Early-Age Autogenous and Drying Shrinkage Behavior of Cellulose Fiber-Reinforced Cementitious Materials

The objective of this study was to determine how the early-age shrinkage behavior of cementitious materials is affected by the addition of saturated cellulose fibers under sealed and unsealed conditions. The sealed condition simulates autogenous shrinkage exclusively while the unsealed condition introduces drying shrinkage, as well. Although the primary focus was to determine whether saturated cellulose fibers are suitable to mitigate autogenous shrinkage as an internal curing agent, evaluating their effect under drying conditions provided further insight into the overall shrinkage behavior of the composite material. At additions of 1% by mass of cement, the cellulose fibers were found to show significant drying shrinkage cracking control while providing some internal curing. In addition, early-age shrinkage test results were supplemented with a quantitative measure of fiber dispersion based on comparing theoretical and experimental values of the fiber volume fraction in hardened cementitious samples. Results indicated that improved dispersion leads to improved properties

Studying Initiation and Growth of Shear Cracks in Reinforced Concrete Beams Using Full-Field Digital Imaging

The mechanisms which lead to shear failure of longitudinally reinforced concrete beams are complex and often misunderstood. In this paper, a combination of a special closed loop test technique and machine vision are applied to tests on two reinforced concrete beams failing in shear: one made of normal strength concrete and the second of high strength concrete. To control the progression of the shear crack, a closed-loop test technique was devised and employed such that the crack propagation was stable even during the post peak. To locate the crack and to measure the crack displacements, machine vision, a nondestructive measuring technique, was employed which gives full-field displacements of a viewed area. The displacement measurements are obtained by first grabbing digital images of the observed area at different stages of loading during the test. By comparing discrete subsets of the reference and target images using normalized cross correlation matching, the displacement fields of the images are calculated. Shear crack initiation and progression are monitored for the two beams tested through complete failure, and differences in the shear failures between reinforced beams made of normal and high strength concretes are noted

Influence of Clays on the Shrinkage and Cracking Tendency of SCC

The influence of different types of clay on the shrinkage and cracking tendency of fly ash modified self-consolidating concrete (SCCF) for the application of slipform paving were investigated in this study. The mortar phase of each mix was tested for autogenous shrinkage, total free shrinkage under drying and restrained shrinkage cracking. The mechanical properties (flexural strength, compressive strength, and modulus) were studied to supplement the results of the shrinkage and cracking tests. The plain SCCF mix was compared against the clay-modified SCCF mixes, as well as conventional SCC and slipform concrete (SFC) mixes. The results showed that the very early-age autogenous shrinkage of SCCF mortar was increased by the addition of clays due to adsorption effects. The effects of the clays on total shrinkage under long-term drying were found to depend mainly on the pozzolanic reactivity, but these effects were very slight at low dosages of about 1% by mass of binder. The early-age cracking tendency was aggravated by the clays composed of purified magnesium alumino silicate and metakaolin, but little influenced by the clay composed of kaolinite, illite and silica. Overall, the SCC mixture modified with both fly ash and a small amount of clay showed comparable shrinkage and early-age cracking performances as conventional SFC

Influence of Kaolinite Clay on the Chloride Diffusion Property of Cement-Based Materials

To constitute blended cement concrete with high chloride diffusion resistivity, the effects of kaolinite clay on the mechanical properties and chloride diffusivity of cement paste, mortar and concrete were investigated. Ordinary Portland cement was partially replaced by kaolinite clay at 0%, 1%, 3%, 5%, 7% and 9% by weight of cement. All blended cement-based samples were prepared using a w/c ratio of 0.5. The microstructure, workability, early-age and long-term flexural strength of pastes were tested. The chloride diffusivity of mortars was measured. And the compressive strength and chloride diffusivity of concrete were measured. Mercury Intrusion Porosimetry (MIP) was employed to evaluate porosity characteristics. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectra (EDS) were applied to investigate the micro morphology and chemical element distribution inside the cement matrix, and the rapid chloride migration (RCM) method was applied to test chloride diffusivity. The MIP test results show that the addition of clay improves the micro-pore structure in the cement paste and limits the introduction of chloride ions. SEM imaging suggests that the kaolinite clay is acting as both filler and accelerator of cement hydration. It is found that the addition of clay alters the water requirement of normal consistency and the setting time of cement, whereas it has little influence on the soundness. Compared to the control, the flexural strength of cement paste with 1% kaolinite clay increased by 30.41%, 39.04%, 36.27% and 38.32% at 1, 3, 7 and 90 days, respectively. The 28-day flexural strength only increased slightly. It is observed that the cement mortar with clay has lower chloride diffusion coefficient values compared to the plain mortar, the 28-day diffusion coefficient of chloride ion View the MathML source(DCl-) of cement mortar is decreased by 53.03% with 5% clay. The increase in compressive strength of the cement concrete with clay is 12%, 13.5%, and 28.4% compared to the control at 1%, 3% and 5% addition, respectively. The chloride diffusion coefficient of cement concrete decreases exponentially with the clay addition. The reduction of chloride diffusion coefficient of cement concrete is 8.68% and 18.87% at 1% and 5% clay, respectively. The 28-day compressive strength increases linearly with the chloride diffusion coefficient of the concrete

