5. Concrete Testing

The purpose of the concrete measurements was to validate the conclusions drawn from the data obtained in the cement paste for the best type and dosage of mineral admixture. The concrete rheological behavior was tested using the standard slump cone test. Therefore, only an indication of the yield stress was available for comparison. The effect of the addition of a mineral admixture was detected by an increase in the slump or a reduction of the water content and/or a reduction of HRWR dosage needed to obtain the same slump.

Two different concrete test programs were conducted. These are identified as Test A and Test B. Both of these test programs involved determining both fresh and hardened concrete properties. Only the rheological results will be discussed here. More details are published elsewhere [22]. In all the concrete tests only silica fume or UFFA were used. None of the other mineral admixtures were tested because these two seemed to encompass the range of rheological behaviors. Also, it would have been too expensive to test all the mineral admixtures in concrete. The cement used was the same as described on Table 1.

Test A was divided in two series: the 360 series and the 420 series (Table 3). The series are named after the cementitious (cement and mineral admixtures) material contents of 360 kg/m³ and 420 kg/m³, respectively. The UFFA dosage and the w/c ratio were varied to obtain about the same slump and to determine the influence of these factors on the slump. Table 3 gives the details of the concrete composition and the slump and air content of the concrete.

Test B concretes had 370 kg/m³ of cementitious materials. The w/c ratio was kept to 0.4 for all but two mixes. The UFFA dosage was varied between 8 % and 16 % by mass of cement. Two mixtures were made with silica fume replacements of 8 % and 12 % by replacement by mass of cement. Table 4 gives the composition, slump, and air content of the concretes.

5.1 Concrete Workability

To determine the influence of the mineral admixtures in concrete, the HRWR dosages and w/c ratio were compared. The HRWR dosage was reported as the ratio between the dosage used for mixtures with mineral admixtures and the dosage used for the control, with no mineral admixtures. A value below 1 indicates that the mineral admixture addition improved flow compared to the control, while a value above 1 indicates the opposite. A reduction in the w/c ratio implies that the addition of mineral admixture was beneficial. Therefore, the results in Figure 8 should be interpreted to determine the composition that reduces both the HRWR and the w/c ratio. It can be seen from Figure 8 that for all three series the addition of SF never leads to a value below the control. However, the addition of UFFA leads to a reduction of both HRWR dosage and w/c in almost all mixtures. The only exception to this improvement was when the w/c was less than 0.31 corresponding to a reduction in water content of more than 20 % compared to the control with a w/c of 0.40 (plot 3 in Figure 8).

Figure 8: Comparison of water and HRWR requirement of SF and UFFA concretes at different cementitious contents: (a) 360, (b) 420, (c) test B. The relative HRWR dosage is the ratio between the dosage used in the mixes with mineral admixtures and the control. The dotted line indicates the value of the control for the relative HRWR dosage, i.e., equal to 1. These are unique tests, therefore no error can be estimated.

In more detail, it can be observed that for the 360 series, with slumps ranging from 180 mm to 210 mm, the UFFA mixtures needed only 50 % to 63 % as much HRWR dosage as did the SF concrete mixture. This was achieved even with 10% less water than was used in the silica fume concrete mixture. When the UFFA concrete mixture had 16% less water, it still needed only 78 % of the HRWR dosage of the SF concrete mixture. For the 420 series, at slumps ranging from 210 mm to 250 mm the UFFA mixtures needed only 56 % and 83 % of the HRWR dosage and had 15 % and 23 % less water as compared to the SF concrete mixture. In test B, at the same water and cementitious amounts, silica fume increased the HRWR dosage by 30 % and 50 % over the control, when used at 8 % and 12 % replacements, respectively. In contrast, UFFA reduced the HRWR dosage by 30 %, 42 %, and 50 % when used at 8 %, 12 %, and 16 % replacements, respectively. The HRWR demand decreased as the amount of cement replaced by UFFA increased. It should be noted that the slump was significantly lower for a 12 % addition than for 8 % or 16 % (Table 4).

These concrete tests have shown that if the goal was to reduce any or all of the slump, w/c ratio and HRWR dosage in a mixture containing a mineral admixture, the best selection would be to use UFFA and not SF. These tests are limited as they did not address other types of mineral admixtures, but they are enough to be used as a validation of some of the cement paste tests.

Table 3: Mixture Proportions (Test A)

360 series
Material	Control	SF	UFFA 3	UFFA 4	UFFA 5	UFFA 6
Cement	360	331	331	331	331	331
SF	0	29	0	0	0	0
UFFA	0	0	43	29	43	43
CA	1035	1035	1035	1035	1035	1035
FA	745	735	733	778	764	789
Water	142	143	141	130	129	120
W/C	0.39	0.4	0.38	0.36	0.34	0.32
HRWR	655	1047	524	589	655	818
HRWRmix/ HRWRcontrol	1.00	1.60	0.80	0.90	1.00	1.25
AEA	33	39	52	46	52	52
Slump, mm	200	190	185	165	210	190
Air Content, % By volume	6.4	5.0	5.6	5.4	5.4	6.5

Cement, SF, UFFA, water, are in kg/m³
CM stands for cementitious materials, i.e., Cement and SF or UFFA
HRWR dosage expressed as ml/100 kg of cementitious materials
The first number in the mixture I.D indicates the dosage in mineral admixture and the second number (after the slash "/") indicates the w/c ratio