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The coarse aggregates and the sand used in this study were a siliceous-limestone alluvial material from the area of Washington, D.C. The aggregates are rather rounded in shape with a maximum size of 10 mm for the coarse aggregate. To assure continuity in the particle size distribution of the mixtures, a single-size rounded fine sand (density 2.65, water absorption 0%) was used to correct the deficiencies in gradation of the alluvial sand. An ASTM Type I/II Portland Cement, Standard Specifications for Portland Cement (ASTM C 150-95), was used in all of the tested concrete mixtures. A silica fume provided in the form of slurry containing 54% solids, was also used. The specific gravity of the silica fume was taken as 2.2. A sulfonated naphthalene, in an aqueous solution with 40% dry extract by mass, was used as a high-range water reducing admixture (HWRA) in half of the mixtures. The material properties and various particle size distributions are given in (Ferraris et. al. 1997 [6] ).
The concretes were mixed in a vertical-axis pan mixer with a maximum capacity of 16 L. The aggregates were oven dried before being used. Details of the mixing procedure are given elsewhere (Ferraris et al. 1997 [6] ). In brief, the total mixing time was 3 min 30 s for the concretes without HWRA, and five minutes for concretes with HWRA. The mixing time for the concretes with HWRA was longer because of the addition of some admixtures was delayed.
The slump was measured in accordance with ASTM Test Method C 143-90, except that the apparatus was provided with an axial vertical rod and a stainless steel disk that could slide down the rod. This plate was placed on top of the concrete cone (see section on Design and Dimension of Modified Slump Test Apparatus). The effect of these modifications on the final slump was found to be negligible. To measure the rheological parameters, yield stress and plastic viscosity in a more fundamental way, a BTRHEOM rheometer was used. Details of the rheometer operation and data interpretation are beyond the scope of this paper and are described by (Ferraris et al. 1997 [6] ).
The experimental plan (Ferraris et al. 1997 [6] ) consisted of systematically testing most of the mixtures that could be designed using the three basic materials: gravel, corrected sand (a mixture of alluvial sand and fine sand in fixed proportions) and cement. Based on a certain number of "dry" compositions, three fresh concrete mixtures with different water/solid ratio were selected so as to cover the range of consistencies that could be evaluated with the rheometer. The lower limit was a slump of about 100 mm for mixtures without HWRA (Hu et al. 1995[7] ), while the upper limit was reached when bleeding or segregation was considered to be excessive.
Since the goal was to develop a test that was above all simple, robust and inexpensive, it was not practical to record the slump as a function of time. To do a complete recording, it would have been necessary to use an electronic data acquisition. The interpretation of the resulting curve would also have been too complex. Therefore, it was decided to try to characterize the plastic viscosity based on an average rate of slumping in the slump test. Thus, measurement of the time necessary to reach an intermediate height between the initial and final values appeared a priori to be a good means of discriminating among the concretes according to their plastic viscosity.
The choice of this partial slump took into consideration two potential problems: (1) a height that was too small would lead to very small slump times and thus poor relative precision of measurement; (2) a partial slump that was too large would have ruled out all concretes with a smaller final slump. Since the range of concretes that can be characterized by the rheometer is, as already stated, approximately that for which the slump is greater than 100 mm, this value was chosen for the partial slump.
The Tanigawa setup for measuring slump as a function of time would be too fragile for a work-site environment (Tanigawa 1991 [9] ). Therefore, we adopted the use of a plate, allowed to slide on a centrally-located rod as the means for monitoring the time to reach the 100 mm-slump. The rod was at the axis of symmetry of the conic frustum. Since the axis of symmetry of the concrete did not change significantly during the flow of the concrete, it was assumed that the rod would not greatly disturb the slump. This point was later verified. The dimensions of the apparatus and the test setup are shown in Figure 2 and Figure 3.
Figure 2 - Rod and top plate in the modified slump apparatus.
In order to measure the partial slump time, it was found satisfactory to use a stopwatch controlled by the operator on the basis of a visual criterion (such as in the VEBE test). The stopwatch is started as soon as the slump cone is lifted, and is stopped when the sliding plate placed on the fresh concrete had fallen 100 mm and reached the stop on the rod (Figure 4).
Figure 4. Schematics of the modified slump test. T is the slump time.
The following components are needed to conduct the modified slump test:
The concrete was placed in the same manner as in the standard slump test (ASTM C 143-90). The various steps are as follows.
