Low slump concrete is desirable when concrete is placed in large open forms, or when the form is placed on a slope. The concrete mix must be designed for these special applications.
Very low slump concretes are typically too stiff to be measured with test methods that consider the ability of concrete to flow. Instead, tests for very low slump concretes generally attempt to simulate the actual placement conditions for low slump concretes and measure more relevant properties like compactability. The proctor test and the Kango hammer test utilize vibration to compact samples whereas the intensive compaction test uses compression and shear forces.
These tests are generally simple to perform, although none can be used as a simple field quality control device.
1.Proctor Test
The proctor test used for soils can also be used for lean, dry concrete mixes The test procedure for concrete is the same test procedure commonly used for soils. Either the standard Proctor test or the modified Proctor test can be used. Four to six samples, each with varying moisture content, are compacted in a cylindrical mold using a drop hammer. The unit weight of each compacted sample is plotted against moisture content to determine the maximum dry unit weight and corresponding moisture content.
Advantages:
• The test is appropriate for low slump concrete mixtures that cannot be tested with conventional workability tests.
• The test is simple and well known.
Disadvantages:
• The test does not incorporate vibration, which is commonly used to compact low slump concretes.
• The test is time consuming: performing the test requires four to six samples to be prepared to define the unit weight versus moisture content curve.
Very low slump concretes are typically too stiff to be measured with test methods that consider the ability of concrete to flow. Instead, tests for very low slump concretes generally attempt to simulate the actual placement conditions for low slump concretes and measure more relevant properties like compactability. The proctor test and the Kango hammer test utilize vibration to compact samples whereas the intensive compaction test uses compression and shear forces.
These tests are generally simple to perform, although none can be used as a simple field quality control device.
1.Proctor Test
The proctor test used for soils can also be used for lean, dry concrete mixes The test procedure for concrete is the same test procedure commonly used for soils. Either the standard Proctor test or the modified Proctor test can be used. Four to six samples, each with varying moisture content, are compacted in a cylindrical mold using a drop hammer. The unit weight of each compacted sample is plotted against moisture content to determine the maximum dry unit weight and corresponding moisture content.
Advantages:
• The test is appropriate for low slump concrete mixtures that cannot be tested with conventional workability tests.
• The test is simple and well known.
Disadvantages:
• The test does not incorporate vibration, which is commonly used to compact low slump concretes.
• The test is time consuming: performing the test requires four to six samples to be prepared to define the unit weight versus moisture content curve.
2.Kango Hammer Test
The Kango hammer test attempts to measure workability by simulating the effect of vibration and pressing on low-slump concretes. Concrete is placed in a cubic or cylinder mold in two to three separate layers. A demolition hammer, which is mounted in a frame and equipped with a special bit that fits the shape of the mold, applies a constant pressure and vibration to each layer of concrete. After compaction of all layers, the density of the concrete specimen is
determined. The greater the density of the compacted concrete specimen, the greater will be the compactability and workability of the concrete mix.
The particular demolition hammer typically used for this test method is manufactured by Kango
Advantages:
• By using both vibration and pressure, the test accurately simulates field placement conditions.
• The test is simple and easy to perform.
Disadvantages:
• The particular hammer is not specified, making comparisons of the test results difficult.
• The apparatus is larger than the proctor test and requires electricity.
The Kango hammer test attempts to measure workability by simulating the effect of vibration and pressing on low-slump concretes. Concrete is placed in a cubic or cylinder mold in two to three separate layers. A demolition hammer, which is mounted in a frame and equipped with a special bit that fits the shape of the mold, applies a constant pressure and vibration to each layer of concrete. After compaction of all layers, the density of the concrete specimen is
determined. The greater the density of the compacted concrete specimen, the greater will be the compactability and workability of the concrete mix.
The particular demolition hammer typically used for this test method is manufactured by Kango
Advantages:
• By using both vibration and pressure, the test accurately simulates field placement conditions.
• The test is simple and easy to perform.
Disadvantages:
• The particular hammer is not specified, making comparisons of the test results difficult.
• The apparatus is larger than the proctor test and requires electricity.
