Reduction In Strength Of Concrete Research Articles

The Influence of Atmospheric Pressure on Air Content and Pore Structure of Air-entrained Concrete

To study the effect of atmospheric pressure on the properties of fresh and hardened air-entrained concrete, three kinds of air entraining agents were used for preparing air-entrained concrete in the plateaus (Lhasa, 61 kPa) and the plains (Beijing, 101 kPa). Air content, slump, compressive strength and pore structure of the three air-entrained concretes were tested in these two places. It is found that the air content of concrete under low atmospheric pressure (LAP) is 4%-36% lower than that of concrete under normal atmospheric pressure (NAP), which explaines the decrease of slump for air-entrained concrete under LAP. Pore number of hardened concrete under LAP is reduced by 48%-69%. While, the proportion of big pores (pore diameter >1 200 μm) and air void spacing factor are increased by 1.5%-7.3% and 51%-92%, respectively. The deterioration of pore structure results in a 3%-9% reduction in the compressive strength of concrete. From the results we have obtained, it can be concluded that the increase of critical nucleation energy of air bubbles and the decrease of volumetric compressibility coefficient of air in the concrete are responsible for the variation of air content and pore structure of concrete under LAP.

Assessment of high-strength concrete encased steel composite columns subject to axial compression

This paper investigates the axial load capacity of concrete encased steel composite stub columns with high strength concrete and steel materials. A total of 14 column specimens with varying material strengths and different steel section shapes were tested under concentric compression load. The test results revealed that the design methods in EN 1994-1-1 and JGJ 138-2016 overestimate the axial load capacity of high strength concrete encased steel columns. Adding a small percentage of steel fiber in the high strength concrete was found to improve the compression resistance of the composite section. A new test database consisting of 51 partially encased composite sections and 82 fully encased composite sections was established, covering a wide range of section geometric and material grades. Parametric study was then carried out to assess current design methods in predicting the axial capacity of such composite columns with respect to material strengths, steel contribution ratio, reinforcement ratio, section slenderness ratio, confined concrete area ratio, and concrete confinement efficiency. The effectiveness of concrete confinement in partially encased composite column was evaluated and a simplified method was proposed to compute the enhanced concrete strength based on regression analysis. For fully concrete encased composite columns, a concrete strength reduction factor was proposed to be used with EN 1994-1-1 to predict the compression resistance. Design recommendation was made considering the early cover spalling of high strength concrete and the material compatibility between steel and concrete in composite column design.

Cyclic shear resistance for seismic design, based on monotonic shear models in fib Model Code 2010 and in the 2018 draft of Eurocode 2

AbstractShear resistance decays during cycling of inelastic deformations, causing flexural plastic hinges to fail in cyclic shear before reaching their ultimate flexural deformation. The monotonic shear model in MC2010 does not work for shear failure after flexural yielding. Accurate, unbiased, lack‐of‐fit‐free prediction of cyclic shear resistance, before or after flexural yielding, is possible, if: (a) inelasticity is considered in the calculation of longitudinal strains at section mid‐depth, through the “equal displacement rule” of Earthquake Engineering; (b) axial forces are taken to contribute to shear resistance with the transverse component of the diagonal strut between member ends; (c) the Eurocode 2 shear model for loads near supports is used for squat members; and (d) minor changes are made to the minimum compression field angle and the concrete strength reduction along it in MC2010, Levels II or III of approximation.

Effect of Heat Intensity and Duration on the Compressive Strength of Concrete

The objective of this paper is to evaluate the effect of high temperatures and durations on the strength of concrete using concrete specimens produced from two mix ratios of 1:2:4 and 1:3:6 and exposing them to different heat conditions. In the first condition the specimens are subjected to one hour heat intensities of 100 OC, 200 OC, 300 OC & 400OC and in the second condition they are subjected to heat periods of 30, 60, 90 and 120 minutes under a constant 600OC temperature. The 1:2:4 concrete mix specimens lost 85.96% of its strength after 2 hours of heating at 600OC and also lost 16.04% of its strength after 1 hour duration of 400OC. The 1:3:6 concrete mix specimens recorded a 73.61% maximum strength reduction after 2 hours of 600OC heat and a 19.63% strength loss under heat from ambient temperature to 400OC. Though the results showed an overall reduction in concrete strength it however did not establish a clear cut relationship between concrete strength and heat intensity or heat duration.Keywords: Concrete, Compressive strength, Heat Intensity, Heat duration, Fire.

