Field-oriented test methods to evaluate structural build-up at rest of flowable mortar and concrete

Abstract

Thixotropy of flowable mortar and concrete is an important property that affects stability and form pressure characteristics. The increase in thixotropy can reduce lateral pressure on formwork systems. On the other hand, low thixotropy or a continuous casting is required to eliminate the formation of weak interface between lifts in multilayer casting. Thixotropy can be assessed by determining the rate of structural build-up at rest, which necessitates the use of simple and robust test methods to be quantified. Five field-oriented test methods that can be used for the determination of structural build-up at rest of mortar and concrete are proposed in this paper in an attempt to select a reliable field-oriented test. This includes the inclined plane (IP), portable vane (PV), undisturbed slump spread (USS), cone penetration (CP), and K-slump test methods. The repeatability of these test methods was determined four times using two concrete-equivalent mortars and two self-consolidating concretes (SCC) of different thixotropy levels. The IP, PV, and USS tests showed relative error (RE) values of 0.5–37 %. The CP test was successfully used to determine structural build-up of mortar; however, it was difficult to assess the thixotropy of concrete. The K-slump test exhibited a RE, less than 12 % for SCC mixtures with low thixotropy, but up to 76 % for highly thixotropic SCC. Good correlations were established among the various structural build-up indices determined from the proposed test methods and those determined by rheometric tests using various concrete.

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Acknowledgments

The authors would like to acknowledge their colleagues and technicians in the Cement and Concrete Research Group at the Department of Civil Engineering at the Université de Sherbrooke for their help in conducting parallel thixotropic testing, in particular Ms. J. Roby and Dr. T. Pavate. The authors acknowledge the financial support of the National Ready-Mix Concrete Education Research Foundation and the Strategic Development Council (SDC) of the American Concrete Institute (ACI), and member companies of SDC.

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Correspondence to A. F. Omran.

Appendices

Appendix 1: Test protocol for IP test

  1. a.

    Arrange four sets of the IP on a level table. Fix waterproof sand papers of grit No. 600 (according to CAMI, which is approximately equivalent to grade P1150 according to FEPA) onto the top surfaces of each incline plane (IP) plate. In order to position the sample in the center of the plane.

  2. b.

    Spray a small layer of water onto the four sand paper surfaces. Fill each cylinder measuring 60 mm in diameter and 120 mm in height to the 100-mm mark in the case of mortar, and up to the top in the case of SCC. The filling time is noted with respect to the initial cement–water contact time.

  3. c.

    One by one, slowly lift the cylinders so that the mixtures flow uniformly onto the flat surfaces of the upper planes. Cover the mixtures with wide cylindrical containers covered by a wet cloth so as to avoid any evaporation from the mixtures during the rest time.

  4. d.

    Determine the density of the mixture.

  5. e.

    The mixture’s first time of rest on the plane, under covered conditions, can be about 10 min after the inclination of first spread. One minute before the time of rest is over, remove the wet cloth and the covering container, and measure the spread of the sample. Find the height of the spread by averaging five measurements near the central area. At the given rest time, slowly lift the first IP until the flow of the mixture starts.

  6. f.

    Measure the angle of inclination of the IP with a protractor.

  7. g.

    Repeat steps (e) to (f) at other rest periods using other IP set-ups with undisturbed samples. The time duration between subsequent tests can vary between 5 and 15 min, depending on the degree of structural build-up of the material.

  8. h.

    Clean the sandpaper using moist towel at the conclusion of each test.

  9. i.

    Knowing the critical angle (α) (the angle at which downward flow commences), the static yield stress of the IP test (IPτ0rest) in Pa can be calculated, as given in Eq. (6).

    $$ {\text{IP}}\tau_{{0{\text{rest}}}} = \rho gh{ \sin }\alpha $$
    (6)

    where ρ is the density of mixture (concrete or mortar) (g/cm3); g is the gravitational acceleration (=9.81 m/s2); h is the characteristic mean height of slumped sample at horizontal position (average of five measurements taken prior to lifting the plate near the center of the spread) in (mm); α is the critical angle of the plane when the sample starts to flow (degree).

  10. j.

    Plot the IPτ0rest as a function of rest time to determine the rate of structural build-up of the tested material with rest time, IPτ0rest(t).

Appendix 2: Test protocol for PV test

  1. 1.

    Prepare four square plastic buckets of about 2 mm in thickness, 200 mm in length and width, and 400 mm in height. Fix a screw bolt of 4 mm in length and 2 mm in diameter in the middle of the bucket base going from the outer to the inner direction of the bucket. Tighten a 4-mm thick nut to the appeared part of screw bolt inside the bucket. Therefore, 2 mm of the nut thickness is fastened to the bolt, and the other 2 mm is hollow to hold the vane’s shaft centered [Fig. 2 in [19] for more details].

  2. 2.

    Place the buckets (1–4) on a flat surface with the vanes set in vertical positions with the aid of the nut described in step 1 and a plastic cover with central hole. Care should be taken to keep the buckets in place without disturbance. Position the vane of large dimension at the center of bucket # 1 with the aid of the nut at the base of the bucket, described in step 1. Proceed with the same approach with the smaller vanes placed in buckets # 2, 3 and 4.

  3. 3.

    Fill up the four containers with cement-based material up to a height h which should not exceed the total vane’s height H. The filling height (h) depends on the thixotropy level of the tested mixture. Shorter h value of about 50 mm can be used for highly thixotropic mixtures, and the maximum h (i.e., H) can be employed for relatively low thixotropic mixtures. Cover each bucket by plastic cover with a central hole of 2 mm greater diameter opening than the vane’s shaft diameter to align the vane’s shaft in the center of the bucket.

