Relation Between Density and Compressive Strength of Hardened Concrete

Concrete must has to ensure satisfactory compressive strength and durability. The mechanical properties of concrete are highly influenced by its density. A denser concrete generally provides higher strength and fewer amount of voids and porosity. Smaller the voids in concrete, it becomes less permeable to water and soluble elements. So water absorption will also be less and better durability is expected from this type of concrete. In this paper an experimental program conducting on compressive strength, density, absorption capacity and percent voids of hardened concrete is described. The variation of these properties with maturity of concrete was main focus of this experiment. Comparison is made between two types of concrete’s property test results. One of them is lightweight concrete made with crushed brick (BC) as primary coarse aggregate. Crushed brick is a locally available construction material in Indian subcontinent. Another type of concrete is a denser one, made with crushed stone (SC) as primary coarse aggregate. The comparisons on test results are presented with respect to time. It was observed from the experiment that, strength and density increases with maturity of concrete and percent void and absorption capacity decreases with time. Better results were obtained from stone aggregate concrete than brick aggregate concrete in cases of all of the tests

Oxidized Graphitic Nano-Amendment of Cement Composites: Exploring Truly Low Concentrations and Novel Particle Morphologies

Oxidized graphitic nanoparticles such as multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs), can produce enhancements in physicomechanical properties of cement composites that are relevant to structural and durability performance, provided that said nanoparticles are well dispersed in the composite. Lower-bound MWCNT concentrations are reported in the literature in the range 0.01-0.05% in weight of cement (wt%). Such concentrations may not be costeffective for practical applications and may also be excessive since they would result in a nanoparticle surface area at or above 105 m2 for 1 m3 of cement paste, or 104.5 m2 for 1 m 3 of concrete, which may be large enough to facilitate nanoparticle agglomeration in cement matrices. These agglomerates may contribute to the inhomogeneity of the cement matrix and introduce defect sites that negatively contribute to mechanical properties associated with strength, stiffness, and toughness. In this research, low concentrations of MWCNTs in the range 0.001-0.05 wt% were explored as amendments in cement paste, thus one order of magnitude smaller than lower-bound concentrations reported in the literature. It was found that such decrease in MWCNT concentration continues to provide significant enhancement to compressive strength and stiffness compared to plain cement paste. In fact, neither significant enhancements in the 28-day compressive strength and stiffness, nor noticeable changes in the nano- and micro-structure, were observed by increasing the MWCNT concentration by 50 times, from 0.001 wt% to 0.05 wt%. Instead, as originally hypothesized, MWCNT agglomerates were consistently observed in the cement matrix when using a 0.05 wt% MWCNT concentration. The results above highlighted the merits of exploring more effective means of leveraging the specific surface area (SSA) of graphitic nanoparticles by considering alternative morphologies, in addition to truly low concentrations. To this end, partially unzipped carbon nanotubes (PUCNTs) are introduced in this research as nanoamendments for cement composites. Oxidized PUCNTs are of interest because they combine the aspect ratio and mechanical strength of MWCNTs, and the high graphene edge content and dispersibility of GNPs. In addition, PUCNTs offer a greater specific surface area compared to MWCNTs, which makes them suitable for exploring reduced concentrations. In this part of the research, truly low concentrations of PUCNTs in the range 0.001-0.05 wt% were utilized in cement paste. It was found that reducing the PUCNT concentration by one order of magnitude, from 0.05 wt% to 0.005 wt%, led to significantly enhanced nano- and micro-structure as well as compressive strength and elastic stiffness. These results were supported by SEM micrographs, which consistently showed PUCNT agglomerates for a concentration of 0.05 wt%; instead, for a concentration of 0.005 wt%, well dispersed PUCNTs were consistently observed, together with preferential and accelerated formation of C-S-H. This evidence agrees with dynamic light scattering test results on PUCNT suspensions where hydrodynamic size and zeta potential values indicate less dispersibility of PUCNTs at concentrations equivalent to 0.05 wt% in the cement paste manufactured with these suspensions. The research into more effective graphitic particle morphologies continued by investigating graphene oxide nanoribbons (GONRs), which are obtained by fully unzipping MWCNTs. GONRs combine the properties of carbon nanotubes and graphene, i.e., high surface area, graphene edge content, and aspect ratio. In fact, GONRs are easy to disperse in stable aqueous suspensions without compromising the sp2 basal plane to defects. Due to their edge content resulting from oxidation, GONRs have higher contents –COOH edge groups compared to MWCNTs and GNPs, and to a lesser extent PUCNTs. A dispersibility study in aqueous solution was conducted using GONRs with different oxidation level (i.e., oxygen functional group content), and in different concentrations. Uniform GONR dispersions were quantitatively verified through DLS testing, based on hydrodynamic size and zeta potential measurements, for concentrations in the range 0.0125-1.25 g/L (equivalent to 0.0005 to 0.05 wt% of cement in concrete with water-to-cement ratio of 0.4), and for oxygen content in the range ~30-40 wt% for up to 7 days after initial sonication. These dispersed and stable aqueous suspensions of GONRs are suitable to explore novel graphitic-nanoamendment solutions for cement composites, including actual concrete, using more rational and practical concentrations

Optimum Proportion of Masonry Chip Aggregate for Internally Cured Concrete

Abstract Proper curing of concrete is essential for achieving desirable mechanical properties. However, in a developing country like Bangladesh, curing is often neglected due to lack of proper knowledge and skill of local contractors. Consequently, general concreting work of the country has been found to have both strength and durability issues. Under such scenario, internal curing could be adopted using masonry chip aggregate (MCA) which is quite common in this region. It is observed that saturated MCA desorbs water under favorable relative humidity and temperature. This paper presents the effectiveness of MCA as internal curing medium and recommends a tentative optimum mix proportion to produce such concrete. The experimental study was conducted in two phases. It was found that 20% replacement of stone chips with MCA produced better performing internally cured concrete both in terms of strength and durability. Performance of internally cured concrete with recommended proportion of MCA is comparable to that of normally cured control concrete samples with conventional stone chips. In addition, internally cured concrete performed significantly better than control samples when kept under similar adverse curing conditions. In the absence of supply of external water for curing, compressive strength of internally cured concrete for 20% replacement can be as high as 1.5 times the strength of the control concrete samples. Significant better performance in permeability than that of control samples was also observed for this percent replacement under such adverse curing conditions