Synergetic effect of MEA and SAP on the deformation and mechanical properties of concrete

This paper presents an experimental investigation on the synergetic effect of magnesium oxide expansion additive (MEA) and superabsorbent polymer (SAP) on the long-term drying shrinkage, autogenous shrinkage, internal humidity and mechanical strength of concrete. Concrete with varying dosages of MEA (0, 4, 6, 8 and 10%) and SAP dosage (0, 0·2, 0·4 and 0·8%) was studied to assess the synergetic effect of MEA and SAP on concrete. The results revealed that the long-term drying shrinkage of concrete could be effectively inhibited by a hybrid system based on SAP and MEA addition. SAP had a significant effect on maintaining the internal relative humidity of concrete. The hybrid system of 0·4% SAP and 6% MEA not only eliminated autogenous shrinkage but also controlled the deformation of expansion to a minimum. The addition of SAP and MEA generally resulted in a reduction in the cube strength of concrete. Scanning electron microscopy and stereomicroscopy results showed that larger voids were formed by desorbed SAP, which had a negative effect on the cube strength of concrete.

Concrete strength reduction due to over compaction

The purpose of this study was to evaluate the effect of over-compaction on the strength of concrete with regards to commercial and industry practices. Normal concrete, which is commonly used in construction was chosen. Samples were cast using a varying vibration time and testing was performed to determine concrete compressive strength, air content, density and a measure for segregation. These parameters were used to determine the relative sensitivity of concrete to under- versus over-compaction. If segregation was present, it was hypothesized the results would indicate a mathematical relationship between the segregation and strength reduction. This relationship would allow for determination of an optimum vibration time range. It was found in this study there was no segregation or strength reduction in vibrated samples. This result proved the concrete was far more sensitive to under-compaction, with over-compaction not being a practical concern. It was also determined that concrete compressive strength and vibration time had a basically logarithmic relationship and 30 s of continuous internal vibration provided the most efficient result of approximately 92% of maximum strength. The study recommends adjustment of standard concrete vibration practice to accommodate the more efficient 30 s duration. Alternatively, a 1.4x 'vibration factor' can be applied during design of compressive strength of concrete to accommodate the current 5–15 s vibration time window.

Influence of poly-lactic acid on the properties of perlite concrete

This paper is about the development of a stable and reproducible ultra-lightweight concrete with the inclusion of expanded poly-lactic acid aggregate. In total, nine mixtures were prepared by partially replacing expanded perlite (EP) aggregate with expanded poly-lactic acid (EPLA) and expanded polystyrene (EPS) to 0%, 30%, 40% and 60% by aggregate volume. The resutls of EPLA concrete were compared to the EPS concrete with identical mix proportions to evaluate the performance of the proposed concretes. The mechanical properties, thermal conductivity, fire resistance, electrical resistivity, setting time, exothermic reaction, and microstructural properties of the concrete were studied and discussed. The results reveal that the replacement of EP with EPLA causes a considerable reduction in mechanical strength of concrete. It was found that the chemical reaction of EPLA in the highly alkaline environment of cement causes significant changes in the microstructure of concrete. A large amount of calcium carbonate was found as hydration products of EPLA concrete. The carbon dioxide production and heat release rate of EPLA concrete in a fire is much lower than that of concrete containing expanded polystyrene (EPS). Contrary, a higher carbon monoxide production was observed in samples containing EPLA. The electrical resistivity and water absorption ratio of concrete increased with an increase in EPLA volume.

Influence of Rice Straw Fibers on Concrete Strength and Drying Shrinkage

Fibers have been used in construction materials for centuries. This study investigated the impact of the addition of rice straw fibers (RSF) on the compressive and flexural strengths of concrete, drying shrinkage, and on the heat of cement hydration. RSF was saturated before being added to concrete. Addition of RSF in concrete reduced concrete strength, increased concrete drying shrinkage, and increased the induction period of cement hydration. It was suggested that water squeezed out of RSF during mixing and sample consolidation increased effective water-to-cement ratios (w/c) and resulted in reduction of concrete strength and increase of concrete drying shrinkage. The increase of retardation time was attributed to leaching of organic and inorganic compounds out of RSF into the pore solution. It was shown that samples containing washed RSF did not have noticeable improvement in compressive strength over samples containing unwashed (as received) RSF. However, samples containing washed RSF had lower drying shrinkage and shorter induction period compared to those containing unwashed RSF.

