Showing papers on "Compressive strength published in 2015"

Reinforcing effects of graphene oxide on portland cement paste

Abstract: In this experimental study, the reinforcing effects of graphene oxide (GO) on portland cement paste are investigated. It is discovered that the introduction of 0.03% by weight GO sheets into the cement paste can increase the compressive strength and tensile strength of the cement composite by more than 40% due to the reduction of the pore structure of the cement paste. Moreover, the inclusion of the GO sheets enhances the degree of hydration of the cement paste. However, the workability of the GO-cement composite becomes somewhat reduced. The overall results indicate that GO could be a promising nanofillers for reinforcing the engineering properties of portland cement paste.

Mechanical properties and microstructure of a graphene oxide-cement composite

Abstract: Graphene oxide (GO) is the product of chemical exfoliation of graphite. Due to its good dispersibility in water, high aspect ratio and excellent mechanical properties, GO is a potential candidate for use as nanoreinforcements in cement-based materials. In this paper, GO was used to enhance the mechanical properties of ordinary Portland cement paste. The introduction of 0.05 wt% GO can increase the GO-cement composite compressive strength by 15-33% and the flexural strength by 41-59%, respectively. Scanning electron microscope imaging of the GO-cement composite shows the high crack tortuosity, indicating that the two-dimensional GO sheet may form a barrier to crack propagation. Consequently, the GO-cement composite shows a broader stress-strain curve within the post-peak zone, leading to a less sudden failure. The addition of GO also increases the surface area of the GO-cement composite. This is attributed to increasing the production of calcium silicate hydrate. The results obtained in this investigation suggest that GO has potential for being used as nano-reinforcements in cement-based composite materials.

Mechanical, thermal insulation, thermal resistance and acoustic absorption properties of geopolymer foam concrete

Abstract: This study reports the synthesis and characterization of geopolymer foam concrete (GFC). A Class F fly ash with partial slag substitution was used for GFC synthesis by mechanical mixing of preformed foam. The GFCs exhibited 28 d compressive strengths ranging from 3 to 48 MPa with demolded densities from 720 to 1600 kg/m3 (105 °C oven-dried densities from 585 to 1370 kg/m3), with the different densities achieved through alteration of the foam content. The thermal conductivity of GFCs was in the range 0.15–0.48 W/m K, showing better thermal insulation properties than normal Portland cement foam concrete at the same density and/or at the same strength. The GFC derived from alkali activation of fly ash as a sole precursor showed excellent strength retention after heating to temperatures from 100 to 800 °C, and the post-cooling compressive strength increased by as much as 100% after exposure at 800 °C due to densification and phase transformations. Partial substitution of slag for fly ash increased the strength of GFC at room temperature, but led to notable shrinkage and strength loss at high temperature. Thin GFC panels (20–25 mm) exhibited acoustic absorption coefficients of 0.7–1.0 at 40–150 Hz, and 0.1–0.3 at 800–1600 Hz.

Use of OPC to improve setting and early strength properties of low calcium fly ash geopolymer concrete cured at room temperature

Abstract: Most previous works on fly ash based geopolymer concrete focused on concretes subjected to heat curing. Development of geopolymer concrete that can set and harden at normal temperature will widen its application beyond precast concrete. This paper has focused on a study of fly ash based geopolymer concrete suitable for ambient curing condition. A small proportion of ordinary Portland cement (OPC) was added with low calcium fly ash to accelerate the curing of geopolymer concrete instead of using elevated heat. Samples were cured in room environment (about 23 °C and RH 65 ± 10%) until tested. Inclusion of OPC as little as 5% of total binder reduced the setting time to acceptable ranges and caused slight decrease of workability. The early-age compressive strength improved significantly with higher strength at the age of 28 days. Geopolymer microstructure showed considerable portion of calcium-rich aluminosilicate gel resulting from the addition of OPC.

Hydration of quaternary Portland cement blends containing blast-furnace slag, siliceous fly ash and limestone powder

Abstract: In this study the hydration of quaternary Portland cements containing blast-furnace slag, type V fly ash and limestone and the relationship between the types and contents of supplementary cementitious materials and the hydrate assemblage were investigated at ages of up to 182 days using X-ray diffraction and thermogravimetric analysis. In addition thermodynamic modeling was used to calculate the total volume of hydrates. Two blast-furnace slag contents of 20 and 30 wt.% were studied in blends containing fly ash and/or limestone at a cement replacement of 50 wt.%. In all cases the experiments showed the presence of C–S–H, portlandite and ettringite. In samples without limestone, monosulfate was formed; in the presence of limestone monocarbonate was present instead. The addition of 5 wt.% of limestone resulted in a higher compressive strength after 28 days than observed for cements with lower or higher limestone content. Overall the presence of fly ash exerts little influence on the hydrate assemblage. The strength development reveals that amounts of up to 30 wt.% fly ash can be used in quaternary cements without significant loss in compressive strength.

