ULTRA HIGH PERFORMANCE AND HIGH EARLY STRENGTH CONCRETE

Mehdi Sadeghi e Habashi, Seraj Higher Education Institue, Iran

36th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 14 - 16 August 2011, Singapore

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36th

Conference on Our World in Concrete & Structures Singapore, August 14-16, 2011

ULTRA HIGH PERFORMANCE

AND HIGH EARLY STRENGTH CONCRETE

Mehdi Sadeghi e Habashi

Seraj Higher Education Institue 3rd Floor, LR.B6-Sahand AP. Zafaraniye

Tabriz, Iran E-mail: <youngelites09@gmail.com>

Keywords: Ultra ultra-high performance concrete; autogenously shrinkage; stress-strain curve

Abstract. Ultra-High-Performance Concrete (UHPC) with compressive strength higher than 150 N/mm2 and other perfect properties is a new type of cementations materials. Basic principle to improve concrete properties is advancing a matrix as dense as possible and a good transition zone between matrix and aggregate. In this work an original UHPC was modified through partial replacement of cement or silica fume by fine quartz powder. A self-compacting UHPC with a cylinder compressive strength of 155 N/mm² can be produced without heat treatment or any other special measures. Mechanical properties and autogenous shrinkage of the modified UHPC were presented in this work.

1. INTRODUCTION:

Ultra-high performance concrete is a new cementitious material with strength more than 150 N/mm² and other perfect properties. Basic principle to improve the concrete properties is the reduction of defect places, such as micro cracks and capillary pores, in concrete. In [1] some measures have been preferred for the production of UHPC: • Enhance the homogeneity of the concrete by elimination of coarse aggregate. It is well known that the transition zone between coarse aggregate and matrix is often the source of micro cracks in concrete, due to their different mechanical and physical properties. It was suggested that maximal aggregate size in UHPC should be less than 600 µm [1] • Improve the properties of matrix by addition of pozzolanic admixture, i.e. silica fume. The modifying effects of silica fume in concrete are attributed to its pozzolanic reaction with Ca (OH) 2 and filler effect in voids among cement or other components particles. In concrete containing typical Portland cement 18% silica fume, in the weight of cement, is enough for total consumption of Ca (OH) 2 released from cement hydration [2]. However, considering the filler effect the optimal share of the silica fume is about 30% of cement [1] [3]. Therefore the silica fume content in UHPC is normally 25-30% of cement • Improve the properties of matrix by reducing water to binder ratio • Enhance the packing density of powder mixture. According to the results in [3] a mixture with wide size distribution has a low avoid among their particles. This means powder mixture should be composed of number classes of granular powder • Enhance the microstructure by post-set heat-treatment since 1994 intensive researches have been carried out in France and Canada. Cement content in these original UHPC ranged between 900-1000

Mehdi Sadeghi e Habashi

kg/m³. In this paper a modified UHPC and its mechanical properties and autogenous shrinkage are presented.

2. MATERIALS:

Based on the principle for ultra-high performance mentioned above quartz sand with the size of 0.3-0.8 mm was used as aggregate. An ordinary Portland cement CEM I 42.5 R was used as binder. A white silica fume was added as pozzolanic admixture in concrete. Its particle size lies between 0.1-1.0 µm. A quartz powder with a diameter smaller than 10 µm was used as micro filler. Its particle fill the lack between the cement particle and the silica fume and make the grading curve of the mixture composed of cement, silica fume and quartz powder continuous. Super plasticizer on the basis of polyethercarboxylate was used to ensure the concrete flowing ability.

3. MODIFY THE REFERENCE CONCRETE COMPOSITION:

In literatures UHPC was characterized with very high cement content about 950 kg/m³ [4]. Because of the low water to cement ratio, only a part of the cement has hydrated. Unhydrated cement particles lie in matrix as fine aggregate. In this work cement was stepwise replaced by inert quartz powder with same volume. The slump flow of fresh concretes was measured according to DIN 1048 but without shock. It can be seen in Table 1 that even 30% cement was replaced compressive strength was not suffered. Furthermore, with the cement replacement by quartz powder the flowing ability of the concrete was improved. When 30% cement was replaced by quartz powder the slump flow increased from 510 mm (mixing 1) to 620 mm (mixing 6). This can be resulted from that the incorporation of fine quartz powder reduced the voids in the original mixture (in mixing 1) containing only cement and silica fume. Otherwise ,with the cement replacement less hydrate have been produced in the first few minutes. There were not enough hydration products to bridge various particles together. Some particles were still free and could move easily.

Table 1: Compressive strength of UHPC with quartz powder

Compressive Strength

fc1 4*4*16 cm (N/mm2) W

/ C

Ce

me

nt

Re

pla

ce

me

nt

Sili

ca

/

Ce

me

nt

Ce

me

nt

14d/20˚ C+3d/90˚C

28d /20˚C

189.7 149.2 0.187 0% 25% 950 1

195.5 141.6 0.199 6% 26.6% 893 2

186.3 145.9 0.213 12% 28.4% 836 3

177.5 151.1 0.229 18% 30.5% 779 4

190.1 143.0 0.247 24% 32.9% 722 5

201.7 148.1 0.268 30% 35.7% 665 6

*All of those concretes were cost without Vibration

Finally a part of silica fume was replaced also by quartz powder. The modified composition and compressive strength of self-compacting or vibrated concretes are shown in Table 2.

