Modified Slump Test

     The modified slump test (Ferraris and de Larrard 1998; Ferraris 1999; Ferraris and Brower 2001) is intended for use as a field test to measure both the plastic viscosity and yield stress of concrete mixtures. The test adds the parameter of time to the standard slump test in order to measure plastic viscosity.
      The apparatus for the modified slump test consists of a vertical rod that extends from a horizontal base plate through the center of the standard slump cone. The slump cone is filled in accordance with ASTM C143 and a sliding disk is placed atop the fresh concrete. Once the slump cone is removed, the time for the disk to slide a distance of 100 mm is measured. The sliding disk comes to rest on a stop located on the vertical rod. After the disk comes to rest, the concrete continues to subside to its final position. The final slump measurement is recorded no later than 60 seconds after the slump cone is removed. A schematic of the test procedure is shown in Figure below.

      The rheological parameters of yield stress and plastic viscosity can be expressed in fundamental units using equations based on the results of the test. The yield stress (  τ₀ , Pa) is expressed in terms of final slump (s, mm) and concrete density ( ρ , kg/m3); while the plastic viscosity (μ ,Pa.s) is a function of final slump, slump time (T, sec), and concrete density. The proposed expression for yield stress is given below:

                                            τ₀=ρ/347(300-s)+212

The equation for plastic viscosity is based on a semi-empirical model developed by using the results of the modified slump test, as shown in Equation [1]:

     For 200 mm < s < 260 mm: μ = ρT .1.08 .10⁻³ (s −175)
     For s < 200 mm: μ = 25 .10⁻³ρT                                                            [1]

      Nomographs have been developed based on the above equations to allow quick determination of yield stress and plastic viscosity in the field. Due to the need to measure the time for a slump of 100 mm to be achieved, the test only applies to concrete with slumps ranging from 120 to 260 mm. It has been shown that the rod has a negligible effect on the final slump and that there is no risk of the concrete falling faster than the plate. Other researchers have eliminated the sliding plate and shortened the rod so that it terminates 100 mm below the top of the slump cone (Ferraris 1999). There is a possibility of operator error in determining the precise instances to start and stop the measurement of the slump time.
       Additional experimental testing needs to be carried out on a wider range of concrete mixtures in order to verify the validity of the test. Ferraris and Brower (2001) found poor correlation between the results of the modified slump test and plastic viscosity measured with five rotational rheometers.
Advantages:
• The test is simple to conduct and only requires slightly more equipment than the slump test.
• The test gives an indication of both yield stress and plastic viscosity.
Disadvantages:
• The test is not a dynamic test and does not account for the thixotropy of concrete or the ability of concrete to flow under vibration.
• Further testing is required to verify the validity of the test.

Inverted Slump Cone Test

     The inverted slump cone test (Tattersall and Banfill 1983; McWhannell 1994; Johnston 1994; ASTM C995-01; Bartos, Sonebi, and Tamimi 2002) was developed as a simple and inexpensive field test to measure the workability of fiber-reinforced concrete. Although fiber-reinforced concrete can show increased workability, the individual fibers act to increase concrete thixotropy. McWhannell (1994) has shown that mixes incorporating polypropylene fibers show a slight decrease in slump but an increase in workability as measured with the compacting factor test. Indeed, SI Concrete Systems, a large manufacturer of steel and synthetic fibers for concrete, advises against using the slump test for measuring the workability of fiber-reinforced concrete.
         The test apparatus is comprised of readily available jobsite equipment—an internal vibrator, slump cone, and bucket. The test is standardized in ASTM C995: “Standard Test Method for Time of Flow of Fiber-Reinforced Concrete through Inverted Slump Cone.” A specially constructed wood frame, shown in Figure 13, holds the slump cone in an inverted position above the standard bucket described in ASTM C29/C29M for determination of unit weight. A 4 inch gap is left between the bottom of the inverted slump cone and the bottom of the bucket. The  dampened slump cone is then filled with concrete in three layers. Although the concrete should not be compacted, each layer of concrete should be leveled off to minimize entrapped air. To keep the concrete from falling through the bottom of the slump cone, the ASTM standard recommends placing a sufficiently large volume of concrete in the bottom of the cone to bridge the opening. With the slump cone full and leveled off at the top, a one-inch diameter internal vibrator is inserted into the top of the concrete and allowed to descend at a rate such that the vibrator comes into contact with the bottom of the bucket in 3 +/- 1s. The vibrator is then held in a vertical position and the total elapsed time from the insertion of the vibrator until all the concrete has passed out of the slump cone is recorded.

