1. 4.7.2 High-strength Concrete (HSC) Introduction –Definition –Classifications Choice of HSC raw material –Binding material –Excellent aggregate –Superplasticizer Decisive factors in concrete strength

2. Definition In our country, it refers to the concrete not lower than C60. It is obtained by excellent aggregate, cement whose mark is no lower than 42.5 and low W/C with strong vibration. But high efficiency water reducing is helpful in making high strength and good mobility concrete.

3. Tab.4.7.1 ClassificationstypesW/C F c28 ， MPa notes HSC with good mobility 0.25 ～ 0.40 40.0 ～ 70.0 150 ～ 200mm slump large amount of cement high-strength with normal consistency 0.35 ～ 0.45 45.0 ～ 80.0 50 ～ 100mm slump large amount of cement high-strength without slump 0.30 ～ 0.40 45.0 ～ 80.0 lower than 25mm slump normal amount of cement high-strength with low W/C 0.20 ～ 0.35 100.0 ～ 170.0 admixture impact HSC 0.05 ～ 0.30 70.0 ～ 240.0 70.0Mpa or above

4. Binding Material Characteristics of cement – kinds and grades – mineral composites and fineness Quantity of cement Choice of HSC Raw Material

5. Kinds and Grades It is not necessary to use rapid-hardening cement to mix high-strength concrete. Normally, these kinds of cement, can obtain higher final strength is applied in improving consistent strength. such as –high-mark Portland cement –ordinary Portland cement –slag Portland cement –pozzolan Portland cement

6. In the past, it was difficult to mix high-strength concrete. The cement is usually 0.9~1.5 times as much as its corresponding strength grade of concrete. That is, the mark of cement should be higher than strength grade of concrete, or sometimes be slightly lower than strength grade of concrete. In our country, at present, it is much easier to mix high-strength concrete with the modification of material properties and craftworks especially the wide use of admixture. Kinds and Grades

7. Mineral Composites and Fineness Fineness of cement can influence strength of concrete(Fig.4.7.6). Therefore, the cement is required with high content of C3S and certain fineness characteristic (surface area is 3500-4000cm 2 ／ g). Therefore, if cement is used before another twice vibration and levigating, strength can be enhanced largely. The finer levigating, the large surface area, the more sufficient hydration reaction, the higher strength

8. Although the content and fineness of C 3 S are important influence factors, these two are not the only decisive factors. Proper cement should also be determined by testing. Maybe uniformity of cement is not as important as strength, at least uniformity is as important as potential strength. Mineral Composites and Fineness Fig.4.7.6 Influence of concrete strength by fineness of cement

9. Quantity of Cement The quantity of binding material is so important to produce HSC that it directly influences the cohesion between hardened cement paste and border surface. Construction requirement also needs certain feasibility. In order to increase the proportion of binding material in mortar, the usage of cement is large usually in the range of 500-700kg ／ m 3. But it is recommended not out of this range otherwise it is easy to cause slow heat radiation and large shrinkage in hydration.

10. If technically permitted, we’d better reduce the amount of cement but mix with some high quality fly ash and other silicon ashes with an aim to bringing the minimum side effects of hydration heat release and shrinkage. Based on experience, HSC should be bled by experimenting with various cement in terms of kind and quantity. Cement selection should meet the requirements of low cost in the required compressive strength. It is suggested that aggregate should be in the best gradation and bleed evenly to obtain the minimum amount cement. Quantity of Cement

11. Excellent Aggregate fine aggregate –Concrete admixture contains more sand so middle sand or coarse sand will avoid over drying in concrete. Usually it is recommended to use sand modulus of fineness lower than 3.0 and reduce sand rate as much as possible. Concrete admixture should not be too dry or hard. –Fine admixture gradation should meet the requirements. In the composites of HSC, fine admixture percentage is smaller than that of the ordinary concrete. It is better to use quartz river sand as the chemical composite of sand.

12. Excellent Aggregate coarse aggregate –Concrete strength is influenced by coarse aggregate by the following factors, cement paste, cohesion of mortar and aggregate, aggregate elasticity, inner levels below aggregate when water rising in concrete admixture,stress concentration around the aggregate. For HSC coarse aggregate has good performance in compressive strength, surface, maximum grain-size.

13. Superplasticizer HSC can be obtained by high grade cement and superplasticizer.

14. Decisive factors in concrete strength Cement Paste –The high strength concrete is made by strong vibration of high grade cement in larger quantity with new superplsaticizer. Aggregate –It is better to use qualified aggregate of high strength and density, coarse surface, good gradation, and strong resistance. Fine aggregate quantity is smaller.

15. Decisive factors in concrete strength Cement paste – aggregate adhension –Broken stone has good adhension to produce strong strength(Fig.4.7.7). –The ratio of apparent area and volume is bigger than that with gravel. –Therefore, broken stone should have clean surface. Fig.4.7.7 Influence of concrete strength by coarse aggregate in same gradation