Building and Environment

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Building and Environment 44 (2009) 688 693 Contents lists available at ScienceDirect Building and Environment journal homepage: www.elsevier.

Institute of Sustainable Energy Technology, School of the Built Environment, University of Nottingham, University Park, Nottingham NG7 2RD, UK article info abstract Article history: Received 18 March

1 Building and Environment 44 (2009) Contents lists available at ScienceDirect Building and Environment journal homepage: Influence of cementitious binder content on moisture transport in stabilised earth materials analysed using 1-dimensional sharp wet front theory Matthew Hall *, David Allinson Institute of Sustainable Energy Technology, School of the Built Environment, University of Nottingham, University Park, Nottingham NG7 2RD, UK article info abstract Article history: Received 18 March 2008 Accepted 13 May 2008 Keywords: Granular materials Stabilised earth Moisture sorption Partially-saturated flow Sharp wet front model Experimental work was conducted to determine the influence of cementitious binder content on the moisture transport in partially-saturated stabilised earth materials. Data is presented for the influence of binder content on the initial rate of suction and the sorptivity. The partially-saturated sorption obeys the i/t 0.5 linearity rule demonstrating dependence on the s/h 0.5 relationship. The transfer of moisture has been analysed and explained using 1-dimensional sharp wet front theory. The increased porosity, as a result of increasing binder content, gives significant increase in sorptivity. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Stabilised earth materials can, in engineering terms, be described as multiphase granular composites whose components (whether chemically inert or active) are typically defined by particle diameter and can therefore be classified by their particlesize distribution. The term stabilisation refers to the application of a process and/or additive component that enhances the cohesion, Young s modulus or another physical property. By far the most common forms of stabilisation are (i) dynamic compaction and (ii) addition of hydraulic binders, e.g. cementitious materials. Dynamic compaction increases the inter-particle friction/interlock whilst lowering the void content. The addition of hydraulic binders increases the internal cohesion of the material and enhances durability and toughness [1 4]. Stabilised earth materials commonly exist in civil engineering applications such as pavement construction, embankments and contaminated land treatment. However, stabilised earth has also been effectively used to provide low-cost housing in developing countries as well as for a modern alternative building material [1,4 6]. It offers the advantages of rapid, low-energy construction combined with operation energy savings and enhancement of indoor thermal comfort due to its hygrothermal behaviour, i.e. simultaneous relative humidity and temperature buffering [3,6,7]. The addition of cementitious materials for soil stabilisation can be as high as 20% wt of dry material, however, it is commonly * Corresponding author. Tel.: þ44 (0) ; fax: þ44 (0) address: matthew.hall@nottingham.ac.uk (M. Hall). applied at 10%, and one of the most common stabilisation methodologies is the addition of 6% wt ordinary Portland Cement followed by dynamic compaction close to the Proctor optimum moisture content [1,3,8]. In predominantly granular materials, the hardened cement paste bonds particles together by surface adhesion between the paste and particle surfaces. Cement stabilisation is therefore most effective on low-cohesion soils, partly because it is difficult to ensure good distribution of stabiliser amongst clays [9,10]. Non-cohesive soils typically have particle sizes larger than cement grains and can therefore be coated with cement. The hydrated cement bonds soil particles at their points of contact, thereby leading to significant strength increase of the soil. The increase in strength is a function of the number of points of contact between soil particles (which itself is a function of the gradation of the soil) and of compaction to bring soil particles into the closest possible contact. In cohesive soils, many particles are finer than cement grains and thus cannot be coated by cement. A three-phase reaction with the clay occurs: 1. Hydration reactions form cement gels on the surface of clay aggregations, and the hydrated lime (Calcium Hydroxide) that is freed during hydration reacts with the clay. 2. Clay agglomerations are disaggregated by hydration products and are penetrated by the resultant cement gels. 3. Cement gels and the clay aggregates become intimately bonded. This results in both an inert sand cement matrix and a matrix of stabilised clay in the new structure. Hydraulic conductivity is normally measured under saturated conditions. This is only part of the story since suction is negligible at /$ see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi: /j.buildenv

M. Hall, D. Allinson / Building and Environment 44 (2009) 688 693 689 saturation and flow occurs due to external motivation force such as temperature or pressure differential.

