Category Archives: Concrete Workability

Since the early 20th century, the concrete industry has recognized the need to monitor concrete workability to ensure that concrete can be properly placed and can achieve adequate hardened strength. A myriad of test procedures for determining workability have been developed for research, mix proportioning, and field use. The vast majority of these test methods have never found any use beyond one or two initial studies. With the exception of the widely used slump test, the few methods that have been studied extensively have generally failed to gain widespread acceptance. Even with the increase in knowledge of concrete rheology, the slump test remains the predominately used test method for measuring concrete workability.

ViscoCorder

The ViscoCorder is a single-point device used in Germany to measure the consistency of fresh mortar. Banfill (1990) modified the test to measure both the yield stress and plastic viscosity of mortar.

The device consists of a metal cylinder mounted on a rotating turntable. A paddle inserted in the cylinder is connected to a calibrated spring that measures the torque on the paddle. As the cylinder is rotated, the mortar applies a torque to the paddle. Traditionally, the device was operated at only one rotation speed. Banfill (1990) modified the device to measure torque at multiple rotation speeds. To obtain a plot of torque versus speed, the speed of the cylinder is changed in steps from zero to a maximum speed and back to zero. Unlike other rheometers, the device is not automated to change rotation speed and continuously record torque versus time. The device can be calibrated to correlate values for torque and speed to yield stress and plastic viscosity.

Banfill (1990) found that the ViscoCorder works well for fluid mortars; however, stiff mortars slip on the wall of the container, resulting in torque readings that are not an accurate representation of rheology. The container does not include any protrusions to prevent slip.

Vicat Needle Test

The setting time of concrete, mortar, or paste can be measured as an indication of workability (Ferraris 1999). One of the most common tests is the Vicat needle test for testing cement paste (ASTM C191). The Vicat needle test is also used in ASTM C953 for grout for pre-placed aggregate concrete.

The Vicat needle apparatus consists of a 300 g moveable rod with a 1 mm diameter needle at one end. The rod slides through a frame, where an indicator on the rod moves over a scale mounted to the frame. A specimen of fresh cement paste prepared in a certain proscribed manner is placed in a conical ring below the frame. After thirty minutes the needle is placed on the cement paste specimen and allowed to settle under its own weight. The depth of penetration is recorded from the scale. The test is repeated every 15 minutes (10 minutes for Type III cement) until a penetration depth of less than 25 mm is obtained. Each subsequent reading is taken at a different location on the paste specimen.

Similarly, the penetration test method described in ASTM C403 is used to determine the setting time of concrete by measuring the penetration resistance of mortar specimens sieved from concrete samples. Unlike the Vicat needle test, the apparatus used in ASTM C403 measures the force required to cause penetration, not the depth of penetration.

Mini-Flow Test

The mini-flow test (Zhor and Bremner 1998) is a variation of the mini-slump test described in the above subsection. The plexiglass sheet used in the modified version of the mini-slump test is mounted to a standard flow table, as described in ASTM C230. After the mini-slump cone is lifted from the sample of cement paste, the table is dropped 15 times in 15 seconds. The rest of the test procedure is unchanged. The mass of the cement paste is measured in order to determine air content. The results of the mini-flow test reflect the addition of energy to the cement paste. The mini-flow test is more appropriate than the mini-slump test for stiff mixes.

Mini-Slump Test

The mini-slump test, which was originally developed by Kantro (1980) and later modified by Zhor and Bremner (1998), measures the consistency of cement paste.

The mini-slump cone is simply a small version of the slump cone. The mini-slump cone has a bottom diameter of 38 mm, a top diameter of 19 mm, and a height of 57 mm. The cone is placed in the center of a square piece of glass on which the diagonals and medians are traced. The cone is lifted and after one minute, the average spread of the paste, as measured along the two diagonals and two medians, is recorded.

Zhor and Bremner (1998) modified the device in order to measure more effectively the air entraining and plasticizing effects of admixtures on cement pastes. A clear plexiglass sheet, which is used instead of glass, is set on a balance. After the cone is removed, the mass of the concrete is measured and used to determine the air content of the paste in accordance with ASTM C185. The paste is left to harden on the plexiglass for two days. A planimeter is then used to measure the area of the hardened paste on the plexiglass sheet. Like the conventional slump test, the results of the mini-slump test should be related to yield stress. Research conducted by Ferraris, Obla, and Hill (2001) into the influence of mineral admixtures on the rheology of cement paste showed poor correlation between the results of the mini-slump test and yield stress, as measured with a sophisticated, laboratory grade parallel plate rheometer.

