Effect Of Pore Pressure Research Articles

An experimental study on the damage characteristics of mechanical properties of anthracite induced by supercritical CO2 injection

Carbon dioxide (CO2) is a kind of greenhouse gas affects atmospheric temperature. The deep coal seam is a potentially favorable geological body for CO2 storage, as it can be coupled with enhanced coalbed methane production, which can offset partly the cost of storage. CO2 injection to the deep coal seam would be in supercritical state, i.e., ScCO2, which affects the mechanical properties of coal seam more significantly. In this paper, a study was initiated to investigate the effect of ScCO2 adsorption and pore pressure on the mechanical properties of anthracite by the mechanical testing system, and observations were conducted to analyze the micro-damage of anthracite before and after ScCO2 injection by the scanning electron microscope. The results indicated that the strength of anthracite would be reduced with the pore pressure increases. Corresponding to the He injection pressure with 4 MPa and 8 MPa, under the condition of confining pressure 10 MPa and temperature 40 °C, the compressive strength reductions were 10.2% and 35.9%, the elastic modulus reductions were 10.9% and 24.7%, and Poisson’s ratio increases were 16% and 49%, respectively. The strength of anthracite would also be reduced by ScCO2 adsorption. Corresponding to the ScCO2 injection time for 3 h and 6 h, the compressive strength reductions were up to 42.6% and 61.1%, the elastic modulus reductions were up to 10.4% and 41.3%, and Poisson’s ratio increases were up to 8.92% and 35.71%, respectively. The mechanical weak planes were produced by ScCO2 injection, which could reduce the mechanical strength of anthracite. Compared to anthracite samples with the He injection, the elastic modulus of anthracite with ScCO2 injection was reduced by 21.9%, the compressive strength was reduced by 23.9%, and the Poisson's ratio was increased by 5.3%. The results also indicated that the mechanical strength reduction of anthracite was more significantly caused by ScCO2 injection for the superimposed effect of adsorption and pore pressure.

Multiphysics Lattice Discrete Particle Model for the simulation of concrete thermal spalling

Explosive thermal spalling behavior during fire exposure is one of the major issues in the design of modern reinforced concrete structures. Previous experience on fire disasters indicates that spalling of concrete can have serious structural and economic consequences and must be taken into account in the design for fire. However, spalling mechanisms and their interaction still remain in dispute in the scientific community. In order to shed some light on this phenomenon, a discrete hygro-thermal model of concrete at high temperature called DTemPor3 is proposed and a full coupling scheme between DTemPor3 and the Lattice Discrete Particle Model (LDPM) is performed. The proposed multi-physical coupled model features the effect of pore pressure and temperature on the mechanical response as well as the impact of cracking on moisture mass transport and heat transfer. Simulations of typical spalling experiments show good agreements with data gathered from the literature for both high-performance concrete and ordinary concrete, demonstrating the accuracy of the proposed approach. Cracking localization is found to significantly impede the local pore pressure build-up due to the increase of pore or crack volume. The numerical simulations demonstrate that the spalling phenomenon can be successfully reproduced, only when the effect of thermal stresses is taken in account along with the effect of pore pressure on crack initiation.

Analysis of instability mechanisms of natural fractures during the approach of a hydraulic fracture

A fluid-driven fracture propagation model dominated by viscosity-leak-off regime was presented to investigate the mechanical stability of natural fracture (NF) when hydraulic fracture (HF) approaches. The induced stresses generated by the approaching HF were calculated by a fully coupled integro-differential equation. The pore pressure effect caused by fracturing fluid leak-off was simulated based on dual porosity medium theory. The critical failure conditions of NF were determined based on maximum tensile criterion and Barton-Bandis criterion. The sensitivity of the stability of NF to in-situ stress anisotropy, approaching angle and distance, injection rate, fracturing fluid viscosity, and NF properties was analyzed in detail. Simulation results reveal that the shearing-mode failure of NF is obviously easier than the opening-mode failure, and the shear slip zone is much larger than the tensile failure zone. As HF approaches NF non-orthogonally, the induced debonding process is unstable, and the debonding zone is asymmetric with respect to the center of NF. The tensile-induced expansion zone is primarily located in the portion of NF ahead of the HF tip, however the shear-induced slip zone can even occur on NF behind of the HF tip. Induced stresses alone have negligible effect on the stability of the NF that is unconnected to the HF. The change in pore pressure due to leak-off effect dominates the final stimulated reservoir volume during large-scale fracturing. The instability of the NF dominates the propagation trajectory of the subsequent HF. A more suitable arrested/crossing condition by extending Gu-Weng criterion was established to predict the path of HF propagation.

