Mechanical Properties of Expansive High-Strength Concrete under

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Free shrinkage test and simulated-completely restrained ... free shrinkage and restrained shrinkage. ..... Concrete”, ACI Materials Journal, March-April, 211-217.
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Mechanical Properties of Expansive High-Strength Concrete under Simulated-Completely Restrained Condition at Early Age Park Sun-Gyu 1 , Ippei Maruyama 2 , Kim Jeong-Jin, Takafumi Noguchi 3

ABSTRACT: This paper presents a study of the efficiency of expansive additive in controlling restrained shrinkage cracking of high-strength concrete at early age. Free shrinkage test and simulated-completely restrained test were performed. Creep and shrinkage of high-strength concrete with and without expansive additive under restrained conditions were characterized by experiments that provided data on free shrinkage and restrained shrinkage. The results showed that the addition of expansive additive effectively reduced autogenous shrinkage and tensile stress. Also, it was found that the shrinkage stress was relaxed by 70% in ordinary high-strength concrete and 80% in expansive high-strength concrete. KEYWORDS: Shrinkage cracking, Creep, Shrinkage stress, Expansion additive.

1. INTRODICTION Development of high-strength and improved durability in concrete has brought new opportunities to the construction industry. In recent years, some attention was particularly given to cracking sensitivity of such concrete. It has been argued and demonstrated experimentally that such concrete undergoes autogenous shrinkage due to self-desiccation at early age, and, as a result, internal tensile stress may develop, leading to micro cracking and macro cracking1). Such premature deterioration affects integrity, durability, and long-term service life of concrete structure. One possible method to reduce cracking due to autogenous shrinkage is the addition of expansive additive. Tests conducted by many researches have shown the beneficial effects of addition of expansive additive for reducing the risk of shrinkage-introduced cracking2, 3). However, past researches on shrinkage of concrete containing expansive additive have typically focused on stress development and critical cracking condition of concrete. This paper aims at evaluating stress and creep strain of restrained high strength concrete with and with out expansive additive in early age using VRTM (Variable Restraint Testing Machine) which can simulate the behavior of concrete member in real structure. Also, the risk of cracking due to autogenous shrinkage of expansive high-strength concrete is discussed.

1 2

Graduate Student, Dept. of Arch., Graduate School of Eng., Univ. of Tokyo, M. Eng Research Assoc., Dept. of Social and Environmental Eng., Graduate School of Eng., Hiroshima University, Dr. Eng

3

Assoc. Prof., Dept. of Arch., Graduate School of Eng., Univ. of Tokyo, Dr. Eng.

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2. VRTM AND PROGRAM FLOW OF SIMULATED-COMPLETELY RESTRAINED TEST 2.1 Description of VRTM A general view of the VRTM is shown in Figure 1. A specimen with length of 1500mm and cross-section of 100×100mm in the center part is mounted horizontally in a frame. The VRTM system is a modification of TSTM (Temperature Stress Testing Machine) developed by Springenshmid4), 5). Two rigid steel claws hold the concrete specimen and are able to exert tensile or compressive force. The right end of the specimen is fixed. The left cross head can move while monitoring the distance of embedded bars in the center of concrete specimen. The cross head movement is controlled by converting a rotation of screw bar. The load in the restrained specimen is measured by means of a load cell with an accuracy of ±1N.

Figure1. Detail of VRTM

2.2 Program flow of simulated-completely restrained test The restrained condition is simulated by maintaining the total deformation of the restrained specimen within a threshold, which is defined as the permissible change in the length of the specimen. The computer monitors the shrinkage deformation continuously, and when the deformation exceeds the threshold compressive or tensile load was applied by the cross head to restore the specimen to its original length. In the meantime, when the load reaches a certain fixed threshold, the cross head moves in order to enable the specimen deform freely until the deformation approaches threshold. While repeating this process in VRTM, a simulated- completely restrained condition is achieved and the stress generated by shrinkage is measured. The threshold values in load and deformation are called triggers and set as parameters for the investigation on creep behavior of the specimen.

