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RESULTS

The modified slump test as described was performed on all of the mixtures and the partial slump times (which will hereafter be called the slump times) are given by (Ferraris et al. 1997 [6] ). After the mortars and concretes for which the final slump was less than 100 mm are excluded, the measured times range between 0.63 and 15.97 s.

One question was to find out if the minimum time was controlled by the slump of the concrete or whether, the disk separated from the concrete during the fall. The theoretical drop time of a body subject to gravity to fall a distance h of 100 mm is , or 0.14 seconds. Two measurements of this time (without concrete) gave values of 0.16 and 0.15 seconds. Hence, it was concluded that any separation is unlikely (at least with the concretes tested). In addition, the precision of measurement is on the order of 1/10 of a second due to the reaction times of the operator. Also, the precision reflects the fact that the cone lifting is not precisely controlled (Bartos 1992 [1] ).

The coherence of the measurements was examined by examining the variation in slump time within each mixture group. With rare exceptions, the times are arranged very well as a function of the volume of water: they decrease regularly as the water dosage increases. On the other hand, comparison of the average values of series of measurements is equally instructive and encouraging. The slump times of all mixtures without the HWRA average 1.51 seconds (range ± 0.54 seconds), while the values for all mixtures with HWRA are generally greater and more widely spread (average of 4.80 seconds, range of ± 4.66 seconds). Therefore, this test will be more useful in determining the plastic viscosities of concretes containing HWRA.

Comparison of final slumps obtained with the standard test and the modified test

A check was done to determine whether the modification to the standard slump test affected the final slump measurement. This was necessary to have complete compatibility with the unmodified test.

The mass of the disk (212 g) increases the vertical stress on the sample by a maximum value equal to its weight divided by the upper area of the frustum, this was 0.27 kPa. When the disk reaches the stop, the height, h, of the concrete is 200 mm. Hence, the vertical compression stress at the base of the sample equals gh (where  =  2400 kg/m³ is the density of the fresh concrete and g is the acceleration due to gravity), that is, about 4.8 kPa. Thus it is seen that the vertical stress due to the disk is at most on the order of 6% of the stress due to the concrete. Moreover, the friction of the concrete along the rod would tend to reduce the final slump. To verify that these effects are negligible, a comparative study was done with six compositions chosen because they are representative of the range of slumps obtained. The two tests are representative of the range of slumps obtained. The two tests (the standard slump test and the modified test) were conducted in parallel. Figure 5 shows the comparison between the two tests. The best fit line passing throug the origin has a slope of 1.01 ± 0.03. This regression leads to a standard deviation of 17 mm. Therefore, the slumps measured with the two tests can be considered identical.

Figure 5. - Comparison of the slump values between the standard slump test and the modified test. The slope of the best fit straight line passing through the origin is 1.01 with R²= 0.95.