Influence of Transverse Flexural Crack on Chloride Penetration in Concrete

The mechanism for chloride penetration in cracked concrete and its major impact factors were analyzed. As a result, a revised chloride diffusion model based on Fick's Law was built by dual porous medium model. Then several cracked reinforced concrete beams self-anchored with sustained flexural loads were immersed in the 5% NaCl solution with the condition of dry-wet cycles. After 15 times of dry-wet cycles, the rapid chloride testing (RCT) was used for the determination of chloride ion content of the powder at each cracked sections. The test results show that: 1) with the condition of dry-wet cycles, the chloride content will occur a peak in the surface 20mm concrete, so the depth for surface convection zone can be assumed to be 15~20 mm; 2) when the crack width is less than 0.3mm, the equivalent chloride diffusion coefficient increases steadily, which agrees well with model's prediction; when the crack width is larger than 0.3mm, the equivalent chloride diffusion coefficient augments rapidly and influence of convection on chloride penetration becomes more significant; 3) the deterioration factor for equivalent chloride diffusion coefficient in flexural cracked concrete is directly correlative with crack width, which can be expressed by second order power function or separate function.     

Experimental Study on Concrete Crack Repairing by Grouting through Embedded Pipe

Repairing concrete crack through embedded pipe is a new attempt in the field of structure repairing and a preparation for a kind of intelligent concrete with self-healing capability.In this paper,the properties of fluid-modified epoxy resin containing different thinners,ethylamine and plasticizer were tested and the effect of fluid-modified epoxy resin in repairing concrete crack through embedded pipe was compared with that of conventional grouting.     

Influence of Shrinkage Cracks on Chloride Penetration and Crabonation of Concrete

The influence of shrinkage crack on chloride ion penetration and carbonation of concrete is investigated. The concrete samples with shrinkage crack of 0.07 mm,0.1 mm,0.2 mm,0.4 mm and 0.62 mm are prepared, and both an accelerated carbonation test and Qingdao sea water corrosion test are conducted on sound concrete and cracked concretes. The depths of carbonation, the content of free chloride and total chloride in crack zone and perimeter zone of concrete are analyzed. The enperimental result shows that both free chloride and total chloride content in cracked concrete increase with the rise of the shrinkage crack width, and they are related in quadratic function. The chloride penetration velocity in crack zone is higher than that in perimeter zone of concrete. However, the increased specific surface in shrinkage crack tip improves chloride binding capacity and decreases the free chloride content in this zone. When the shrinkage crack is less than 0.07mm, the carbonation depth in crack zone is same as that of sound concrete. The chloride diffusion coefficient and the carbonation depth of cracked concrete increase limited as the shrinkage crack is less than 0.1mm, and then increase significantly with the increasing of shrinkage cracked width. Moreover, the carbonation depth in crack zone is increased by 3mm in comparison with that in perimeter zone of cracked concrete.     

Microcracking and Chloride Penetration of Concrete under Uniaxial Compression

A non-destructive method of evaluation of specific crack area was used to characterise microcracking in concrete prisms during uniaxial compression loading and unloading. Chloride profiles were also measured after the same concretes were exposed to chloride environment. The relation among microcracking, stress-strength ratio and chloride penetration of concrete were analysed based on the experimental results. Under compressive load, the apparent chloride diffusion coefficient decreases until about one third of the ultimate load. Further increase of compressive load improves the chloride diffusion coefficient again. There is a close link between microcracking and stress-strength ratio. The index of specific crack area can be used to indicate the microcracking and study chloride transport behaviour into concrete under the influence of microcracking.     

Chloride Transport in Splitting Cracked Concrete at Marine Tidal Zone

The chloride ion transport in concretes with different size of splitting cracks at marine tidal zone is investigated. The splitting crack widths of concrete are about 003 mm, 005 mm, 010 mm, 015 mm and 030 mm. The cracked concrete and sound concrete are placed at marine tidal zone for 30 d, and the chloride ion content in crack zone and perimeter zone of concrete, chloride ion penetration depth and steel bar corrosion area are quantitatively determined. It is shown that splitting crack will be recovered when the concrete is unloaded in split test. And the splitting crack width of concrete should be characterized by the value of displacement sensor when the concrete is unloaded in split test. The chloride ion content of cracked concrete decreases with increased depth, and then gets to remain steady from 10 mm to the inner. The relationship between chloride ion content and crack width in the steady zone can be regressed as the exponential function and the linear function for the crack zone and the perimeter zone of concrete, respectively. Due to away from the crack zone, the influence of splitting crack width on accelerated chloride ion ingression in the perimeter zone is less than that in the crack zone. When the splitting crack width is more than 005 mm, the chloride ion penetration depth in crack plane and in vertical crack plane of concrete is 50 mm and 20 mm, respectively. And the steel corrosion area enlarges with the increasing crack width. And the chloride diffusion coefficient of cracked concrete is closely related to the resistance capacity to chloride penetration of concrete, crack density (matrix width, crack width), and chloride binding capacity of concrete. When the crack density of concrete is less than 70, the chloride diffusion coefficient of cracked concrete increases with decreased crack density linearly. The chloride penetration will accelerate near the crack zone. And the raised splitting crack width will lead to its influence area in concrete increased quickly.     

