1, Introduction
Baiguodu Jialing River Bridge is a super-large bridge on Jialing River from Vu Thang, Sichuan to expressway, Hechuan, Chongqing on National Highway 2 12. The total length of the bridge is 1433m, the main bridge is (130230 130)m single-box single-cell prestressed concrete continuous rigid frame, and the substructure is 16 24m long Ф 232; . The structural size of a single pile cap is 18.7mx 10.2mx5m, and the concrete volume of a single pile cap is 953.7m3, which is completed at one time.
2. Introduction
2. 1, the main reason of temperature stress:
2. 1. 1. During the hardening process of mass concrete, a large amount of heat is released after cement hydration, which makes the temperature in the central area of concrete rise, while the temperature on the surface and boundary of concrete decreases due to the influence of air temperature, thus forming a large temperature difference on the cross section, causing compressive stress inside the concrete and tensile stress on the surface (called internal constraint stress).
2. 1.2. When the hydration heat of concrete reaches the highest temperature in 3 ~ 7 days, it gradually cools down due to heat dissipation, and the shrinkage is intensified due to water loss. This shrinkage is restrained by bedrock, resulting in tensile stress (called external restraint stress).
2.2. Temperature stress distribution of concrete in pile caps;
To sum up, before the construction of mass concrete of pile caps, it is necessary to estimate the temperature change and stress change of concrete to determine the maintenance measures, layered thickness, pouring temperature and other construction measures, so as to guide the construction.
Construction Calculation of Cracking Control for Mass Concrete of 3C30 Bearing Platform
3. 1, related information:
3. 1. 1, mix ratio
Cement, fly ash, sand, gravel, water, NNO-II water reducer.
369:50:677: 1 148: 176:3.66
1:0. 136: 1.835:3. 1 1 1:0.48: 1%
3. 1.2, material:
Cement: Tenghui F.032.5 cement.
Gravel: Caojie continuously graded gravel (5~3 1.5mm).
Mixed medium sand: 40% machine-made sand and 60% canal fine sand.
Fly ash: Grade II fly ash of Huaneng Power Plant.
Admixture: Dahua NNO-Ⅱ retarding water reducer.
3. 1.3, meteorological data
The relative humidity is 80-82%; The annual average temperature is 17.5~ 17.6℃, the highest temperature is 40.5℃, and the average temperature is 20℃ in the summer heat period (May to September).
3. 1.4, automatic dosing machine feeding, loader feeding, centralized mixing in the mixing station, concrete delivery pump conveying concrete into the mold.
3.2. Maximum hydration heat temperature of concrete and adiabatic temperature of hydration heat in 3d and 7d.
c = 369kg/m3； Fly ash 32.5 cement: hydration heat Q7d=257J/kg, Q28d=222J/kg (data provided by Tenghui Cement Factory); c = 0.96j/kg . k； ρ= 2400 kg/m3.
3.2. 1, adiabatic temperature rise of the highest hydration heat of concrete
tmax = CQ/cρ=(366 * 257)/(0.96 * 2400)= 40.83℃
3.2.2 Three-dimensional adiabatic temperature rise
t(3)= 40.83 *( 1-e-0.3 * 3)= 24.23℃
δT(3)= 24.23-0 = 24.23℃
3. 2. 3 7d adiabatic temperature rise
t(7)= 40.83 *( 1-e-0.3 * 7)= 35.83℃
δT(7)= 35.83-24.23 = 1 1.6℃
(4) adiabatic temperature rise 4) 15d
t( 15)= 40.83 *( 1-e-0.3 * 15)= 40.38℃
T( 15)=40.38-35.83=4.55℃
3.3. Calculation of shrinkage deformation value of concrete at different ages
εy(t)=εy0( 1-e-0.0 1t)* m 1 * M2 *……* m 10
Look-up table: M 1= 1. 10, M2= 1.0, M3= 1.0, M4= 1.2 1, M5 =
Yes: m1mm 23m 4m 5m 7m 8m 9m10.
= 1. 10* 1.0* 1.0* 1.2 1* 1.2*0.7* 1.4* 1.0*0.895= 1.40 1
3.3. 1 and 3d shrinkage deformation value
εy(3)=εy0*( 1-e-0..03)* 1.40 1*M6
=3.24* 10-4*( 1-e-0..03)* 1.40 1* 1.09=0. 146* 10-4
3.3.2, 7d shrinkage deformation value
εy(7)=εy0*( 1-e-0..07)* 1.40 1*M6
=3.24* 10-4*( 1-e-0..07)* 1.40 1* 1.0=0.307* 10-4
3. 3. 3 15d shrinkage deformation value
εy( 15)=εy0 *( 1-e-0. 15)* 1.40 1 * M6
=3.24* 10-4*( 1-e-0.. 15)* 1.40 1*0.93=0.588* 10-4
3.4, concrete shrinkage deformation is converted into equivalent temperature difference.
3.4. 1、3d
t(y)(3)=-εy(3)/α=(-0. 146 * 10-4)/( 1.0 * 10-5)=- 1.46℃
3.4.2、7d
t(y)(7)=-εy(7)/α=(-0.307 * 10-4)/( 1.0 * 10-5)=-3.07℃
15d
t(y)( 15)=-εy( 15)/α=(-0.588 * 10-4)/( 1.0 * 10-5)=-5.88℃
3.5. Calculation of concrete modulus at different ages E (t) = EC * (1-E-0...09 tons)
3.5. 1, 3d age
e(3)= 3.0 * 104 *( 1-e-0..09*3)
= 7. 1 * 103 N/mm2
Day 7 age
e(7)= 3.0 * 104 *( 1-e-0..09*7)
= 1.40 * 104 Newton/mm2
3.5.3, 15d age
e( 15)= 3.0 * 104 *( 1-e-0..09* 15)
= 2.22 * 104 Newton/mm2
3.6, concrete temperature shrinkage stress calculation
Concrete strength conversion f(n)=f(28)*lgn/lg28, concrete tensile strength ft=0.23*f2/3cu for C30 concrete f(28)= 15N/mm2.
3d age: f (3) = f (28) * lg3/lg28 =15 * lg3/lg28 = 8.76n/mm2.
Ft = 0.23f2/3 (3) = 0.23 * 4.952/3 = 0.668 N/mm2.
7d age: f (7) = f (28) * lg7/lg28 =15 * lg7/lg28 = 8.76n/mm2.
FT = 0.23F2/3 (7) = 0.23 * 8.762/3 = 0.98N/mm2.
Due to the high temperature when pouring the concrete of the bearing platform in July, it is assumed that the mold entry temperature To=30℃ and Th=25℃.
3.6. 1, 3d age: H(t)=0.57, R=0.35, V=0. 15.
δT = To2/3T(T)Ty(T)-Th = 302/3 * 24.23 1.46-25 = 22.6 1℃
σ=-(7. 1* 103* 10* 10-6*22.6 1*0.57*0.35)/( 1-0. 15)
= 0.377 Newton/mm2