Numerical simulation of hydro-mechanical constraints on the geometry of a critically tapered accretionary wedge

A critically tapered active accretionary wedge was simulated using a numerical analysis of plastic slip-line theory to understand the mechanics of morphologic evolution. The concept of critical state soil mechanics was applied to describe the entire wedge area overlying a basal decollement fault. Presuming a condition of two-dimensional plane strain along the compressional direction, we obtained the numerical solution of conjugate plastic slip lines at a critical state of stress defined by the Coulomb yield criterion. The velocity vectors were obtained by applying the associate flow rule with the boundary conditions at the upper surface of the wedge. Finally, the detachment was determined from the effective stress condition inside the wedge and the sliding friction coefficient along the fault. Our numerical simulations demonstrate that the morphology of a critically tapered wedge is dependent on the frictional strengths of both the wedge materials and the basal fault. The critical taper angle decreases with increasing internal friction angle and decreasing basal friction coefficient. The results also revealed that the pore pressure controls the morphology of the accretionary wedge for cohesive sediments but not for non-cohesive materials. The effect of pore pressure on the morphology of a critically tapered accretionary wedge becomes more significant as the cohesion increases. Assuming that the cohesion is very low, we could infer the ranges of strengths that most observed wedge geometry data have 0.3–0.6 for the basal friction coefficient and ~35–45° for the internal friction angle of the wedge materials.

Synergetics of geological mediums and its impact on the effectiveness of the exploitation and exploration for mineral deposits

The mechanisms of joint influence of mountain and reservoir pressures, saturating fluid, structure elements of rocks and external dynamic effects on their behavior in natural conditions, in particular near of the well, are investigated. With specific examples, it is shown that the behavior of rocks with such a set of influencing factors is determined by the laws of synergetics and the combined action of external influences, uneven stress-strain state of the rocks, the pore pressure and chemo mechanical effects. Examples are the results of gas-flow and gas-metric studies of closed wells, as well as the results of explosive perforation and intensification of producing wellbores at different depths. Defects occurrence in minerals with a high modulus of elasticity is initiated by an external dynamic effect and independently under the action of the saturating fluid. Then, under volumetric non-uniform compression and reservoir pressure, gradual fracturing of terrigenous rocks occurs at the micro and macro level. The result of these processes is the formation of areas of the improved permeability near the wells during drilling, production and suspending. When drilling on traditional technology they will impair formation reservoir properties via infiltration of water and solid phase. In oil and gas wells and in closed wells - improve these properties. Analysis of the behavior of rocks from the synergetic position shows that the best mode of loading on the reservoir during wells drilling, wells completion and oil and gas production is depression (reduced pressure) on the reservoir. The known and new promising technologies for the intensification of oil and gas production are determined.

A coupled mathematical model for accumulation of wave-induced pore water pressure and its application

Prediction of the wave-induced instability of seabed due to the build-up of pore water pressure is essential for coastal engineers involved in the foundation design of marine infrastructure. Most Previous studies have been limited to decoupled residual mechanism for the rise in excess pore water pressures. In this study, the existing decoupled approach has been improved to consider the coupling effect between the development of pore water pressures and evolution of seabed stresses. Comparisons with the existing wave flume tests and geotechnical centrifuge wave tests have demonstrated that the developed coupling approach is capable of reproducing the accumulation of pore water pressure under cyclic wave loadings, and shows more promising predictions compared to the existing decoupled model, especially for the case of standing waves. The validated framework is further extended to a field scale numerical model to investigate the development of pore water pressures and the corresponding liquefaction susceptibility of seabed to water waves. The numerical results have revealed the different mechanism for wave-induced pore pressure build-up in marine soils between the developed model and the existing decoupled model. The coupling effect of residual pore pressure and seabed stress could accelerate pore pressure accumulation, which implies that the existing decoupled model may underestimate the liquefaction potential of seabed, particularly under standing wave loadings. Furthermore, results of the developed model have shown that the effect of wave nonlinearity on advancing seabed liquefaction is more noticeable for progressive waves than that for standing waves.

