Characterization of elastic and time-dependent deformations in normal strength and high performance concrete by image analysis

Deformations in normal strength concrete (NSC) and high performance concrete (HPC) were examined using image analysis to better understand the distribution of strain in these materials as related to their composition and microstructure. Elastic strain, creep and shrinkage were shown to occur non-uniformly throughout the NSC and HPC microstructure and creep and shrinkage strain both increased with time, as expected. The non-uniformity in the strain measurements also increased with time, as a consequence of dissimilarities in time-dependent behavior of the paste and the aggregate. However, compared with NSC, the time-dependent strains in the HPC were lower and showed less variation, suggesting a more uniform microstructure. In both NSC and HPC, high-strain sub-regions were evident in the vicinity of the interfacial transition zone (ITZ), likely as a consequence of the strain mismatch between aggregate and paste. The thickness of the high strain sub-regions along the ITZ in HPC were approximately one half of those in NSC.     

Carbonation around near aggregate regions of old hardened concrete cement paste

Analogous with most modern cities, waste disposal is a pressing issue due to limited landfill and public filling (land reclamation) areas in Hong Kong in which construction and demolition (C&D) waste forms the major source. Concrete, apportioning the largest portion of C&D waste, has the greatest potential for recycling. However, the knowledge on micro-structural behavior of concrete waste is immature to give adequate details on the macro-behavior of concrete waste. This paper attempts to examine the problems of recycling old concrete by investigating the microstructure and phase transformation of the concrete samples collected from buildings with 46 and 37 years of services. From the results of Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM) examination, it is found that there are a lot of pores at the near layers of aggregate where carbonation of the hardened cement paste (HCP) is high. The pores may be generated as a result of poor workmanship such as insufficient concrete mixing time, trapping of air voids beneath coarse aggregate, inappropriate water to cement ratio, and the microclimate conditions such as humidity that affects the demand on water from the aggregate during mixing.     

Volume and deviator creep of calcium-leached cement-based materials

This paper provides new experimental evidence of the specific volume creep and deviator creep of cement-based materials. Creep tests conducted on calcium-leached cement pastes and mortars under various triaxial loading conditions provide conclusive evidence of a specific short-term creep and a specific long-term creep. It is found that at least two competing dissipative mechanisms are at work in short-term creep of cement-based materials: (1) a creep relaxation mechanism in the calcium-silicate-hydrate (C-S-H) solid phase activated by microstress concentrations in the heterogeneous microstructure; and (2) stress relaxation by microcracking beyond a relatively small stress threshold. If the first dominates over the second, the short-term volume creep is contracting, and in the inverse case, a short-term dilating behavior is found. Furthermore, provided that microcracking stabilizes during the short-term creep, the long-term creep occurs at constant volume. Based on these results, we argue that the concept of a creep Poisson's ratio, which is commonly employed in engineering practice to extrapolate uniaxial creep tests to multiaxial stress conditions, should be abandoned when triaxial stress states affect the durability performance of concrete structures.     

Influence of lightweight aggregate on the microstructure and durability of mortar

The results of an investigation on the effect of dry and prewetted lightweight aggregates on the microstructure and durability of mortar are presented in this paper. The results are compared with those obtained for normal aggregate mortar. There appears to be only a small difference in the microstructure of the interfacial transition zone (ITZ) between dry and prewetted lightweight aggregate mortars. The porous ITZ surrounding lightweight aggregate appears to extend for about 10 and 15 μm from the aggregate surface for dry and prewetted lightweight aggregates, respectively. The ITZ for dry and prewetted lightweight aggregates seems to be surrounded by dense paste that extends from 10 to about 50 μm from the aggregate surface. This dense paste has lower porosity than that observed in the bulk paste located 50 μm and farther from aggregate surface. The normal aggregate mortar prepared with the same water/cement ratio appears to have porous ITZ that extends beyond 35 μm from the aggregate surface. The dry and prewetted lightweight aggregate mortars seem to have a lower sorptivity and electrical conductivity than does the normal aggregate mortar. Lightweight aggregate mortars also appear to have excellent resistance to sulfate attack as compared with normal aggregate mortar.     

On the mechanism of strength enhancement of cement paste and mortar with triisopropanolamine

Recent work on the strength-enhancing mechanism of triisopropanolamine (TIPA) suggested that TIPA enhances the mechanical properties of mortar and concrete by acting on the interfacial transition zone (ITZ) between paste and sand or aggregate rather than improving the properties of the hydrated binder. This paper presents compressive strength data for 10 Portland cements tested as cement paste as well as two different kinds of mortar after 28 days hydration, so that these two mechanisms could be compared directly. The average strength improvement with TIPA was 10% in the hydrated portland cement paste and 9% in the mortar, clearly showing that the strength enhancement is not dependent on an ITZ mechanism.     

