Self-desiccation Shrinkage Research Articles

Deformation mechanisms of cement paste with ultra-low water-to-cement ratios under different curing conditions at early ages

Cement-based materials with ultra-low water-to-cement ratios (w/c) swell with external water supply, which may cause cracking and strength reduction. Studies on the volumetric stability of cement-based materials ignore the possible expansion occurring in the material before the starting point, standard curing (20 ± 2 ℃ and relative humidity (RH) > 95 %) for 28 d. Thereby, the deformation and mechanical properties of cement paste cured under standard and sealed conditions at early ages were compared. Deformation mechanisms were investigated by thermogravimetric analysis, 1H nuclear magnetic resonance relaxometry, and pore structure measurement. The saturation degree and hydration degree were both estimated. The results show that swelling occurs in cement-based materials with ultra-low w/c under standard curing, with increasing flexural and compressive strength. Gel water and capillary water contribute to large proportions of the total absorbed water. Cement paste with low w/c produces higher swelling when absorbs the same amount of water. Deformation mechanisms are attributed to the mitigation of self-desiccation shrinkage, calcium-silicate-hydrate gel swelling due to water absorbed, and solid volume increase of solids after hydration.

Determination of Concrete Shrinkage Initiation in Internally Cured and Conventional Concrete Decks

Abstract The extended performance of the concrete bridge deck depends on the concrete’s ability to resist early-age cracks, which are primarily induced by self-desiccation shrinkage. Several studies have been conducted to study the effect of shrinkage on the development of early-age cracks. When concrete shrinkage occurs, its magnitude may account for tensile stresses developing in the concrete deck over time. Therefore, determining when shrinkage stress initiates in concrete is critical to precisely predict and analyze early-age cracking. The main objective of this research was to determine the appropriate time at which shrinkage initiates at early age for various concrete mixtures under field conditions. To achieve this objective, two adjacent bridge decks, one with internally cured concrete (ICC) and one with conventional concrete, were constructed and instrumented to measure concrete strains and the temperature in both bridge decks during the first 30 h after concrete placement. The results of the field study were also compared with previous studies. Besides, a new approach to determine shrinkage initiation at early age depending on the strains of the free and restrained shrinkage was used in this research. The results showed that the time of concrete shrinkage initiation in both concrete decks was identical. For each deck, this time was approximately 5 h after concrete casting. The results showed that the time of the shrinkage development initiation in the ICC was consistent with the initial setting time.

Wood ash versus expanded clay aggregate as internal curing water reservoirs in high performance concrete

The performances of expanded clay aggregate and wood ash as internal curing water reservoirs were studied in the mortar phase of a hypothetical high performance concrete with a low water-to-binder ratio. The two materials substituted the 15 and 30% of the sand volume. Two different binders, Portland cement and high-volume fly ash blended cement, were used. The compressive strength and the volume stability of the mortars in sealed and air-drying conditions were studied. Furthermore, the desorption capacity of the internal curing water reservoirs and the internal humidity inside the mortars during the first days after casting were analysed. The results shown that the reduction in the self-desiccation shrinkage was higher when the expanded clay aggregate was used, even in air-drying curing conditions, due to its higher desorption capacity in low-relative-humidity environments in comparison to that of the wood ash. However, wood ash had a stronger beneficial effect on early age autogenous shrinkage without significantly increasing the drying shrinkage. The two alternative aggregates influenced the strength moderately. Considering the technical, economic, and environmental implications of using the two lightweight aggregates, wood ash is recommended.

Autogenous deformation and coefficient of thermal expansion of early-age concrete: Initial outcomes of a study using a newly-developed Temperature Stress Testing Machine

Thermal and autogenous deformations of high-performance concrete at early ages have significant impact on the performance, serviceability, and durability of concrete structures. While substantial past research has significantly improved our fundamental understanding of the autogenous shrinkage and of the coefficient of thermal expansion (CTE) of cement paste and mortar, such understanding for concrete, especially modern types of high-performance concrete at early ages, remains inadequate. Accordingly, a new Temperature Stress Testing Machine (TSTM) capable of generating reliable data on autogenous shrinkage and CTE of concrete from very early ages has been built. After a review of relevant major aspects, key features of the newly-built TSTM are reported, with a focus on advanced thermal regulation and deformation capturing systems. The outcomes of a study using the newly-developed TSTM on autogenous deformation and CTE of early-age concrete (two mixtures with water-to-binder ratio of 0.35 and 0.42) subject to different curing temperatures (23, 35 and 45 °C) are then presented. Higher curing temperatures significantly accelerate the development of autogenous shrinkage at very early ages, whereas after the initial period, the magnitude of autogenous shrinkage cured at the intermediate temperature of 35 °C gradually becomes the largest. Despite this cross-over, the maturity concept appears appropriate to approximate the effect of curing temperature on autogenous shrinkage of concrete at early ages. The linear CTE of early-age concrete, obtained using stepped temperature profiles, follows a clear rising trend after setting. Based on the newly-measured CTE, an attempt was made to separate thermal strain and self-desiccation shrinkage, in which the presence of non-negligible delayed thermal strain became evident.