Capillary pore pressure effects on acoustic to seismic coupling detection of buried containers

The effects of rainfall on one type of buried container has been investigated at a test site near the University of Mississippi. Simulated and natural rainfall experiments were carried out over a three-year time period. The measurements allow for weathering effects on the buried container seismic to acoustic ratio detection contrast. The container was a filled 20 l plastic fuel container buried 30 cm in the natural loess soil. Six containers were buried, two were controls, two were exposed to the natural weather and two were used for simulated rainfall experiments. A loudspeaker was used to excite vibrations in the ground and a co-located vertical component geophone and a microphone measured the out of plane seismic/acoustic ratio in the vicinity of the containers. The rain induced wetting phase causes the container’s acoustic-to-seismic coupling contrast to increase and resonant frequency to decrease. The water drying phase restores the original frequency responses of smaller amplitude and higher resonant frequency. The phenomenon is cyclical. The capillary pressure in the pore spaces of soil grains reduces with wetting and increases with drying. These pore pressure changes are controlling the soil stiffness and acoustic response of the buried container.The effects of rainfall on one type of buried container has been investigated at a test site near the University of Mississippi. Simulated and natural rainfall experiments were carried out over a three-year time period. The measurements allow for weathering effects on the buried container seismic to acoustic ratio detection contrast. The container was a filled 20 l plastic fuel container buried 30 cm in the natural loess soil. Six containers were buried, two were controls, two were exposed to the natural weather and two were used for simulated rainfall experiments. A loudspeaker was used to excite vibrations in the ground and a co-located vertical component geophone and a microphone measured the out of plane seismic/acoustic ratio in the vicinity of the containers. The rain induced wetting phase causes the container’s acoustic-to-seismic coupling contrast to increase and resonant frequency to decrease. The water drying phase restores the original frequency responses of smaller amplitude and higher resonan...

Water-Gas Two-Phase Flow Behavior of Multi-Fractured Horizontal Wells in Shale Gas Reservoirs

Hydraulic fracturing is a necessary method to develop shale gas reservoirs effectively and economically. However, the flow behavior in multi-porosity fractured reservoirs is difficult to characterize by conventional methods. In this paper, combined with apparent porosity/permeability model of organic matter, inorganic matter and induced fractures, considering the water film in unstimulated reservoir volume (USRV) region water and bulk water in effectively stimulated reservoir volume (ESRV) region, a multi-media water-gas two-phase flow model was established. The finite difference is used to solve the model and the water-gas two-phase flow behavior of multi-fractured horizontal wells is obtained. Mass transfer between different-scale media, the effects of pore pressure on reservoirs and fluid properties at different production stages were considered in this model. The influence of the dynamic reservoir physical parameters on flow behavior and gas production in multi-fractured horizontal wells is studied. The results show that the properties of the total organic content (TOC) and the inherent porosity of the organic matter affect gas production after 40 days. With the gradual increase of production time, the gas production rate decreases rapidly compared with the water production rate, and the gas saturation in the inorganic matter of the ESRV region gradually decreases. The ignorance of stress sensitivity would cause the gas production increase, and the ignorance of organic matter shrinkage decrease the gas production gradually. The water film mainly affects gas production after 100 days, while the bulk water has a greater impact on gas production throughout the whole period. The research provides a new method to accurately describe the two-phase fluid flow behavior in different scale media of fractured shale gas reservoirs.

Slip length of methane flow under shale reservoir conditions: Effect of pore size and pressure

The conventional hydrodynamic equations (Navier-Stokes, Hagen-Poiseuille equations and Darcy’s law) are no longer applicable in shale gas recovery due to strong surface adsorption and slip flow effect on gas transport in nanopores. Understanding the shale gas transport behavior is important for reservoir evaluation and production optimization. Herein, we report a molecular simulation study of methane flow in organic nanopores under shale reservoir conditions (temperature: 300–450 K, pressure: 10–60 MPa), with pore sizes ranging from 2 to 20 nm. We use the grand-canonical Monte Carlo simulations to determine the methane content in nanopores. We analyze the density distribution using equilibrium molecular dynamics. The surface adsorption effect is almost negligible under high pressure (60 MPa) conditions on average density, while it plays an important role under low-pressure conditions. Finally, we use nonequilibrium molecular dynamics simulation to study the transport behavior of nano-confined methane molecules. We find that the pore size has a significant effect on the slip length under low pressure (10 MPa) conditions. In contrast, the slip length is almost constant under high-pressure conditions. Under most conditions, the slip length decreases with the increase of pressures. Such a trend is more obvious in small pores under high-temperature conditions. Our work should provide important insights into the quantification of slip length in shale reservoir conditions.