  4. 4.

    Record the rest time after the end of concrete placement in the buckets. The first rest time before conducting the first shear test can be on the order of 15 min. The remaining three rest times can be 30, 45, and 60 min, or lower for highly thixotropic mixtures. This is needed to obtain the excessive structural build-up of the material.

  5. 5.

    At the end of each rest time, remove the plastic cover and attach the torque-meter (the torque-meter indicator arrows should be coincided at zero) to the top tip of the vane’s shaft. Turn the toque-meter slowly (10–15 s per quarter turn) until the mixture starts to move (that is when the peak torque is clearly passed).

  6. 6.

    Record the maximum torque (T max) value and corresponding rest time.

  7. 7.

    Repeat steps (4–6) at other rest times.

  8. 8.

    The T max values at the four rest periods are converted to static yield stress values (PVτ0rest) using Eq. (7).

    $$ {\text{PV}}\tau_{{0{\text{rest}}}} = T_{ \max } /K $$
    (7)

    where T max is the maximum torque (Nm), K = 2πr 2(h + 1/3r),  and h and r are the filling height and radius of the vane (m), respectively.

  9. 9.

    Plot the PVτ0rest versus rest time to determine the structural build-up of the concrete with the rest time, PVτ0rest(t).

Appendix 3: Test protocol for USS test

  1. 1.

    Prepare five cylindrical moulds of 78 mm in diameter and 110 mm in height.

  2. 2.

    Lubricate the interior surface of the moulds with form releasing oil.

  3. 3.

    Moisten a horizontal surface measuring approximately 1.2 × 1.2 m in dimensions with water, then arrange the testing moulds on it at appropriate distances from each other.

  4. 4.

    Fill four moulds with mortar and tap the sides five times in 5 s, then cover the moulds with plexiglass covers.

  5. 5.

    Keep approximately 4 L of mortar a side in a bucket under a moistened towel for measuring the disturbed spread.

  6. 6.

    At the first rest time, remove the plexiglass cover and lift the mould slowly (over 5 s) allowing the rested mortar to spread. Wait until the spread completely stops, and measure two perpendiculars spread diameters for the spread sample. The average value represents the USS.

  7. 7.

    Immediately after step 6, stir the mortar in the bucket with a steel rod for 30 s and cast the fifth mould. Similar to step 6, lift the mould slowly (5 s) allowing the mortar to spread. Wait until the spread completely stops, and measure two perpendiculars spread diameters for the tested sample. The average value represents the DSS. Return the disturbed sample to the bucket for subsequent measurements.

  8. 8.

    Calculate the difference between disturbed and undisturbed slump values, as follow:

    $$ \Updelta {\text{SS}} = {\text{DSS}} - {\text{USS}} $$
  9. 9.

    Repeat steps 6, 7, and 8 for the other three rest times.

  10. 10.

    Plot the results of DSS, USS, and ∆SS with rest time to determine structural build-up rate.

The USS test protocol for flowable concrete testing can be summarized, as follows:

  1. 1.

    Prepare four cylindrical moulds measuring 100 mm in diameter and 200 mm in height.

  2. 2.

    Repeat steps 2, 3, 4, and 6 similar to mortar testing.

  3. 3.

    Repeat step 9 similar to mortar testing for the other three rest times.

  4. 4.

    Plot the results of DSS with rest time to determine the structural build-up rate.

Appendix 4: Test protocol CP test (ASTM D 3441)

  1. 1.

    Cast four beakers (100 mm in diameter and 50 mm in depth) simultaneously with mortar or extracted mortar from concrete using 5-mm opening sieve. Guard the covered samples under a moisturized cover.

  2. 2.

    At the first rest time, set flush the tip of a cone to the top surface of the mortar, then leave the cone to sink into the mortar under its own weight. When the cone stops moving, measure the PD. Conduct the CP tests three times for each cylinder with each measurement, move the cone about 20 mm from the previous location and from the edge of the container. The mean of the three measurements gives the value of the PD at the first rest time (PD@ti min).

  3. 3.

    Repeat step 2 for the three other rest times.

Appendix 5: Test protocol for K-slump tester (ASTM C 1362)

  1. 1.

    Fill four buckets (buckets have at least 300 mm in lateral dimensions and 200 mm in height) with SCC. Cover the buckets to avoid water evaporation.

  2. 2.

    At the first rest time, soak the K-slump tester in water and shake it well to remove any excess water.

  3. 3.

    Insert the tester vertically into the concrete until the disc rests on the concrete surface, and then leave it for 60 s in the concrete. During the 60-s insertion, the fresh mortar of the concrete enters the shaft of the tester through the side openings causing the rise of the floating plunger.

  4. 4.

    Note the raised distance of the plunger to give concrete slump in centimetres.

  5. 5.

    Repeat steps 2–4 at the other three rest periods.

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Khayat, K.H., Omran, A.F., Naji, S. et al. Field-oriented test methods to evaluate structural build-up at rest of flowable mortar and concrete. Mater Struct 45, 1547–1564 (2012). https://doi.org/10.1617/s11527-012-9856-8

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Keywords

  • Concrete-equivalent mortar
  • Self-consolidating concrete
  • Structural build-up at rest
  • Static yield stress
  • Thixotropy