Yuko Ogawa, Ryoichi Sato and Kenji Kawai Institute of Engineering, Hiroshima University, Japan

Transcription

1 Early Age Deformation, its Resultant Stress and Creep Properties of Concrete with and without Internal Curing Subjected to High Temperature History at an Early Age Yuko Ogawa, Ryoichi Sato and Kenji Kawai Institute of Engineering, Hiroshima University, Japan 1

2 CONTENTS 1. Introduction 2.Experimental programs 3. Results and discussion -Compressive strength and modulus of elasticity -Free strain in concrete -Shrinkage and temperature changed-induced strain in reinforcement -Effective modulus of elasticity -Evaluation of creep coefficient 4. Conclusions 2

3 Introduction JCI Guidelines for Control of Cracking of Mass Concrete 2016 (JCI Guidelines) Temperature E e (t e ) = Φ(t e ) E c (t e ) Φ=0.42 Tension linear interpolation Φ=0.65 Inner part Surface Main Target: newly placed members such as wall-type structures restrained predominantly EXTERNALLY by existing members σ c Compression Schematic diagram of stress in concrete 3

4 Introduction A surface layer of newly placed members such as cap beams supported by piles is predominantly restrained due to nonlinear distribution of temperature in sections. Temperature In this case, tensile stress develops up to a time at the peak temperature and thereafter compressive stress develops. σ c Tension Inner part Compression Surface Schematic diagram of stress in concrete 4

5 Introduction-Objective JCI Guidelines Cracks due to internal restraint are not addressed in the verification because the cracks can be prevented in construction stages by using appropriate curing methods. HOWEVER, structures could not always be cured sufficiently due to requirement of shorter construction period and so on. In that case, it is important to verify the superficial cracks beforehand. Objective to investigate creep behavior of concrete, in which tensile stress develops up to a time at the peak temperature and thereafter the tensile stress decreases, by measuring concrete deformation, rebar strain in a reinforced concrete prism subjected to high temperature history with a maximum of 70 o C at an early age. 5

6 Experimental Programs 6

7 Materials and Mix proportions Name BB BB-PCFA W/C s/a Unit content (kg/m 3 Slump Target: 12±2.5 (cm) Air content Target: 4.5±1.0 (%) W C S PCFA G C(cement): Portland blast furnace slag cement-type B Porous ceramic fine aggregate derived from roof tile waste 7

8 Item of Investigation Mechanical properties -Compressive strength and modulus of elasticity under compression -at the age of 1, 3, 7, 28 and 91 days -with a diameter of 100 mm and a height of 200 mm Length change of concrete under temperature change -with an embedded gauge including thermocouple -at the center of a prism with a size of 100x100x400mm Embedded gauge with thermocouple Elastic strain in reinforcement induced by shrinkage and temperature change 8

9 Item of Investigation Elastic strain in reinforcement induced by shrinkage and temperature change a D19-bar located at the center of the cross section of prismatic specimen -two self-temperature-compensation gauges -Strain of each gage in bare rebar was measured in a temperature-controlled room ranging from 10 o C to 70 o C automatically and repeatedly until relationship between temperature rise and the measured strain overlapped that between temperature drop and the measured strain. -Elastic strain in reinforcement was obtained by subtracting the strain in bare rebar from measured strain in reinforced concrete prism. Strain in bare reinforcement (x10/ o C) D19-bar Temp. rise 5mm-gauge Temperature ( o C)

10 The top surface of all specimens were sealed after casting >Stored in a temperature-controlled room >High temperature history with the maximum temperature of 70 o C >Remolded at the age of 7 days, and sealed, and then stored at 20 o C 70 BB 60 BB-PCFA 50 Temperature ( o C) Room temperature Curing Conditions Remolded, sealed and stored in a room at 20 o C Age (day) Temperature inside of both concrete specimens were almost the same as that in the temperature-controlled room. 10

11 Results and Discussion 11

12 Compressive Strength and Modulus of Elasticity JCI Guidelines Temperature-adjusted concrete age, t e n 4000 t e = Δt i exp T (Δt i )/T j=1 0 Compressive strength strength, f c (t e ) where, Δt i is the period of constant temperature continuing in concrete (day); T(Δt i ) is concrete temperature for Δt i ( o C); T 0 is 1 o C. f ' c (t e ) = t e S f a + b(t e S f ) f ' c(t n ) Modulus of elasticity, E c (t e ) E c (t e ) = C 3 f ' c (t e ) C 4 where, t n is the strength control age of concrete cured under water at 20 o C (day); a and b are experimental constants; S f is the temperature adjusted concrete age corresponding to initiation of hardening (day); f c (t n ) is the compressive strength of concrete at t n (N/mm 2 ). where, C 3 and C 4 are constants. 12

