1002 9IMC-LPOpaper Final

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9thInternational Masonry Conference 2014 in Guimares

9thInternational Masonry Conference, Guimares 2014 1

The modelling of water grout transfer into masonry walls units

PEREIRA-DE-OLIVEIRA, LUIZ1

ABSTRACT:The questions of masonry grout infill workability could be a problem to be solved, when the narrowdimensions of the masonry unit holes needs to be filled. Normally, two solutions are used: the W/C

ratio is increased or by the use of superplasticizer. The masonry units, as porous media, are stronglyhygroscopic and capable to reduce the grout infill W/C ratio. This reduction may undermine thecement paste hydration if even an adequate grout consistency is attained. By consequence, theprediction of resultant W/C ratio in the grout mixtures is an important parameter to help the mix designand the grout placing. This study is focussed on the grout water transfer into the masonry units andabout the effect of the variables concerning this mechanism. With this goal, an analytical model,based on Darcy, Laplace and Poiseuille laws, was first developed and justified by an experimentalstudy where some variables as grout W/C ratio, temperature and masonry unit porosity were put inevidence. As conclusion, it was possible to define a model to preview the mixing grout water resultingin the grout infill as a function of time and the variables studied.

Keywords: masonry grout infill, water transfer, water loss

1 INTRODUCTION

In a typical reinforced hollow unit masonry construction, the steel reinforcing provides a strongstructure that can be tied together and better resist the lateral dynamic forces of wind andearthquakes. To assure the necessary consolidation between a hollow unit and reinforcement, inpractice, a concrete infill, or grout, is used to fill cells of hollow masonry units and relatively narrowspaces in masonry walls [1]. The difficulty in consolidating the masonry grout by vibration requiresthat this material must have a high flowability to flow through the hollow space around the reinforcingbars and completely surround and bond to the steel and masonry unit. While the normal concrete is

placed with a minimum of water into nonporous forms, the masonry grout is placed with considerablymore water, as the masonry units creates absorptive forms [2]. To obtain a self consolidating grout(SCG) it is necessary to apply water - cement ratio around 0.60 to 0.80 or combine a more reducedwater amount with the action of a superplasticizer to provide a necessary flowability [3].

The concrete masonry unit, as a porous medium, induces a grout water loss at the early age. Thewater loss makes the grout susceptible to increased shrinkage and may prevent or weaken thecontact with the concrete block wall surfaces. Thus, the knowledge about variables influents on themasonry grout water loss could be an important tool to help the practitioner on the grout mix designand its workability optimization.

This paper presents a theoretical and experimental study about the grout water movementhighlighting the variables affecting the mechanism of water transfer from the grout to the masonryunits.

1)Associate professor, University of Beira Interior, Centre of Materials and Building Technologies C-MADE, [email protected]


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Pereira de Oliveira, Luiz A.

9thInternational Masonry Conference, Guimares 20142

2 THE MECHANISM OF THE GROUT WATER ABSORPTION BY THE MASONRY UNITS

The grout flowability is necessary to fill the masonry wall when a certain wall height is reached.When the grout is poured, this fresh mixture is confined inside the hollow of masonry units. Theseunits have a relative high porosity favourable to be impregnated by water that try to escape to outside.The porous media are generally characterized by a number of interconnected capillaries of differentshapes and dimensions that make the determination of their porous structure practically impossible.Under these conditions the study of the movement of a liquid in a porous medium can only be donethrough a simplified model. In order to simplify the reasoning, it was assumed that the masonry unitporous are, at the beginning, empty and have a constant size, while the grout porous vary accordingits set and hardening. The movement of water between the grout and the masonry unit is firstgoverned by the masonry unit action and then by the grout action.

2.1. Masonry unit action

The masonry unit pores (empty), in the beginning, are smaller than those of the grout. Thus, themasonry unit pores act on water grout due to the pressure capillary forces differences. They will be

impregnated by grout water. It is noticed that grout water is composed of an ionic solution and asuspension of colloidal particles.

