Age Of 28d Research Articles

Experimental study of light transmitting cement-based material (LTCM)

Light transmitting cement-based material (LTCM) was prepared by polymethylmethacrylate (PMMA) fibers, Portland cement, aggregates, deforming agent and water reducers in a given proportion. The compressive and flexural strengths of LTCM prepared with different diameters and volume fractions of optical fibers were measured at the age of 28d. The compressive and flexural strengths of LTCM specimens decreased with an increasing volume fraction of fiber. Factors such as distance from the light source to specimen, diameter and volume fraction of fiber, and the variety of light sources that affect the optical power of LTCM were studied in this paper. The experimental results demonstrate that the optical power was significantly affected by the distance from light source and specimen, and the diameter and number of fibers. The matrix material, cross section of optical fiber, and the interface between fiber and matrix were analyzed by SEM.

Effect of magnesia on properties and microstructure of alkali-activated slag cement

The effect of magnesia burnt at 800-950°C on the properties, especially the shrinkage, of alkali-activated slag cement (AASC) was experimentally studied. Experimental results show that, although adding 4%-8% lightly-burnt magnesia may shorten the setting time and slightly reduce the compressive strength of AASC, it can remarkably reduce the shrinkage of AASC. The results also show that the setting time of AASC with a certain amount of magnesia increases with the burning temperature, and that the flexural and compressive strengths of AASC decrease with the increase of the additive amount of magnesia. Generally, the adverse effect of magnesia decreases with the increase of the burning temperature, and the shrinkage-reducing effect of magnesia increases with the additive amount of magnesia. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses show that some magnesia particles in the hardened AASC paste at a 28-d age remained unhydrated, and that the compactness decreased a little as magnesia was added. We can also conclude that magnesia burnt at 850-950°C can be used to reduce the shrinkage of AASC only when its additive amount does not exceed 8%; otherwise, the setting time may be too short, and the flexural and compressive strengths may severely decrease.

Effects of dielectric heating of fresh concrete on its microstructure and strength in the hardened state

By means of radio-wave technology it is possible to allow the uniform, rapid and accurate, temperature-controlled heating of fresh concrete. This results in the significantly accelerated release of hydration heat. The heat release from this second source can be easily compensated by the temperature-controlled reduction of RF energy input. The consequences are accelerated hydration and higher early-age compressive strength. Early-age concrete, heat-treated by radio waves, have a denser microstructure with a higher gel-pore content and more C-S-H phases than untreated concrete. When cured at the maximum temperature of 80°C, secondary ettringite can be formed in concrete, whereas this is not the case for the treatment temperatures of 40°C and 60°C. At the age of 28d, heat-treated concretes exhibit a looser microstructure than does untreated concrete. Larger portlandite crystals and secondary ettringite are its characteristic features as a consequence of the much faster reaction rates in the early-age concrete. This results in lower values of the compressive strength of the heat-treated concrete.

PRODUCTS OF ALKALI-ACTIVATED CALCAREOUS FLY ASH AND GLASS CULLET

Geopolymerisation comprises a promising field of research, which deals with the rich in aluminosilicates solid wastes and byproducts (such as fly ash, recycled glass cullet), which are converted into useful products via alkaline activation process and heat curing. The fly ash used in this study is a calcareous one, which is the only type produced in Greece from the combustion of lignite. In this paper, mixtures of fly ash with glass cullet, deriving from windshields, are investigated and in particular, the mechanical characteristics and physical properties of colored mixtures that may be used for the manufacture of precast products Four trial mixes were studied experimentally, by forming prismatic (40x40x160 mm) specimens and small slabs (200x200x25mm). The tests of the prismatic specimens include the determination of the compressive and flexural strength, the elastic properties and the porosity at 7-d, 28-d and 90-d age. Volume deformation measurements were also carried out at 28-d age. The mechanism of strength development was monitored by using SEM microscopy, X-ray diffractometer and FTIR spectroscopy. The small slabs were tested in relation to their resistance to abrasion, impact strength, flexural strength and physico-chemical properties.