Turning Tube Viscometer

The turning tube viscometer (Hopkins and Cabrera 1985; Ferraris 1999) is based on the same principle as the moving sphere viscometer—namely, Stoke’s Law—but is only considered appropriate for testing mortar.

An 800 mm long, 60 mm diameter tube is attached to a rotating arm, which allows the tube to be rotated in the vertical plane. A metal ball is allowed to fall through the fresh mortar in the tube.

A magnet can be placed on the specially milled end caps to ensure that the ball starts in the center of the tube. Inductance coils wrapped around the tube at two locations detect when the ball passes in order to determine the time for the ball to fall a known distance.

The test is conducted with different ball diameters and the results of the test are plotted on a graph of the inverse of the ball diameter squared versus time. The apparent viscosity of the concrete can be calculated based on Stoke’s law. Since the assumption in Stoke’s law that the ball is moving slowly through a fluid of infinite size is not valid for the test apparatus, correction factors are applied to provide a more accurate result.

The dimensions of the device are not large enough to permit the turning tube viscometer to be used for concrete. The ball diameter should be significantly greater than the maximum aggregate size so that the fluid can be considered a uniform medium. Further, the diameter of the tube should be sufficiently large to avoid interlocking of aggregate particles, which could interfere with the ball’s descent.

Wuerpel Device

The Wuerpel device (Maultzsch 1990) measures the consistency of mortars by applying a shear force to a mortar specimen and measuring deformation energy.

The apparatus consists of a quadratic mold with side lengths of 100 mm and a height of 50 mm. The corners of the mold are hinged to allow the mold, which is filled with compacted mortar, to deform from a square shape to a rhombus shape. The operation of the device is depicted conceptually in Figure 31. A load cell and a displacement transducer continuously measure the deformation force and the displacement of the mold, respectively. The area under the resulting  force-displacement curve represents the deformation energy, which is used to characterize workability.

The test method was developed in Germany and was briefly included in German standards in the late 1960s. Maultzsch (1990) used the test to measure the change in workability with time for mortars with a maximum aggregate size of 4 mm and found that the test device works particularly well for stiff mortars, although it is applicable to a wide range of concrete workability. The results of the test are dependant on the deformation speed of the device.

Flow Cone Test

Several versions of a funnel test are used to measure the workability of pastes and grouts. These devices differ in dimensions and intended uses; however, they all work on the principle of measuring the time for fresh paste or grout to flow through the opening of a funnel.

The flow cone test (Scanlon 1994) is intended for use in measuring the flow properties of grout for preplaced-aggregate concrete, but can also be used for other highly flowable grouts. The test is standardized in ASTM C939 and is considered appropriate for use in both the field and the lab. To perform the test, grout is poured into the flow cone, which is shown in Figure 29. The level indicator is used to ensure that a standard volume of grout is used for each test. The opening at the bottom of the cone is opened and the time for the grout to flow out of the cone is recorded. The test is not considered applicable to grouts that become clogged in the cone and do not continuously flow out the opening. Test results for such mixtures should be discarded.

The Marsh cone test (Zhor and Bremner 1998; Ferraris, Obla, and Hill 2001) is a non-standard test most typically used for oil well cements. The Marsh cone is a funnel with a long neck and an opening of 5 mm. A stand holds the Marsh cone in place above a glass graduated cylinder. After one liter of cement paste is placed in the cone, the orifice at the bottom of the neck is opened. The time for various volumes of paste to flow out of the orifice is measured. Since the weight of the cement paste in the funnel should be sufficient to overcome the yield stress, the time of flow should be related to viscosity. However, Ferraris, Obla, and Hill (2001) showed that the flow time from the Marsh cone test was not correlated to the viscosity measured with a laboratory parallel plate rheometer and hypothesized that the lack of correlation was related to factors such as friction and sedimentation in the Marsh cone.

The flow cone test has been adapted for measuring concrete (Ferraris 1999). The larger funnel used for concrete is 615 mm long with a 150 mm long outlet. The upper diameter of 230 mm narrows to 75 mm at the orifice. The device can be used for concretes with aggregate up to 20 mm.