13 Compressive Strength and odulus of Elasticity Compressive strength (N/mm 2 ) ε s,e JCI Guidelines(BB W/C=0.4) 100 Sf: Initiation BB BB-PCFA Temperature adjusted age (day) Modulus of elasticity (N/mm 2 ) JCI Guidelines (BB W/C=0.4) BB40 BB40PCFA Temperature adjusted age (day) f c and E c with BB-PCFA were almost the same as those of BB They were higher during ages of 1 day to 10 days, and lower after the age of 10 days compared with those recommended by the JCI Guidelines. 13

14 Strain in concrete (x10-6 ) Free Strain in Concrete, ε as +ε ΔT ε 70 as +ε c, ΔT increased and decreased corresponding to the temperature 60 rise and drop up to the age of 7 days. 50 Temperature in specimen εsh+εδtin BB-PCFA εas+εδt in BB Assumed εas in BB-PCFA (TEC10.4) Assumed εas in BB (TEC11.4) Assumed εas in JCI guidlines Age (+1day) Temperature in specimen ( o C) ε s,e 100 Sf: Initiation Thermal expansion coefficient (TEC) was assumed to be a constant of BB: 11.4 x10-6 / o C BB-PCFA:10.9 x10-6 / o C measured with the cylindrical specimen after shrinkage was negligible. 14

15 Shrinkage and Temperature Change-Induced Strain in Reinforcement, ε s,e The elastic strain ε s,e in reinforcement increased with the temperature rise and decreased with the temperature drop, regardless of mix proportion. After reaching a constant temperature, the strain increased monotonically with time. The change in ε s,e with age during temperature rise and drop obtained from BB-PCFA is slightly larger than that from BB. Strain, temperature (x10-6, o C) Sf: Initiation of hardening Induced by autogenous shrinkage and the difference in TEC Temperature BB-PCFA BB Elastic strain ε s,e Induced by autogenous shrinkage Age (+1 day) >This difference in the change could be explained by the difference in TEC between both concrete samples. That is, the TEC gap between BB-PCFA and reinforcement can be larger than that between reinforcement and BB. 15

16 Effective Modulus of Elasticity E e Strain in concrete and reinforcement can be expressed as follows. ε c = ε c,e + ε cr + ε sh + ε c,δt ε s = ε s,e + ε s,δt Based on the compatibility of real strain ε c =ε s, stress related strain (ε c +ε cr ) in concrete can be obtained as follows. ε c,e + ε cr = ε s,e + ε s,δt (ε as + ε c,δt ) = ε s,e + α s ΔT (ε as + ε c,δt ) Measured elastic 11.8x10-6 / o C Measured free strain strain in reinforcement in concrete Stress in concrete can be given by the following equation using E e. σ c = E e (ε c ε as ε c,δt ) = E e (ε c,e + ε cr ) Stress in concrete can be obtained by measuring elastic strain in reinforcement. σ c = A s A c σ s = pe s ε s,e E e can be evaluated as follows. E e == σ c ε c,e + ε cr = pe sε s,e ε c,e + ε cr 16

17 Strain, Temperature (x10-6 o C) Stress and Stress related strain in concrete with age Stress related strain ε c,e + ε cr = ε s,e + ε s,δt (ε sh + ε c,δt ) Stress related strain BB Stress related strain BB-PCFA Temperature (1) (3) (4) Age (day) σ c BB σ c BB-PCFA Stress in concrete N/mm 2 Tensile stresses in both concretes were generated during temp. rise, decreased during temp. fall, and increased after reaching a constant temperature of 20 o C. The stress related strain in BB increased at a higher rate than that of BB-PCFA in temperature rise, whereas the BB decreased in roughly parallel with the BB-PCFA in temperature drop.

18 Relationship between stress related strain and stress in concrete Stress in concrete, σ c (N/mm 2 ) BB (1) days (2) days (3) days (4) days (3)y = x R² = days (1)y = x R² = (4)y = x R² = days (2) Tmax 1.4days Stress related strain ε c,e +ε cr (x10-6 ) Stress in concrete, σ c (N/mm 2 ) BB-PCFA 50 days (4)y = x R² = days (2) Tmax 1.4days (1)y = x R² = (3)y = x R² = days (1) days (2) days (3) days days (4)50-200days Stress related strain ε c,e +ε c,cr (x10-6 ) (1) from initiation of hardening to the age at T max (2) from the age at T max to the age at T max +1day in temperature adjusted age (3) from the age of at T max +1day in temperature adjusted age to the age of 7 days (4) from the age of 7 days to the age of around 200 days Effective modulus of elasticity E e = σ c ε c,e + ε cr Reduction coefficient Φ = E e E c 18