Analysis of the Influence of Clay Dosage and Curing Age on the Strength of Plastic Concrete

This test is sand coarse aggregate ratio, ratio of water to cementitious material, cement dosage, bentonite dosage, type of sand and type of bentonite in certain cases, studied the influence of clay dosage and curing age on the basic mechanical properties of plastic concrete, it is concluded that the best clay dosage; the clay dosage has a great influence on all the strength of plastic concrete (compressive strength, splitting tensile strength, rupture strength), with the increase of clay dosage and all kinds of strength decreased; when clay dosage is certain, various strengths of plastic concrete are increased with the increase of curing age, curing age has a great influence on various strengths of plastic concrete. If the long curing age (540d) intensity values as the actual strength of plastic concrete, growth rate of strength of 540d long age relative to the 28d age about 120%, this shows that plastic concrete has a higher late strength, plastic concrete curing age longer than concrete curing age.

An Experimental Study on Mechanical Properties of Basalt Fiber Reinforced Concrete

As a new environment-friendly fiber material, the basalt fiber has been used in engineering construction to a certain extent. Through the tests for compressive strength and split tensile strength of concrete matrices in different ages, the reinforcement of basalt fiber for concrete was explored in this paper. As shown by the test results, the mixture with basalt fiber would efficiently enhance the ductility of the concrete being broken, and improve the compressive strength and split tensile strength of concrete matrices in different ages, in which the compressive strength of concrete in the age of 3d and the split tensile strength of concrete in the age of 28d were improved more significantly, by 9% and 19% respectively. The test analysis results can be used as a reference for application of basalt fiber in concrete projects.

Experiment Study on Mechanical Properties of Shotcrete under Heat Injury Environment of High Ground Temperature Tunnel

With the tunnel growing more long and deep,heat injury problems have become increasingly prominent.In order to analyze how would the environment of high ground temperature impact on compressive strength of shotcrete, experiment was designed to explore the compressive strength of C25 shotcrete at 28d age by simulating environmental. The test results show that: water tank and the oven can more accurately simulate the natural hot-humid and hot-dry environment; Compared to the standard curing conditions, Whether in the hot-humid environment or hot-dry environment, the compressive strength of shotcrete is decreased. Mixed with 25% fly ash can improve the compressive strength of shotcrete in hot-humid environment.

The Effect of Cementation on the Mechanical Properties of Muddy Soil of East-South Coastal Area of China

Based on characteristic of the muddy soil residing in east-south coastal area of China, the effect of cement grade, cement-mixing ration and cement type on mechanical properties of cement-soil was carried out in this paper. The results show that cement-soil increases obviously with increasing of hydration age for different cement types. At early hydration age, strength of cement-soil increases quickly. And at the age of 28d~90d, strength of cement-soil still show quite obvious increasing, but increase rate is slow. Increasing cement-mixing ratio improve strength of cement-soil obviously and 0.15~0.20 cement-mixing ratio is suitable to use. Higher cement grade can bring higher strength of cement-soil. Early strength of cement-soil solidified with P.S32.5 increases slowly, but the middle and late stage strength of cement-soil solidified with P.S32.5 increase quickly. The solidification effect of P.S 32.5 is better than that of AEC32.5 and P.MSR32.5.

Effect of Freezing Temperature on the Microstructure of Negative Temperature Concrete

The pore size distribution and the microstructure of negative temperature concrete was studied with different temperature, combining with some testing methods, such as MIP and SEM. Moreover, the change of the compressive strength was also studied with different ages. In addition, the relationship between the microstructure and the macro-mechanical properties on negative temperature concrete was explored further with different freezing temperature. It indicated that the lower the early curing temperature, the looser the original structure of cement paste; the total volume of gel pore whose pore size was less than 20nm was decreasing apparently, and the compressive strength declined. When changing to standard curing, the pore size trended to be thinner, the compressive strength was increasing sharply. The concrete was cured from -5°C to standard curing, the volume of pore that was less than 200nm was equal to that of the concrete with the standard curing in the age of 28d, so was the compressive strength. However, the volume of the macro pore of the concrete curing under -10°C and -15°C was greater than the concrete curing the standard condition, the compressive strength was less.

