Square Root Of Compressive Strength Research Articles

Behavior of mortar exposed to different exposure conditions of sulfate attack

The sulfate deterioration of cement-based materials has become severe durability problems for the offshore structures. Considering the various service conditions such as splash and tidal zone with/without sulfate attack, the behavior of mortar samples under full immersion and dry-wetting cycles with 0% and 5% Na2SO4 solution, i.e., four exposure conditions, was investigated in this research. Compressive strength, elastic modulus, permeability (water permeability coefficient), expansion behavior and SEM images were tested and analyzed. The results showed that the behavior of mortar evolved in two stages, initial improvement followed by later degradation, determined by two competitive effects under each condition. Different deterioration mechanisms worked in the different exposure conditions. We proposed binomial formulas to describe the evolutions of the compressive strength and water permeability, and we further applied them in other researches well. Meanwhile, there was a linear relationship between elastic modulus and square root of compressive strength. Furthermore, the relation between water permeability and compressive strength was highly dependent on exposure types and time. Study showed that compressive strength and water permeability coefficient can be selected as controlling parameters in in-situ tests, and a possible approach to evaluate the sulfate deterioration level of concrete structures in-situ was represented.

Mechanical properties of concrete containing recycled coarse aggregate at and after exposure to elevated temperatures

This paper presents the effect of recycled coarse aggregate (RCA) of 50% and 100% by wt. on compressive strength and elastic modulus of concrete at elevated temperatures of 600 and 800 °C. The residual compressive strengths of above recycled aggregate concretes after exposure to elevated temperatures are also evaluated and compared to those measured during heating. The effect of polypropylene (PP) fibers on residual compressive strength and cracking behavior of above recycled aggregate concretes after exposure to elevated temperatures is also evaluated in this study. Results show that the loss of compressive strength of recycled aggregate concretes at 600 and 800 °C is slightly higher (about 10%) than its counterpart control concrete containing natural coarse aggregates. Concrete containing 100% RCA exhibited about 50% loss of its ambient temperature compressive strength at 800 °C, which is about 25% less in concrete containing 50% RCA. The residual compressive strength, which is measured after cooling of heated specimens, of both recycled aggregate concretes and control concrete is higher at both elevated temperatures. The measured elastic modulus of both recycled aggregate concretes show no significant reduction compared to their counterpart control concrete at both elevated temperatures. A strong correlation between elastic modulus and square root of compressive strength of recycled aggregate concretes is observed at elevated temperatures. The model proposed in Eurocode to predict the elevated temperature compressive strength and elastic modulus agrees well with the measured values for recycled aggregate concretes. The addition of PP fibers reduced the residual compressive strength of concrete containing 100% RCA at both temperatures with significant reduction at 800 °C. However, mixed results are obtained in the case of concrete containing 50% RCA. The cracking patterns of concrete containing RCA and those containing PP fibers also correlated well with the observed residual compressive strength results.

Effect of early-age subfreezing temperature on grouted dowel precast concrete wall connections

During cold weather precast wall construction, in-situ heating of the grout used in grouted dowel connections is usually conducted for short periods of time. Hence, early-age exposure to subfreezing conditions may affect the quality of the grout and subsequently the bond strength of the connection, which can compromise structural integrity. In this study, grout specimens typical of that used in precast wall construction were initially cured at ambient conditions for one day and then placed inside a walk-in environmental chamber at subfreezing temperatures. The hardened grout properties and bond strength of the connection were examined and compared to that of specimens cured at ambient temperature. The compressive strength of the grout was monitored at temperatures of 1°, −10° and −20°C. The effect of subfreezing exposure on the mechanical properties, hydration process and pore size distribution of the grout were examined. It was found that early-age subfreezing curing temperatures reduced the compressive strength of the grout, leading to increased dowel embedment length to achieve bar fracture. The bond strength of the connection remained proportional to the square root of compressive strength, even subsequent to early-age exposure to subfreezing temperature.

