value. In other words, the higher the flow value, the greater its ability to fill the formwork. For Self-Compacting Concrete (SCC), a minimum flow value of 650 mm is required. This ensures that the concrete can easily flow and settle into the formwork without any additional compaction efforts.
In situations where there is severe segregation, the coarse aggregate will tend to remain in the center of the pool of concrete, while the mortar and paste settle at the periphery of the concrete. This can lead to an uneven distribution of the components within the concrete, which can affect its overall quality and strength.
The T50 time, which is the time taken for the concrete to reach 50% of its ultimate height after the release of the slump cone, is an important indicator of the flowability of concrete. A lower T50 time indicates greater flowability, as the concrete settles into the formwork more quickly and efficiently. Therefore, it is important to monitor the T50 time during the concrete pouring process to ensure that the concrete has the desired flowability and can settle into the formwork properly.
2. J-Ring Test
The J-ring test is used to determine the passing ability of concrete and is commonly performed on construction sites. The test involves placing bars and sections at varying distances apart to mimic the reinforcement congestion at the site. Typically, the sections are spaced at a distance of 3 times the maximum size of aggregate. During the test, the vertical sections form a ring with a diameter of 300 mm and a height of 100 mm.
Equipment
The equipment required for a slump cone test includes a slump cone without foot pieces, a base plate with a minimum size of 700 mm square, a trowel, a scoop, and tape. Additionally, a J-ring with a rectangular section measuring 30mm x 25mm should be planted vertically to form a ring with a diameter of 300 mm. The spacing between J-rings should be around 48 ± 2mm. These tools are essential for conducting a slump cone test, which is used to determine the workability of concrete. The J-ring is particularly important in ensuring that the slump test results are accurate and reliable. With these tools, engineers and construction professionals can ensure that their concrete mixes meet the necessary quality standards and are suitable for use in various building projects.
Procedure
To conduct a concrete test, approximately 6 liters of concrete is required. Before beginning the test, it is important to moisten the inside of both the slump cone and base plate. Once this is done, the J-Ring should be placed centrally on the base plate, and the slump cone should be positioned centrally inside the J-Ring.
To fill the slump cone, a scoop should be used to transfer the concrete without tamping. Once the cone is full, the concrete should be leveled off using a trowel, and any excess material should be removed. The next step is to raise the cone vertically, allowing the concrete to flow out through the J-Ring.
Once the concrete has settled, the final diameter should be measured in two perpendicular directions. It is important to note if there is a border of mortar or cement paste without coarse aggregate at the edge of the concrete.
Interpretation
It is commonly understood that concrete with good passing ability should exhibit similar spread when tested with and without a J-ring. In other words, the use of a J-ring should not significantly impact the flow of the concrete. Typically, any differences observed in the spread of concrete between the two tests should not exceed a maximum difference of 50mm. This is an important consideration in the testing of concrete, as it helps ensure that the material will flow consistently and evenly during construction. By evaluating the spread of concrete under these conditions, builders and engineers can gain valuable insights into its properties and suitability for use in various construction applications.
3. V-Funnel Test and V-Funnel Test at T5 Min.
The V-Funnel test is a method to evaluate the flowability or filling ability of concrete that contains aggregate with a maximum size of 20 mm. The test involves filling a funnel with approximately 12 liters of concrete, and then recording the time it takes for the concrete to flow down through the funnel. The flow time measurement provides an indication of the concrete’s flowability, as the more time it takes for the concrete to flow through the funnel, the less flowable it is.
After the initial flow time measurement, the same funnel can be filled with concrete and allowed to settle for 5 minutes. If the concrete shows any sign of segregation, such as coarse aggregate settling to the bottom or bleeding of water to the surface, the flow time will increase significantly. This is because segregation reduces the homogeneity of the mixture, making it less flowable. The increase in flow time due to segregation is an indicator of the potential for blockages or issues during placement of the concrete in a construction project. Therefore, the V-Funnel test is an essential tool for assessing the flowability of concrete before its use in construction.