2.2. Grout action

After the first grout water movement to the masonry unit wall, the grout has a tendency to shrink.This shrinkage is independent of the grout settlement, but it is the result of grout water internaldrainage and bleeding due to the hydrostatic pressure effects. During a first period, the bleedingwater is provoked by the action of capillaries forces carried by the masonry unit pores, while the groutcapillaries are still saturated. After the bleeding water elimination, the grout capillaries start to emptyand meniscuses in vicinity of grout-masonry unit surface are formed.

The water movement will be dependent of resultant capillaries forces:

a) At the beginning the mean radius of the masonry unit capillaries may be lower than the groutcapillaries: it follows that the water movement is from the grout towards the masonry unit:

b) under the action of cement hydration, the mean radius of the grout capillaries decreases until itbecomes equal to the masonry unit capillaries: consequently the movement of water stops;

c) if the mean radius of the grout capillaries becomes smaller than of the masonry unit capillaries,the direction of water movement changes, in other words the grout receives water from themasonry unit. This is possible only after the grout hardening. This eventual process that has asecondary interest is not taken into account in this study.

3 MODELLING THE GROUT WATER MOVEMENT

The capillary movement of liquid through a porous medium is usually described par Darcy's law [4,5], equation 1.

L

hKQ (1)

Where, Q is the liquid flow rate through a unit area of the porous medium, L is the thickness of theporous medium through which the liquid flows, h is the pressure variation expressed in liquid heightand K is the permeability coefficient. One can still write Darcy's law in the following differential form:

gradKv (2)

Where, v is the flow rate in the porous medium and is the hydraulic loading. Using the equation ofcontinuity:


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0vdiv (3)

One finds that the hydraulic loading should verify the Laplace equation with different boundary

conditions:0

(4)

Taking into account that this equation can only be solved analytically in number of special casesusing numerical methods, it was estimated that a more simple reasoning to model the materialsporosity could be sufficient to represent the water grout transfer into masonry wall units.

The flow in a porous medium is done through the voids left and the solid grains. These voids in theporous mass form a network of very fine channels. One can think that the flow is similar to thatobserved in capillary tubes. That is why we often refer to the capillary models, and the simplestamong them is the one that represent a porous medium by a set of parallel cylindrical capillaries with

equal radii R. The application of this model in this study led to the representation shown in Fig1.

Figure 1. Porous media represented by an ensemble of parallels capillaries tubes.

The flow through these capillaries is given by Poiseuille's law:

grad

8

2

R

Q (5)

Where, R is the capillary radii, is the liquid density and is the liquid viscosity.If n is the number of capillaries by surface unit, the flow through capillaries is given by:

grad

8

2

2

Rn

R

Qnv (6)

Comparing equations (5) e (6), one can deduce that the permeability coefficient K can be definedby:

8

2

Rn

K (7)

Now the driving pressure in the capillaries has to be determined. It is known that in a capillary tube,the liquid raise due to surface tension. If the tube wetting is not perfect, a contact angle between thefluid and the tube wall is formed [6]. In this case, the capillary rise H can be determined at equilibrium,as shown in the equation 8.

cos2R

H

(8)


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Where, H is the capillary rise, is the surface tension and is the wetting angle.

As we have seen previously, the masonry unit as a hollow concrete block is treated as a porousmedium represented by parallel capillaries. After filling the hollow concrete block column in the

masonry wall, the grout is in contact with the inner wall of masonry unit. This inner wall can also beconsidered as a porous medium consisted of parallel capillaries, as shown in Fig.1, but withsignificantly larger radii. The Rb and Rgare defined as the average radii of the cylindrical capillaryblock and grout respectively. The extremities of these capillaries finish at the air and at the grout.When the grout contact with the block inner wall, it is saturated with water, and the block is dry. As Rgis substantially larger than Rb, the grout water is absorbed by the block capillaries. The amount ofwater absorbed depends on the difference between the capillary action of the masonry unit and thegrout water retention capacity, which generates a pressure driving given by the Laplace laws(equation 9).