Experimental Study on Temperature Data of Mass Concrete Construction in Hot Weather Conditions

Through studying on temperature data of mass concrete poured completely after 20 days, the temperature time-history curves were plotted in different heights section of the raft foundation. Moreover, the characteristic parameters such as solid concrete temperature, temperature difference between inside and outside , also between surface and atmosphere, and its cooling rate were studied. Then by checking cracks of the mass concrete, the most likely period of temperature stress cracks appearing is pointed out. The useful experience data of the mass concrete construction in conditions of hot weather is accumulated, whose concrete strength standard is evaluated by 28d age.

Experimental Study on Compressive Strength of Recycled Aggregate Concrete with Different Replacement Ratios

Compressive strengths of recycled aggregate concrete(RAC) with different recycled aggregates(RA) replacement ratios at 7d, 28d, 60d ages are investigated respectively. Failure process and failure mode of RAC are analyzed, influences on compressive strength with same mix ratio and different RA replacement ratios are analyzed, and the reason is investigated in this paper. The experimental results indicate that compressive strength of recycled concrete at 28d age can reach the standard generally, it is feasible to mix concrete with recycled aggregates, compressive strength with 50% replacement ratio is relatively high.

Research on Mechanical Properties of Geopolymer Concrete under early Stage Curing System

Mechanical properties of geopolymer concrete under early stage curing system were studied. The results showed that at the early stage of curing time, compressive strength was improved significantly with the increasing of curing temperature and curing time. The compressive strength decreased and was close to that of standard curing condition at the age of 28d as the curing age increased. In addition, prolonging the storage time at room temperature before the step of high temperature curing could increase the long-term strength.

Mechanical and Durability Properties of Recycled Aggregate Concretes Containing Fly Ash

The reutilization of waste concrete and fly ash produced by power plant is one way to develop green concrete. The addition of fly ash in recycled aggregate concrete has two methods, named equivalent replace and super-substitute technique. The workability of fresh concrete, compression strength, spilt and flexural strength of recycled aggregate concrete with different content of fly ash at different curing age are tested. Durability performance index such as sulfate attack, drying shrinkage and wear resistance is acquired base on the corresponding test. The result shows that the fluidity of fresh concrete is improved by the addition of fly ash. Before the age of 28d, the compression and flexural strength of recycled aggregate concrete with the contain of fly ash is lower than concrete without fly ash, after the age of 60d, the compression and flexural strength for FRC-10, FRC-10-2, FRC-10-4 are even higher than that of concrete without fly ash. The optimum equivalent replace ratio is 10%, and super-substitute ratio is 20% according to the compression and flexural strength. The performance of sulfate attack resistance, drying shrinking and wearing resistance of recycled aggregate concrete are significant improved as the containing of fly ash.

The Application of Blast Furnace Slag in Cementitious Self-Leveling Mortar

This article presents a study on the adequate application of blast furnace slag to replace quartz sand in cementitious self-leveling mortar. The results show that the strength of mortar decreases slightly, because slag sand can drag in some air bubbles which can form holes in the harden mortar. And compared with quartz sand, its compression strength loses only 4.6 % on 28d age. the proportion of 75 % slag sand and 25 % quartz sand was the best proportion as coarse filler.

