*3.6. Compressive Strength*

The compressive strength results shown in Table 6 are average values of three 100 mm cubes. The rate of gaining strength is different between concrete types; the ratio of gaining strength at earlier ages (1 day) was 8% for NSVC, but it was 21% and 25% respectively for HPSCC and HPVC. At 28 days NSVC gained two-thirds of its 90 days' strength while the ratio was 72%, 79%, and 85% respectively for HSSCC, HPSCC, and HPVC type mixes.


**Table 6.** Compressive strength of 100 mm cubes at 7 ages, and Ø100 mm cylinders.

No considerable differences were observed at 180-days. The water/binder ratio has a great influence on the compressive strength of SCC and VC, whereas, the subject is still controversial and the authors got different conclusions. Some studies on the mechanical behavior of SCC showed that for the same w/b ratio, SCC has generally lower mechanical strengths than traditional vibrated concrete [2]. However, other studies stated: Compared with the majority of the published test results the tendency becomes obvious that at the same w/c ratio, higher compressive strengths were reached for SCC [12]. Three cylinders with Ø100 mm had also been tested for each of the mixes, to study the influence of the shape of the specimen on compressive strength of concrete mixes. Results showed that the higher compressive strength mixes were less affected by specimen shape since the cube to cylinder factor for NSVC mix was 0.79, but it was 0.90 and 0.96 for HPSCC and HPVC mixes, respectively. The shape of specimens and loading direction during tests, were the two factors that controlling (fcy/fcu) ratio. Other authors have reported that the compressive strength ratio of cylinders to cubes is 0.80–0.85 for VC, but it is 0.90–1.00 for SCC [7]. Silica fume is the most commonly used admixture for the production of HPSCC. It has been reported that adding 10% silica fume to the mixtures can increase the compressive strength by 30–100%, 6–57%, or 5–24%, by different authors [15,29,43]. The ratio of the fly ash used for HSSCC was negatively affecting the compressive strength of the mix, since, it is determined that the optimum fly ash content is 25–35%. In essence, fly ashes with 10% do have positive influence on overall quality of SCC, which increases the workability, frost durability and an acceptable level of strength. Further increase in FA% led to reducing of the CaO content, which led to a lower level of hydration [10,25,33].
