Influences of Heat of Hydration on Autogenouse Shrinkage induced Stresses in Reinforced High Strength Concrete Columns at Early Ages.

Transcription

1 Influences of Heat of Hydration on Autogenouse Shrinkage induced Stresses in Reinforced High Strength Concrete Columns at Early Ages. Keiichi IMAMOTO Ashikaga Institute of Technology, JAPAN Abstract This paper deals with autogenouse shrinkage induced stresses in reinforced high-strength concrete columns. Two full sized reinforced concrete columns were constructed; one was constructed in summer and the other was constructed in winter. The influence of heat of hydration on the maginitude of the shrinkage induced stress was investigated, measuring steel strains of reinforcing bars of the column. In addition, elastic modulus and creep of concrete at early ages were tested. Test results showed that the magnitude of autogenouse shrinkage induced stress in summer was higher than that in winter. On the other hands, elastic modulus of concrete in summer developed more rapidly than that in winter. It was found that the heat of hydration strongly affected the autogenouse shrinkage induced stress as well as elastic modulus and creep of concrete. Using of the mechanical informations of concrete at early ages, a step-by-step analytical method well simulated the autogenouse shrinkage induced stresses of these columns. 1. Introduction High strength concretes over 100MPa produced by utilizing of superplasticizer and silica fume have been actualized high raise buildings and other super structures. However, recent researches indicate that early age cracking have become significant problem in these structures. J.C.I. (Japan Concrete Institute) Technical Committee on Autogenous Shrinkage of Concrete reports that autogenous shrinkage is caused by self desiccation due to cement hydration and might be the main reason of early age cracking in high strength concrete[1]. Temperature conditions strongly affect the cement hydration. Hence, it is easy to understand that evaluating the influence of hydration heat of cement on stress generation due to autogenous shrinkage is quite significant for crack control of the concrete structures. This paper deals with measurements and analysis of autogenous shrinkage induced stresses in reinforced concrete columns cast in summer and in winter. Based on tests results for creep and elastic modulus carried out at early ages, the shrinkage induced stresses of the columns were analyzed by a step-by-step method assuming a superposition of virgin creep curves. Keiichi IMAMOTO, Autogenouse Shrinkage induced Stresses in HSC, Page 1 of 10

2 2. Outline of experiments 2.1 Materials, mix proportions and properties of concrete Concretes were produced using belite-rich cement and silica fume with replaced 10% in mass of cement, with the water to binder ratio (W/B) being 0.23 and Quartzite crushed stone was used as coarse aggregate. In order to obtain high fluidity of fresh concrete, polycarboxylic acid type superplasticizer was used. The compressive strengths of concretes in normal curing at 28 days were 104 MPa and 111 MPa in W/B 0.23 and 0.215, respectively. Also, the elastic moduli of concretes in normal curing at 28 days were 37.2 GPa and 42.4 GPa in W/B 0.23 and 0.215, respectively. Materials, mix proportions and properties of concrete are shown in Table 1, 2 and 3. Table 1 Material properties Cement Belite-rich cement, density 3.20g/cm 3 Fine aggregate Pit sand, density 2.64 g/cm 3, absorption 1.16% Coarse aggregate Quartszite crushed stone, density 2.63 g/cm 3, absorption 0.58% Minaral admixture Silica fume, S i O % average particle diameter µ Chemical admixture Polycarboxylic acid type superplasticizer Table 2 Mix proportions Cast season W/B Unit content (kg/m 3 ) (%) Water Cement Silica fume Fine agg. Coarse agg. Superplasticizer Winter Summer Table 3 Properties of concrete Cast Concrete Flow Air content Comp. Elastic season temperature () (cm) (%) Strength* (MPa) modulus* (GPa) Winter Summer * 28 days in normal curing 2.2 Outline of full sized column In this study, two full sized reinforced concrete columns with 850*850 mm in cross section and 2200 mm in height were constructed. A reinforcement ratio of the column was 2.97%, and steel strains and temperatures were measured with strain gauges and thermocouples, respectively. In addition, two plain concrete blocks having the same cross section as the columns and 90cm in height were also prepared in order to measure free shrinkages, i.e. autogenous shrinkages in both seasons. The strains and temperatures were measured at the centre of the blocks with embedded gauges and thermocouples. The tops and bottoms of the blocks were covered with 20 cm thickness of insulation boards. The details of the column and block are shown in Fig.1. Keiichi IMAMOTO, Autogenouse Shrinkage induced Stresses in HSC, 2 of 10