Performance evaluation of sugarcane bagasse ash blended cement in concrete

Abstract: By-products from a number of industrial processes are used as alternative supplementary cementitious materials in concrete. Sugarcane bagasse ash is mainly composed of amorphous silica and can be used as a pozzolanic material in concrete. Production of sugarcane bagasse ash (SCBA) based blended cements with different replacement levels of SCBA, and the performance of concrete with these cements in terms of compressive strength, heat of hydration, drying shrinkage and durability are discussed in this paper. Durability performance was investigated by five different methods in this study, namely oxygen permeability test, rapid chloride penetration test, chloride conductivity test, water sorptivity test, DIN water permeability test and Torrent air permeability test. The results from this study show that use of sugarcane bagasse ash in concrete prominently enhances its performance. Low heat of hydration, additional strength gain due to pozzolanic reaction, significant reduction in permeability because of pore refinement and similar drying shrinkage behavior were observed for bagasse ash blended concrete compared to control concrete.

Performance Enhancement of Recycled Concrete Aggregates through Carbonation

Abstract: The cement paste attached to natural aggregates has a significant effect on the quality of recycled concrete aggregates (RCAs) because it usually has higher porosity and lower strength than natural aggregates. This work attempted to improve the quality of RCAs through carbonation of the attached cement paste. During carbonation reactions, CO2 reacted with Ca(OH)2 and calcium silicate hydrate (C─S─H) to form CaCO3 and silica gel, which filled the pores in the attached cement paste. Thus, carbonation increased the density, and decreased the water absorption and crushing values of the RCAs. It, thus, increased the flowability and compressive strength and decreased drying shrinkage of the recycled aggregate mortars.

The influence of the use of recycled aggregates on the compressive strength of concrete: a review

Abstract: This paper provides a systematic review of 119 publications, selected from 235, published over a period of 36 years from 1978 to 2014, relating to the effect on concrete compressive strength of the various aspects related to the use of recycled aggregates (RA) such as replacement level, size, origin, moisture content, exposure of the resulting concrete to different environmental conditions, use of chemical admixtures and additions, and strength development over time The data were collectively subjected to a statistical analysis, the results of which allowed producing a model for predicting concrete strength, based on the quality and content of the RA

Mechanical Behavior of Fiber Reinforced Engineered Cementitious Composites in Uniaxial Compression

Abstract: Polyvinyl alcohol (PVA) fiber reinforced engineered cementitious composite (ECC) is a class of high performance cementitious composites with pseudo strain-hardening behavior and excellent crack control when subjected to uniaxial tension. However, the compressive behavior of ECC has not been well characterized in the literature. In this paper, uniaxial compression tests were carried out on ECC with five different mix proportions and compressive strength ranging from 35 MPa to 60 MPa. Complete stress-strain curves were obtained. Based on the test results, the compressive parameters, such as the elastic modulus, engineering strain at the peak stress, the Poisson's ratio and the toughness index, were studied. A new constitutive model was proposed to express the pre- and post-peak mechanical behavior of ECC under uniaxial compression. The proposed model showed a good agreement with the experimental curves. The model proposed should be a valuable reference for the nonlinear analysis of ECC material in the part of structures under uniaxial compression.

Durability properties of high volume fly ash concrete containing nano-silica

Abstract: This paper reports the results of an experimental study on durability of ordinary concrete and high volume fly ash (HVFA) concrete containing nano silica (NS). In this study, concretes containing 2 and 4 % of NS are prepared at a constant water/binder ratio of 0.4 for testing ages of 3, 7, 28, 56 and 90 days. The concrete containing 2 % NS exhibited similar compressive strength of that containing 4 % NS and is used to evaluate its effectiveness in HVFA concrete. The compressive strength and durability tests including water sorptivity, volume of permeable voids (VPV), chloride permeability and porosity are investigated experimentally. Results show that besides improvement in compressive strength, the addition of 2 % NS significantly reduced the water sorptivity, VPV, chloride permeability and porosity of HVFA concretes. It is also revealed that the above durability properties of concretes containing 38 % class F fly ash and 2 % NS as partial replacement of cement are superior than ordinary concrete containing 100 % cement.