Study on Toughening Cement-Based Composites with CaCO<sub>3</sub> Whisker and Fiber

Low tensile strength and high brittleness are the inherent defects of cement-based composites. So improving the toughness effectively is an urgent need for developing high-performance cement materials. The aim of this study is to study the toughening effect by mixing CaCO3 whiskers and polypropylene fibers in cement mortar. The results showed that CaCO3 whiskers could increase the split tensile strength of cement mortar of 28d age by 26.2% at maximum with the whisker content of 10wt.%; when CaCO3 whiskers content was 10wt.% and polypropylene fibers content was 0.20vol.%, the split tensile strength of cement mortar was increased by 21.1% at 28d age; when the fiber length was 19mm, the composite toughening effect of whiskers and fibers was more effective. The toughening mechanisms of CaCO3 whiskers and polypropylene fibers in cement mortar were also discussed.

Research on Influences of Activity Mineral Admixture on Workability and Mechanical Properties of High Strength Lightweight Aggregate Concrete

Aiming at the high expense of high strength lightweight aggregate concrete (HSLWAC), low-cost HSLWAC was produced making use of the industrial wastes including fly-ash (FA) and ground granulated blast furnace slag (GGBS) as admixture, and the influences of these admixtures on the workability and mechanical properties of HSLWAC were studies. Result shows that HSLWAC whose cubic compressive strength is high to 70MPa at the age of 28d can be produced with mixing admixtures; the effect of mixing GGBS to the strength of HSLWAC is better than that of mixing FA; and mixing admixtures is useful for the improvement of the workability of HSLWAC; However, the brittleness of HSLWAC rises with the increase of its strength.

Experimental Study on CaCO<sub>3</sub> Whisker-Reinforced Fiber Cement Mortar

As a kind of monocrystal reinforcing material, CaCO3 whisker has good characteristics of high strength, high module and high thermal stability. This research aims to optimize the microscopic structure of fiber cement mortar by adding CaCO3 whiskers and improve the macroscopic mechanical properties. The results showed that CaCO3 whiskers increased the flexural and compressive strength of cement mortar at 3d age by 29.5% and 19.8% at maximum respectively, which fell to 7.2% and 13.0% at 28d age; when CaCO3 whiskers content was 10% and polypropylene fibers content was 0.05%, compared with the pure cement mortar, the flexural and compressive strength were increased by 32.4% and 29.4% at 3d age and 7.6% and 4.6% at 28d age respectively; when the fiber length was 9mm, the composite effect of whiskers and fibers was more ideal; through the SEM analysis, the reinforcing mechanisms of CaCO3 whiskers and polypropylene fibers in cement mortar were discussed.

The hydration characteristics and expansion machanism of expansive cement at lowW/B ratio

The hydration characteristics and expansion impetus of three kinds of cement paste under free-and confined-curing conditions were investigated, which were respectively mixed with three different kinds of expansive agent at low W/B ratio. The results show that the hydration products of pure cement paste and paste mixed with expansive agent are same, but the amount of hydration products, un-hydrated C3S and C2S are obviously different at the same hydration age. At 3 d age, the amount of CH in pure cement paste is less than that of paste mixed with expansive agent, but it is reverse when at 28 d age. The amount of AFt at 3d and 28d age in pure cement paste is less than those of paste mixed with expansive agent. Regardless of under free- or confined-curing condition, the amount of ettringite produced varies little since 3d age. The joint effect of the tumefaction of gel-ettringite due to water absorption and the expansive pressure on the pore caused by the crystalloid ettringite is the cause of the volume expansion of cement paste, and the former effect is much greater than the latter.

Physical Properties of Heat Insulators Prepared from Blast Furnace Slag Mixed with Hemihydrate and Portlandite

Utilizing blast furnace slag, porous light weight materials have been prepared at room temperature in order to apply them as heat insulators of general purpose for buildings and residences. For 28-d age specimens prepared from slurries of blast furnace slag mixed with hemihydrate and portlandite, physical properties such as thermal conductivities, bulk densities, porosities and compressive strengths were measured, each showing 0.22-0.30 W/m.K, 780-1100 kg/m 3 , 45.2-66.6% and 16-81 MPa depending on W/S (water/ solid) ratio. These results indicate that present materials have a potential to contribute to saving air conditioning energies as well as reducing green house effect on earth, especially for high W/S materials.