Pumping effect of wave-induced pore pressure on the development of fluid mud layer

Fluid mud widely exists in the movement of fine sediment, which is an important potential cause of marine geological disasters. It is important to understand the generation and migration mechanism of fluid mud. The purpose of this study is to investigate the vertical migration response of fine-grained sediments by wave-fluid test of pore pressure response of silty seabed under wave action. The results show that the adhesion between the silt particles and the additional pressure of the film water have a significant effect on the liquefaction characteristics of the silt. Even if the liquefaction degree is far more than 100%, the soil does not completely liquefy. The experiment result show that, there is concentrated movement of the split channel, and mesoscopic migration of fine particles between the gaps of the particles, pumping effect provides an effective source of supply for the development of the floating mud layer, it also exacerbates modern sedimentation when strengthening the vertical exchange of shallow soil in the seabed. Besides, the blending of the middle and lower layers of the previous soil with the recent sediments has increased the difficulty of identifying the time series of the modern sedimentary layer to some extent.

Contribution of Pumping Action of Wave-Induced Pore-Pressure Response to Development of Fluid Mud Layer

To investigate the vertical migration response of fine sediments, the pore pressure response of the silty seabed under the action of waves was tested. Under the action of waves, there is an obvious pumping phenomenon in the sludge accumulated by pore pressure. The excess pore water pressure caused by the waves in the seabed is unevenly distributed with respect to depth and there is an extreme value of up to 1.19 kPa. The pressure affects the liquefaction properties of the sludge. According to instantaneous-liquefaction judgment, the liquefaction of surface soil occurs, but the soil is not completely liquefied. Using theoretical calculations, the vertical source supply of floating mud development was analyzed. The pumping effect of the wave-induced excess pore pressure manifests in two aspects, as follows: (1) The centralized migration of splitting channels, which is visible to the naked eye, and (2) the general migration of fine particles between particle gaps at the mesoscopic level, which accounts for up to 22.2% of the migration of fine particles.

Experimental and numerical analyses of apparent gas diffusion coefficient in gas shales

A unified model of apparent gas diffusion coefficient in gas shales has been developed by considering the combined effects of Knudsen diffusion, Fick diffusion, and surface diffusion. Results showed that the model predictions matched the measured apparent gas diffusion coefficient in Longmaxi black shale reasonably well when the correction factor for real surface diffusion was approximately equal to 0.01.Results of sensitivity analyses revealed that Knudsen diffusion and Fick diffusion control the bulk of the methane diffusion in shale nanopores. The governing mechanism of methane molecules transport through shale nanopores varied depending on the overburden pressure, pore pressure, and resultant effective stress (i.e., contributions of Fick diffusion and Knudsen diffusion to the apparent gas diffusion coefficient varied depending on the level of pore pressure and effective stress).The apparent methane diffusion coefficient also varied depending on the size of the pore space. The effective radius of pores in a formation with a relatively high effective stress is smaller than that of the one with a relatively low effective stress. Greater apparent methane diffusion coefficient is, therefore, observed in a relatively lower effective stress formation because of the increasing contributions of Fick and Knudsen diffusion.Considering the combined effects of pore pressure and effective stress, we have found that the overall impact of gas production from shale formations (i.e. the decreasing pore pressure and the corresponding increase in effective stress) was to increase apparent methane diffusion coefficient, which would result into further stimulation of the gas production from shale reservoirs.

