@phdthesis{oai:ynu.repo.nii.ac.jp:00005594,
 author = {Huynh, Phuong Nam},
 month = {Sep},
 note = {The durability of concrete is one of the most concerns of researchers andengineers. Early ages are the time concrete suffers from many factors those inducecracking in the concrete. Microcracking in concrete adversely affects macroscopic tensilestrength of concrete that will leads to the severe cracking in later ages. Especially with theaddition of mineral additives such as ground granulated blast furnace slag or fly ash,concrete is susceptible to microcracking particularly under temperature variation. Thesemicrocracks must be one of the causes of severe macrocracks observed in real structuresusing slag concrete.Not only microcracking, chloride attack is one of the main factors lowering thedurability of concrete structures, particularly for the structures in very cold region, wheredeicing agent is frequently used in winter. Deicing agent can cause severe deterioration ofconcrete structures such as scaling, corrosion of steels, and ASR.This study deals with improvement of cracking and chloride penetration resistanceof slag and fly ash concrete by using a new type of cement named high alite cement (HAC). HAC is newly developed Portland cement with very high alite (3CaO.SiO2) content andalmost no belite (2CaO.SiO2).Due to its sensitivity and the rich information it yields from collected parameters,the acoustic emission (AE) technique has been widely used to detect cracking in hardenedconcrete. Nevertheless, AE measurement at a very early age is difficult because AE sensorscannot be directly attached on the surface of unhardened concrete. Therefore, a waveguideembedded inside concrete has been employed. Unfortunately, attenuation of acousticwaves in high moisture content ambient led to the diminished effectiveness of thewaveguide in detecting AE signals in concrete with high water to binder ratio (W/B). One part of the research is to solve the problem of attenuation. An addition of twoperpendicular bars on main rod of the waveguide was proposed. Due to these wings, thedistance from cracking points in concrete to waveguide was significantly reduced. It wasproved that the redesigned waveguide worked more effectively than the previous one,especially in concrete with high W/B. By using this redesigned waveguide, microcracking was intensively investigatedin slag concrete with W/B of 0.3 and 0.5 subjected to temperature history simulating steamcuring. In all types of concrete, the replacement ratio of slag for cement was 50%. Becausethe coefficient of thermal expansion (CTE) of materials is one of the important parametersaffecting microcracking in concrete, two types of coarse aggregate with the samemaximum particle size of 19 mm but remarkably different CTEs, namely limestone andandesite were used in the study.The test results showed that net shrinkage of HAC mortar with W/B of 0.3 wasmuch larger than that of OPC mortar because of its larger autogenous shrinkage. Normally,larger shrinkage of mortar results in more extensive cracking in concrete. However, thenumber and the degree of microcracks in HAC slag concrete with W/B of 0.3 were smallerthan those in OPC slag concrete. This means that HAC slag concrete with W/B of 0.3obviously achieved larger resistance against microcrack than OPC slag concrete. On theother hand, net shrinkage of HAC mortar with W/B of 0.5 was a bit smaller than that ofOPC mortar due to its smaller thermal contraction. Therefore, microcracking in HAC slagconcrete with W/B of 0.5 was also smaller than that in OPC slag concrete.The interesting characteristic of HAC explored in the research is that it couldimprove resistance against microcracking in slag concrete. AE data revealed HACconcretes could disperse tensile stress, leading to the formation of many small cracks ratherthan concentrating tensile stress to create a severe crack. Evenly distributed calciumhydroxide (CH) crystals acting as a kind of buffer that prevents the propagation ofmicrocracks in HAC slag concrete might be one of the reasons. Another reason for the highcracking resistance of HAC slag concrete was the strong bond between mortar and coarseaggregate. This high bond strength was more clearly observed in concrete with low W/Bthan in concrete with high W/B. The high bond strength of HAC slag concrete was verifiedthrough tensile strength test, AE test, visual observation, and scanning electron microscopeimages. The strong bond in HAC slag concretes might be due to the formation ofsecondary CSH gel from the reaction of CH and active SiO2 in slag at pores near theinterface transition zone.The effects of fly ash on the microcracking resistance of concrete subjected totemperature variation at very young ages were also investigated by AE, physical and mechanical tests. Fly ash was used as a cementitious material as well as fine aggregate. Inaddition, a combination of fly ash and HAC, which was effective in improving resistanceagainst microcracking in slag concrete, was studied.Concrete containing fly ash was not so weak against elevated temperature.Nevertheless, fly ash reduced tensile strength of concrete remarkably. The reason is that flyash is not so active in early ages even under steam curing.It is found that HAC can significantly improve tensile strength of concretecontaining fly ash due to high hydration rate of HAC. However, high bond strength in HAC fly ash concrete was not observed from the direct tensile test as in HAC slag concrete.Further research is necessary to clarify the cracking resistance of fly ash concrete.Another important part of the research is to show the effectiveness of HAC toimprove the resistance against chloride ingress of concrete containing additives by using HAC. The effectiveness was analyzed by Surface Water Absorption Test (SWAT) andwater/chloride penetration depth test. Four types of binder were used: OPC, HAC, slag,and fly ash. Concrete with OPC only was the control mix. In other mixes, the cementswere replaced by slag or fly ash with the replacement ratios of 40% or 15% by mass,respectively. Coarse aggregate of andesite with maximum particle size of 19 mm was used.Air content of concrete was controlled at 6 ? 0.5% to prevent concrete from scaling underfreeze/thaw cycles. In order to obtain a wide range of concrete quality, three W/B of 0.4,0.5, and 0.6 and five curing conditions covering from very good to very poor conditionwere applied. Three-binder concretes containing slag and fly ash with W/B of 0.4 werealso prepared. To investigate the effect of bond on mass transfer in concrete, someconcretes made with limestone were added in the cases of concrete with W/B of 0.4 and0.5.Experimental results presented that there were good correlations between waterabsorption rate at 10 minutes (p600) and penetration depth of water and chloride ions in allkinds of concrete. It means that SWAT can be applied to evaluate the resistance againstmass transfer into concrete. The same linear relationship was observed between the waterpenetration depth and p600 regardless of the type of binder. It means that p600 is related to akind of governing characteristic regarding the microstructure of concrete, which should be studied further. The effects of curing conditions were fully reflected in SWAT results. Iflinear lines expressing the correlations between SWAT index and parameters related todurability of concrete such as penetration depth of water or chloride ion, carbonation depth,etc. are established, SWAT index can be used as an indicator of durability of concretestructures. This is very significant because SWAT is a simple, automatic, and rapid methodand it is easy to be applied for concrete structures in actual sites.It is found that slag or fly ash could improve the resistance against chlorideingress of concrete remarkably due to the chloride binding ability of C3A componentexisting in slag and fly ash. However, OPC concretes containing additives were much moresensitive to curing conditions than concretes with OPC only.Owing to the high hydration rate of C3S, HAC slag/fly ash concrete was lesssensitive to curing conditions than OPC slag/fly ash. Thus, HAC should be utilized withslag or fly ash in concrete structures subjected to severe conditions.},
 school = {横浜国立大学},
 title = {Improvement of cracking and chloride penetration resistance of slag concrete by utilizing high alite cement},
 year = {2014}
}