2 M. Hall, D. Allinson / Building and Environment 44 (2009) saturation and flow occurs due to external motivation force such as temperature or pressure differential. Partially-saturated flow is known to be typically dominated by suction gradients [11]. Concrete is also a granular hydraulically-bound composite and presents unusual pore size distributions. Previous research shows that these do not normally obey the extended Darcy equation for partially-saturated flow within the pore structure [11,12]. Porosity occurs in (i) the cement gel, (ii) the porous aggregate soil constituents, and (iii) the inter-particle voids due to particle geometry. Pore sizes can range from less than 10 nm in the case of cement gel pores through the micron scale up to millimetres in the case of sand or gravel inter-particle pores [13]. Compacted cement-stabilised soils present an unusual problem due to presence of clay and binder clay interactions and are the topic of this paper Dimensional sharp wet front (SF) theory In saturated porous media the permeability is determined by measuring the flow velocity of water (induced by pressure differential) and referring to Darcy s law. For partially-saturated porous media the capillary potential (or suction), J, is introduced such that vector flow velocity q can be written [12]: q ¼ KðqÞ dj (1) dx where dj/dx ¼ gradient of capillary potential and K(q) ¼ moisturecontent dependent hydraulic conductivity. The water content, q, of a porous material can be calculated using [14]: q ¼ðw w min Þ=ðw max w min Þ 0 < q < 1 (2) where w ¼ the relative moisture content, w min ¼ the relative moisture content in equilibrium at 0% relative humidity, and w max ¼ relative moisture content at capillary saturation. Note that a fully dried material can only exist after drying to constant mass at 105 C, and a fully saturated material can only occur under vacuum. When liquid moisture is absorbed by a porous body, the sharp front (SF) approximation is that the advancing wet front of the liquid can be represented by a rectangular profile, as shown in Fig. 1. It is assumed that for the wetted region w w w max, and that this is uniform and constant. The wetting front itself has a hypothetical constant capillary potential, J f which differs from the capillary potential of the partially-saturated region ahead by the amount J J f. 1-Dimensional absorption into an infinite dry porous medium of length L begins at the interface between a static water source and the porous body where x ¼ 0 at a time t ¼ 0 [11]. Note that the advancing wet front is located at x f ¼ l(t). The porous medium has an effective permeability coefficient, K e, and an effective porosity, f e, which in the wetted region is assumed to be at capillary saturation with an effective water content of q f. In relation to the cumulative (volume) per unit inflow surface area, I (mm), f e ¼ i/l. Using the partial differential expression for Darcy s Law, the flow rate can be expressed as [11]: Fig. 1. Sharp wet front 1-dimensional water sorption into an infinite porous medium adapted from Hall & Hoff [11]. By recalling that u ¼ di/dt, and that I ¼ f e l, then the following differential can be derived: di dt ¼ K J f ef e i Using integration this becomes: i 2 ¼ 2K e f e J f t þ constant (6) For the point at which spontaneous absorption of water begins x ¼ x f ¼ I ¼ t ¼ 0, therefore the constant in Equation (6) is zero. This gives a new expression for the cumulative volume of absorbed water per unit inflow surface area, I (mm 3 /mm 2, or simply mm), as a function of elapsed time t (min) [11]: 0:5t 0:5 i ¼ 2f e K e jj f j (7) In the case of absorption by capillary suction, I can be determined experimentally as I ¼ Dm/Ar w [11,15]. Since the sorptivity, S, is the linear regression slope of the straight line i/t 0.5, then in the 1-dimensional case S can also be expressed as: S ¼ i 0:5 t 0:5 ¼ 2f e K e jj f j (8) Viscosity, h, and surface tension, s, are inversely proportional when the linearity rule for partially-saturated flow applies such that (s/h) 0.5 is proportional to S, and hence the chief motivational force in the sharp wet front model is the gradient of capillary potential. (5) df u ¼ K e dx ¼ K J f e (3) l Note that at x ¼ 0 the total potential F ¼ P 0,whereP is the hydrostatic fluid pressure. Also, at x ¼ x f the total potential F ¼ P 0 þ J f.therefore, referring back to Equation (3) df/dx ¼ dj f /dl, and so because the effective moisture content is uniform in the wetted region [11]: du dx ¼ K d 2 F e dx 2 ¼ 0 (4) Fig. 2. The three-phase soil model adapted from Barnes [19].