A state-dependent constitutive model for methane hydrate-bearing sediments inside the stability region

The presence of methane hydrate in soil pores alters the stress–strain and volumetric behaviour of soil. The extent of this alteration is affected by initial temperature and pore pressure when hydrate is formed inside the thermodynamically stable region. In this paper, a state-dependent critical state model is developed for methane hydrate-bearing sediments (MHBS) within the theoretical framework of bounding surface plasticity. A phase parameter is newly introduced into the constitutive model to account for the coupled effects of temperature and pore pressure on the mechanical behaviour of MHBS. This unique feature of the proposed model enables it to capture the behaviour of MHBS inside the methane hydrate stability region. A non-associated flow rule is adopted and a modified dilatancy expression is proposed considering the degree of hydrate saturation, phase parameter and stress state of MHBS. To verify the new model, computed results are compared to measured results of drained triaxial tests on MHBS with different morphologies and at different effective confining stresses, degrees of hydrate saturation and phase states inside the stability region. The comparison reveals that the model is capable of capturing the key features such as the evident strain-softening behaviour due to hydrate degradation and the change in stress–strain and volumetric behaviour of MHBS at different initial conditions inside the stability region.

Accumulation of Pore Pressure in a Soft Clay Seabed around a Suction Anchor Subjected to Cyclic Loads

A suction anchor is an appealing anchoring solution for floating production. However, the possible effects of residual pore pressure can be rarely found any report so far in term of the research and design. In this study, the residual pore pressure distribution characteristics around the suction anchor subjected to vertical cyclic loads are investigated in a soft clay seabed, and a three-dimensional damage-dependent bounding surface model is also proposed. This model adopts the combined isotropic-kinematic hardening rule to achieve isotropic hardening and kinematic hardening of the boundary surface. The proposed model is validated against triaxial tests on anisotropically consolidated saturated clays and normally consolidated saturated clays. The analytical results show that the excess pore water pressure accumulates primarily on the outside of the suction anchor, whereas negative pore water pressure mainly on the inside. The maximum values of both sides appear in the lower part of the seabed. According to the distribution characteristics of the residual pore pressure, a perforated anchor is proposed to reduce the accumulation of excess pore water pressure. A comparative study generally shows that the perforated anchor can effectively reduce the accumulation of excess pore water pressure.

Study on the Influence of Pore Water Pressure on the Stability of Slope

The influence of pore water pressure on slope stability under water drop and seismic action is studied in this paper, taking an embankment as the research object. The temporal changes of seepage field are analyzed first, and then the stability of dam slope is investigated by coupling the stress field with the seepage field. During the process of water drop, the minimum safety factor of slope stability appears at the moment when it stops falling. The greater the falling speed is, the faster the pore water pressure dissipates, and the smaller the safety factor is. Seismic action stimulates the increase of excess pore pressure, and whether to consider the coupling effect of seismic load and pore pressure change will make the results of stability analysis quite different. The cumulative effect of pore pressure may be the crucial factor of the dynamic instability of two-phase media slopes.

Nonlinear creep damage model considering effect of pore pressure and analysis of long-term stability of rock structure

Water changes natural seepage in the surrounding geological environment and weaken rock mass of valley during reservoir impounding, which result in large time-dependent deformation of foundation of dam. A nonlinear creep damage model considering water effect is proposed by taking into account the influence of pore pressure on the spherical stress tensor and yield surface. The elastic-viscoelastic model is consisted of a spring and several cascaded Kelvin models. The model is implemented in the ABAQUS user material subroutine. The accuracy of the model is verified by the triaxial creep test of saturated sandstone and dry sandstone. Then it is applied in the valley contraction analysis during impounding of Xiluodu high arch dam.

Evaluation of moisture susceptibility of asphalt-aggregate constituents subjected to direct tensile test using imaging technique

The bond strength of asphalt binder and aggregate is an important parameter to evaluate the mixture ability to resist moisture damage. An innovative approach was utilized where adhesive failures of asphalt-aggregate constituents were computed in direct tension using the Pneumatic Adhesion Tensile Testing Instrument (PATTI). Hydrated Lime (HL) and Pavement Modifier (PMD) were used as anti-stripping fillers in asphalt mastics, while Evotherm (EV) was incorporated as the Warm Mix Asphalt (WMA) additive. Three long-term conditioning levels were performed namely; unconditioned, conditioned long-term aged (LTA) + 1 freeze-thaw (F-T) cycle and LTA + 3F-T cycles prior to testing. The Accelerated Laboratory Vacuum Saturator (ALVS) was integrated to subject asphalt constituents to the combined effects of pore pressure and high temperature. The conventional moisture conditioning and aging procedures were also modified to better mimic the actual field condition. An interesting observation pertaining to the moisture-damage mechanism was made in this study where moisture penetrated into the asphalt-aggregate interface via holes on the asphalt film. Compared to asphalt mastics, more holes were observed to develop on the binder specimens. The percentage adhesive failure of asphalt constituent test specimens was quantified using 2-D imaging technique. The percentage adhesive failure of asphalt specimens increased with LTA and F-T cycle, while tensile strength reduced with moisture conditioning. Asphalt mastics specimens exhibited relatively lower percentage adhesive failure than binder specimens attributed to the addition of fillers that stiffened and reduced binder moisture susceptibility. The image analysis results showed that the percentage adhesive failure for specimens prepared with polymer modified binders was lower than unmodified binder specimens. Samples using PMD exhibited slightly lower percentage adhesive failure than HL specimens. The percentage of failure reduced further with the incorporation of EV, regardless of aggregate type. Specimens using limestone also showed superior moisture resistance than granite specimens in moist condition.