        ACI Committee 544 (1989) on fiber-reinforced concrete recommends the use of the inverted slump cone test. The use of vibration has been deemed appropriate since the fiber-reinforced concretes that are tested with the inverted slump cone test are commonly vibrated during placement. Research has shown that the inverted slump cone test can successfully detect changes in coarse aggregate fraction, fiber content, fiber length, and fiber aspect ratio (Johnston 1994).
       Although the test is improvements on static tests that do not take into account the higher thixotropy of fiber-reinforced concrete, the inverted slump cone test has several important restrictions on its usefulness. The test applies only to concretes with flow times greater than 8 seconds and slumps less than 2 inches. More fluid concretes can flow through the bottom of the cone without vibration and cannot be measured with sufficient precision. The size of the apparatus also restricts the use of some concretes. The small gap of 1 ½ inches around the vibrator at the bottom of the cone limits the maximum aggregate size and the use of long, stiff fibers with high aspect ratios. Tattersall and Banfill (1983) state that the gap between the cone and vibrator should be 10 times the maximum aggregate size. Additionally, long fibrillated and monofilament fibers can wrap around the vibrator and distort results. In order to allow the use of readily available job equipment to conduct the test, the ASTM standard only specifies that the internal vibrator be 1 +/- 1/8 inch in diameter. Variations in the diameter, frequency, and amplitude of the vibrator prevent the direct comparison of test results and the development of specifications for fiber-reinforced concrete in terms of inverted slump cone time.   The precision of the test is influenced by operator error in properly inserting the vibrator and determining the correct start and stop times for the test. Since the concrete is not consolidated prior to the start of the test, the cone can contain large volumes of entrapped air.

Advantages:
• The inverted slump cone test is a dynamic test that takes into account the high thixotropy of fiber-reinforced concrete.
• The test is simple and provides a direct result.
• The test apparatus consists of readily available equipment.

Disadvantages:
• The test is only appropriate for concrete mixes with a slump of less than 2 inches.
• The test is difficult to perform. Filling the inverted slump cone with concrete so that no concrete falls through the hole is tricky. Further, the vibrator must be inserted directly down the center of the inverted slump cone in a certain period of time.
• The gap at the bottom of the inverted slump cone is too small based on typical aggregate sizes and some fiber lengths.
• Some long fibers may wrap around the vibrator.
• Important test parameters are not standardized; therefore, tests conducted with different vibrators cannot be compared. Likewise, it is difficult to write specifications in terms of inverted slump cone time.
• Operator error is introduced in determining the exact stopping point of the test.

SLUMP TEST

       The slump test is the most well-known and widely used test method to characterize the workability of fresh concrete. The inexpensive test, which measures consistency, is used on job sites to determine rapidly whether a concrete batch should be accepted or rejected. The test method is widely standardized throughout the world, including in ASTM C143 in the United States and EN 12350-2 in Europe.
        The apparatus consists of a mold in the shape of a frustum of a cone with a base diameter of 8 inches, a top diameter of 4 inches, and a height of 12 inches. The mold is filled with concrete in three layers of equal volume. Each layer is compacted with 25 strokes of a tamping rod. The slump cone mold is lifted vertically upward and the change in height of the concrete is measured.
Four types of slumps are commonly encountered, as shown in Figure 3. The only type of slump permissible under ASTM C143 is frequently referred to as the “true” slump, where the concrete remains intact and retains a symmetric shape. A zero slump and a collapsed slump are both outside the range of workability that can be measured with the slump test. Specifically, ASTM C143 advises caution in interpreting test results less than ½ inch and greater than 9 inches. If part of the concrete shears from the mass, the test must be repeated with a different sample of concrete. A concrete that exhibits a shear slump in a second test is not sufficiently cohesive and should be rejected.

       The slump test is not considered applicable for concretes with a maximum coarse aggregate size greater than 1.5 inches. For concrete with aggregate greater than 1.5 inches in size, such larger particles can be removed by wet sieving.
        Additional qualitative information on the mobility of fresh concrete can be obtained after reading the slump measurement. Concretes with the same slump can exhibit different behavior when tapped with a tamping rod. A harsh concrete with few fines will tend to fall apart when tapped and be appropriate only for applications such as pavements or mass concrete. Alternatively, the
concrete may be very cohesive when tapped, and thus be suitable for difficult placement conditions.

       Slump is influenced by both yield stress and plastic viscosity; however, for most cases the effect of plastic viscosity on slump is negligible. Equations have been developed for calculating yield stress in terms of slump, based on either analytical or experimental analyses. Since different  rheometers measure different absolute values for the yield stress of identical samples of concrete, the experimental equations are largely depended on the specific device used to measure yield stress. 

Advantages: • The slump test is the most widely used device worldwide. In fact, the test is so well known that often the terms workability and slump are used interchangeably, even though they have different meanings.
• Specifications are typically written in terms of slump.
• The slump test is simple, rugged, and inexpensive to perform. Results are obtained immediately.
• The results of the slump test can be converted to yield stress in fundamental units based on various analytical treatments and experimental studies of the slump test.
• Compared to other commonly used concrete tests, such as for air content and
compressive strength, the slump test provides acceptable precision.
Disadvantages:
• The slump test does not give an indication of plastic viscosity.
• The slump test is a static, not dynamic, test; therefore, results are influenced by concrete thixotropy. The test does not provide an indication of the ease with which concrete can be moved under dynamic placing conditions, such as vibration.
• The slump test is less relevant for newer advanced concrete mixes than for more conventional mixes.