Controlling Technology for Concrete Cracking

1. Controlling Technology for Concrete Cracking Changwen Miao Southeast university Jiangsu Research Institute of Building Science State Key Laboratory of High Performance Civil Engineering Materials July, 2012

2. Outlines • Harmfulness of concrete cracking • Main reasons for concrete cracking • New technologies for concrete cracking controlling

3. Harmfulness of concrete cracking

4. Cracking is still a common problem of concrete Concrete is the cornerstone for civil engineering, hydraulic and construction projects Concrete cracking is a common problem in civil construction projects Affect the use of safety ! Shorten the service life ! Huge economic loss !

5. Concrete cracking declines structural capacity • Changing the force condition of the concrete structure, leading to local and even the overall failure of buildings • Weakening the stiffness of the concrete buildings with the dynamic changes of environment and loads • Reducing the structural seismic capacity, threatening the overall stability and safety of concrete structures

6. Concrete cracking deteriorates structural durability • Stage Ⅰ—Reducing the effective thickness of protective layer • Stage Ⅱ—Accelerating the transmission of environmental aggressive media, air and moisture within the concrete structure • Stage Ⅲ —Shortening activation and corrosion time of reinforcement, reducing the service life of concrete structures Stage Ⅰ Stage Ⅱ Stage Ⅲ Concrete cracking Aggressive media, air, moisture intrusion Reinforced steel bar initial corroding Reinforced steel bar volume expansion

7. Main reasons for concrete cracking

8. The causes of cracks • Load-induced cracks ( structural cracks, about 5%-10%) Cracks induced by direct stress of external loads (static , dynamic load) , and Structural secondary stress • Deformation-induced cracks ( non-structural cracks, 80% or more) • Coupling ( deformation and load ) effect-induced cracks (5 % to 10% ) • Cracks induced by alkali-aggregate reaction, freezing and thawing, uneven expansion, bad soundness and so on • Plastic shrinkage • Self-desiccation shrinkage • Drying shrinkage • Temperature shrinkage • Carbonation shrinkage Humidity changes Temperature changes

9. Settlement cracks Shrinkage (drying shrinkage, autogenous shrinkage) cracks plastic cracks Shear cracks Temperature cracks Corrosion cracks Categories of concrete cracking

10. Shrinkage and cracking of concrete in plastic stage • The plastic phase can be divided into two stages Stage 1 Stage 2 No internal stress generated Internal pore negative pressure rising Capillary negative pressure Time Shrinkage stress (pore negative pressure) generatedMainly in the forms of horizontal plastic deformation and settlement No shrinkage stress inside the paste (saturated), mainly in the forms of bleeding and plastic settlement

11. Shrinkage and cracking of concrete in plastic stage • Settlement and bleeding crack bleeding 钢筋 Aggregate water • Micro-crack formation caused by internal bleeding. • Cracks formation due to constraint of subsidence by reinforcement.

12. Shrinkage and cracking of concrete in plastic stage • Mechanics of plastic shrinkage and cracking The shrinkage driving force is greater than the tensile strength between particles Bleeding rate≧Evaporation rate Bleeding rate < Evaporation rate

13. Shrinkage and cracking of concrete in plastic stage • Drying shrinkage cracking—Evaluation methods Ring method—double ring • Can test out the expansion and shrinkage stresses • Suitable for evaluation of expansive concrete • (Patent Application No. : 201010100449.7 )

14. Autogenous shrinkage (Apparent volume decreases) Chemical shrinkage Pore Cement + Water Volume Hydration products Before hydration After hydration Autogenous shrinkage of concrete • Autogenous shrinkage-mechanics Root reason: the total volume decreases during cement hydration process (chemical shrinkage) . • Chemical shrinkage is about: • 6.4×10-2 mL/g； • Autogenous shrinkage is one of the manifestations of chemical shrinkage, chemical shrinkage equals to the sum of autogenous shrinkage and pore volume formed

15. Autogenous shrinkage-mechanics Autogenous shrinkage of concrete Direct reason: Initial state Before structure formation After structure formation After structure formation, further hydration cause to the meniscus generation inside the paste and the shrinkage stress