3 Test results and discussions Fig.1Details of column and block 3.1 Temperature histories of columns Temperature histories, i.e. temperature-age relationships, of the columns cast in summer and in winter are shown in Fig.2. The initial of the age was defined as the moment when steel strain occurred. The maximum concrete temperatures in summer were 62.0at center and 55.2at edge in the cross section of middle height of the column. On the other hands, those in winter were 30.0at the center and 24.6at the edge. The temperatures at center of the plain concrete block were similar to those of full sized reinforced concrete columns. The slight difference of the maximum temperature between center and edge means that internal restraint stresses due to temperature gradient might be negligible. This would be because that wooden form which has low heat conductivity was used in this experiment. Fig. 2 Temperature histories of concretes (left: winter, right: summer) Keiichi IMAMOTO, Autogenouse Shrinkage induced Stresses in HSC, 3 of 10

4 3.2 Autogeneous shrinkage and its induced stress Because the forms were removed at 7 days, up to this age, moisture evaporation from the concrete would be slight. Hence, the measured strains in plain concrete blocks are expected to be sum of autogeneous shrinkage (A.S.) strain and thermal strain. The thermal strain ( t (t i )) is caused by the difference of thermal expansion coefficient (T.E.C.) between the embedded gauge and the concrete as expressed in Eq.(1). t (t T ) = (α em -α con (t T )) T (1) α em : T.E.C. of embedded gaege (10-6 /) α con (t T ) : T.E.C. of concrete (10-6 /) t T : Effective age (days) (see 3.3 (1)) T : Temperature () While the T.E.C. of embedded gauge i.e. reinfocing bar is constant, that of concrete might change at eraly ages. The changes of the T.E.C. of high strength concrete can be drawn as Fig.3, based on recent researches[2],[3]. Fig. 3 Change of the T.E.C. of concrete Taking into account of the change of T.E.C., the developments of autogenous shrinkage strains and the thermal strains at center of the plain concrete blocks are shown in Fig.4. It is clear that the magnitudes of the thermal strains are slight and the autogenous shrinkage strain accounts for the most part of the total strain in this experiment. The autogeneous shrinkage strains reached approximately at 2 days in both plain blocks cast in summer and in winter. In full sized high strength concrete members at early ages, self desiccation which cause autgeneous shrinkage might generate rapidly not only in summer but also in winter. Keiichi IMAMOTO, Autogenouse Shrinkage induced Stresses in HSC, 4 of 10

5 Fig. 4 Strains of concrete block Fig.5 shows changes of steel strains in full sized reinforced concrete columns. Nevertheless the autogeneous shrinkage strains in both seasons were almost equal, the magnitude of steel strain in summer was twice of that in winter. Assuming the force equilibrium between the steel and the concrete, average stress of concrete i,e, autogeneous shrinkage induced stress can be calculated by Eq (2). The shrinkage induced stresses of concrete reached about 2.0 MPa in summer and 1.0 MPa in winter as shown in Fig.6. It is clear that the autogenous shrinkage plays a significant role in early age cracking of high strength concrete structures and temperature history due to hydration heat of cement strongly affect its behaviour. c(t i )= s (t i )EsAs/Ac (2) c(ti): Average concrete stress at time t i s (t i ) : Steel strain at time t i Es: Elastic modulus of reinforcing bar As: Cross section area of reinforcing bars Ac: Cross section areas of concrete Keiichi IMAMOTO, Autogenouse Shrinkage induced Stresses in HSC, 5 of 10