11cos2

gb

RRp

(9)

Poiseuille's law written for one capillary permits to determine the amount of liquid that penetrates acertain length L in the block from the contact surface. For this case the flow velocity is expressed by:

L

p

8

2

bRv (10)

In which the quantity grad has been replaced by the finite quantity Lp . Then, replacingpin equation (10), give:

L

1

R

R-1R

4

cos

g

bb

v

(11)

As, dtdLv and if we put:

R

R-1R

4

cos

g

bb

(12)

We have:

Ldt

dL (13)

Assuming that is a constant, which will be discussed below, we get:

21

).2( tL (14)

If n is the number of capillaries per unit area of the masonry unit, and if V cis the volume of waterabsorbed at time t per unit area of masonry unit, one has:

21

2 ).2( tLRnV bc (15)

Replacing by its value defined in equation (12), we get:

2

1

2

1

2

521

)1(2

costR

RLRnV

g

bbc

(16)


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The Rbcould be considered during the suction phase having a constant radius, while Rgvaries withthe time. In this phase, Rgdecreases as a result of the contraction due to the capillary pressure andthe cement hydration. As the grout normally has much water, one can consider that Rgvaries little for

a defined time interval. It should also be noted that the term

21

1

g

b

RR is not very sensitive to

variations of Rgas long as Rb/Rgis sufficient small. One can consider that we have approximately:

21

tVc (17)

Where is approximately constant. If a slightly more sophisticated modelling is desired, this is

possible if the time is divided into intervals ti, during which it is assumed that the Rgradius remainsconstant and equal to Rgi. Then, the volume of water absorbed between the initial time t0and thesaturation time tscould be express by:

21

0

2

1

2

5

)1( i

s

t

t

t g

bbc

RRRV

(18)

Where

21

2

cos

n

(19)

It is assumed now that the masonry unit walls are saturated. The water flows to the masonry unitwill be possible if their outer face is directly exposed to atmospheric effects (sun, wind, environmenthumidity), which promote evaporation of the water contained in the masonry unit. It is alwayspossible, theoretically, to model this phenomenon, for example by applying the general diffusion

equation. However, this model is less attractive, since the water movements become much lesssignificant and therefore less influential in the subsequent grout behaviour.

4 EXPERIMENTAL STUDY ABOUT GROUT MIXING WATER LOSS

The problem to be investigated is represented by a wall built at a certain height when the hollowsare grouting. It is concern the mixing water loss.

4.1. Materials

Different types of structural hollow concrete blocks (fbk > 6.0 MPa), whose characteristics arepresented in Table 1, were used. These physical characteristics were determined by standardized

tests according EN 771-3:2003.A Portland cement type CEM I 42.5R with density of 3140 kg/m3, a natural sand with maximum

size 0.3 mm and density of 2635 kg/m3, a limestone coarse aggregates with maximum size 9.5 mmand a density of 2699 kg/m3were used to composed a standard grout mix. A modified polycarboxylatesuperplasticizer was used to obtain a slump flow higher than 600 mm.


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Table 1. Physical properties of hollow concrete masonry units

Block

type

SSbagr

(m2/kg)

Water

absorption

(%)

Capillary

absorption

(kg/m2)

Density

(kg/m3)

Bulk

density

(kg/m3)

Voids

ratio

BI 174 7.55 5.05 2678 1972 0.264

BII 246 4.4 1.82 2643 2154 0.185

BIII 289 4.7 0.29 2720 2094 0.230

BIV 261 5.9 0.86 2749 2092 0.239

In this study, the masonry grout mixtures were composed with a mass proportions 1.00:2.50:2.50of cement : sand : coarse aggregates. The grout mixtures were produced with different water/cementratios, i.e: 0.55; 0.60; 0.75 and 0.85. The superplasticizer dosage for mixtures with W/C = 0.55 and0.60 was 1.0% of cement mass and 0.6% for mixtures with W/C = 0.75 and 0.85.

