Contents What is durability of concrete Why study it What is the situation now What to study How to enhance durability
Durability It can meet high security, function and appearance requirement in the estimated period as designed without much additional maintenance, taken natural environment engineering environment and structural internal factors into consideration. Global topic and focus in civil engineering.
Necessary of Research Infrastructure constructions mostly use concrete Capital construction in three phases Phase I new construction Phase II new construction and maintenance Phase III maintenance Most developed countries in Phase III It is reported that we are in Phase II. Durability of concrete structure is more serious than security problem. Durability study is very important for new structure and maintenance.
What is the international situation The US It is reported that 130 billion dollars are needed for over 29% bridges, more than 1/3 roadsand and 2100 unstable dam, by American Civil Engineering Society in1998. There are one million bridges in the US, of which a quarter need maintenance by the end of the 20th Century. In the US, 10% of the total infrastructure construction capital goes to maintenance per year.
What is the international situation In Britain, it is reported that 1/3 bridges need maintenance. Britain lost 600 million pounds in 1996. Canada spends about 500 billion dollars in maintaining degradation destruction. Japan spends 40 billion yuan in structure maintenance every year.
Our Situation Our situation is what developed countries experienced decades ago. Concrete beam of Beilun Harbor in Ning Bo-11 years since bulit. Fig.1 Concrete beam of Beilun Harbor in Ning Bo-11 years since bulit
Fig.2 Pictures of degradation in foreign countries
Aspects of Durability Study Environment atmospheric environment sea environment soil environment industry environment Material corrosion of reinforcing bar alkali-aggregate reaction carbonation 、 fire damage 、 soundness hydrate － chemical corrosion 、 fire 、 dimensional stability pore structure － permeability 、 chlorine ion permeation 、 frost resistance
Aspects of Durability Study Model model of rupture due to corrosion expansion model of reducing cohesion model of bearing strength change Structure design evaluation
Corrosion of Reinforcing Bars Reinforcing bars in concrete with high alkali can form oxidized passive film, which can separate water and oxygen from metal to prevent corrosion. Reinforcing bars are corroded due to permeation of chlorine and carbonation which destroy passive film. Chlorine ions disperse from surface of concrete to surface of bars. When the concentration of chlorine ions is formulated to the critical state, the passive film destroyed. When carbonation of concrete is developed to the surface of bars, the passive film is destroyed. Corrosion caused by chlorine more serious than corrosion by carbonation.
Corrosion of reinforcing bars (Cl - ) Chlorine ions penetrate to the surface of reinforcing bars from the protective layer,destroy passive film, and change bars from passive state into active state. Fig.3 Destroyed passive film
Critical concentration of chlorine ions It refers to concentration which is related to solidity of concrete(w/c), moisture of concrete (saturated water), when reinforcing bars change from passive state. Resistance of concrete It is related with moisture and solidity. Supplement degree of oxygen and water It is related with concrete solidity and water percentage in concrete. Factors affecting corrosion of reinforcing bars
Tab.1 Relative humidity effecting on corrosion ratio of reinforcing bars (µm/a) Relative humidity Corrosion caused by carbonation Corrosion caused by chlorine ions 50 ％ 09 70 ％ 036 80 ％ 161 90 ％ 1298 95 ％ 50122 99 ％ 233
Methods based on model of degradation Report of RILEM-130 Committee Durability design of concrete structure 1996 DuraCrete Concrete Durability Design 2000 ACI Life-365 Computer Program 2000 Japanese Civil Engineering Society Standards 2002 So far,degradation model of carbonation and chlorine corrosion can be used for reference in constructions.