16. Autogenous shrinkage-testing methods Autogenous shrinkage of concrete Autogenous shrinkage (apparent volume decreases） • Solving the defect that test ends of pipe debond with internal concrete in the vertical length measurement; • Solving the interference of the probe to the early test results by using non-contact sensor technology; • Realizing the staged and whole process testing since casting and molding, improving data reliability and continuity. Concrete Cement paste Corrugate pipe testing method

17. Autogenous shrinkage-testing methods Autogenous shrinkage of concrete Pore negative pressure testing method Early shrinkage driving force test since self-drying zero Physical and mechanical characteristic parameters time • Application of semi permeable membrane characteristics of the water-saturated porous ceramic probe • Realizing the characterization of initial structure formation and self- drying 0:00 • Solving the leak problem of traditional testing method, with test range upgrading 1 times • Overcoming the international problem that traditional method is difficult to test the shrinkage driving force in the humidity stage self-drying “time-zero” • Corresponding to the initial structure formation; • Corresponding to the starting point of autogenous shrinkage

18. Temperature deformation and cracking of concrete • Temperature deformation—mechanics 1.Material inherent properties Causing expansion Temperature rising 2.Capillary pore stress relaxation Additional expansion 3.Liquid phase migration Delayed shrinkage （ignored usually） Thermal expansion deformation properties is significantly affected by humidity

19. thickness Temperature distribution Adiabatic temperature rise Compression zone Stress distribution Tension zone Temperature deformation and cracking of concrete • Temperature deformation and cracking Cracking reason-Thermal stress caused by temperature difference between inside and outside Center temperature rise increases with the cross-sectional dimensions of the structure Surface tension, internal compression

20. Temperature deformation and cracking of concrete • Tempreature deformation cracking Crack criterion ℃ temperature T1 T2 Tl Ta Cracking when tensile stress caused by temperature deformation and creep is greater than the tensile strength of concrete elel e % t e3 e4 strain creep e1pl e2pl t1 t2 t3 t t s time stress t Temperature rises stage Temperature decreases stage Strength curve compression tension destroy

21. Maximum temperature Tmax T The first zero stress temperature TZ,1 t Temperature rises stage Constant temperature stage Rapid cooling stage cracking temperature Tc s σc,max t Cracking stress sc Specimen stress (or center temperature) changes with age Temperature deformation and cracking of concrete • Temperature deformation cracking-Evaluationmethod Temperature-stress testing machine • Crack resistance parameters • Temperature rising time, Stress occurring time, • The first zero stress temperature TZ,1 , • Temperature peak occurring time, • Maximum temperatureTmax, • The second zero stress temperatureTZ,2, • Cracking temperature Tc , • Maximum compressive stress σc,max

22. Free-form deformation creep Elastic stress（Hooker Theorem） 12 12 strain（me） Relaxation stress stress（MPa） 8 8 Constraint + creep (cumulative effect) Threshold Deformation recovery 4 4 strength 0 Measured stress(after relaxation) 0 0 age (d) 0 0 0 0 7 7 14 14 21 21 28 28 age (d) Temperature deformation and cracking of concrete • Temperature deformation cracking-Evaluation method Temperature-stress testing machine Through controlling the total strain of the constrained specimen at 0, and combining with the reference specimen, functions describing parameters such as restraint stress, elastic modulus, creep coefficient and so on changing with time could be obtained.

