Key Engineering Materials Vols. 400-402

Abstract: Workability, mechanical properties and durability of concrete can be greatly improved with the advanced mineral admixtures such as steel slag powder. The workability and mechanical properties of steel slag concrete with different types of steel slag and different dosage of admixtures are studied. The experimental results showed the effect of steel slag powder on concrete performance. When compound mineral admixtures with steel slag powder and blast-furnace slag powder are mixed into concrete, the performance of concrete can be improved further due to the synergistic effect and mutual activation.

Abstract: A two-part experimental program is presented in this paper. In the first part, four self-consolidating mortar mixes were designed with different compositions of cementitious materials including cement, pulverized fuel ash and condensed silica fume. For each SCM mix composition, the superplasticizer dosage was varied and a total of 30 batches of mortar were produced. For every batch of mortar, the rheology was determined by a rheometer and the workability was measured by the mini slump flow test and the mini V-funnel test. From the test results, the saturation SP dosage of each SCM mix was determined. In the second part, four self-consolidating concrete mixes were produced, each comprising a SCM mix with saturation SP dosage and a fixed coarse aggregate content. The workability, filling and passing abilities and segregation stability were measured by the slump flow, U-box and sieve segregation tests, respectively. It was found that the performance of all the SCC mixes was satisfactory. The test results suggested that using a SCM mix with saturation SP dosage as the mortar phase can produce SCC with high performance and therefore is a good starting point to optimize the performance of SCC mixes.

Abstract: In this paper a new sort of high performance concrete is introduced which combines most advantages of prestressed concrete and steel fiber concrete, named steel fiber reinforced self-stressing concrete(SFFRSSC for short). Self-stressing concrete is actually a kind of expansive concrete which self-stresses, namely pre-compressive stresses, are induced by dint of some restrictions generally provided by steel bars to concrete expansion after hydration of expansive cement. As a result of chemical reaction, concrete archived prestresses by itself different from mechanical prestressed concrete, so called self-stressing concrete. By distributing short-cut steel fibers into self-stressing concrete at random, self-stresses are generated in concrete under combined restriction of steel bars as well as steel fibers. Thank to the pre-stresses tensile strength of concrete are significantly increased as well as cracking strength. In addition, on the one hand, expansive deformation of SFFRSSC can compensate the shrinkage of concrete to decrease non-loaded cracks resulting from shrinkage, and even when cracking, the steel fibers play an important role in resistance to crack development. On the other hand, self-stressing concrete can avoid the troubles of construction compared with conventional mechanical prestressed concrete. Therefore, above-mentioned advantages of SFFRSSC over ordinary concrete imply a better prospect in using SFFRSSC in civil engineering. For purpose of understanding the properties of SFFRSSC, in this paper some researches were carried out to investigate the special expansive behaviors with ages and tensile strength. The test results indicated that at early age the expansion of SFFRSSC developed rapidly but 14 day the 90% of overall expansive deformation basically fulfilled and subsequently expansion kept stable. Axial tensile test result showed that tensile strength were improved 2-3 times for self-stressing concrete specimens restrained by steel bars as well as steel fibers.

Abstract: Taking free shrinkage as the key parameter in evaluating cracking of concrete, the cracking properties of self-compacting concrete with strength grade of C35 and C50 were investigated based on ring and slab restraint tests. Meanwhile, the ultimate tensile strain of self-compacting concrete was studied by using flexural test. Experimental results show that the shrinkage of self-compacting concrete is lower than that of normal concrete. Self-compacting concrete cracks later than normal concrete with the same strength grade. Although the ultimate flexural tensile strains of self-compacting concrete in 7 d and 28 d are a little bit lower than that of the normal concrete, it can still be concluded from the research that the cracking property of self-compacting concrete can be greatly improved by using proper mix proportion.

Abstract: By method of artificial simulating acid environment to celebrating corrosion, the effects of boric acid on the physical properties and performance of reinforced concrete were investigated. The experimental study of concrete in nature and in various boric solutions (2000ppm, 8000ppm, 30000ppm) was carried out. The compressive strength, splitting tensile strength and elastic modulus of concrete were measured. The weight losses of steel bars and bond strength of reinforced concrete were calculated to analysis the influence of boric acid on reinforced bars. The ingredient change of concrete submerged in boric acid solution was studied with X-ray diffraction analysis. The experiments show that, in general, the mechanical strength of corrupted concrete increases with time. While comparing to the specimen curried in nature environment, the fundamental properties of concrete in boric acid solution decrease slightly. In this research, the influence of boric acid on the corrosion of the steel bars and the interacting between the concrete and the steel bars are insignificant. Three kinds of borides are detected in the X-ray diffraction analysis. It can be concluded from the research that boric acid will not generate apparent influence on the mechanics performance of concrete and steel bars. The same conclusion can be drawn on the bond properties between concrete and steel bars. Generally, corrosiveness of boric acid to reinforced concrete is quite weak.