Time-zero and deformational characteristics of high performance concrete with and without superabsorbent polymers at early ages

Internal curing with superabsorbent polymers (SAP) can help lower early-age cracking risk of high performance concrete, by reducing both autogenous shrinkage and thermal strain. In this paper, early-age properties of a concrete mixture (with water-to-binder ratio of 0.25) with and without SAP were examined. First, a practical and convenient method for determining time-zero of concrete with and without SAP based on the rate of temperature change induced by cement hydration was proposed. The validity of the proposed method was demonstrated through the good agreement between its determined time-zero values and the corresponding time (i) of divergence between chemical and autogenous shrinkage as well as (ii) when stress due to restrained deformation started to develop. Second, autogenous shrinkage of concrete with different SAP amounts was studied, together with the temperature dependency of autogenous and self-desiccation shrinkage of concrete containing 0.33% SAP. The addition of 0.33% SAP was found to have completely mitigated autogenous shrinkage of early-age concrete. Third, the coefficient of thermal expansion (CTE) of concrete with and without SAP was reliably measured. It is shown that both internal curing (using SAP) and external curing can effectively reduce the CTE, thereby reducing the associated thermal stresses and the risk of thermal cracking. In addition, the inclusion of SAP was found to compromise mechanical properties of concrete during the first 7 days, but such adverse effects became rather small at later ages.

In situ monitoring of early cement hydration by X-ray computed tomography

X-ray computed tomography was used to study the early hydration of cement. The two-dimensional slices and three-dimensional images were investigated, and porosity and unhydrated cement particles were analysed quantitatively. This work confirmed that X-ray computed tomography can be used to observe the early hydration of cement as an in situ and non-destructive technique. The cement paste, holes, unhydrated cement particles, and the mould were observed in two-dimensional slices distinguished by grey values. Self-desiccation shrinkage caused the separation of the specimen from the mould after 8 hours of hydration. After 24 hours, the residual water in the crack disappeared, the holes inside the specimen remained unstable and the number of unhydrated cement particles continued to decrease.

A review on testing methods for autogenous shrinkage measurement of cement-based materials

Autogenous shrinkage is an important factor for cracking of cement-based materials at early ages. This review summarizes different definitions and testing methods for autogenous shrinkage in literature. Autogenous shrinkage should be defined as the sum of chemical shrinkage and self-desiccation shrinkage. All testing methods in the literature can be divided into two categories: direct and indirect testing methods. Direct testing methods usually measure volumetric or linear dimensional changes. Indirect methods usually measure porosity or change in relative humidity of cement-based materials, which can reflect the change in autogenous shrinkage. Indirect methods need to establish correlations between autogenous shrinkage and porosity or relative humidity.

Controlling the coefficient of thermal expansion of cementitious materials – A new application for superabsorbent polymers

Abstract Self-desiccation occurring in low w/c cement paste leads not only to autogenous shrinkage, but also causes an increase of the coefficient of thermal expansion (CTE) at early ages. This increase can reach about 70% or even more in the first days of hydration. The latter effect is due to the internal relative humidity (RH) changes induced by temperature changes, which cause additional deformation acting in the same direction as the pure thermal dilation in a partially saturated cementitious material. Internal curing by means of superabsorbent polymers (SAP) has recently been proven effective for promoting hydration of cement and for limiting self-desiccation and autogenous shrinkage. This study shows that SAP are also able to control or even completely eliminate another negative effect of self-desiccation, the increase of the CTE. This is to the authors’ knowledge the first such application of SAP. In this work, both aspects of SAP use are investigated, with special focus on limiting the thermal expansion coefficient. First, self-desiccation and self-desiccation shrinkage and their reduction with SAP are investigated in cement pastes and mortars using water-activity measurements and linear shrinkage measurements with corrugated tubes. Next, the thermal expansion is studied experimentally using both volumetric and linear methods for a series of cement pastes and mortars of different w/c. Addition of SAP in cement pastes and mortars is proven to practically eliminate the increase of the CTE during the first days of hardening, limiting the CTE to only 60–70% of the value developed for materials without SAP addition. The reduction of the CTE is possible thanks to maintaining high internal RH levels and consequently reducing the temperature effect on RH.