23. cement Modern concrete water Optimizing ratio and processes fine aggregate coarse aggregate • Composition characteristics • complex component • low w/c ratio • more concent of cementitious materials • Performance characteristics • large flowability • excellent mechanical properties • good durability Mineral admixture Chemical admixtures Modern concrete cracking reasons are more complex Conventional concrete

24. Modern concrete cracking reasons are more complex • Impact of cement composition and admixture on the cracking resistance The greater the reduction value of cracking temperature, the better the cracking resistance

25. Impact of superplasticizer Modern concrete cracking reasons are more complex Traditional naphthalene ( condensation polymer ) calcium lignosulfonate blank blank shrinking percentage (10-6) shrinking percentage (10-6) Time / d Time / d • Lower w/c and cement consumption, improving mobility ; • Increasing concrete shrinkage in the same w/c, dry shrinkage at 60d increased by 20%-40%

26. Impact of W/C Autogenous shrinkage (10-6) Time / d Modern concrete cracking reasons are more complex Compared to specimen with w/c 0.6:The autogenous shrinkage of specimens with w/c 0.5,0.45,0.4,0.35,0.3 and 0.25 at 1year increased by 175%, 250%, 275%, 335%,495% and 505%, respectively

27. Modern concrete cracking reasons are more complex • The concrete strength grade is gradually increasing, while tension and compression ratio is gradually decreasing • C30 concrete: Tension/compression, about 1/10-1/12 • C50 concrete: Tension/compression, about 1/16 Brittleness increases, lead to a higher cracking risk

28. New technologies for concrete cracking controlling

29. General train of thought Inhibit water evaporation Shrinkage compensation by expansion Environmental temperature water evaporation Cementitious materials hydration Structure regulation and control Temperature changes Plastic shrinkage Autogenous shrinkage Drying shrinkage crack resistance by fiber Hydration heat Hydration heat regulation In-situ toughening Capillary negative pressure growth Temperature shrinkage Strength, toughness, creep Evaluation methodology Chemical shrinkage Driving force Force resistance Cracking resistance

30. Plastic stage   Hardening stage Hydration degree Curing technologies in early age Initial setting, wiping the surface High performance curing materials with hydrophobic structure Monolayers with high evaporation resistance ability

31. Curing technologies in early age—water evaporation inhibition • Mechanism air water water evaporation controllable structure with hydrophilic main chain and hydrophobic side chains Monolayers Inhibition evaporation by 75% Water evaporation / g bleeding Concrete Time / min Inhibition evaporation by 75%

32. Curing technologies in early age—New conservation materials • Mechanism Water evaporation concrete concrete Particle aggregation Dense membrane with high evaporation resistance ability Membrane formation concrete concrete

33. 3 1 2 4 Curing technologies in early age—water evaporation inhibition Mortar mix proportion Reference Monolayers Monolayers can effectively suppress the plastic cracking risk

34. Curing technologies in early age—water evaporation inhibition Impact of monolayers on pore negative pressure Monolayers can effectively delay the appearing time of pore negative pressure inflection point of the surface mortar

35. Curing technologies in early age—water evaporation inhibition Impact of monolayers on plastic shrinkage The monolayers can reduce about half of the plastic shrinkage in the horizontal direction

36. Curing technologies in early age—water evaporation inhibition • Engineering applications Dingxin Airport in Gansu Province ( the largest in Asia) Xigaze Airport in Thibet Suitable for terrible drying area, it could dissolve the problem of crack and crust on plastic concrete.

37. Curing technologies in early age—New conservation materials • Effect No curing Pore negative pressure (kPa) Curing materials Non-curing crack Curing materials Time / h Delaying the time when capillary negative pressure begin to increase and reduce crack risk.

38. Curing technologies in early age—New conservation materials Taizhou bridge across yangzi river The second double of the Lanxin Railway Suitable for drying and high temperature condition, it could dissolve the problem of drying crack of hardening concrete at early stage.

39. Chemical techniques for shrinkage-reducing • General ideas Water Effect of the reduction of surface tension on additional pressure on the curved surface Water The surface area of the surface tension in the pore solution was significantly reduced, which can effectively reduce autogenous shrinkage and drying shrinkage of concrete. Schematic diagram of evaporation of capillary moisture

40. Chemical techniques for shrinkage-reducing • Traditional low molecular shrinkage reducing agent The effect of SRA on Mechanical Properties of Concrete (same amout of water) contradiction Reduced strength Reduced shrinkage The effect of SRA with different dosages The structure-activity relationship study found that the traditional low molecular shrinkage reducing agent can not fundamentally solve the problem of declining strength of concrete.