A Modified Chloride Ion Diffusion Model in Concrete

Based on the analysis of influencing factors chloride ion diffusion in concrete and the simplified model induced by Fick's second law, a chloride ion diffusion model was established, in which , the influences of water-cement ratio, humidity, time, temperature, chloride ion binding capacity of concrete and freeze-thaw cycle are taken into consideration. The Validity of the new model is verified through the comparison of results between the proposed model and the existing analytic /numerical model. Compared with the analytic model, the proposed model can consider the seasonal change of humidity and temperature,and the time-variant effect of freeze-thaw cycle.Compared with the numerical model, the computational efficiency of the proposed model is higher.     

The Performance of Steel Corrosion of Damaged Concrete Structures with Different Repair Techniques

The reinforced concrete beams which have been bending damaged are repaired by the six kinds of repair materials and its related methods. Based on the linear polarization method, the corrosion situations of the steel bars in the repaired concrete structures were studied, which were immersed in 5% magnesium sulfate. The results show that: different repair materials and its related methods, significantly affect the steel corrosion performance of patched members. The repairing method, cement-based repair mortar with zinc wire connected with the bar, can most effectively improve anti-corrosion ability of reinforced concrete. There is an exponential relationship between the polarization resistance and corrosion current density, and the relationship formula between the polarization resistance and corrosion current density was put forward.     

Diffusion Behavior of Chloride Ions in Concrete Pipe Piles

Concrete pipe piles, whose internal surface is usually closed and external one is exposed to chloride environment, are widely used in marine environment. Based on Fick second law and the boundary condition of pipe pile, the analytical solution to chloride diffusion equation was deduced with the method of separation of variables. The solution comprises two parts: a stationary solution consisting of Bessel functions, and a transient solution. This is different from the traditional analytical solution that is based on semi-infinite boundary condition. Applying this result to a typical PHC pipe pile, the diffusion of chloride concentration was analyzed. Furthermore, the effects of the ratio of outer radius and inner radius, diffusion coefficient and protective concrete cover thickness on diffusion of chloride concentration were also discussed. When the ratio of outer radius and inner radius equals 1.5, the decay rate of secondary segment of transient solution is 4 times larger than that of the primary part. This example provides a computational basis for the corrosion of steel bar and some reference for durability design of pipe piles as well.     

Investigations on Effect of Solution Concentration and Temperature on Rate of Concrete Sulfate Attack

Through many tests and standard analysis,the effects of solution concentration and temperature on rate of concrete sulfate attack were investigated.The test results show that when solution concentration and temperature are lower than certain value,the speed of attack is accelerated with increasing concentration and temperature,however,higher temperature and concentration of solution may decrease the rate of attack.The speed of attack of sodium sulfate is faster than the one of attack of magnesium sulfate under various factors.It is suitable to adopt the coefficient of tensile as the accelerated assessing criterion for the attack of sodium sulfate.The coefficient of tensile and compressive strength should be considered together for the attack of magnesium sulfate.     

Elastic_Brittle Fracture Analysis of Micro_Crackin Concrete Loaded by Stretching Stresses

The concrete is a typical brittle material.The fracture and propagating failure of the micro-crack in loaded concrete are very important problem in civil engineering.In this paper,from the equilibrium differential equations and failure criterion,the elastic-brittle static fracture analysis of the plane stress micro-crack in concrete loaded by stretching stresses was done near crack line.The relations between the stretching stresses and the length of brittle fracture zone on the crack line were obtained.     

Service Life Prediction of Concrete Structures under Chloride Environment

The problems rendered from the conventional time marching, referring to stepwise time marching scheme (STMS) adopted in boundary element method (BEM) for chloride diffusion in concrete structures, were investigated, and a new time marching, referring to initial time marching scheme (ITMS) in BEM, was developed for evaluation of service life of the concrete structures under chloride environment. Results of the numerical examples show that the ITMS-BEM proposed can eliminate domain integral and simplify the computational model, so that the stability in iteration process can be improved, resulting in better efficiency and accuracy, compared with the STMS-BEM. It can also be concluded that the dimensions of the diffusion of chloride can affect the service life of the concrete structure significantly, which should be taken into account in structural design.     