6 Fig.5 Changes of steel strains in full sized reinforced concrete columns Fig.6 Changes of shrinkage induced stress in full sized reinforced concrete columns 3.3 Analysis of concrete stress (1) Elastic modulus and compressive strength Temperature conditions strongly affect developments of mechanical properties of concrete. The influences of hydration heat of cement on the developments of elastic modulus and compressive strength were investigated by comparing the data under three kinds of curing conditions ; column temperature history simulated (C.T.S.) curing, sealed curing in winter and in summer. Fig.7 shows the relationship between age and development ratio of compressive strength and elastic modulus. The development ratio was evaluated as a ratio of each age strength (or elastic modulus) to 28days normal curing strength (or elastic modulus). The development ratio of compressive strength in sealed curing in summer was higher than that in winter. And that in C.T.S. curing was highest among the all curing conditions adopted in this study. The tendency of the development ratio of elastic modulus was similar to that of compressive strength. Keiichi IMAMOTO, Autogenouse Shrinkage induced Stresses in HSC, 6 of 10

7 Fig.7 Development ratio versus age A maturity concept, i.e. effective age, adopted in CEB-FIP model code 1990(MC90)[4], as shown in Eq.(3), was used in order to evaluate the influence of temperature on the development of compressive strength and elastic modulus. Fig.8 shows the relationship between the effective age and the development ratio of compressive strength and elastic modulus. It is shown that good agreements were obtained among the three kinds of curing conditions by using effective age, and their developments could be well expressed by the MC90 equations shown in Eqs.(4) and (5). t T =Σ t i exp /(273+t( t i )/T 0 ) (3) t i : Number of days during which concrete temperature is T( t i ) T 0 : 1 F c (t T )/F c28 =exp(s(1-(28/t T ) 0.5 )) (4) E c (t T )/E c28 ={exp(s(1-(28/t T ) 0.5 ))} 0.5 (5) F c (t T ) : Compressive strength at effective age t T (MPa) F c28 : Compressive strength at 28 days in normal curing (MPa) E c (t T ) : Elastic modulus at effective age t T (GPa) E c28 : Elastic modulus at 28 days in normal curing (GPa) s : Coefficient depending upon type of cement (s=0.25 for low heat cement is used in this study) Fig.8 Development ratio versus effective age Keiichi IMAMOTO, Autogenouse Shrinkage induced Stresses in HSC, 7 of 10

8 (2) Creep Creep behavior plays a significant roles in stress generation of concrete at early ages. Outlines of creep test carried out in this study are shown in Table 4, 5 and 6. Cylindrical specimens, 10 cm in diameter and 20 cm in height, were prepared. Loaded stresses were applied at 3 different ages and their magnitude were about 20% of compressive strength at loded ages. The specimens were sealed with aluminum foil tape and all the tests were perforformed in a temperature-relative humidity-controlled room at 202and 605% R.H.. Kawaguchi indicates that the influence of hydration heat on the creep behaviors at early ages could be evaluated with the effective age at loading[5]. The relationship between effective age and creep coefficient are shown in Fig.9. In order to express the creep curves, equations for notional creep coefficient Φ 0 and the rate β H of MC90(Eq. (6)) were modified to Eqs. (7), (8) and (9) by regression analysis. Table 4 Material properties Fine aggregate Pit sand, density 2.62 g/cm 3, absorption 2.00% Others see Table1 Table 5 Mix condition and properties of concrete w/b Unit water Concrete Flow Air content Comp. (%) content (kg/m 3 ) temperature () (cm) (%) Strength* (MPa) * normal curing Table 6 Test conditions Specime size 1020cm, sealed with aluminum foil tape Loaded stress level 20% of compressive strength at loded ages Method for strain measurement Ebedded gauge Temperature and R.H.condition 202and 605% R.H. * φ(tt o )φ o (t-t o )(β H (t-t o ) 0.3 (6) φ(tt o )creep coefficient at age t loaded at effective age t o φ o : notional creep coefficient t o : up to 0.86 φ o t o (7) t o : φ o -0.25t o (8) to :after 8.5, φo : constant β H 2.66to (9) Keiichi IMAMOTO, Autogenouse Shrinkage induced Stresses in HSC, 8 of 10