Behavior and design of high-strength circular reinforced concrete columns subjected to axial compression

High-strength concrete (HSC) can be of great interest for the construction of lower-stories reinforced-concrete (RC) columns in high-rise buildings. This superior material can reduce the dimensions of the columns, and therefore, can be considered as a more advantageous material than normal-strength concrete (NSC). To compensate for its higher brittleness nature, the confinement reinforcement ratio required for HSC has to be larger than that required for NSC. The aim of this study is to examine the confinement transverse steel reinforcement amounts required for HSC circular columns part of high-rise buildings. The study first compares the confinement reinforcement requirements given by leading standards. Following the comparative research, an experimental study on six HSC columns subjected to axial loading was performed. The results show that in terms of axial capacity, it is recommended to further examine the concrete strength reduction factor and that it is more reasonable to consider a value lower than 0.85 for the design of HSC columns. Moreover, in terms of confinement and ductility, the recommended volumetric transverse steel reinforcement ratio for HSC circular columns not considered as part of the seismic-force-resisting system (SFRS), can be based on the ACI standard requirement and equal to half of the maximum specified volumetric transverse steel reinforcement ratio required for HSC columns considered as part of the SFRS.

Experimental investigation of compressive strength and compressive fracture energy of longitudinally cracked concrete

In this study, compression tests of longitudinally cracked concrete were conducted to clarify the effect of existing crack on reduction of compressive strength and compressive fracture energy. In the specimens, one or two longitudinal cracks were introduced mechanically before compression loading. The experimental parameters were set to specimen shape, size, height to diameter ratio, coarse aggregate size, number of existing crack and the width. Based on experimental evidences, it was clarified that the reduction of both compressive strength and compressive fracture energy due to existing crack was dependent on crack width and those reduction tendencies are clearly influenced by coarse aggregate size without the effect of other parameters. Moreover, in order to discuss the reduction mechanism, compression tests focused on the effect of crack shape such as wave height and length were also conducted. In the specimens, imitation of existing cracks were embedded for neglecting the effect of damage near crack. As a result, it was found that the crack shape was significant for the reduction of concrete strength and compressive fracture energy. Finally, compressive strength reduction model in association with maximum crack width and maximum coarse aggregate size was proposed.

Influence of processing factors over concrete strength.

Construction of facilities of cast in-situ reinforced concrete poses additional requirements to quality of material, peculiarities of the construction process may sometimes lead to appearance of lamination planes and inhomogeneity of concrete, which reduce strength of the material and structure as a whole. Technology compliance while working with cast in-situ concrete has a significant impact onto the concrete strength. Such process factors as concrete curing, vibration and compaction of the concrete mixture, temperature treatment, etc., when they are countered or inadequately followed lead to a significant reduction in concrete strength. Here, the authors experimentally quantitatively determine the loss of strength in in-situ cast concrete structures due to inadequate following of process requirements, in comparison with full compliance.

Evaluation of the Efficiency of Limestone Powder in Concrete and the Effects on the Environment

The major environmental impact of concrete comes from the CO2 emissions, produced during the cement manufacturing process. The main goal of this research project is to evaluate the efficiency of limestone powder as a partial cement replacement, in order to reduce energy consumption and CO2 emissions. This study utilizes limestone powders, with different particle sizes, to replace a portion of Portland cement using various ratios. Due to the dilution effect when partially replacing cement, there is a reduction in the concrete’s physical properties. To assess the dilution effect, a modification to Féret’s equation is used to calculate an efficiency factor for the limestone powder when compared to cement. To measure the environmental impact, a life cycle assessment is conducted on concrete made with limestone powder combined with cement. This allows for an evaluation of the various cement/limestone powder ratios that will maximize the environmental benefit, with minimal reduction in concrete strength. Additional microstructural analysis using petrographic examination was completed to provide a visual understanding of the distribution of the limestone particles within the cement paste. The results indicate that the efficiency of limestone powder in partially replacing cement can be achieved by particle packing and particle distribution in the concrete and the benefits of emission reductions exceed the loss in compressive strength when higher levels of limestone powder is used to replace cement.

The use of granulated plastic waste in structural concrete

This work looked at the ways in which waste plastic can be effectively recycled as an alternative material in concrete production. The granulated plastic waste was used to partially replace fine aggregate (quarry dust) at 0, 5, 10, 15, and 20% and the concrete formed from the replacement was termed P0, P5, P10, P15 and P20, respectively. Slump tests were used to determine the workability of the wet concrete, while compressive strength, tensile strength, bending moment and deflection tests were carried out on the hardened concrete. The slump test results obtained showed that P20 concrete has shear failure, while the test on hardened concrete showed gradual reduction in concrete strength as more granulated plastic was added to the concrete mix. The work concluded that granulated plastic can partially substitute fine aggregate in concrete up to 20%, and recommended the concrete for lightweight concrete based on defined standards.