The influence of pore pressure on fracture behaviour of Normal-Strength and High-Performance Concretes at high temperature

Explosive spalling consists in the sudden separation of concrete layers or pieces from the surface exposed to fire, due to (a) stress caused by thermal gradients and external loads and (b) pore pressure rise induced by water vaporization. As regards the material properties, the phenomenon is affected by some meso- and micro-structural aspects mainly concerning the interaction during heating among cement paste, aggregate and fibre, thus influencing porosity and permeability variation at high temperature. Focusing on the effect of pore pressure in fracture behaviour of Normal-Strength and High-Performance Concretes, an experimental campaign has been planned, aimed at investigating the role played by the mix design. Nine concrete mixes have been studied considering three grades (fc ≥ 40, 60 and 90 MPa), three aggregate types (silico-calcareous, basalt and calcareous aggregates) and different fibre types and contents (monofilament or fibrillated polypropylene fibres). The experimental study, based on indirect tension tests performed under different levels of sustained pore pressure, clearly shows that the influence of pressure on concrete fracture response during heating strongly depends on mix design.

Basement Fault Reactivation by Fluid Injection Into Sedimentary Reservoirs: Poroelastic Effects

AbstractTo investigate mechanisms of seismic fault reactivation in crystalline basement in response to fluid injection in overlying sedimentary reservoirs, we conducted three‐dimensional finite element simulations to assess the effects of direct pore pressure communication and indirect poroelastic stress transfer on the change in Coulomb failure stress of favorably oriented faults of varying permeability structure in normal, strike‐slip, and reverse faulting stress regimes. We demonstrate that the direct pore pressure effect transmitted along a hydraulically conductive fault exceeds the indirect poroelastic effect, but alone is insufficient for fault reactivation in the basement. The poroelastic effect on the Coulomb failure stress results from induced normal tractions and, to a lesser extent, from induced shear tractions that relate to the flexing of the fault as the reservoir expands poroelastically with fluid injection. Assuming a higher Biot coefficient for reservoir over basement rock as previously reported, the combined direct pore pressure and indirect poroelastic effects result in reactivation of hydraulically conductive faults in the basement in normal and strike‐slip faulting stress regimes and in the reservoir in reverse faulting regimes. Sealing normal faults that are not preferentially conductive also preferentially reactivate in the reservoir. These findings apply to injection in either hanging or footwall of normal and reverse faults. Reducing the contrast in Biot coefficient between reservoir and basement favors fault reactivation in the reservoir for injection in the footwall in normal faulting stress regimes. These simulations demonstrate that geomechanical models without coupled poroelasticity underestimate the potential of fault reactivation in crystalline basement.

Design and Evaluation of a Deformable Sensor for Interstitial Pore Pressure Measurement in Concrete under Very High Stress Level

Previous studies have shown the strong influence of free water saturation ratio on the triaxial behavior of concrete at high confinement. This influence of the free water is usually attributed to a pore pressure effect. This article presents an experimental method aiming at measuring the pore pressure of free water into concrete samples under very high mean stress. Two types of deformable pressure sensors were designed and tested. The first one works in hydrostatic compression while the second one acts as a flexible membrane. The two sensors give comparable results and show that pore pressure may reach several hundred MPa in saturated concrete sample under a maximum 400 MPa hydrostatic compression. Such levels of pressure may explain the loss of shear strength and increase in volumetric stiffness observed on the macroscopic behavior of concrete due to the presence of free water.