9 Fig.9 Relationship between age and creep coefficient (3) Step-by-step method The total stresses, i.e. autogenous shrinkage induced stress due to restraint of reinforcing bars were analyzed based on a step-by-step method expressed in Eq. (10)[6]. The analysis procedure for reinforced axial member is given by Eqs. (11) and (12). This method enables to estimate the developments of stress and stress relaxation due to early age concrete creep. The total stresses computed, based on the assumption of linear strain distribution through the cross section, using elastic modulus obtained by Eq.(5) and creep coefficient by Eqs.(6), (7), (8) and (9) are compared with measured values in Fig.10. Good agreements between the computed stresses and the measured values in both seasons indicate that the autogenous shrinkage induced stresses in concrete structures under various temperature conditions might be simulated by the step-by-step method, evaluating the changes of the mechanical properties of early age concrete with the effective age. However, further research should be needed on the influence of hydration heat of cement on the autogenous shrinkage itself. Stress-strain relation of concrete ; 1 σ ( t i / 2 ) = ε ( t i+ 1 / 2 ) ε e ( t J ( t i+ 1 / 2, t i ) 1 φ( ti+ 1/ 2, ti ) J ( ti+ 1/ 2, ti ) = + E( t ) E i { ) ε ( t )} + 1 i 1 / 2 f i+ 1 / 2 ε e( ti 1/ 2) = σ ( t j ) J ( ti+ 1/ 2, t j ) J ( ti+ 1/ 2, ti) σ ( ti 1/ 2) j = 1 i 28 (10) (t i+1/2 ): concrete stress at time t i+1/2 (t i+1/2 ): actual strain of concrete at time t i+1/2 f (t i+1/2 ): free shrinkage strain i.e. autogenous shrinkage strain at time t i+1/2 Equilibruim condition; A c (t i+1/2 )+A s s (t i+1/2 )=0 (11) s (t i+1/2 ): steel stress (MPa) Strain compatibility; (t i+1/2 )= s (t i+1/2 ) (12) Keiichi IMAMOTO, Autogenouse Shrinkage induced Stresses in HSC, 9 of 10

10 Fig.10 Relationship between measured values and computed stresses 4. Concluding remarks Two full scale high strength concrete columns were constructed in summer and in winter and the autogenous shrinkage induced stresses were measured and analyzed. The following conclusions were obtained. 1. Autogenous shrinkages of high strength concretes, produced using belite-rich cement and silica fume with replaced 10% in mass of cement, with the water to binder ratio (W/B) being 0.23 and 0.215, reached about 600 micro strain at 2 days both in summer and in winter. 2. Autogenous shrinkage induced stresses in full sized concrete columns with 2.97% of reinforcement ratio reached about 2.0 MPa in summer and 1.0 MPa in winter. The temperature histories strongly affected the magnitude of the concrete stressesin the columns. 3. The step-by-step method taking into account of early age mechanical properties with effective ages well simulated the developments of autogenous shrinkage induced stresses in columns at early ages. References 1. Autogenous Shrinkage of Concrete, Ed. by Ei-ichi Tazawa, E & FN Spon. London O. Bjontegaard and E. J. Sellevold, Thermal dilation-autogenous shrinkage: how to separate?, Autogenous Shrinkage of Concrete (Ed. by Ei-ichi Tazawa), pp , E & FN Spon. London H. Hashida and N. Yamazaki, A Study on Composed Deformation of Autogenous Shrinkage and Thermal Expansion due to Heat of Hydration in High-strength Concrete, Concrete Research and Technology, pp.25-32, Vol.13, No.1, Jan (in Japanese) 4. CEB-FIP Model Code 1990, Thomas Telford. 5. T. Kawaguchi, Measurement and analysis of thermal stresses in massive concrete foundation, Control of Cracking in Early Age Concrete (Ed. by H. Mihashi and F.H. Wittmann), pp , Balkema, A. M. Neville, W. H. Dilger and J. J. Brooks, Creep of Plain and Structural Concrete, Construction Press, Longman, pp , Keiichi IMAMOTO, Autogenouse Shrinkage induced Stresses in HSC, 10 of 10