Assessing the Performance of Concrete Containing Recycled Glass on Tensile Splitting Strength and Resistance to Alkali Silica Reaction

Using recycled glass in concrete applications decreases the amount of glass in landfills and substitutes for expensive aggregates in the concrete mix. However, there has been a concern on recycled glass with smooth surfaces that would result in a drop in strength and in particular a reduction of an already low ductility. Thus, in many design aspects, the use of recycled glass in concrete is limited up to 30% by weight due to concern on concrete strength reduction. The current manufacturing technology in the recycling glass has been grown and evolved through which recycled glass has been processed to exhibit the following features: basically zero water absorption, excellent hardness (great abrasion resistance), high durability to resist extreme weather conditions, etc. The paper challenges the currently used recycled glass mixtures and presents new mix design principles for concrete mixed with 10%, 20%, 30%, 50%, and 100% recycled glass as replacements of nature sand and Portland cement to assess (1) strength changes and (2) resistance to alkali silica reaction (ASR). Aggregate, water reducer, hydration stabilizer, mid-range water reducer, fiber, and viscosity modifier were prepared with varying dosages of recycled glass. A series of scanning electron microscope (SEM) imaging were performed to evaluate the resistance of recycled glass specimens to ASR. The paper concludes that the use of recycled glass as an alternative aggregate and cement binder in the concrete mixtures show promising performance in both tensile splitting strength and ASR.

Self‐Healing of Concrete Cracks by Ceramsite‐Loaded Microorganisms

Protective carrier is essential for the self‐healing of concrete cracks by microbially induced CaCO3 precipitation, owing to the harsh conditions in concrete. In this paper, porous ceramsite particles are used as microbial carrier. Heat treatment and NaOH soaking are first employed to improve the loading content of the ceramsite. The viability of bacterial spores is assessed by urea decomposition measurements. Then, the self‐healing efficiency of concrete cracks by spores is evaluated by a series of tests including compressive strength regain, water uptake, and visual inspection of cracks. Results indicate that heat treatment can improve the loading content of ceramsite while not leading to a reduction of concrete strength by the ceramsite addition. The optimal heating temperature is 750°C. Ceramsite particles act as a shelter and protect spores from high‐pH environment in concrete. When nutrients and spores are incorporated in ceramsite particles at the same time, nutrients are well accessible to the cells. The regain ratio of the compressive strength increases over 20%, and the water absorption ratio decreases about 30% compared with the control. The healing ratio of cracks reaches 86%, and the maximum crack width healed is near 0.3 mm.

Dynamic structural response of reinforced concrete dry storage casks subjected to impact considering material degradation

The performance of vertical concrete casks (VCC) for storage of spent nuclear fuel (SNF) under hypothetical accident conditions is of particular importance given the planned extensions of service-lives of such structures in the absence of a permanent disposal solution. Numerical models are created in this paper to simulate the structural response of a generic VCC in the cases of end-drop (handling drop) and tip-over considering material degradation. A concrete damage plasticity model (CDP) was calibrated using standard material tests to capture the non-linear behavior of concrete. Similarly, the steel properties and the properties of the soil supporting the cask foundation were based on material test data. The finite element models (FEM) were validated using experimental data from end-drop and tip-over tests of a 1/3-scale VCC performed by the same authors and reported elsewhere. The numerical results are compared with the experimental measurements in terms of velocity, acceleration, damage, and failure modes to gain confidence in the numerical results. Thereafter, a push-over analysis method was proposed and performed on the VCC for the first time to investigate the effect of different levels of material degradation on the structural response of VCC. The reduction in the concrete reinforcing steel area and bond degradation due to corrosion, and concrete strength reduction due to effects such as alkali-silica reactivity were considered as the variables for the parametric analysis. The results are interpreted to understand the changes in the performance of VCC in the long-term when subjected to hypothetical accident conditions in comparison to intact as built conditions.