Mehdi Sadeghi e Habashi

Table 2: The modified concrete composition and compressive strength

665 CEM I 42,5R (Kg/m3)

200 Silica Fume (Kg/m3)

285 Quarts Powder (Kg/m3)

1020 Quarts Sand (Kg/m3)

178 Total water ( Kg/m3)

23.0 Super plasticizer (Kg/m3)

155.5 Without

vibration fC,CYl 100*300 (N/mm

2),

28d/20˚C 174.3 60 sec. vibrated

197.0 >90 sec vibrated

4. MECHANICAL PROPERTIES OF THE CONCRETE:

4.1 Relationship between compressive strength and elastic Modulus

Compressive strength and Modulus of elasticity were determined on concretes at ages of 3, 7, 14, 28 and 90 days. Cylindrical specimens, 300 mm high and 100 mm in diameter, were cast as self-compacting concrete or vibrated with a rod vibrator for 60 or 90 sec. Due to the retarding effect of superplsticizer on cement hydration specimens were demoulded 2 days after casting and then immersed in water at 20°C. 3 days before testing they were taken out from water and stored under relative humidity of 80% at 20°C till to test. Elastic Modulus of concrete increases disproportional with compressive strength. For high performance concrete containing quartz coarse aggregate Modulus of elasticity can be estimated from compressive strength as following [5]:

With this equation the E-Modulus of UHPC would be overestimated, because the paste volume in UHPC is much higher than that in conventional high performance concrete. Results in this work would suggest a new relationship between cylinder strength and E-Modulus as following:

Fig. 1 Relation between compressive strength and E-modulus of UHPC cured at 20°C

4.2 Stress-strain curve under axial compressive loading

Stress-strain curves were investigated on 100x300 mm cylinders. After load-unload cycles specimen was loaded to rupture stress. The minimal and maximal stress in the load-unload cycles was 5 N/mm² and 65% of compressive strength, respectively. After the first cycle the loading und

Mehdi Sadeghi e Habashi

unloading curves were almost identical. This indicates that only a few new micro cracks were generated during loading cycles. Poisson's ratio of UHPC is about 0,18. The low Poisson's ratio can be resulted from the firm bound between fine quartz sand and matrix. This value kept constant till ca.70% of compressive strength. After compressive stress exceeded this level, the Poisson's ratio rose abruptly, this indicated a rapid propagation of micro cracks.

Fig. 2 Stress-strain curve of self-compacting UHPC without fiber (6 loading-unloading cycles)

Fig. 3 Stress-strain curve of UHPC without fiber (60 sec. vibrated, 4 loading-unloading cycles)

Mehdi Sadeghi e Habashi

5. AUTOGENOUS SHRINKAGE

Autogenous shrinkage is the consequence of chemical volume contraction during cement hydration and self-desiccation in concrete. High cement content and low water to binder ratio in UHPC may lead to high autogenous shrinkage, which will induce micro cracks in early ages. Experiments investigating autogenous shrinkage of UHPC were carried out on 150x150x700 mm beams. In order to prevent the friction between concrete and form a plastic foil had been inserted before casting. Concrete was sealed with over-long foil immediately after casting. Length changes and temperature variation in concrete were recorded every 15 min and saved in computer. Deformations of UHPC with different silica fume contents or water to cement ratios are shown in Fig. 4, in which deformation is the sum of autogenous shrinkage and thermal expansion due to temperature increasing during cement hydration.

Fig. 4 Deformation of sealed self-compacting UHPC

Fig. 5 Autogenous shrinkage of self-compacting concrete

Autogenous shrinkages shown in Fig. 5 were calculated by subtracting thermal expansion from the measured deformations in Fig. 4. The Zero-point of the time-axis in Fig. 5 corresponds the time, when highest temperature in concrete was reached. Generally UHPC show a higher autogenous shrinkage than conventional HPC. Similar autogenous shrinkages of UHPC with different w/c-ratio or different silica fume content indicate that the both factors have not great influence on autogenous shrinkage at the age of 28 days. However, they have significant influence on the development of autogenous shrinkage. Curves in Fig. 5 suggest that autogenous shrinkage of UHPC with lower w/c-ratio and higher silica fume content could increase continuously.

6. CONCLUSION

In this paper some results about the properties of UHPC are presented. Through optimizing the composition of powder mixture in matrix a flow able UHPC has been produced. Its strength depends intensively upon air content in concrete. Compressive strength higher than 200 N/mm² can be reached under normal curing condition by reducing the air content less than 1%.

Mehdi Sadeghi e Habashi

Relationship between compressive strength and elastic Modulus of UHPC is quite different from that in conventional high performance concrete. A new empirical equation predicting E-Modulus of UHPC was suggested. Because of the high binder (cement and silica fume) content and low water to binder ratio UHPC shows a higher autogenous shrinkage than conventional high performance concrete.

REFERENCES

[1] PIERRE RICHARD, MARCEL CHEYREZY, “Composition of Reactive Powder Concretes”, Cement and Concrete Research, Vol. 25, No. 7, 1995, pp. 1501-1511.

[2] PAPADAKIS V., “Experimental Investigation and theoretical Modelling of Silica Fume Activity in Concrete”, Cement and Concrete Research, Vol. 29, 1999, pp. 79-86

[3] RESCHKE T. “Einfluss der Granulometrie der Feinstoffe auf die Gefügeentwicklung und die Festigkeit von Beton”, Schriftenreihe der Zementindustrie, Heft 62/2000