Surface Treatment of Cement-Based Materials with NanoSiO2

A dense surface structure of cement-based material is favorable for its resistance to the impacts of environment. In this work, effectiveness and mechanisms of the surface treatment of cement-based materials with nanoSiO2 of different states, that is, colloidal nanoSiO2 (CNS) and the in situ formed nanoSiO2 gel through the hydrolysis of its precursor of tetraethoxysilane (TEOS), by brushing and soaking techniques, were investigated. Results showed that both CNS and TEOS are capable of reducing the liquid and gaseous transport properties of hardened cement-based materials, although at a different extent. It revealed that the pozzolanic reactivity and the filler effect of nanoSiO2 are the main causes for the refining of the threshold size and the reduction of volume of the capillary pores, and they finally lead to a linearly reduction of the transport property. From this study, it can be reflected that surface treatment of cement-based materials with nanoSiO2 would be an optimal alternative of making concrete structure more durable

Rate of Thixotropic Rebuilding of Cement Pastes Modified with Highly Purified Attapulgite Clays

This study investigates the influence of highly purified, nano-sized attapulgite clays on the rate of structural rebuilding of cement pastes. A shear rheological protocol is implemented that measures the rate of rebuilding of pastes after being broken down under shear and maintained under stress corresponding to the weight of the material. This simulates a real casting situation during which the concrete is initially in motion, then cast in place and measures how quickly it gains green strength immediately after placement. The rate of recovery for different resting times and preshear conditions are considered. The strain rate decay curves are fitted with a compressed exponential model to obtain relaxation time. The results show that the purified attapulgite clays significantly accelerate rate of recovery of pastes, especially at early ages. However, this accelerating effect diminishes at longer resting times as hydration mechanisms begin to dominate

Modification of Cement-Based Materials with Nanoparticles

This is a summary paper on the work being done at the Center for Advanced Cement-Based Materials at Northwestern University on the modification of cement-based materials with nanoparticles, specifically nanoclays, calcium carbonate nanoparticles, and nanosilica. The rheological properties of clay-modified cement-based materials are investigated to understand the influence of nanoclays on thixotropy. The influence of the method of dispersion of calcium carbonate nanoparticles on rate of hydration, setting, and compressive strength are evaluated. And an in-depth study on the mechanisms underlying the influence of nanosilica on the compressive strength gain of fly ash–cement systems is discussed. The motivation behind these studies is that with proper processing techniques and fundamental understanding of the mechanisms underlying the effect of the nanoparticles, they can be used to enhance the fresh-state and hardened properties of cement-based materials for various applications. Nanoclays can increase the green strength of self-consolidating concrete for reduced formwork pressure and slipform paving. Calcium carbonate nanoparticles and nanosilica can offset the negative effects of fly ash on early-age properties to facilitate the development of a more environmentally friendly, high-volume fly ash concrete

Influence of Purified Attapulgite Clays on the Adhesive Properties of Cement Pastes as Measured by the Tack Test

This study evaluates the influence of small additions of highly-purified attapulgite clays (0.2% and 0.5% addition by mass of cement) on the adhesive properties of cement pastes. Adhesive properties are measured by the tack test, a novel method of evaluating the rheological properties of granular materials. To better understand the results of the tack test as they pertain to cementitious materials, a highly concentrated material that is evolving due to thixotropic rebuilding and hydration, they are supplemented with a measure of the viscoelastic properties over time obtained through low-amplitude oscillatory shear rheometry. The influence of different preshear conditions and resting times (age of paste) on the adhesive properties are determined. Results show the tack test to be a suitable method for obtaining useful information about the adhesive properties and structural evolution of the material in the fresh state

Study of the Mechanisms Underlying the Fresh-State Response of Cementitious Materials Modified with Nanoclays

The fresh-state properties of concrete can be tailored to meet the needs of specific applications, such as reduced formwork pressure for self-consolidating concrete (SCC), which requires high flowability, and slipform paving concrete, which requires compactibility during casting followed by enhanced green strength immediately after placement. Small additions of clays (especially nanoclays) have been found to be very effective in achieving these properties. The purpose of this study was to examine concentrated cement–clay systems at multiple scales to gain a better understanding of the mechanisms underlying this behavior. This was done by investigating the macroscopic flow properties over time through a shear rheological approach, where the viscosity evolution under a constant applied shear rate and a measure of structural rebuilding over time were obtained. The measured flow parameters were used to describe the microstructure in the plastic state through the Krieger–Dougherty model. This was then tied in with results obtained previously through scanning laser microscopy, a direct measure of the microstructure by focus beam reflectance method (FBRM). The dominating mechanisms behind the stiffening behavior were determined, and ultimately used to explain the response of SCC for lab-scale simulations of formwork pressure. The effective solid volume fraction and maximum packing density were found to be useful parameters in doing so, and helped to explain why nanoclays are an effective mineral admixture in modifying SCC for reducing formwork pressure

Effects of Colloidal NanoSiO2 on Fly Ash Hydration

The influences of colloidal nanoSiO₂ (CNS) addition on fly ash hydration and microstructure development of cement–fly ash pastes were investigated. The results revealed that fly ash hydration is accelerated by CNS at early age thus enhancing the early age strength of the materials. However, the pozzolanic reaction of fly ash at later age is significantly hindered due to the reduced CH content resulting from CNS hydration and the hindered cement hydration, as well as due to a layer of dense, low Ca/Si hydrate coating around fly ash particles. The results and discussions explain why the cementitious materials containing nanoSiO₂ had a lower strength gain at later ages. Methods of mitigating the adverse effect of nanoSiO₂ on cement/FA hydration at later ages were proposed