3 690 M. Hall, D. Allinson / Building and Environment 44 (2009) Table 1 Physical properties of stabilised earth mix designs Soil component proportions (kg/kg 10) Gravel Sand Silty clay CEM IIa OMC (% wt dry) Dry density [r d (kg/m 3 )] Void ratio (e) Porosity (n) Sorptivity [S (mm min 0.5 ] r s (kg/m 3 ) Mix 433 (0%) (3%) (6%) (9%) (0%) (3%) (6%) (9%) (0%) (3%) (6%) (9%) f cu 0 (N/ mm 2 ) This assumes a negligible air pressure differential and/or temperature differential between x ¼ 0 and x ¼ L. 3. Experimental work and specimen physical properties Stabilised earth is a porous, hydraulically-bound granular composite produced by dynamic compaction of aggregate materials and can be described using the geotechnical soil model, as illustrated in Fig. 2. The particle-size distribution for SRE must normally fall within the upper and lower limits designated by the International Centre for Earthen Architecture, CRATerre [16]. A discrete selection of quartzite sub-soil materials were used to manufacture the SRE test specimens which consisted of a mm rounded gravel, a <5 mm medium grit sand, and a silty clay. These individual sub-soil components can then be proportioned and mixed together to form a range of sub-soils that all conform to the required particle grading for SRE materials. The parameters of aggregate mineralogy, particle angularity and clay mineralogy are constant, whilst the only variable is particle-size distribution enabling very careful manipulation of grading parameters. Three mix designs were chosen and their composition and physical properties are given in Table 1. The optimum moisture content (OMC) for dynamic compaction was determined using the standard Proctor test and was found not to vary significantly between binder contents of 0% and 10% wt, as discovered by other researchers [10]. Specimens were prepared as 100 mm cubes dynamically compacted at optimum moisture content. The specimens were cured for a minimum of 28 days in a curing chamber at 20 C(2 C) and 75% RH (5%) prior to testing. Since the material has been compacted to maximum dry density r d (at OMC) then V w ¼ V v at saturation and V v ¼ V a when oven dried to constant mass at 105 C. In reality, complete saturation is rarely achieved under atmospheric conditions as some of the voids may be impermeable. The earth materials used in this study were stabilised using a CEM IIa Portland Cement hydraulic binder. The density of the CEM IIa hardened cement paste (HCP) was assumed to be 3150 kg/m 3 [13], and the values for constituent solid particle density were determined using the gas jar method (see Table 1 for values). The mean particle density r s was calculated as the weighted sum of the particle densities of the constituents: r s ¼ Xn i ¼ 1 x i r si (9) Where r si ¼ densities of each constituent, i ¼ 1 / n (kg/m 3 ), and x i ¼ volume fractions of each constituent (m 3 /m 3 ). Note that void ratio, e, isgivenbyv v /V s and the porosity, n, is calculated from the mean particle density, and the dry density, r d, of the rammed earth where: n ¼ 1 r d rs (10) Fig. 3. Interrelation between capillary potential and theoretical maximum height of capillary rise adapted from Hall [14]. Fig. 4. The capillary sorption wick test apparatus adapted from Hall & Djerbib [17].