Equipment
- V-funnel
- Bucket 12 liters
- Trowel
- Scoop
- Stopwatch
Procedure
To conduct a test for concrete, approximately 12 liters of concrete is required. The V-funnel should be placed on a firm ground and moistened from the inside of the funnel. It is necessary to close the trap door and place a bucket underneath before filling the apparatus completely with concrete. During the filling process, no compaction or tamping should be done. After 10 seconds, the trap door can be opened, and the time taken for the concrete to flow down should be recorded. This can be observed by looking at the light when viewed from the top. It is important to note that the entire test should be completed within 5 minutes.
Procedure for flow time at T5 min.
The given context describes a set of instructions for using a V-funnel apparatus to measure the flow time of concrete. The first instruction advises against cleaning or moistening the inside surface of the funnel. The second instruction is to close the trap door and refill the V-funnel immediately after measuring the flow time, making sure to place the bucket underneath to catch the concrete.
The third instruction is to fill the apparatus completely with concrete without tamping or tapping. Once filled, the concrete should be struck off level with the top using a trowel. After waiting for five minutes, the trap door should be opened again to allow the concrete to flow. The time taken for the concrete to completely discharge is referred to as the flow time at T5 min.
These instructions should be followed carefully to ensure accurate measurement of the flow time of the concrete using the V-funnel apparatus.
Interpretation
The test being conducted is designed to measure the ease of flow of concrete. Shorter flow times are indicative of greater flowability. Specifically, in the case of Self-Compacting Concrete (SCC), a flow time of 10 seconds is considered to be appropriate. During this test, an inverted cone shape is used to restrict flow, and prolonged flow times may give some indication of the mix’s susceptibility to blocking. After 5 minutes of settling, segregation of the concrete will be apparent, and a less continuous flow will result in an increase in flow time.
The V-funnel test is also used to measure the flow time of concrete. It is recommended that the flow time for this test be between 8 and 12 seconds. After 5 minutes and 3 seconds have passed, the flow time for the V-funnel test is still allowed. The aim of the test is to ensure that the concrete has the appropriate flow time, and that it does not segregate or block during use. By conducting this test, concrete manufacturers and users can ensure that their product is of the highest quality, and will perform as expected in real-world applications.
4. L-Box Test Method
When performing concrete work underwater, it is important to evaluate the flow of the concrete and the potential for it to be obstructed by reinforcement. To achieve this, a test is typically conducted to determine the concrete’s flowability and the extent to which it may be blocked by any reinforcement present. This information is crucial in ensuring the successful completion of underwater concreting projects. As a result, it is generally recommended that this type of testing be conducted prior to the commencement of any underwater concrete work. By doing so, potential issues can be identified and addressed beforehand, minimizing the risk of project delays or failures.
The test assesses the flow of concrete and also, the extent to which the concrete is subjected to blocking by reinforcement it is mostly preferred while underwater concreting to be done
Equipment
- L box of a stiff non-absorbing material
- trowel
- scoop
- stopwatch
Procedure
To conduct a test on the passing ability of concrete through bars, a total of 14 liters of concrete is required. The test involves filling the vertical section of a setup with concrete and then lifting the gate to allow the concrete to flow into the horizontal section. Once the flow of concrete has stopped, the height of the concrete in the horizontal section is measured as a proportion of the height remaining in the vertical section, expressed as H2/H1 in the diagram. This proportion indicates the slope of the concrete when it is at rest and provides an indication of the degree to which the passage of concrete through the bars is restricted.
Interpretation
When concrete is in a resting state and flows as freely as water, it naturally settles in a horizontal position. This indicates that the ratio of the height of the concrete (H2) to the height of the slump cone (H1) will be equal to 1. Therefore, the closer the test values are to unity, the better the flow of the concrete. A minimum acceptable value of 0.8 is preferred for the blocking ratio, which refers to the ratio of the height of the concrete after blocking to the initial height of the slump cone.