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3. EXPERIMENTAL PROGRAM

3.1 Materials and composition In the present study, concrete with w/c ratio of 0.30 and 10 kg/ expansive additive was examined. The ordinary portland cement (density: 3.16 g/cm3) was used. The expansive additive has density of 3.16 g/cm3 and specific surface area of 2650 g/cm2. Mix proportion of concrete is shown in Table 1. A polycarboxylate ether type of superplasticizer was used to make the concrete workable. Concrete were mixed in a pan type mixer and cast into φ100×200mm mold for the test on compressive strength, elastic modulus and tensile splitting strength. After casting, concrete specimens were sealed to prevent water evaporation to simulate condition where only self-desiccation occurs and cured at 20. Table 1. Mix Proportion of Concrete with W/C of 30% Mix composition OHC (Ordinary HighEHC (Expansive HighStrength Concrete) Strength Concrete) Portland cement (kg/m3) 550 540 Expansive Additive (kg/m3) 0 10 Water (kg/m3) 165 165 Fine Aggregate (kg/m3) 781 781 Coarse Aggregate (kg/m3) 869 869 SP agent (%)[×Cement weight] 0.7 0.7

3.2 Autogenous shrinkage test and VRTM test Measurement of the free autogenous shrinkage was performed on concrete specimens of 100×100×400mm, which deformation was monitored with contact type sensors. In this research, load and deformation of simulated-completely restrained test were conducted with trigger of 100N and 4µm. The autogenous deformation, and the strain and stress in VRTM up to 5 days after setting time of concrete were monitored on sealed specimens cured at 20 . 4. TEST RESULTS 4.1 Mechanical properties Test result of compressive strength, splitting tensile strength and elastic modulus are shown in Table 2. The results represent the average value of three specimens. OHC shows the fastest strength gain and the highest value at 5days. The compressive strength, splitting tensile strength and elastic modulus of EHC show analogous value with those of OHC. Table 2. Properties of Ordinary and Expansive Concrete Specimens with W/C of 30% Comp. Strength [MPa] Splitting Tensile Strength [MPa] E-modulus[GPa] Age 1day 3days 5days 1day 3days 5days 1day 3days 5days OHC 25.6 56.5 65.4 2.2 4.4 4.9 22.5 30.6 32.3 EHC 29.1 59.1 67.4 2.2 3.5 4.6 24.3 30.6 35.3

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4.2 Free shrinkage

Autogenous Strain(x10-6)

Figure 2 shows the result of the autogenous shrinkage of OHC and EHC. The free autogenous shrinkage was initialized at zero at the time when stresses 40 were first recorded in the VRTM. The autogenous shrinkage of OHC specimen 0 OHC occurred at a rapid rate in the first few hours and EHC the rate decreased afterward. While the -40 autogenous shrinkage of OHC were about 135×10-6 at 5 days, that of EHC is 110×10-6 at -80 same age. The reason why EHC showed smaller -120 autogenous shrinkage was that the autogenous shrinkage of high-strength concrete was reduced -160 with the expansion additive. 0 2 4 6 Age(days) 4.3 VRTM test Figure 2. Autogenous Shrinkage in OHC and EHC

(1) Temperature and Strain

Tem perature( )

The temperature history of OHC and EHC is shown in Figure 3, and develepment of strain under simulated-completely restrainted condition is shown in Figure 4. It can be seen that the deformation is well controlled within the range of threshold 30 value. A positive value of strain demonstrates that the specimen shrinks and the cross-head 25 moves inward to keep the tensile stress constant. 20 The sudden drop of the strain in OHC around 15 the age of 4 days probably demonstrates that a crack occurs during the outward movement of 10 OHC the cross head. 5 EHC (2) Tensile stress

expansion additive.

0

2

4

6

Age(days) Figure 3. Temperature History in OHP and EHC

Expansion Shrinkage -6 Strain(x10 )

Figure 5 shows the stress measured in OHC and EHC under simulated-completely restrained condition. While tensile stresses in OHC and EHC increased rapidly, that in OHC failed at 4 days, demonstrating that a crack occurred on the surface but did not propagate across the specimen. On the other hand, the stress in EHC showed that no cracking occurred, and tensile stress generated was lower than that in OHC. Accordingly, it can be said that the tensile stress generated by autogenous shrinkage in high strength concrete under simulated -completely restrained condition can be decreased by

0

10 5 0 -5 -10 -15

O HC EHC

-20 -25 0

2 Age(days)

4

Figure 4. Development of Strain under Simulated -Completely Restrained Condition Concrete Materials

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(3) Creep

Com pressive Tension Stress(M Pa)