Smart Additives for Self-Curing Concrete

ABSTRACTSelf-curing, or internal curing (IC), technology has been developed to counteract self-desiccation and autogenous shrinkage of high-strength/high-performance concrete (HSC/HPC), which is considered the "Achilles’ hill" of HSC/HPC [1]. According to ACI [2], IC refers to the process by which the hydration of cement continues because of the availability of internal water that is not part of the mixing water; while the internal water is made available by the pore system in structural lightweight aggregate (LWA) that absorbs and releases water. Recently ACI defined internal curing as “supplying water throughout a freshly placed cementitious mixture using reservoirs, via pre-wetted lightweight aggregates, that readily release water as needed for hydration or to replace moisture lost through evaporation or self-desiccation” [3]. Both definitions address the use of pre-wetted LWA as a self-curing (or internal curing) agent.According to the definition of the RILEM Technical Committee TC-196 [4], IC implies introduction to the concrete mixture a component, which serves as a curing agent. This agent can be either a normal aggregate introduced into the concrete mixture in water-saturated state or a new component (for example, an additive or special aggregate). Similarly to the division accepted in external curing, RILEM TC-196 distinguishes between two categories of internal curing: (a) internal water curing (sometimes called “water entrainment”), when the curing agent performs as a water reservoir, which gradually releases water, and (b) internal sealing, when the curing agent is intended to delay/prevent loss of water from the hardening concrete. Although water-saturated porous aggregate is still the most popular material among IC agents, super-absorbent polymers (SAP), ceramic waste, recycled aggregate and wood-derived products show promising properties. In view of this, self-curing covers not only use of pre-wetted LWA, but also other methods of curing: water curing by means of variety of curing agents introduced in the concrete mix, and also the methods based on internal sealing.The recent achievements in methods and materials for self-curing are reviewed, and the future trends in development of self-curing concrete are discussed.

Research on Self-Desiccation at early Age of High Performance Concrete with Polypropylene Fiber

This paper focuses on the impact which polypropylene fiber (PF) has on the self-desiccation effect at early age of high performance concrete (HPC). The experimental results indicate that PF has little influence on the Internal Relative Humidity (IRH) caused by self-desiccation effect of concrete, but can reduce early aged self-desiccation shrinkage of concrete. With the PF dosage increasing, the values of early self-desiccation shrinkage of HPC decrease first and then increase. In the experimental conditions, the value of self-desiccation shrinkage of concrete with 0.6Kg/m3 PF is the lowest one.

A new multi-scale modeling approach based on hygro-Cosserat theory for self-induced stress in hydrating cementitious mortars

The present paper focuses on the modeling of internal stresses induced by the restrained autogenous shrinkage of hydrating cementitious matrix in cement-based mortars. At very early age (0–48h), these self-induced stresses may be relatively high and even critical, especially for cementitious systems with low water-to-cement ratio, since the physico-chemical phenomena involved (hydration and self-desiccation) are particularly intense. To pursue the mentioned objective, an original multi-scale approach based on the application of hygro-Cosserat theory has been developed to model the self-induced stress variation in the cement paste surrounding the aggregates. In fact, the characteristic length scale parameter Lc in the Cosserat theory helps us to reduce the specimen size from macro-scale to micro-scale and even sub-micro-scale due to its explicit size effect features, which is not feasible in the classical theory, i.e. Cauchy–Bolzmann’s theory. The self-shrinkage phenomenon at early age has been observed and modeled via the experiments and a freshly defined Cosserat Size effect number (CS) based upon the Representative Volume Element (RVE) concept. The proposed method is capable of treating the internal stress and could be followed by cracks appearance investigation in the cementitious matrix surrounding the sand inclusions, which should occur inside of the RVE of mortar subjected to self-desiccation shrinkage during the hydration process at early age. The occurrence of these micro-cracking networks are confirmed by Scanning Electronic Microscopy (SEM) observations at the interface cement paste/aggregate performed on different mortars at early age. By taking advantage of the time-dependent Finite Element Analysis (FEA), the numerical outcomes are well agreed with the experimental observations coming from SEM. It concludes that the inclusion creates high hygro-stress concentration around the grains: when the number of inclusions increases, this hygro-stress could lead to a micro-crack network through the matrix.