While the T20 and T40 time measurements can provide some indication of the ease of flow, no suitable values have been suggested. It is important to consider the blocking ratio in determining the flow of concrete. A blocking ratio closer to unity indicates better flow, while a lower ratio suggests difficulty in the concrete’s ability to flow freely. Overall, achieving a satisfactory flow of concrete is essential for ensuring the success of construction projects.
5. U box test method
The purpose of this test is to evaluate the filling ability of self-compacting concrete. The test involves using a vessel that is divided into two compartments by a middle wall. While the test itself is straightforward to perform, constructing the necessary equipment can be challenging. However, it is an effective means of directly assessing the filling ability of the concrete.
The test is used to measure the filling ability of self-compacting concrete. It consists of a vessel that is divided by a middle wall into two compartments This is a simple test to conduct, but the equipment may be difficult to construct. It provides a good direct assessment of filling ability
Equipment
- U box of a stiff non-absorbing material
- trowel
- scoop
- stopwatch
Procedure
To perform the test, approximately 20 liters of concrete are required. Prior to filling the apparatus with the concrete sample, it is important to moisten the inside surfaces and remove any excess water. Once this is done, the concrete sample should be poured into one compartment of the apparatus and left to stand for one minute.
Next, the sliding gate should be lifted to allow the concrete to flow out into the other compartment. After the concrete has come to rest, the height of the concrete in the filled compartment should be measured in two places, and the mean height (H1) should be calculated. Additionally, the height of the concrete in the other compartment (H2) should be measured.
To calculate the filling height, which should be as low as possible, H1 should be subtracted from H2.
Interpretation
The given context pertains to the flow and passing ability of concrete. It states that if concrete flows as freely as water, it will be horizontal when at rest, and therefore, the difference between H1 and H2, representing the filling height, will be zero. This suggests that the nearer the filling height is to zero, the better the flow and passing ability of the concrete.
In essence, the context highlights the importance of measuring the filling height in determining the flow and passing ability of concrete. The filling height is a crucial indicator of the degree to which the concrete flows freely, and the closer it is to zero, the more effectively the concrete is able to pass through tight spaces and narrow gaps. Thus, achieving a low filling height is desirable in concrete applications that require efficient flow and passage through constricted areas.
6. Fill box test method
A test is utilized to assess the filling capacity of self-compacting concrete that has a maximum aggregate size of 20mm. The test involves a transparent container with a smooth and flat surface. The container is filled with concrete through a filling pipe, and the disparity in height between the two sides of the container serves as a gauge for the filling capacity. However, even if a concrete mix has a high filling capacity, it may still perform inadequately if it has poor passing ability and segregation resistance. Despite being an essential test, it is complicated and challenging to conduct on-site, thus it is not widely used.
Equipment
- Fill box of a stiff, transparent, non-absorbing material
- Scoop 1.5 to 2 liter
- Ruler
- Stopwatch
Procedure
To perform a concrete test, approximately 45 liters of concrete is required. The first step is to moisten the inside surfaces of the apparatus and remove any excess water. Next, the apparatus is filled with the concrete sample. The container is then filled gradually by adding fresh concrete in intervals of 5 seconds, with each scoop containing 1.5 to 2 liters of concrete. The process continues until the concrete has just covered the first top obstacle. Once the concrete has come to rest, the height at which the container is filled is measured at two places on either side. The average of these two measurements is calculated and denoted as h1. This process is then repeated on the opposite side, and the average height measurement is calculated and denoted as h2. Finally, the average filling percentage is calculated based on these measurements.
Interpretation
When concrete flows freely like water, it tends to rest in a horizontal position. This results in the average filling percentage being at 100%. The filling height, which is a test value used to measure the self-compacting characteristics of concrete, is considered better when it is closer to 100%. Essentially, the closer the filling height is to 100%, the more efficient the self-compacting features of the concrete.