Influence of aggregate size and volume fraction on shrinkage induced micro-cracking of concrete and mortar

In this paper, the influence of aggregate size and volume fraction on shrinkage induced micro-cracking and permeability of concrete and mortar was investigated. Nonlinear finite element analyses of model concrete and mortar specimens with regular and random aggregate arrangements were performed. The aggregate diameter was varied between 2 and 16 mm. Furthermore, a range of volume fractions between 0.1 and 0.5 was studied. The nonlinear analyses were based on a 2D lattice approach in which aggregates were simplified as monosized cylindrical inclusions. The analysis results were interpreted by means of crack length, crack width and change of permeability. The results show that increasing aggregate diameter (at equal volume fraction) and decreasing volume fraction (at equal aggregate diameter) increase crack width and consequently greatly increases permeability.     

Shrinkage cracking of lightweight concrete made with cold-bonded fly ash aggregates

Shrinkage cracking performance of lightweight concrete (LWC) has been investigated experimentally on ring-type specimens. LWCs with and without silica fume were produced at water-cementitious material ratios (w/cm) of 0.32 to 0.55 with cold-bonded fly ash coarse aggregates and natural sand. Coarse aggregate volume ratios were 30%, 45%, and 60% of the total aggregate volume in the mixtures. A total of 12 lightweight aggregate concrete mixtures was cast and tested for compressive strength, static elastic modulus, split-tensile strength, free shrinkage, weight loss, creep, and restrained shrinkage. It was found that the crack opening on ring specimens was wider than 2 mm for all concretes. Free shrinkage, weight loss, and maximum crack width increased, while compressive and split-tensile strengths, static elastic modulus, and specific creep decreased with increasing coarse aggregate content. The use of silica fume improved the mechanical properties but negatively affected the shrinkage performance of LWCs. Shrinkage cracking performance of LWCs was significantly poorer than normal weight concrete (NWC).     

Autogenous deformations of cement pastes: Part I. Temperature effects at early age and micro-macro correlations

A micro-macro experimental study has been performed, from the end of mixing up to 2 years, on a set of plain cement pastes prepared with the same type I ordinary Portland cement (OPC) and various water-to-cement ratios (W/C), and cured at various constant temperatures. In this part I of the paper, volumetric autogenous shrinkage has been analysed in relation to various parameters characterizing the hydration process: chemical shrinkage, degree of hydration of the cement, Ca(OH)₂ content and Vicat setting times, within the early-age period (≤24 h). The effects of the curing temperature (ranging from 10 up to 50 °C) have in particular been investigated. Its effects recorded on both the rate and the magnitude of volumetric autogenous shrinkage vs. time have pointed out the irrelevance of the usual maturity concept to describe such phenomenon within the whole early-age period. An improved maturity concept has hence been proposed. It is based on separating the early-age period in different phases and on using chemical shrinkage data for the calculation of the apparent activation energy applied to the prediction of autogenous deformations occurring after the setting period. Furthermore, micro-macro relationships have been pointed out, illustrating in particular the determining role of Ca(OH)₂.     

对高性能混凝土的认识及混凝土开裂的问题

高性能混凝土是以耐久性为显著标志，充分考虑环境友好并兼顾可持续发展的高科技混凝土，实现混凝土高性能化，不是简单的材料与配比问题，更重要的是更新观念，精心制备，强化施工。现代混凝土的开裂问题始终是影响混凝土耐久性的重要因素，基于综合分析和评价国内外相关文献，本文归纳总结了混凝土收缩裂缝的不同形式及影响因素，认为混凝土的收缩开裂是不同形式收缩导致开裂的叠加结果。为明晰各种收缩对开裂行为的影响，有必要区分不同形式的收缩开裂，进而有针对性地改善混凝土的抗裂性，同时提出了改善混凝土抗裂性能的措施和建议。     

A numerical model for elastic modulus of concrete considering interfacial transition zone

The effect of interfacial transition zone on mechanical properties of concrete has been found to be significant, thus the interfacial transition zone should be considered in the analysis for better estimation of elastic modulus of concrete. However, it is difficult to estimate elastic modulus of concrete practically using simple models proposed so far. In this study, a numerical concrete model that adopts three-phase model and finite element with material discontinuity was proposed to analyze concrete with complex interface in three dimensions. The validity of the proposed model was verified by comparing the calculated elastic moduli of concrete with those obtained from experiments. The effect of interfacial transition zone on elastic modulus of concrete with either low or high w/c was also investigated. The analysis results suggest that careful selection of characteristics for interfacial transition zone should be made for the accurate estimation of elastic modulus of concrete.     