3.Intensive Compaction Test
The intensive compaction test is a gyratory compactor used to measure the workability of concrete mixtures with slumps less than approximately 1 cm. The test apparatus is a machine that applies compression and shear forces to a concrete specimen while recording the density of the specimen. To perform the test, the concrete to be tested is placed in a cylindrical mold, which is loaded into the test apparatus. The mold is available in two diameters—a 100 mm diameter mold is used for concretes with maximum aggregate sizes of up to 20 mm while a 150 mm diameter mold is appropriate for maximum aggregate sizes up to 32 mm. Two pistons at either end of the cylinder apply a compressive force to the sample.
Simultaneously, the angle of inclination of the pistons rotates to apply a shearing motion to the concrete. This compaction technique is represented in the Figure below . The pressure and speed of rotation can be adjusted for each test; however, these variables are held constant during each test. The volume of the sample, which is used to calculate density, is recorded continuously throughout the test. The test is performed in 3-5 minutes.
The intensive compaction test is a gyratory compactor used to measure the workability of concrete mixtures with slumps less than approximately 1 cm. The test apparatus is a machine that applies compression and shear forces to a concrete specimen while recording the density of the specimen. To perform the test, the concrete to be tested is placed in a cylindrical mold, which is loaded into the test apparatus. The mold is available in two diameters—a 100 mm diameter mold is used for concretes with maximum aggregate sizes of up to 20 mm while a 150 mm diameter mold is appropriate for maximum aggregate sizes up to 32 mm. Two pistons at either end of the cylinder apply a compressive force to the sample.
Simultaneously, the angle of inclination of the pistons rotates to apply a shearing motion to the concrete. This compaction technique is represented in the Figure below . The pressure and speed of rotation can be adjusted for each test; however, these variables are held constant during each test. The volume of the sample, which is used to calculate density, is recorded continuously throughout the test. The test is performed in 3-5 minutes.
Figure : Compaction of Concrete Sample in Intensive Compaction Device.
To determine the workability of a concrete mixture, the density of the concrete is plotted versus the number of working cycles of the pistons. Concrete mixes are evaluated by comparing the density after a certain number of cycles under a given pressure. Additionally, the performance of concrete production machines can be evaluated by comparing the density achieved with a particular machine to the density achieved with the intensive compaction test.
After the test, the sample of concrete can be removed from the cylinder mold and tested for compressive or splitting tensile strength either in the concrete’s fresh or hardened state. The results of the intensive compaction test show good correlation to the results of the Kango
hammer test and the Proctor test.
Although the larger 150 mm diameter model is too heavy and bulky for field use, the lightweight version of the 100 mm diameter model weighs approximately 120 lbs and can be transported to a field site. Electricity and compressed air are required to perform the test.
Advantages:
• Research has shown that the test is capable of accurately measuring even small changes in mixture proportions.
• The test accurately simulates placement conditions for low slump roller-compacted concretes.
• The test is fast and computer controlled.
• The test can be used for research, mix proportioning, or quality control. The smaller 100 mm model is feasible for field use.
Disadvantages:
• The equipment is expensive, especially when compared to the proctor test. The 150 mm diameter model is too heavy for field use.
• The test does not incorporate vibration, which is commonly used in the placement of low slump concrete.
After the test, the sample of concrete can be removed from the cylinder mold and tested for compressive or splitting tensile strength either in the concrete’s fresh or hardened state. The results of the intensive compaction test show good correlation to the results of the Kango
hammer test and the Proctor test.
Although the larger 150 mm diameter model is too heavy and bulky for field use, the lightweight version of the 100 mm diameter model weighs approximately 120 lbs and can be transported to a field site. Electricity and compressed air are required to perform the test.
Advantages:
• Research has shown that the test is capable of accurately measuring even small changes in mixture proportions.
• The test accurately simulates placement conditions for low slump roller-compacted concretes.
• The test is fast and computer controlled.
• The test can be used for research, mix proportioning, or quality control. The smaller 100 mm model is feasible for field use.
Disadvantages:
• The equipment is expensive, especially when compared to the proctor test. The 150 mm diameter model is too heavy for field use.
• The test does not incorporate vibration, which is commonly used in the placement of low slump concrete.