Evaluation of Rate of Deformation for Early-Age Concrete Shrinkage Analysis and Time Zero Determination

Early age cracking is usually the result of internal tensile stresses induced by self-desiccation shrinkage (SDS) rather than external loading. Hence, the prediction of early age cracking risk is strongly linked to autogenous shrinkage development. Both the ultimate magnitude of shrinkage and the time zero (TZ), at which shrinkage starts to develop an internal stress, could be decisive for high-performance concrete (HPC) durability. The moment TZ can be considered as the borderline between autoplastic shrinkage and effective shrinkage. Deformation rate curves might be used as a framework to identify the three main phases of hardening that occur in cement paste as hydration progresses, and the development rate of autogenous shrinkage (AS). The present study proposes starting shrinkage measurement (autogenous or total) from the moment when the rate of deformation reaches its maximum value (the first peak in the curve) and the shrinkage strain rate curve pattern changes sharply, or at the end of plastic-shri...

New Approach of Multi Scale Modeling Based on the Hygro-Cosserat Theory for Self Desiccation Deformation of Cement Mortars at Early Age

In the present paper, we concentrate on the heterogeneous cement mortars and we treat them as Cosserat-based media. The autogenous shrinkage phenomenon at early age (from 1 up to 3 days after mixing) has been analyzed by means of Cosserat theory. The characteristic length scale parameter Lc in this theory helps us to change the size specimen from macro-scale to micro-scale using the theoretical size effect aspects. This methodology is also capable of treating cracks initiation and their appearance in the cementitious matrix surrounding the sand-inclusions, which should occurred inside of the Representative Volume Elementary (RVE) of mortar subjected to self-desiccation shrinkage during hydration at early age. By taking advantage of the Nonlinear Finite Element Analysis (NFEA), the numerical experiments have been performed. The numerical outcomes are well agreed with the experimental observations coming from Scanning Electronic Microscopy (SEM) images. It concludes that the inclusions create not only a hygro stress concentration around the grains but also the number of inclusions should influence the network in cementitous matrix.

Physico-chemical deformations of solidifying cementitious systems: multiscale modelling

At early stages of hydration and in autogenous conditions (no mass transfer with the outside), solidifying cementitious systems exhibit dimensional variations following two main processes: Le Chatelier contraction (also called chemical shrinkage) and self-desiccation shrinkage causing autogenous shrinkage. Chemical shrinkage results from the difference between the specific volumes of reactants (anhydrous cement and water) and hydration products. Early-age autogenous shrinkage is generally attributed to the development of a negative capillary pressure in the porous network related to the water consumption by the hydration reactions. If restrained, deformations associated to these shrinkages can induce the development of internal stresses high enough to generate cracking of the hardening material. The purpose of this study is to propose a multiscale approach to model the rate of self-desiccation shrinkage of cementitious materials at very early-age, between 0 and 48 h. Within the first hours, Le Chatelier contraction is computed from a formulation suggested in a later work which is based on the chemical equations of hydration and the specific volume of each phase. Then, when the setting of the cement paste takes place, the autogenous shrinkage is calculated according to the evolution of the capillary pressure and the stiffness of the cement paste. The stiffness is calculated by applying a classical homogenization method. Computed results are discussed and analyzed. Good agreements between experiments and simulations are achieved and a sensitivity study is performed to assess the influence of the cement fineness and the aggregate volume fraction on early-age autogenous strain.

Pore Structure of Concrete with Mineral Admixtures and Its Effect on Self-Desiccation Shrinkage

High-performance concrete (HPC) is known for the specificity of its pore system compared with that of conventional concrete. Indeed, its capillaries are characterized by very fine pores. Refinement of these pores mainly depends on the type, proportion, and fineness of the pozzolanic materials incorporated. The finest HPC pores, however, induce two opposite phenomena: on the one hand, better mechanical performances, which lead to considerable improvement in durability; and on the other hand, increased self-desiccation shrinkage caused by the high capillary depression in these finer pores. This study highlights both fundamental aspects resulting from the use of mineral admixtures in HPC. Concrete mixtures with two water-binder ratios (w/b) of 0.35 and 0.30, and three types of binders were used in this research. Shrinkage was measured on massive blocks that somewhat simulated a real concrete structure. The pore size distribution was measured at different ages by mercury intrusion porosimetry (MIP). The results obtained show that refinement of the capillary network and its evolution with time are closely related to mineral addition type and fineness. Moreover, refinement of the pores evolves significantly at a very early age depending on the type of binder used. This change in the pore structure of capillaries induces an increase in the magnitude of self desiccation shrinkage during this lapse of time. Also, the lower the w/b of the mixtures, the higher the effect of refinement of the capillary network on self desiccation shrinkage development. It was also found that the incorporation of mineral admixtures considerably reduces the average pore radius while it increases the amount of pores smaller than 15 nm (5.9 x 10 ―7 in.). In general, the smaller the average pore radius is, the higher the shrinkage will be.