Penetration Test for Segregation

The penetration test for segregation (Bartos, Sonebi, and Tamimi 2002; Bui, Akkaya, and Shah 2002) measures the penetration resistance of highly fluid and self-compacting concretes.

The test apparatus consists of a hollow cylindrical penetration head that is allowed to sink under its own weight into a sample of concrete. The penetration head has an inside diameter of 75 mm, a thickness of 1 mm, and a height of 50 mm. The mass of the penetration head is 56 grams. A rod attached to the penetration head slides through a frame, which includes a graduated scale for measuring penetration depth. Several different dimensions for the concrete container have been used by different researchers. Bui, Akkaya, and Shah (2002) set the apparatus on top of an Lbox with cross sectional dimensions of 200 mm by 200 mm (instead of the more commonly used dimensions of 100 mm by 200 mm). The container must be placed on level ground and must not be moved during the test. The concrete, which is not consolidated with vibration or tamping, is allowed to sit for two minutes after being placed into the container. The cylinder is then placed on top of the concrete surface and the depth of penetration is measured after 45 seconds. This measurement is performed at a total of three different locations of the concrete surface. When the concrete mixture is susceptible to segregation, the coarse aggregate particles will settle from  the top surface of the concrete and the penetration depth will increase. If the average depth of penetration is greater than 8 mm, the concrete is considered to have poor segregation resistance.

The test method is simple and inexpensive; however, little data exists to relate test results to actual field performance.

Wet Sieving Stability Test (GTM Screen Stability Test)

The wet sieving stability test (EFNARC 2002; Bartos, Sonebi, and Tamimi 2002) was developed by a French contractor to measure the segregation resistance of self-compacting concrete. To perform the test, a 10 liter sample of concrete is placed inside a bucket and allowed to sit for 15 minutes to allow any internal segregation to occur. The container is sealed to prevent evaporation. After sitting for 15 minutes, the top approximately 2 liters of the concrete is poured from the bucket into a smaller pouring container. This 2 liter sample of concrete is then poured from a height of 500 mm onto a 5 mm sieve. Mortar from the sample is allowed to flow through the sieve into a lower sieve pan for a period of 2 minutes. The mass of the concrete poured onto the sieve, Ma, and the mass of mortar in the sieve pan, Mb, are measured and used to calculate the segregation ratio:

Mb/Ma * 100%

The segregation ratio should be between 5-15% for acceptable segregation resistance. Concretes with a segregation ratio above 15% will exhibit too much segregation. Severe segregation is suspected if the segregation ratio is above 30%. If the segregation ratio is less than 5%, the sample is too harsh and will result in a poor surface finish.

Although the test results are valuable and accurate, the test is slow and requires an accurate scale, making it unsuitable for field use. Additionally, poor repeatability of the test results has been reported.

Fill Box Test (Simulated Filling Test, Filling Capacity Box, Kajima Test)

The fill box test (EFNARC 2002; Bartos, Sonebi, and Tamimi 2002) measures the passing ability and segregation resistance of self-compacting concrete.

The apparatus consists of a clear plastic box with 35 plastic 20 mm diameter bars, as shown in Figure 28. An early version of the test featured a wedge shaped box instead of a rectangular box and did not include a funnel. Concrete is poured at a constant rate into the funnel and allowed to flow into the box until the height of the concrete reaches the height of the top row of bars. After the concrete comes to rest, the height of the concrete at the two ends of the box is measured.

These measurements of the height of the concrete at the side nearest the funnel, h1, and the height at the opposite end, h2, are used to calculate the average filling percentage:

The closer the filling percentage is to 100%, the greater the filling ability of the concrete. The test is a good representation of actual placement conditions. However, the test is bulky and difficult to perform on site.

A similar simulated soffit test (Bartos, Sonebi, and Tamimi 2002) consists of a rectangular box with reinforcing bars placed in the box in an arrangement that simulates actual placement conditions for a given job. The reinforcing bars can be both horizontal and vertical. Concrete is placed in the box in a similar manner as with the simulated filling apparatus. After the concrete is allowed to harden, saw-cut sections of hardened concrete are removed to judge how well the concrete filled the box and moved around reinforcing bars. Since each apparatus is constructed based on actual field conditions, the test is not standardized and results from different apparatuses cannot be directly compared.