4 M. Hall, D. Allinson / Building and Environment 44 (2009) Fig. 5. A graph to show the influence of cement content on the initial rate of suction for the 433 mix design. Fig. 8. A graph to show the influence of cement content on the sorptivity for the 433 mix design. (kg/m 2 min). It provides experimental validation for the moisturecontent dependent nature of partially-saturated flow in porous media, as explained previously. Since the phenomenon of capillarity is itself caused by the attraction between the media surface and the fluid (determined by surface tension), it follows that the maximum height of capillary rise, h c, is dependent upon the pore radius, r, such that: h c ¼ 2s rr w (11) Fig. 6. A graph to show the influence of cement content on the initial rate of suction for the 613 mix design. Since water content is fixed by the soil OMC value, the free water/ cement (w f /c) ratio is variable and therefore decreases as binder concentration is increased. This obviously limits the degree of hydration which can occur, as is the case with most stabilised soils. For each of the three soil types, if the dynamic compaction input energy and OMC energy are kept constant then increasing the binder content increases the characteristic unconfined compressive strength, f cu 0, and the void ratio, e (see Table 1). 4. Analysis of moisture sorption The initial rate of suction (IRS) is given as the mass of sorbed moisture per unit inflow surface area against the elapsed time Therefore in the 1-dimensional case, the sharp wet front can theoretically be positioned at x ¼ h c, and the decrease in capillary potential J occurs as we move from x ¼ 0 towards x ¼ h c as illustrated in Fig. 3. The initial rate of suction is measured experimentally by gravimetric determination of sorbed water (due to capillary suction) by the porous solid at particular time intervals. The water temperature is kept constant, normally w20 C, with a 1 C variation about the mean in order to minimise significant variation in h. The procedure used was the IRS wick test (devised by the author Hall [17]) which uses a 30 mm high OasisÔ wick which is 80 mm in diameter and partially immersed in the water reservoir, as illustrated in Fig. 4. The initially dry specimen is placed on top of the saturated wick and moisture transfer occurs by capillary suction. The wick has previously been observed to offer negligible capillary resistance to hygric transfer [17]. The cumulative mass of sorbed water is measured at 1 min time intervals and the measurement period is a maximum of 30 s [15,17]. Fig. 7. A graph to show the influence of cement content on the initial rate of suction for the 703 mix design. Fig. 9. A graph to show the influence of cement content on the sorptivity for the 613 mix design.

5 692 M. Hall, D. Allinson / Building and Environment 44 (2009) Fig. 10. A graph to show the influence of cement content on the sorptivity for the 703 mix design. The effect of increasing binder content on the initial rate of suction, for each of the three soil types, is shown in Figs In all cases, the initial rate of suction is considerably higher with increased concentration of cement stabilisation, and subsequently followed by a more rapid decrease in suction over elapsed time. As we have shown for the rate of moisture inflow, q, the gradient dj/ dx is related to the term K(q), therefore the more rapid decrease in IRS over elapsed time is caused by the greater volume of sorbed water per unit inflow surface area, i, seen in specimens with higher binder content. Although the intrinsic sorptivities of the three different soil types are different due to changes in void ratio and pore size distribution, the same patterns for increased cement addition are observed in all cases. This indicates that it is the cement microstructure and/or the cement soil interactions that influence sorption by causing increased porosity. The influence of binder content on sorption rates is shown by the sorptivity plots for each of the three soil types in Figs In all cases the sorptivity is linear against the square root of elapsed time indicating that the simple dependence of (s/h 0.5 ) presides in stabilised earth materials regardless of binder content. This is not usually the case in concrete materials, which is also a hydraulicallybound granular composite, where deviation from the linearity rule has often been observed [11,12]. Since the addition of cement appears to increase sorptivity, the reduction in capillary potential is most likely created through enlargement of the effective mean pore diameter. The sorptivity appears to exhibit a positive relationship with void ratio (Fig. 11), which suggests that the addition of cement has the effect of increasing the quantity of permeable pore space within the material. In fact, pores are quite simply the voids that exist between elementary particles and/or assemblages of particles. The various ways through which pores can be created within a soil structure is illustrated in Fig. 12 [18]. The simplest forms of intra-elemental pores occur due to geometric arrangement of inert granular elementary particles, e.g. silt, sand or gravel. The size of these interparticle pores is generally greater in soils that have a correspondingly larger number of rounded granular particles (e.g. beach sand). Intra-elemental pore spaces also occur inside clustered groups of cohesive elemental particles such as clays. Within the context of an assemblage, pores can occur inside the matrix of the material (intra-assemblage), around the points of contact between two or more assemblages (inter-assemblage), or even as a passage between assemblages separating them entirely (trans-assemblage) [18]. Clay minerals may normally act as expansive pore blockers that swell in the presence of moisture and inhibit sorptivity. It is therefore hypothesised that the three-phase reaction occurring between clay minerals and hydraulic binder reaction products (Calcium Silicate Hydrates and Calcium Hydroxides), especially the disaggregation of clay agglomerates, produces the increased sorptivity observed in stabilised earth materials. Indeed, at higher concentrations the cement gel may begin to produce its own pore blocking effect even if the hydration reaction is incomplete. 5. Conclusions In dynamically compacted stabilised granular media, the addition of cementitious binders increases porosity and sorptivity significantly with increasing binder content. The increase in porosity results in higher initial rates of suction but that decreases very sharply over time due to the moisture-content dependent nature of the capillary potential. The non-linearity of partiallysaturated capillary sorption normally present in concretes does not appear to apply to cement-stabilised soils. Conformity to the linearity rule demonstrates dependence on s/h 0.5 and so sharp wet front theory can be used to explain and model partially-saturated Fig. 11. A graph to show the relationship between void ratio and sorptivity for each mix design at varying cement contents.