41. Chemical techniques for shrinkage-reducing • High dispersed comb copolymer shrinkage reducing agent • The molecular tailoring technology was adopted to make the alkyl polyether with shrinkage reducing function and steric effect graft to the main chain of copolymer, then the structure-activity relationship between molecular structure and shrinkage performance of the grafted copolymer/cement/water composite system was studied, as a result, a new type amphiphilic and high dispersion comb copolymer class concrete shrinkage reducing agent has been invented, which realized the unity of shrinkage reducing and water reducing and dispersion. Short side chain (shrinkage reducing group)-shrinkage reducing, dispersion Side chain Long side chain Long polyether side chain – steric effect Adsorption behavior regulation - dispersion

42. Chemical techniques for shrinkage-reducing • High dispersed comb copolymer shrinkage reducing agent • Impact on shrinkage at early ages (a) condensed shrinkage(b) autogenous shrinkage before 1d 43% lower than the naphthalene series 53% lower than the naphthalene series

43. Chemical techniques for shrinkage-reducing • High dispersed comb copolymer shrinkage reducing agent • Impact on shrinkage in the mid- or late period (a) drying shrinkage (b) autogenous shrinkage 53% lower than the naphthalene series at 28d 42% lower than the naphthalene series at 28d

44. Chemical techniques for shrinkage-reducing • High dispersed comb copolymer shrinkage reducing agent • Impact on plastic cracking Tablet plastic cracking experimental results The cracking area is 13% of that of ​naphthalene series, with crack width only 0.27mm.

45. Chemical techniques for shrinkage-reducing • High dispersed comb copolymer shrinkage reducing agent • Impact on shrinkage in the mid- or late period The ring cracking test results Crack width reduced more than 45% compared with the naphthalene series Realizing a unified effect of water reducing and shrinkage reducing at a lower dosage, effectively reducing the plastic shrinkage, early and late autogenous shrinkage and drying shrinkage .

46. Side chain (SRA) Long side chain _ _ _ _ _ Chemical techniques for shrinkage-reducing • Shrinkage-reducingtype polycarboxylate superplasticizer Lower content，unified function between dispersion and reduced shrinkage shrinkage reducing dispersion 53% lower than the naphthalene 53% lower than the naphthalene after 28 days (a) Autogenous shrinkage before 1st day(b) Autogenous shrinkage

47. Chemical reduction techniques • Engineering applications • Applications of shrinkage-reducing type polycarboxylate superplasticizer • Shrinkage deformation and the maximum temperature rise are controlled within a reasonable range • The main structures were not cracked and leaking Model road tunnel Suzhou Dushu Lake Tunnel Gongboxia Hydropower Station • Applications of shrinkage reducing admixtures(SRA) • CFRD concrete , the effects of reduced cracking is significant Wuxi Lihu Tunnel

48. Shrinkage-compensating technology by expansion • Thought Controlled and regulated expansive history Composition characteristics:low W/B，low porosity Large expansive performance, low dehydration shrinkage Shrinkage cracking characteristics: shrinkagecaused by autogenous, dry and thermal factor Compensating autogenous and thermal shrinkage , and reducing dry shrinkage Mutiple complex between Caand Mg Stable hydration products Improve concrete anti-cracking capacity Characteristics of the environment: temperature and humidity history Small water requirement Early expansion CaO by light burning MgO with high activity Medium-term expansion MgO with high activity Later expansion

49. Shrinkage-compensating technology by expansion • Deformation performance Waterproof curing Water curing • Expansion rate can be controlled • Autogenous shrinkage can be inhibit effectively • Dry shrinkage can be significantly reduced so that stable period can be in advance. In the standard dry condition without curing, dry shrinkage rate for C50 with the content of 8% was only 45% of the reference concrete after 120 days. Dry curing

50. Shrinkage-compensating technology by expansion • Crack resistance performance---Ring method Cracking time and initial crack width C30 （a）reference （b）with expansive agent C50（a）reference （b）with expansive agent