4.2. Methods

The water absorption, the water capillary absorption, the real density and bulk density weredetermined according to EN 772 standards.

To simplify the study a water flow model in one direction was adopted [7]. A parallelepipedicalPlexiglas cell shown in the Fig. 2 with internal dimensions of 100 mm x 100 mm x 110 mm was built.This cell has two open sides, which allows the placement of a wall masonry unit piece measuring 100mm x 100 mm x 40 mm, so as to obtain a void volume which is then filled with grout. The grout waterloss is determined by successive weighing of the cell with grout and the masonry unit piece andseparated weighing of the masonry unit piece. After each weighing the masonry unit piece wasreturned to the cell. A sheet of filter paper was interposed between the masonry unit piece and grout

facilitating their separation. Preliminary results showed that the interposition of filter paper has aninsignificant influence on the results.

Figure 2. Plexiglas cell with the masonry unit sample

5 RESULTS AND DISCUSSION

In Table 1, the water absorption gives the percentage of voids accessible by water, while the voidsratio expresses the amount of pores in the masonry unit.

Since the masonry unit porous structure is dependent of their aggregates specific surface area orvoids, one can also think that this structure influences the masonry unit hygroscopic properties. In this

regard, Fig. 3 shows a more significant correlation between the masonry unit aggregates specificsurface area and water capillary absorption than water absorption.


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Figure 3. Water absorption and water capillary absorption as function of masonry unit aggregatesspecific surface area (SSagr)

In fact, the right graph of Fig. 3 shows a significant linear correlation. It is observed that increasing

SSagr reduces the water capillary absorption by the masonry unit. It is resultant of a more compactmasonry unit structure found in the concrete blocks BIII and BIV.

When the water absorption and water capillary absorption results, shows in Fig. 4, are correlatedwith the masonry unit voids ratio a nonlinear significant correlation with the water absorption wasfound. This outcome was expected, once the water absorption by immersion is dependent only of theaccessible volume of voids.

Figure 4. Water absorption and water capillary absorption as function of masonry unit voids ratio

Fig. 5 shows the results of grout mixing water loss obtained at temperatures of 20C and 40C.These results represent the values of water absorption caused by one type of masonry unit in contactwith one grout mix produced with different W/C ratios. It was observed that the evolution of the groutwater loss as function of the time square root had the same characteristics for all mixtures. Thesecharacteristics identify a bilinear trend evolution, which can be represented by the model shown inFig. 6.


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Figure 5. Grout water loss as function of time at the different environment temperatures 20C and40C

Analysing the grout water loss on the time, at 20C, an increase of the water loss with the W/C

ratio was observed.The environmental temperature influence was more evident for the W/C ratio lower than 0.85. The

water loss at 20C is similar for 7 and 24 hours while at 40C a slight increase is observed during thisperiod modifying then the grout hardening kinetics.

Taking into account the grout water loss results on the time, a model presented at Fig. 6 could bedesigned. This model is composed of two zones where the grout water loss by masonry unit surfacearea increases linearly with the time square root. The straight line 0 -1 essentially describes the groutwater absorption by the masonry unit (zone I) and the line 1 2 represents the water loss byabsorption and evaporation (zone II). In this zone the water loss velocity is reduced by the effect ofthe cement hydration that modifies the grout porosity structure. After the time t2 the water lossbecome insignificant and sometimes can oscillate around an equilibrium value that depends of

environmental conditions. Taking into account such behaviors, it was taken the bilinear model asrepresentative of the water loss, here identified by zones I and II.

