Analytical Methods for Prediction of Water Absorption in Cement-Based Material

China 0cean Engnering，Vol.23, No.4, pp.719 - 728C 2009 Chinese Ocean Engineering Sociely, ISSN 0890-5487Analytical Methods for Prediction of Water Absorption inCement-Based Material*WANG Li-cheng (王立成)1State Key Laboratory of Coastal and Ofshore Engineering ，Dalian University of Technology ，Dalian 116024, China(Received 9 September 2008; received revised form 5 May 2009; accepled 24 August 2009)ABSTRACTThe capillary absorption of water by unsaturated cement-based material is the main reason of degradation of thestructures subjected to an aggressive environment since water often acts as the transporting medium for damaging contami-nants. It is well known that the capillarity coficient and sorptivity are two important parameters to characterize the waterabsorption of porous materials. Generally, the former is used to describe the penetration depth or height of water trans-port, which must be measured by special and advanced equipment. In contrast，the sorptivity represents the relationshipbetween cumulative volume of water uptake and the square root of the elapsed time, which can be easily measured by thegravimetric method in a normal laboratory condition. In the present study, an analytical method is developed to build upa bridge between these two parameters, with the purpose that the sorptivity or the gravimetric method can be used to prdict the penetration depth of water absorption. Additionally, a new model to explain the dependence of sorptivity on initialwater content of the material is developed in order to fit the in situ condition. The comparison of predicted results by theanalytical method with experimental data or numerical calculation results, as well as some previous models, validates thefeasibility of the methods presented in this paper.Key words: uater absorption ; capillarity cofficient ; sorptivity; cement- based material ; initial uwater content1. IntroductionThe cement- based materials, such as the mortar and concrete, are often subjected to deteriorationprocesses in which water usually acts as the principal medium to transmit aggressive agents (e.g. chlo-ride or sulfate ions) (Yang et al., 2006). It is typical for the structures that are subjected to repeatedwetting and drying regimes, for example, the tidal zone of marine structures ，fluctuating groundwater,exterior facades, tunnel linings, and the materials are rarely in a state of full saturation ( Zhuang andLin, 1989; Saetta et al.，1993; Leech et al., 2003; He et al., 2005 ). The ingress process ofchloride ions ，largely depends on the water absorption process and water content, because under satu-rated conditions ions can diffuse through the continuously water-illed channels. However, when thewater content decreases, the diffusion process is hindered bv the decrease of mumber of the water-filledporous channels (Satta et al.，1993). Under中国煤化Iwalter uplake by cpillayMHCNMHG.* This work was financially supported by the Key Programs of Minitry of Education of China (Grant No. 109046), theScientific Research Foundation for the Returned Overseas Chinese Scholars， State Eduation Ministry, and the Pro-gram of Innovative Research Team of Education of China (Grant No. IRT 0518)1 Corresponding author. E-mail: wanglicheng2000@ 163 . com720WANG Li-cheng/ China Ocean Engineering, 23(4), 2009, 719- 728absorption plays a more dominant role in the ingress of chloride ions and also in the durability of mate-rials (Lunk, 1998).Several methods are available to measure the water absorption property of cement-based materials .Among them, the gravimetric method is the simplest one and therefore has been widely used in someprevious researches and applications although some other advanced techniques have also been utilized(Carpenter et al., 1993; Hanzic and lic, 2003). It is known that the shortcoming of gravimeticmethod is that it can not provide the information of moisture distribution in a sample. As a result it isimpossible to determine the position and movement of the advancing wetting front. However, in somecases, the profile of moisture content as well as the position of wetting front is of great importance toprecisely predict the material durability ( Hanzic and lic, 2003; Parlange et al., 1984).The sorptivity has been widely used to characterize the water absorption property of building ma-terials since it is a well-defined physical parameter and can be easily measured by means of the gravi-metric method. Hence it is also regarded as a measure of the potential durability of cement-based mate-rials ( Hall, 1989; Lockington and Parlange, 2003). The sorptivity defines the rate of water uptake byany porous material when it is exposed to a free water source. When the gravimetric method is applied,the cumulative volume of the absorbed water per unit area of the inflow surface，i, is expressed as :i = Stl,(1)where S is the sorptivity. Usually, S is experimentally determined in a defined initial state of dryness，but it has been revealed that the sorptivity also strongly depends on the initial and final water content ofthe material (Hall, 1994).The purpose of the present work is to propose an analytical method to predict the moisture distri-bution in a specimen during the water absorption process and the front wetting position based on thesorptivity obtained by the traditional gravimetric method. In addition， the dependence of sorptivity oninitial water content is also under consideration.2. Unsaturated Flow Theory2.1 Fundamental Governing EquationAccording to the moisture content, water transport in porous materials can be classified into twotypes, i.e. permeability and sorptivity. Permeability describes the flow of a fluid (e.g, water or air)under the action of gravity or a pressure gradient; sorptivity ( also defined as penetration) is linked to .characterize an unsaturated material's ability to absorb and transmit water by capillary suction forces(McCarter et al.，1992; Sabir et al.， 1998)_hat the permeabilty is awrong parameter for modeling the capillary flows中国煤化工because most of them are0YHCNMHGrarely saturated in practical use. Therefore, the urusatuatbu Luw uiouly ias ween proved to be a realis-tic method to investigate the water movement in cement-based materials ( Hall, 1989). The equationfor unsaturated flow through porous media is expressed as:= v.[ D(0)Vθ- K(0)] .(2)WANG Li-cheng/ China Ocean Engineering, 23(4), 2009, 719- 72872 lHere and elsewhere in the context, θ is the normalized water content, scaled to be zero and one for theinitial and saturated volumetric water contents (i.e. volume of water/ bulk volume of concrete)，日;and日。，respectively. Thus θ can be written as:θ=日-0;(3)日。-日;in which日is the volumetric water content under any state.In Eq. (2)，D(θ) represents the hydraulic diffusivity function, which depends on both the typeof material and the water content, and K(θ) is the hydraulic conductivity which is usually neglectedunless gravity is expected to have a significant efect (Carpenter et al., 1993). Therefore, for a one-dimensional semi- infinite system subjected to a boundary condition θ=1 at x = 0 and an initial condi-tionθ=0, x>0, t=0 (uniform initial water content within the sample of material), Eq. (2) maybe described mathematically by:Jθ=我(D(0)和)0