Effect of micro-silica on mechanical and durability properties of high volume fly ash recycled aggregate concretes (HVFA-RAC)

This paper presents the effect of different micro-silica (MS) contents of 5, 10 and 15 wt.% as partial replacement of cement on mechanical and durability properties of high volume fly ash - recycled aggregate concretes (HVFA-RAC) containing 50% class F fly ash (FA) and 35% recycled coarse aggregate (RCA) as partial replacement of cement and natural coarse aggregate (NCA), respectively. The measured mechanical and durability properties are compressive strength, indirect tensile strength, elastic modulus, drying shrinkage, water sorptivity and hloride permeability. The effects of different curing ages of 7, 28, 56 and 91 days on above properties are also considered in this study. The results show that the addition of MS up to 10% improved the early age (7 days) strength properties of HVFA-RAC, however, at later ages (e.g. 28-91 days) the above mechanical properties are improved for all MS contents. The 5% MS exhibited the best performance among all MS contents for all mechanical properties of HVFA-RAC. In the case of measured durability properties, mix results are obtained, where 10% and 5% MS exhibited the lowest sorptivity and drying shrinkage, respectively at all ages. However, in the case of chloride ion permeability a decreasing trend is observed with increase in MS contents and curing ages. Strong correlations of indirect tensile strength and modulus of elasticity with square root of compressive strength are also observed in HVFA-RAC. Nevertheless, it is established in this study that MS contributes to the sustainability of HVFA-RAC significantly by improving the mechanical and durability properties of concrete containing 50% less cement and 35% less natural coarse aggregates.

횡보강근이 없는 100 MPa 이하 콘크리트의 철근 압축이음 강도와 이음길이

Although the compression splice needs not be longer than the tension slice due to existence of end bearing, current design codes impose a longer compression lap splice than a tension lap splice in high strength concrete. Hence, new criteria for the compression lap splice including the effects of concrete strength need to be sought for economical design involving ultra-high strength concrete. An experimental study has been conducted with column specimens in concrete strength of 80 and 100 MPa. Test results show that the splice strength can be evaluated to be proportional to square root of compressive strength of concrete. Bar stress developed by end bearing is not affected by splice length and is expressed with a function of the square root of concrete strength. Mean value of stresses developed by end bearing is 16.5 square root of fck. The stresses developed by bond in compression splices are nearly identical to those in tension splices and, therefore, strength increment of compression splices is attributed to end bearing only. From regression analysis of 58 tests, a design equation is proposed for compression lap splice in 40 to 100 MPa of compressive strength of concrete. By the proposed equation, the anomaly of lap lengths in tension and compression is got rid of. In addition, the equation has a reliability equivalent to those of the specified strengths of materials.

Reliability, Brittleness, Covert Understrength Factors, and Fringe Formulas in Concrete Design Codes

The paper analyzes the reliability consequences of the fact that the current design codes for concrete structure contain covert (or hidden) understrength (or capacity reduction) factors. This prevents distinguishing between different combinations of separate risks due to the statistical scatter of material properties, the error of the design formula, and the degree of brittleness of failure mode, and also makes any prediction of structural reliability (or survival probability) impossible. The covert formula error factor is implied by the fact that the design formula was calibrated to pass not through the mean but through the fringe (or periphery, margin) of the supporting experimental data. The covert material randomness factor is the ratio of the reduced concrete strength required for design to the mean of the strength tests. As a remedy, the covert understrength factor of design formula should be made overt, its coefficient of variation (based on the supporting test data) should be specified, and the type of probability distribution (e.g., Gaussian or Weibull) indicated (which then also implies the probability cutoff). Alternatively, the code could give the mean formula, specify its coefficient of variation and type of distribution, and either prescribe the probability cutoff or overtly declare the understrength factor. The mean of strength tests required for quality control should be figured out from the required design strength on the basis of a specified probability cutoff and the coefficient of variation of these tests. Furthermore, it is proposed that the currently used empirical understrength factor, which accounts mainly for the risks of structural brittleness (or lack of ductility), should be based on the expected maximum kinetic energy that could be imparted to the structure. The reliability integral taking into account the randomness of both the load and structural resistance is generalized for the case of multiple (statistically independent) understrength factors. Finally, it is pointed out that the currently assumed proportionality of the tensile and shear strengths to the square root of compressive strength of concrete is realistic only for the mean, but grossly underestimates the scatter of tensile and shear strengths.