Figure 6. Grout water loss model

The line 0 - 1 inclination can be expressed by the coefficient as follows:

21

2

min

kg/m

i

i

t

M (20)

coefficient identifies the water loss velocity in a certain absorption period that depend of thewater concentration C. The diffusion velocity in the zone I is proportional to the volumetricconcentration gradient, that is:


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v

CD

dt

dM

(21)

where D is the diffusion coefficient calculated as function of the water mass transferred and themasonry unit volume in contact with the grout. Thus, if one can express:

wbMD (22)

So,

21min mM

Dwb

(23)

where, Mwbis the weight of the water absorbed by masonry unit volume at instant t I.

The model describing the grout mixing water loss is represented in the two zones as follows:

tbMtM EA and (24)

Where MAis the water loss, in the zone I, by surface unit due exclusively to the absorption and M Eisthe water loss influenced by grout set and hardening reactions. It was found that the proposed modelto predict the grout water loss was confirmed experimentally. Indeed, the grout water absorption bythe masonry unit is characterized as follows:

21tMA (25)

The equation (25) is nothing else than equation (17), where the water loss is expressed in mass.Observing the Fig. 7, one can verify that the coefficient increases with the W/C ratio. It is also

observed that the coefficient is not significantly affected by the environmental temperature.

Figure 7. The coefficient as function of W/C ratio at temperatures of 20C and 40C

In all grouts here studied, a separation between zones I and II was observed. These zones aredelimitated by the time t1/2 between 15 and 20 min1/2which correspond to the period between 4 and 6hours. This indicates that the phenomena are well defined: first by the absorption by the masonry unit,that is little dependent on temperature; second by the absorption and evaporation plus hydrationkinetics, depending on the temperature. These observations allowed us to understand themechanisms of mixing grout water loss from the moment it comes into contact with the masonry unitwalls. This however, should not be an end in itself. Indeed, in a cement based mixture, the strength

depends largely of the W/C ratio. In the case of grout, the available water quantity for the cementhydration is a key element, and it is important to translate the previous findings in terms of W/C ratiofor the purpose of the grout mix design.


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6 CONCLUSIONS

The transfer of water from grout to masonry unit is as function of a surface wetting and capillaryactions.

Different masonry concrete blocks were characterized and a strong linear correlation was foundbetween the blocks water capillary absorption and their aggregates specific surface areas (SSagr). Theincrease of blocks SSagr reduces the water capillary absorption. On the other hand, it was observedthat water absorption of concrete blocks is more influenced by their voids ratio. Taking into accountthat the mixing grout water transfer to the masonry unit is governed by capillarity, a theoretical modelwas developed and experimentally agreed.

The experimental results show that, on the same porous medium, a bilinear model can explain twodifferent behaviours. One defined as zone I where the coefficient is only dependent of W/C ratiosand another, zone II, where the coefficient is also influenced by the environment temperature.

Finally, the prediction of grout water loss can be used to control the mixing water amount in themasonry grout mix design.

REFERENCES

[1] Amrhein, J.E.: Reinforced Masonry Engineering Handbook, Masonry Institute of America 7thEdition (2012), 469p

[2] Pereira de Oliveira, L.A. Evaluation of early shrinkage of structural masonry concrete infill,Masonry International, UK, 17 (2004), 2, 66-70.

[3] National Concrete Masonry Association: Self-Consolidating Grout Investigation: Making andTesting Prototype SCG Mix Designs, Report of Phase II Research, Project No. 05-330,Publication No. MR3, February 2007, Herndon, VA, USA

[4] Scheidegger, A.E.: The physics of flow through porous media. University of Toronto Press,Canada, 1974.

[5] Szymkiewicz, A.: Modelling water flow in unsaturated porous media. Springer, 2012, XXI, 237p.[6] Bories S.: Transferts de chaleur et de masse dans les matriaux, analyse critique des diffrents

modles mathmatiques utiliss, lhumidit dans le btiment, Sminaire de lUNESCO, 23-25Novembre 1982, France,

[7] Pereira de Oliveira, L. A.: The influence of masonry grout and constructions systems on thestructural masonry behaviour. (In French). PhD thesis, University of Lige, Belgium, 1992, 189p

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1002 9IMC-LPOpaper Final