Abstract: Recycling old concrete as an alternative source for production new concrete can solve the environment produced by old concrete. To make such recycling feasible, the strength and durability of the recycled aggregate concrete (RAC) must be assured. Prepared recycled coarse and fine aggregate using natural concrete specimens produced by compressive strength test of cubic concrete. Prepared recycled aggregate concrete which replacement ratios is 50% and 70%. Processed compressive strength test of cubic RAC specimens. Carried through the test for resistance of concrete to rapid freezing and thawing on both RAC and natural concrete. As a result of the experiment, the strength of the RAC can reach the design strength, however, the index for the resistance of concrete to rapid freezing and thawing depressed to a certainty degree relative to that of common concrete. Replacement ratio of aggregate influenced relative dynamic modulus of elasticity of RAC seriously. The weight change increased as long as the number of freezing and thawing cycles increased, and the weight change increased when replacement ratio elevation. RAC showed a sharp weight loss during the testing period, and reduce the water cement ratio can increase the durability characteristics of RAC. Also, absorption of the recycled aggregate influenced durability properties of RAC badly.

Abstract: The polymer sulphoaluminate cement is composed of styrene-acrylic emulsion and sulphoaluminate cement. The resistance to sulfate attack, impermeability and mechanical performance are studied in the paper. The hardened paste microstructure of the polymer sulphoaluminate cement is studied by means of SEM. The experiment shows that the impermeability of the cement is improved with the increase of styrene-acrylic emulsion. When the ratio of polymer to cement increases from 0% to 15%, the permeate height of the hardened cement paste decreases by 70%. The resistance to sulfate attack of the cement is also improved with the increase of styrene-acrylic emulsion. Styrene-acrylic emulsion has little effect on the flexural strength of sulphoaluminate cement. When the ratio of polymer to cement is less than 15%, the flexural strength of the cement at 28d curing ages are all above 7.1MPa. As shown from SEM photographs, the cement becomes more close-grained when the ratio of polymer to cement increases.

Abstract: A restrained ring setup with a clapboard was developed. Because the clapboard can cause a stress concentration of concrete annulus placed in the ring setup, a crack of the concrete annulus can rise at the tip of the clapboard at an early age of curing. Then the time-span of the evaluation of concrete cracking behavior is shortened. The new evaluation method can avoid the disadvantages of applying the conventinal restrained ring setup without a clapboard when evaluating the cracking resistant behavior of concrete, such as the longer time-span, randomicity and chanciness of concrete cracking. Based on experimental data, in which the effect of polypropylene fiber and expansive agent on the cracking resistant behavior of concrete was investigated, the cracking age of concrete loops and the ratio of tensile strength to compressive strength of concrete were obtained, and the validity of the proposed ring setup has been verified.

Abstract: Deterioration significantly affects the structure performance and safety. A signal-based pattern-recognition procedure is applied for structural damage detection with a limited number of input/output signals. The method is based on extracting and selecting the sensitive features of the structure response to form a unique pattern for any particular damage scenario, and recognizing the unknown damage pattern against the known database to identify the damage location and level (severity). In this study, two types of transformation algorithms are implemented separately for feature extraction: (1) Continuous Wavelet Transform (CWT); and (2) Wavelet Packet Transform (WPT). Three pattern-matching algorithms are also implemented separately for pattern recognition: (1) correlation, (2) least square distance, and (3) Cosh spectral distance. To demonstrate the validity and accuracy of the procedure, experimental studies are conducted on a simple three-story steel structure. The results show that the features of the signal for different damage scenarios can be uniquely identified by these transformations, and correlation algorithms can best perform pattern recognition to identify the unknown damage pattern. The proposed method can also be used to possibly detect the type of damage. It is suitable for structural health monitoring, especially for online monitoring applications.

Abstract: Concrete structures durability is affected by a number of factors which have different sensitivity. According to level of sensitivity classified the influence factors could promote concrete structures durability design. The disadvantage of traditional methods of sensitivity analysis in concrete structures durability is the larger number of samples. The grey correlation theory was used to analyze the degree of the sensitivity of the influence factors and comparison with traditional methods of calculation. The results show that the grey correlation analysis method could quantify the levels of the sensitive factors and with higher reliability. A new way to study the durability of concrete structures was provided.