Water consumption of the early-age paste and the determination of “time-zero” of self-desiccation shrinkage

Self-desiccation shrinkage (SDS) is closely related to the interior water consumption and the relative humidity (IRH) drop in the cement paste. Substantial self-desiccation shrinkage has been observed at very early-age for high performance concrete. However, it is difficult to investigate the IRH by conventional method of hygrometer at this time because the materials are still in the superhygroscopic range. In this paper, an automatically measuring system of meniscus depression is developed on the base of the mechanism of tensiometer and Laplace formula. The interior water consumption and the IRH changing within the paste could be automatically monitored at the very early-age (here specially refers to the stage from the beginning of casting till several hours after final setting). By using this system, the effects of water to binder ratio and replacement of cement by fly ash and ground granulated blast furnace slag on the self-desiccation were investigated for the very early-age cement paste. Experimental results could potentially explain the mechanism of the SDS at very early-age as well as determine the “time-zero” of SDS corresponding to its definition.

Techniques and materials for internal water curing of concrete

This paper gives an overview of different techniques for incorporation of internal curing water in concrete. Internal water curing can be used to mitigate self-desiccation and self-desiccation shrinkage. Some concretes may need 50 kg/m3 of internal curing water for this purpose. The price of the internal curing water is in the approximate range 0.1–1 €/kg.

Measurement of water transport from saturated pumice aggregates to hardening cement paste

In internal water curing of High Performance Concrete, it is fundamental to know how and when the water contained in the internal curing agent is released into the hydrating cement paste. In this study, X-ray absorption measurements showed that considerable transport of water from saturated pumice stone to hydrating cement paste with water/cement ratio 0.3 took place in the first days after casting and covered a distance of at least 4 mm. As a consequence, the total amount of water released by the lightweight aggregates, rather than the spatial distribution of the aggregates, is in this case the crucial factor to avoid self-desiccation shrinkage at early-age.

Pre-Soaked Lightweight Aggregates as Additives for Internal Curing of High-Strength Concretes

Abstract Self-desiccation and autogenous shrinkage of high-strength concretes is one of their drawbacks which cannot be readily accommodated by conventional curing. The concept of internal curing using lightweight soaked aggregates, to provide internal reservoirs of water which enable uniform curing of the whole cross-section, has been advanced by several groups. The present study is intended to develop this approach further, by optimizing the porosity and size of the aggregates to enable successful internal curing with only a small amount of aggregates, which could be viewed as additives rather than bulk replacement of conventional aggregates. The approach taken was to increase the porosity of the aggregates and reduce their size, to obtain a system with numerous internal reservoirs of sufficiently small spacing to allow water to be readily discharged from the aggregate and transported over the whole range of the matrix. It was shown that aggregates with porosities of about 50% by volume, a size of a few millimeters, and contents of less than 50 kg/m3 could provide full elimination of autogenous shrinkage in concretes having w/cm as low as 0.25, with only a small affect on strength. The parameters controlling the efficiency of the aggregates were assessed, indicating that their pore structure is the most important one, and that water from within the aggregates can be readily transported into the matrix to a distance of few millimeters.

Effects of microstructure on restrained autogenous shrinkage behavior in high strength concretes at early ages

Fluorescence microscopic examinations were conducted to identify damages induced by restraining autogenous shrinkage. Characteristics of fluorescent areas and their correspondence to autogeneous shrinkage behavior of high strength concretes were discussed. Silica fume concrete exhibited a greater creep potential when loaded at very early ages. The microstructure in sealed concretes with an extremely low water/binder ratio was porous. The vicinity of aggregate grains was more porous and weaker than the bulk matrix in sealed concretes. In addition, sealed silica fume concretes contained many unhydrated cement particles which were profiled by thin gaps between the core cement particles and the surrounding cement paste matrix. These features of microstructure were not observed in water ponded concretes. The detected fluorescent areas may be defects caused by selfdesiccation and autogenous shrinkage. The flaws had little effects on the development of strength. However, the presence of thin gaps around remnant cement particles may increase creep deformation to relieve internal stresses.