19 Modulus of elasticity N/mm Effective modulus of elasticity and reduction coefficient BB Ec (this study) Reduction constant (JCI) Ee (JCI) Reduction coefficient (this study) Age (+1day) Reduction constrant Modulus of elasticity (N/mm 2 ) Reduction coeffient (this study) 0.6 Ee (this study) Modulus of elasticity, Ec Modulus of elasticity, Ec Effective modulus of elasticity, Ee 0.2 Effective modulus of elasticity, Ee Effective modulus of elasticity, Ee by JCI Effective modulus of elasticity 0.1 by JCI Reduction coefficient Reduction coefficient Reduction constant by JCI Reduction constant by JCI 0.0 (1) (3) (4) 0 (1) (3) Excluded (4) BB-PCFA Ec (this study) Reduction constant (JCI) Ee (JCI) Ee (this study) Age (+1day) (1)Temp. rise E e as well as Φ of both concrete in this study < those in JCI guidelines (3) Temp. fall BB : E e as well as Φ in this study < those in JCI guidelines BB-PCFA : E e as well as Φ in this study slightly< those in JCI guidelines (4) 20 o C constant after high temperature history E e as well as Φ of both concrete in this study those in JCI guidelines Reduction constant

20 Evaluation of Creep Coefficient Stress related strain can be expressed as follows by using the expression of E e. E e == σ c ε c,e + ε cr = pe sε s,e ε c,e + ε cr ε c,e + ε cr = pe sε s,e E e = n e pε s,e where, n e is ratio of modulus of elasticity of reinforcement (E s ) to E e of concrete. The ratio n e can be described the following equation from ε c,e + ε cr = ε s,e + ε s,δt (ε as + ε c,δt ) n e pε s,e = ε s,e + ε s,δt (ε as + ε c,δt ) n e = 1 p 1+ ε as + ε c,δt ε s,δt ε s,e n e = Es Ec(1 + χϕ) = n(1 + χϕ) Based on, creep coefficient (φ) can be obtained. ϕ = 1 χ 1+ n e n = 1 χ 1+ 1 np 1+ ε + ε ε as c,δt s,δt ε s,e where, χ is aging coefficient; n is ratio of E s to E c. Increment of creep coefficient at the step of j can be expressed as follows. Δϕ j = 1 χ 1+ 1 n j p 1+ Δε + Δε Δε as, j c,δt, j s,δt, j Δε s,e, j 20

21 BB Ratio of modulus of elasticity Aging coefficient Δfree strain in concrete Δthermal strain in RB Δelastic strain in RB Step Period Creep coefficient Evaluation of creep coefficient Temp. rise (1) (2) (3) (4) (4') day 7days 200days s days Temp. fall 20 o C days n j = E s /E c,avg,j χ Δε sh,j Δε c,δt,j Δε s,δt,j Δε s,e,j χφ φ BB-PCFA Ratio of modulus of elasticity Aging coefficient Δfree strain in concrete Δthermal strain in RB Δelastic strain in RB (1) (2) (3) (4) (4') day 7days 200days s days days n j = E s /E c,avg,j χ Δε sh,j Δε c,δt,j Δε s,δt,j Δε s,e,j χφ φ Step Period Assuming χ of 0.8 constant, Creep coefficient Temp. rise Temp. fall The evaluated creep coefficients of BB-PCFA were around 1.0 during temperature rise and fall, and those of BB were 2.0 and 3.7 for the same periods, respectively. PCFA may decrease the effect of creep due to its internal curing effect. 20 o C

22 Conclusion 22

23 Conclusion The present study investigated the creep properties of sealed concrete restrained with steel bar subjected to temperature variation assuming that in mass concrete, which were made of blast furnace slag cement Type B with and without internal curing agent PCFA (BB-PCFA and BB). The following main conclusions are drawn within the limit of the study. The effective modulus of elasticity considering creep effect (E e ) during temperature rise, which was obtained from regression line for measurements, was significantly smaller than that recommended by the 2016 Guidelines for each of the above mentioned both concretes. The E e during temperature drop of BB was also significantly smaller than that by the guideline, and however, that of BB-PCFA was slightly smaller than that by the guideline. The creep coefficient of BB was remarkably larger than those of BB-PCFA during both temperature rise and drop corresponding to E e, which may be due to the effect of internal curing with PCFA. Further experiments are needed to improve the reliability of the present data. Thank you for your kind attention. 23