Alkali-aggregate reaction It refers to self chemical corrosion. Sand and gravel aggregate mixed with active mineral composites react with alkali. When meeting water, it forms a certain kind of gelled paste leading to volume expansion. Conditions of alkali-aggregate reaction ： potentially active aggregate enough content of alkali enough water
Destruction of freezing and thawing of concrete Capillary pore water in concrete expands after freezing, and produces strong pressure which causes failure, rupture and scaling. Saturation of water is formulated due to repeated freezing and thawing. When it reaches the critical saturation, concrete will be destroyed by freezing. The effective way to prevent freezing and thawing destruction is to add chemical air-entraining agent. Concrete can absorb enough water when froze with salty water and cement paste will produce osmotic pressure and water pressure, which will make concrete coating scaling or eroding with holes.
Chemical corrosion of concrete Sulfate mixing with Ca(OH) 2 and C 3 AH 6 in cement hydration produces calcium sulfate and ettringite. It produces volume expansion and rupture. The concentration of sulfate in capillary pore water increases continuously till it reaches excessive saturation with crystals, the pressure is so strong that concrete is destroyed. The resistance to acidity of silicate cement concrete is weak. If the touched water shows acid(PH<6.5), there will be problems.
Degradation of concrete and reinforcing bars all need water or as media. To prevent water, gas such as oxygen, carbon dioxide, harmful molecules such as salt, and acid and ions to concrete, the basic way is as follows: － to enhance self solidity and resistance to rupture － to increase the thickness of protective layer and delay harmful materials reaching reinforcing bars
Reasons of Insufficient Durability low durability standard of engineering design Improper construction arrangement Lack of regular detection and repair Low safety standard of component strength design Significant changes in cement properties, concrete curing and environment conditions over the past ten years
Tab.2 Comparison between China (in the 1980s)and other countries Japan sets higher standards (Min=C35,C45 for 100 years) Reinforced concrete ChinaThe USBritain The lowest strength grade C15C25 C30 （ C25 ） Protective layer in carbonation corrosion and exposed in the open air Plate 2cm Beam 3cm 3.8cm （ d<16 ） 5.1cm （ d>16 ） C35, 3.5cm in dry and wet conditions respectively C40 ， 4cm Serious freezing and thawing Air- entraining
Improper construction arrangement Poor curing can reduce permeability of concrete surface by times, and reduce limitation when reinforcing bars begin with corrosion. One-day and seven-day curing can reduced the corrosion limitation into one quarter of that of before. Rush construction will miss the required checking steps, which also makes steels leaving their original positions. If the coating width reduces to half of the original, it will cause the corrosion limitation into one quarter of that of before. The permitted 5-10mm coating width difference in the construction can bring out huge differences in the corrosion limitation by times. Various construction, seam and waterproof layer are the weakest point in durability. They can hardly be guaranteed in the quick construction.
Fig.4 Curing influence on W/C varied concrete to air permeability
Measures to enhance durability of concrete Basic protection to enhance solidity to increase protective thickness to use chemical admixture to use mineral admixture Adjunct measures protection of bar coating reinforcing corrosion-resisted bar electrochemical protection sacrificial anode protection with external electric current protection of concrete coating
Epoxy coating of reinforcing bar surface and corrosion-resisted reinforcing bar Reinforcing bar with epoxy coating Applied the most widely in construction One of sufficient measures to prevent corrosion Reinforcing corrosion-resisted bar Japanese alloyed steel reinforcing bar Stainless reinforcing bar in the US, Canada and Europe High cost New materials in recent years-- inert material: fibre- plastic bar Kinds: carbon fibre, poly fibre, glass fibre Excellent characteristics ： resistance to corrosion, light weight, high strength, no magnetic
Tab.3 Purposes and methods of international durability maintenance purposesdefinitionmethods to prevent erosion of media to reduce or prevent erosion of media injection surface coating supplementary cracks control of moisture to keep or modulate moisture of concrete in stipulated rage injecting hydrophilically surface coating repair of concrete to repair structure unit of old concrete to recover the primitive shape and function mortar 、 modified concrete clad material or coating reinforcing to increase or recover bearing strength of structure unit binding reinforcing bars or steel plate mixing cracks or pores physical and chemical resistance to increase resistance to erosion of physical and chemical media clad material or coating injection