Chemical interactions between siliceous aggregates and low-Ca alkali-activated cements

The chemical interactions between natural siliceous aggregates and a low-Ca alkali-activated cement (geopolymer) were studied. By leaching ideal aluminosilicate minerals such as kaolinite and albite in various alkaline solutions with or without soluble silicates, it was found that addition of 0.5 M SiO₂ to a highly alkaline activating solution ([OH⁻]₀ = 5 to 10 M) was responsible for the formation of an Al-enriched aluminosilicate surface through the initial non-stoichiometric and Si preferential dissolution of the parent aluminosilicates. This then facilitated soluble silicate deposition from the activating solutions onto the Al-rich surfaces, which resulted in the formation of a dense deposited aluminosilicate gel interfacial layer. This aluminosilicate interface formed during albite leaching ([OH⁻]₀ = 5 to 10 M and [SiO₂]₀ = 0.5 M) was found to possess a similar Si/Al ratio to the real interface between a siliceous aggregate slice (basalt or siltstone) and a low-Ca fly ash/kaolinite geopolymer, activated with an activating solution of [OH⁻]₀ = 10 M and [SiO₂]₀ = 2.5 M. Due to the similarities between the activating solutions used and the interfaces formed, it is postulated that a similar formation mechanism is shared between the deposited aluminosilicate interface formed from leaching and a ‘real’ geopolymer concrete synthesis. Without soluble silicate addition, or if a solution of low alkalinity ([OH⁻]₀ = 0.6 M and [SiO₂]₀ = 0 and 0.5 M) was used, the Al-enriched reacting surface was not formed, and no deposited aluminosilicate interface was observed in these systems.     

Water absorption in internally cured mortar made with water-filled lightweight aggregate

The increased propensity for shrinkage cracking in low water-to-cement ratio (w/c) concrete has inspired the development of new technologies that can reduce the risk of early-age cracking. One of these is internal curing. Internal curing uses saturated lightweight aggregate to supply ‘curing water’ to low w/c paste as it hydrates. Significant research has been performed to determine the effects of internal curing on shrinkage and stress development; however, relatively little detailed information exists about the effects of internal curing on fluid transport properties such as water absorption or diffusivity. This study examines the absorption of water into mortar specimens made with saturated lightweight aggregates (SLWA). These results indicate that the inclusion of SLWA can reduce the water absorption of mortar specimens. This observation was reinforced with electrical conductivity measurements that exhibited similar reductions.     

Contribution of granular interactions to self compacting concrete stability: Development of a new device

Understanding the SCC behaviour is essential for resolving placement and consolidation problems in the field. As far as segregation is concerned, one of the main remaining obstacles is the design of a concrete mixture suitable for given casting conditions. Physical approaches which consist in studying the sedimentation of a single particle in a yield stress fluid failed to describe SCC static segregation. Segregation is a more complex phenomenon and the interactions between coarse aggregates have to be taken into account. They contribute to the stability of fresh SCC and this contribution should only depend on the solid fraction of the granular skeleton. A new experimental device has been developed in order to highlight and quantify the combined effects of coarse aggregates. This device allows studying lattices of particles. These latter are organised according to a cubic centred pattern and are immersed in a yield stress fluid. The experimental device and the test procedure are described in this paper. The validity of measurements has been demonstrated by performing a first series of tests and numerical simulations. Repeatability is quite satisfactory and “wall effects” can be limited.     

Water-cement ratio gradients in mortars and corresponding effective elastic properties

Water-cement ratio gradients are modeled through the interfacial transition zone (ITZ) of a mortar with spherical inclusions. The model is a function of the over-all water-cement ratio, volume fraction and radius of sand, specific gravity of cement and thickness of ITZ. Based on experimental data from the literature, the dependence of saturated, homogeneous cement paste is modeled as a function of water-cement ratio. Subsequently, the effective bulk and shear moduli for mortars are determined using a generalized self-consistent method. Finally, application of the model to data in the literature pertaining to elastic wave speeds in saturated mortars composed of 20-30 screened sand with an overall water-cement ratio of 0.3 yielded a mean ITZ thickness of 48.3 μm.     