6 M. Hall, D. Allinson / Building and Environment 44 (2009) Fig. 12. An illustration of the geometry and formation of pore spaces inside granular and soil materials adapted from Mitchell [18]. moisture transfer in stabilised granular media. Compliance with linearity may be due to the low water/cement ratios in stabilised soils where moisture content is fixed by the OMC for dynamic compaction. Since the OMC is the maximum added water, and this is needed for particle lubrication, it follows that much is adsorbed by the solid particles leaving little free water for hydration reactions. Unlike a dense HCP matrix as seen in concrete, stabilised soils appear to have incomplete hydration of cement at localised areas around the inter-particle points of contact. More research of clay cement interactions is needed to better understand the relative influence on mean pore size distributions and the reasons for conformity with the linearity rule. Acknowledgements The authors wish to acknowledge the support of the Engineering and Physical Sciences Research Council, and the assistance of Mr Mark Tuck with the experimental testing. References [1] Jayasinghe C, Kamaladasa N. Compressive strength characteristics of cement stabilized rammed earth walls. Construction and Building Materials 2007;21(11): [2] Hall M. Assessing the environmental performance of stabilised rammed earth walls using a climatic simulation chamber. Building and Environment 2006;42(1): [3] Heathcote KA. Durability of earth wall buildings. Construction and Building Materials 1995;9(3): [4] Walker P, Standards Australia. HB195: the Australian earth building handbook. Sydney: Standards Australia International; [5] Jagadish KS, Reddy BVV, Rao KSN. Alternative building materials and technologies. Bangalore: New Age International Publishers; [6] Walker P, Keable R, Martin J, Maniatidis V. Rammed earth: design and construction guidelines. Watford: BRE Bookshop; [7] Taylor P, Luther MB. Evaluating rammed earth walls: a case study. Solar Energy 2004;76: [8] Dobson S. Continuity of tradition: new earth building. Keynote speech: Terra 2000: Eighth international conference on the study and conservation of earthen architecture, Torquay, Devon, UK; May [9] Bryan AJ. Criteria for the suitability of soil for cement stabilisation. Building and Environment 1988;23(4): [10] Bryan AJ. Soil/cement as a walling material I: stress/strain properties. Building and Environment 1988;23(4): [11] Hall C, Hoff WD. Water transport in brick, stone and concrete. London: Taylor & Francis; [12] Hall C, Yau MHR. Water movement in porous building materials IX: the water absorption and sorptivity of concretes. Building and Environment 1987;22(1): [13] Domone PL. Chapter 13 constituent materials of concrete. In: Illston JM, editor. Construction materials: their nature and behaviour. 2nd ed. London: E & FN Spon; p [14] Hall C. Water movement in porous building materials I: unsaturated flow theory and its applications. Building and Environment 1977;12: [15] Hall C, Kam-Ming T. Water movement in porous building materials VII: the sorptivity of mortars. Building and Environment 1986;21(2): [16] Houben H, Guillaud H. Earth construction: a comprehensive guide. 2nd ed. London: Intermediate Technology Publications; [17] Hall M, Djerbib Y. Moisture ingress in rammed earth: part 1 the effect of particle-size distribution on the rate of capillary suction. Construction and Building Materials 2004;18(4): [18] Mitchell JK. Fundamentals of soil behaviour. London: John Wiley & Sons Inc., [19] Barnes GE. Soil mechanics: principles and practice. 2nd ed. Basingstoke, UK: Macmillan Press Ltd.; 2000.

Chapter I Basic Characteristics of Soils

Chapter I Basic Characteristics of Soils Outline 1. The Nature of Soils (section 1.1 Craig) 2. Soil Texture (section 1.1 Craig) 3. Grain Size and Grain Size Distribution (section 1.2 Craig) 4. Particle