Comparison of the free shrinkage shown in Figure 2 with the shrinkage of the restrained specimen shown in Figure 4 enables the discrimination of creep 1.4 Invisible Crack strain from shrinkage strain, that is, the 1.2 separation of the strain associated with creep in 1.0 the restrained specimen from the strain 0.8 generated by shrinkage and elastic deformation. 0.6 Figure 6 shows how creep strain can be 0.4 calculated from the restrained test and the free 0.2 shrinkage test. The creep calculation is based on 0.0 OHC the hypothesis that free shrinkage can be -0.2 EHC subtracted from the deformation of the -0.4 0 2 4 6 restrained specimen to calculate the creep6), 7), 8), Age(days) 9) . Creep strain in each step is calculated as follows: Figure 5. Development of Tensile Stress under

ε i ,creep = ε i , fr − ε i ,e ------------- (1)

Simulated -Completely Restraint

ε i,creep : Creep strain in each step ε i,fr : Free shrinkage strain in each step ε i,e : Elastic strain in each step Figure 7 shows the development of the creep strain in OHC and EHC calculated according to equation 1. The creep strain showed the tendency to increase rapidly from immediately after the beginning of measurement up to 20 hours, and increased slightly afterwards. The creep strain corresponded to about 70% of the strain of free shrinkage in OHC, while about 80% in EHC. It is assumed that a considerable tensile stress in high- strength concrete can be relaxed under the restrained condition at early age.

Figure 6. Creep Strain Calculated from the Data of Restrained Test and Free Shrinkage Test

40 Elastic Strain

(4) Specific creep strain Figure 8 shows development of specific creep strain in OHC and EHC. The specific creep strain is calculated as follow:

ε i , s −creep = ε i ,creep / σ i

--------------------- (2)

ε i s-creep Specific creep strain in each step ε i,creep Creep strain in each step

σi

Stress in each step

Strain(x10-6)

0 -40 Creep ofOHC Creep ofEHC

-80 -120

Autogenous Strain -160 0

2

4

6

Age(days) Figure 7. Development of Creep Strain

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The values of specific creep strain in OHC and EHC were 100-130(×10-6/MPa) at 1day, and they tended to gradually decrease with time. 5. CONCLUTION

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Specific Creep Strain(x10- 6/M Pa)

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1000

800 600

OHC EHC

400

The following conclusions were obtained in this research. 200 (1) The expansion additive can control the crack 0 risk due to autogenous shrinkage of high 0 2 4 6 strength concrete under restrained condition. Age(days) (2) The creep strain at early age, which forms a substantial portion of the time dependent Figure 8. Development of Specific Creep Strain in deformation, reduces the shrinkage strain and OHC and EHC the restrained stresses, e.g. 70% reduction in ordinary high-strength concrete and 80% in high-strength expansive concrete. 6. REFERENCES Japan Concrete Institute (2002), “Concrete Autogenous Shrinkage Study Group Report “ Ryoichi Satou et al. (2000), “An Investigation of Reducing Shrinkage of High Strength Concrete”, Proceeding of Japan Concrete Institute, Vol.22, No.2, 991-996. Mako Tanimura et al. (2001), “Experimental Study on Reduction of Shrinkage Stress of High Strength Concrete”, Proceeding of Japan Concrete Institute, Vol.23, No.2, 1075-1080 Springenshmid, R., Gierlinger, E., Kienozycki, W. (1985), “Thermal Stresses in Mass Concrete: New Testing Method and the Influence of Different Cements”, 5th International Conference on Large Dams (ICOLD), Lausanne, CH, pp57-72. Van Breugel, K., Vries, J. (1999), “Mixture optimization of HPC in view of autogenous shrinkage”, Proceeding 5th International Symposium on Utilization of High Strength/High Performance Concrete, Sandefjord, pp. 1041-1050. K.Kovler (1994),”Testing system for determining the mechanical behavior of early age concrete under restrained and free uniaxial shrinkage”, Materials and Structures, 324-330 Roint Bloom and Arnon Bentur (1995), “Free and Restrained Shrinkage of Normal and High-Strength Concrete”, ACI Materials Journal, March-April, 211-217 Salah A. Altoubat and David A. Lange (2001), “Creep, Shrinkage, and Cracking of Restrained Concrete at Early Age”, ACI Materials Journal, July-August, 323-331. Ippei Maruyama, Park Sun-Gyu, Takafumi Noguchi (2002), “Time-dependent mechanical properties of concrete under simulated complete restrained at early age”, Proceeding of Japan Concrete Institute, Vol.24, No.1, 357-362.

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