The interface between natural siliceous aggregates and geopolymers

The reaction products as well as the formation mechanisms of alkali-activated binders, or geopolymers, have been studied intensively. However, the interface between mineral aggregates, such as sand and/or natural rocks, and geopolymers has not been studied. This paper reports the microstructure and the bonding strength (Mode I bending) of the interface between natural siliceous aggregates and fly ash-based geopolymers. It was found that when the activating solution that contained no or little soluble silicates, the compressive strengths of the geopolymeric binders, mortars and concretes were significantly weaker than those activated with high dosages of soluble silicates. The presence of soluble silicates in the initial activating solution was also effective in improving the interfacial bonding strengths between rock aggregates and geopolymeric mortars. No apparent interfacial transition zone (ITZ) could be identified near the aggregates if the systems were free from chloride contamination. Chloride (KCl) was found to decrease the interfacial bonding strength between the aggregates and the binders probably by causing gel crystallisation near the aggregate surfaces, which resulted in debonding.     

Measuring permeability and stress relaxation of young cement paste by beam bending

When a saturated rod of a porous material is deflected in three-point bending, two types of time-dependent relaxation processes occur simultaneously: hydrodynamic relaxation, caused by the flow of liquid in the porous body, and viscoelastic (VE) relaxation of the solid network. By measuring the decrease in the force required to sustain a constant deflection, it is possible to obtain the permeability from the hydrodynamic relaxation function, in addition to the VE stress relaxation function of the sample. We report the early-age evolution of permeability, elastic modulus, and stress relaxation function for Type III Portland cement paste with water–cement (w/c) ratios of 0.45, 0.50, and 0.55. The stress relaxation function is shown to preserve its shape during aging; that function is numerically transformed into the creep function.     

Autogenous shrinkage in high-performance cement paste: An evaluation of basic mechanisms

In this paper, various mechanisms suggested to cause autogenous shrinkage are presented. The mechanisms are evaluated from the point of view of their soundness and applicability to quantitative modeling of autogenous shrinkage. The capillary tension approach is advantageous, because it has a sound mechanical and thermodynamical basis. Furthermore, this mechanism is easily applicable in a numerical model when dealing with a continuously changing microstructure. In order to test the numerical model, autogenous deformation and internal relative humidity (RH) of a Portland cement paste were measured during the first week of hardening. The isothermal heat evolution was also recorded to monitor the progress of hydration and the elastic modulus in compression was measured. RH change, degree of hydration and elastic modulus were used as input data for the calculation of autogenous deformation based on the capillary tension approach. Because a part of the RH drop in the cement paste is due to dissolved salts in the pore solution, a method is suggested to separate this effect from self-desiccation and to calculate the actual stress in the pore fluid associated with menisci formation.     

A study on characteristics of interfacial transition zone in concrete

The aim of this study was to observe the behavior of the interfacial transition zone (ITZ) of high-performance concrete that was under curing in saturated lime water. From the Scanning Electron Microscope (SEM), it was found that the pores and hydration products at the ITZ, within 100 μm between the paste and aggregate, permuted each other during the early hydration stage, and then appeared as a large lump or strip. They gradually became irregular and small lumps for the further curing age. At the curing age of 56 days, the pores almost concentrated within an area of 0-15 μm from the aggregate edge. The hydration products were much denser with the increase in its distance from the aggregate edge.     

Early-age deformation, drying shrinkage and thermal dilation in a new type of dental restorative material based on calcium aluminate cement

Dimensional changes during the first hour of hydration for small specimens of a dental material based on calcium aluminate cement (CAC) was examined. The study was conducted on specimens prepared in two different ways. First, intact tablets (three pieces per test) dipped in water were measured. Second, compacted specimens from four tablets were measured after 10 min of hydration. The dimensional changes were studied in both wet and dry conditions at 37 °C and in a dry condition at 25 °C. In the wet environment at 37 °C no dimensional change of the samples was observed. At normal room humidity (RH 55%) at both temperatures, shrinkage of 0.35-0.40% was observed. For comparison to the early-age drying shrinkage, a study of the drying shrinkage in mature material, hydrated for 50 and 100 days, respectively, was conducted. Furthermore the thermal expansion coefficient was determined and found to be close to that of tooth substance.     

Numerical simulation of hydration-driven moisture transport in bulk and interface paste in hardening concrete

In real concrete two types of cement paste can be distinguished, i.e., bulk paste and interface paste. Initially the paste in the interface zone will generally contain more water than the bulk paste and will therefore hydrate differently. Differences in relative humidity and associated differences in pore water pressure will result as well. If the interface paste and the bulk paste could hydrate individually, a situation will result where a relatively porous water-rich interfacial zone coexists with a relatively dry bulk paste. However, due to gradients in porosity, permeability, relative humidity and pore water pressure, a flow of moisture will start from the water-rich interfacial zone to the bulk paste. It will be shown how the moisture transport can be simulated numerically and how this transport phenomenon influences the overall rate of hydration of cement in concrete. Numerical results are compared with experimental data presented in literature. The relevance of modelling of this kind of transport phenomena is briefly dealt with.