Performance of modified self-healing concrete with calcium nitrate microencapsulation

This study investigates the strength reduction associated with incorporating calcium nitrate microcapsules in concrete. It also proposes modifications to the calcium nitrate micro encapsulation procedure to minimize the concrete strength reduction. These modifications consist of altering the continuous phase composition and keeping that of the aqueous phase the same. Amounts of 1%–10% of low Hydrophilic-Lipophilic Balance (HLB) emulsifier and 0.1%–1.0% of oil-soluble sulfonic acid catalyst (by weight of water in the aqueous phase) were dissolved in an organic solvent to prepare the continuous phase. The average diameter and shell thickness of the produced microcapsules were characterized using Scanning Electron Microscopy (SEM). Mortar mixes were prepared for various calcium nitrate concentrations of microcapsules that were encapsulated using the modified procedure. The compressive and flexural strengths and the elastic modulus of the mortar mixes were determined. The results show that the use of the modified encapsulation procedure resulted in a statically insignificant reduction of both compressive and flexural strengths compared to the original encapsulation method. The SEM micrographs of the fracture surface of the samples containing microcapsules showed that the strength reduction may be due to the agglomeration of the un-hydrated particles on the surface (shell) of the microcapsules. The compressive and flexural strengths of samples prepared using the proposed encapsulation procedure were enhanced compared to those prepared using previous encapsulation techniques.

THE RESTORATION OF THE BEARING CAPACITY OF REINFORCED CONCRETE BEAMS DAMAGED BY CORROSION OF THE HANDLING UNIT OF THE RAILWAY SIDING

Purpose. Assessment of load-bearing structures of the building and development of solutions for its further safe operation. Methodology. The authors have developed technology and materials which give the reliable results of the repair work to the design of the strength and reliability of concrete structures including injection techniques and materials. In particular, the strengthening monolithic reinforced concrete structures. Originality. During research it was established that the cause of the reduction of concrete strength according to the theory of reinforced concrete resulted in the reduction of the bearing capacity of reinforced concrete beams and slabs. Practical value. To enhance the restored beams, the application of composite materials: ribbons and mats of carbon fiber were proposed

Bonding in cementitious materials with asphalt-coated particles: Part I – The interfacial transition zone

Reclaimed asphalt pavement (RAP), when used as a coarse aggregate, has been shown to reduce bulk concrete strength and modulus. Part I of this study quantifies and compares the interfacial transition zone (ITZ) for mortar with RAP aggregates relative to dolomite aggregates through image analysis of backscattered electron micrographs. The ITZ with RAP aggregates was larger and more porous with less calcium silicate hydrate (C-S-H) and calcium hydroxide (CH) at the asphalt interface compared with dolomite aggregates. The CH morphology was not significantly affected, although the presence of the asphalt layer may be affecting the CH growth. The addition of silica fume reduced the porosity and size of CH particles in the ITZ with RAP, but not sufficiently to be similar to the ITZ of the dolomite mortar. The microstructural changes caused by RAP aggregates, primarily the larger, more porous ITZ, provide strong evidence for the observed reduction in concrete strength and modulus.

PERFORMANCE OF REINFORCED CONCRETE BEAM COLUMN CONNECTIONS EXPOSED TO FIRE UNDER CYCLIC LOADING

ABSTRACTFire could dramatically reduces the characteristic strength of the reinforced concrete elements [1,2]. A lot of efforts has been devoted to study the effect of fire on the reinforced concrete elements during past decades. These previous studies focused on evaluating fire resistance for concrete elements, [3] and provide some recommendations of structural fire-resistance [4] such as cover thickness, reinforcement ratio and coating materials. A lot of research focused on RC columns [5,6,7], beams [8] and slabs [9] which has been exposed to fire, where less efforts aimed to study the effect of fire on RC beam column connections [10,11,12].The main objective of the present work is to study the structural behavior of reinforced concrete beam-column connections exposed to fire under cyclic loading [13,14] and to evaluate the reduction in concrete strength during fire process. To achieve these goals, this research consists of two parts, the first part is experimental program, and the second is the theoretical analysis. The experimental program include 12 one- third scale specimens half of them casted from normal concrete and other half from SCC. The effect of some parameters on the connection's behavior like reinforcement ratio, type of concrete (normal or self-compacted concrete) [15] and exposure fire time were studied. Experimental program extended also to study heat distribution inside RC beam–column connections by measuring temperature at 65 point Distributors at 5 sections all over the RC connection. Theoretical analysis was done by using finite element program (ANSYS12.1) and a comparison between experimental program and theoretical analysis was done SCC showed good properties in hardening state when exposed to high temperature compared with normal concrete [16]. Increasing reinforcement ratio of RC connections increases its fire resistance Good agreement between analytical and experimental work was observed.