Near Crack Line Elastoplastic Solution for an Eccentric Cracks Loaded by the Pore Water Pressure

By using near crack line analysis method and Mohr-Coulomb strength criterion,the elastic-plastic solution for near crack line of an eccentric cracks loaded by the pore water pressure under large scale yielding condition is analyzed.The elastic stress fields,the plastic stress field,the unit normal vector of the elastic-plastic boundary near the crack line,the elastic-plastic stress fields near crack line are identified.The change pattern of the length of the plastic zone and the pore water pressure is presented.Besides,the elastic-plastic analytical solution for an eccentric cracks loaded by the pore water pressure is obtained.     

Boundary Element Method for Time-dependent Chloride Diffusion and the Chloride Distribution in Concrete

The process of chloride diffusion in concrete is time-dependent.The boundary element method (BEM) with a time-dependent diffusion coefficient is presented for chloride diffusion in concrete based on the suitable transformation of variables.The fundamental solution of the partial differential equation for time-dependent chloride diffusion in concrete is developed,and the compensation length of the diffusion field is defined as well as the compensation coefficient.The scheme of BEM with a time-dependent diffusion coefficient is developed.Two examples are given to demonstrate the accuracy and efficiency of the presented method and the rationality and the importance of the compensation length for the method.     

Influence of Electrochemical Chloride Extraction on the Microstructure of Desalinated Pastes of Concrete

A set of installation is designed for electrochemical chloride extracting circularly on reinforced concrete.The removal efficiency is discussed using saturated Ca(OH)2 solution,saturated Ca(OH)2 0.001 mol/L Li2B4O7 solution and distilled water as electrolyte,respectively.And the microstructure of cement paste is analyzed by scanning electron microscopy(SEM) and energy dispersive X-ray spectroscopy(EDS).The results prove that using saturated Ca(OH)2 0.001 mol/L Li2B4O7 solution as electrolyte is more favorable to removing chloride in concrete.Treated by electrochemical desalination,the hydrated product C-S-H resolves and Ca(OH)2 in the concrete increases,too.It deposites on the wall of pores and fills in pores,and then causes the porosity to decrease,while the ratio of Ca /Si of C-S-H increases in the interface between the steel and the concrete.     

Study on Influence of Mineral Admixture on Chloride Ion Penetration and Diffusion in Concrete

In this paper, the influences of different kinds and different amount of mineral admixture to Cl-penetration and diffusion in concrete are studied. The results of experiment show that separate addition of mineral admixture (ground fly ash, slag fines, silica fume) into concrete can improve its resistance to chloride ion penetration and diffusion. Furthermore,the effect of silica fume is the best, fly ash second, and then follows slag fines. Mechanism analysis shows that the pozzolanic effect of mineral admixture improves the interface between the cement and aggregate in concrete, reduces the total porosity, makes pores more smaller, on the other hand it increases the Cl-consolidation capability of concrete with low alkaline C-S-H gel produced. So the resistance to chloride ion penetration of concrete is improved.     

Technology of Mixing High Property Concrete with Large Dosage Fly Ash

Being the largest mass of man-maded material, concrete demands much to resource and energy as well as affects much to environment. Whether concrete can be the dominating constructive material, the key is whether it can become green material and protect the environment at the same time. The important approach of developing is using industrial castoff largely in concrete. The tests note, that high performance concrete used of large mass of fly ash is doable. By grinding fly ash and adding high-efficient water reducer, when the dosage of cement clinker is only 25% and the dosage of fly ash is 70%, concrete with a good workability that 3 day and 28 day strength are bigger than 20 MPa and 50 MPa respectively can be achieved, which later strength develop very well. The large mass of fly ash concrete can utilize fly ash largely and reduce the dosage of clinker, that is very advantageous to protect environmental.     

Preparing Technique of Super High StrengthHigh Performance Concrete with Superfine Sand

With the understanding that there will be a great demand for superhigh performance concrete (SHPC) in the future construction engineering,while the resources of coarse and intermediate sand commony used for SHPC are scarce,so it is very important to replace coarse and intermediate sand by plentiful superfine one; First in China and abroad as well a superfine sand superhigh performance concrete with max. slump 255mm and max.28d compressive strength 126.8 MPa was prepared.It was compared with superhigh performance concrete with intermediate sand.     

3D BE Analysis on Stress Intensity Factor of a Fatigue Crack in a Welded T-joint

In this paper, based on multi region boundary element method (BEM), a general procedure to calculate stress intensity factors(SIF) of a 3D mixed mode fatigue surface crack is detailed. A BEM model for a welded T joint is constructed referring to the test specimen. The crack corner points are indirectly estimated to circumvent the treatment of the different crack tip singularity at the corner regions. Numerical results from the welded T joint show that this procedure is reliable.