Shrinkage Development in High Performance Concrete. Ammar Yahia, P.Eng., Ph.D.,

Transcription

1 Shrinkage Development in High Performance Concrete Ammar Yahia, P.Eng., Ph.D.,

2 Outlines 1. Introduction 2. High-performance concrete 3. Autogenous shrinkage 4. Autogenous shrinkage stress 5. Autogenous Shrinkage measurements 6. Perspectives Questions & discussions

3 Introduction The middle east is currently one of the greatest construction location in the word Trend in concrete technology towards low w/cm - HPC. Increased tendency to undergo early-age cracking - may or may not compromise the (higher) f c, it likely does compromise their long-term performance.

4 Structural & Economical benefits HPC become a crucial element of the viability of tall building in the region High strength to reduce the size of sections Stiffness provided by high modulus limiting movements Durability of HPC - achieving service life requirements

5 Durability requirements

6 Severe environment Confederation Bridge - Canada w/cm = 0.27, 90 MPa, Ternary binder 100 years service life!! Ternary binder, fly ash + silica fume

7 Durability improvement Durability improvement of HPC is achieved by eliminating the transport mechanisms into the concrete Modification of the mixture proportions Higher cement content Supplementary cementitious materials (GGBS, SF, FA) Lower w/cm and extensive use of superplasticizer

8 Consequences Due to the mixture proportion changes HPC develop higher early-age shrinkage cracking that can reduce the service life of concrete structures

9 Moisture-related shrinkage Plastic shrinkage Drying shrinkage loss of moisture Carbonation shrinkage carbonation effect Chemical shrinkage Autogenous shrinkage Self-desiccation shrinkage

10 Le Chatelier's Experiment When cement paste hardening: In air underwater Level water V Before After Before After Apparent volume shrinks Apparent volume swells Cement hydration creates a certain porosity

11 Jensen and Hansen's Model W/C = 0.42 (0,1) Relative volume Capillary water Anhydrous cement Water gel Cement gel (1,1) Pores About 8% α Degree of hydration (0,0) (1,0)

12 Jensen and Hansen's Model W/C = 0.42 (0,1) External water (1,1) Relative volume Capillary water Anhydrous cement Water gel Cement gel α Degree of hydration (0,0) (1,0)

13 Normal concrete vs. HPC The shrinkage behavior of HPC is quite different from the shrinkage of normal concrete (NC) NC: drying shrinkage is the main component HPC: autogensous shrinkage is the main component This difference can be explained in the size of the pore and capillary network

14 HPC critical characteristics W/C equal or smaller than 0.36 Missing water to fully hydrate cement particles Elastic modulus depends on elastic modulus of the coarse aggregate Very sensitive to autogenous shrinkage

15 Autogenous shrinkage Autogenous shrinkage = macro-volume reduction observed after the initial set. It is induced by the selfdesiccation that occurs during hydration under sealed or partially saturated conditions As the cementitious materials hydrates under sealed conditions, empty porosity is created within the set microstructure hydration product occupy less volume

16 Autogenous shrinkage The creation of empty capillary pore spaces has two major effects on the evolving cement paste system: The chemical shrinkage results in a reduction in the system internal relative humidity (quantified by Gauss and Tucker in 1940) 70% for w/c < 0.30 Based on the Kelvin-Laplace equation, this reduced RH will induce pressure σ cappilary in the pore water

17 Autogenous shrinkage The magnitude of these stresses is influenced by both the surface tension of the pore solution and the meniscus radius of the largest water-filled pore water within the microstructure

18 Kelvin-Laplace Equation σ cap 2γ = = r RH Ln RT 100 Vm RH = relative humidity expressed as percentage γ = surface tension r = radius of the largest water-filled pore, R = universal gas constant (8.314 J/(mol.K)) T = absolute temperature Vm = molar volume of pore solution What will happen if RH decreases from 95% to 70%?

19 Kelvin-Laplace Equation σ cap 2γ = = r RH Ln RT 100 Vm S cap 1 1 ε = σ ( ) 3 K Ks S = degree of saturation K = bulk modulus of elasticity (porous materials) Ks = bulk modulus of solid framework within porous materials

20 Autogenous shrinkage The second effect of the creation of empty capillary pores is a change in the hydration kinetics of the cement paste cement hydration proceed by dissolution/precipitation mechanisms The empty pore space created is no longer available to be filled with hydration product slow down

21 Measurements

22 Water Curing (Aticin, 1998)

23 Effect of Binder Type Ternary Binder 2 Concrete prisms Humidity loss (%) Ternary Binder 1 Ternary Binder 3 w/c = Time (hours) Yahia and Khayat (not published)

24 Effect of Binder Type Time (hours) Ternary binder 2 Ternary binder 3 Concrete prisms w/cm = 0.35 Micro-Strains Ternary Binder Yahia and Khayat (not published)

25 External Water Curing Does not penetrate very far into low water/binder concretes - HPC It should be done very early to reduce autogenous shrinkage But maximum efficiency is 1 to 2 days when capillary pores are still interconnected. Otherwise, external curing is no more efficient!!

26 The issue Shrinkage is a weakness of concrete when it induce more or less severe cracking, because cracking can reduce concrete service life and durability Any micro- and/or macro-cracks network that is developing in concrete due to shrinkage offers pathways to aggressive agent - will attack steel reinforcing bars and result in a weakening of the structure

27 Perspectives Use internal water sources to reduce self-desiccation - Saturated lightweight aggregates (20% - 25%) (Teaming-up with colleagues from Canada) - Chemical reducing admixtures - Super-absorbent polymers (SAP)

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