Magazine of Concrete Research: Vol. 40, No. 143: June 1988

A water absorption test for concrete S. Kelham

BLUE CIRCLE INDUSTRIES PLC*

S Y N O P S I S t time from contact of water with

The durability of concrete near an exposed surface is largely determined by the rate at which harmful agents can penetrate into the concrete. This paper presents a simple analysis of the penetration of' water into unsat- urated concrete and describes a method by which the material constant that determines the rate of penetra- tion, the sorptivity, can be measured, together with the effective porosity. The sorptivity is found to depend on the permeability and porosity of the concrete and the strength of capillary forces. The experimental results conform well to the predictions of the analysis and signlJcant variations in sorptivity have been observed for concretes containing cement replacement materials and subject to dzfferent curing regimes.

Notation A cross-sectional area of flow (m' 1

K intrinsic permeability of sample

1 length of sample (m>

dp/dx pressure gradient (Palm>

material (m')

M mass of sample (kg)

sample (S) V volume flow rate (m3w X depth of penetration (m) Z effective porosity (% v/v)

rl dynamic viscosity of fluid (Pa S> CJ density of fluid (kg/m3> e angle of contact (rad)

Ap capillary pressure difference (Pa>

Introduction The effect of the quality of concrete near an exposed

surface on the durability of concrete structures is an area of current interest"). Those properties controlling the transport of aggressive materials into the bulk of the concrete or to the reinforcement, such as the permeability and sorptivity, are of particular imp~rtance'~,~' . The rate at which unsaturated con- crete absorbs water in contact with its surface is relevant to various aspects of durability. A large number of test methods have been published (e.g. References 4-8) but none can easily provide values for the two basic parameters associated with absorption:

e air pressure at sample surface (Pa) Pcap mean effective capillary pressure (Pa) ea air pressure at air/water interface (Pa)

the mass of water which is required to saturate the concrete-the effective porosity, and

6, water pressure at air/water the rate of penetration-the sorptivity.

p, Q r

S S

interface (Pa) water pressure at sample surface (Pa) mass flow rate of fluid (kg/s) capillary radius (m) surface tension (Pa m> sorptivity (m/s''2)

The ISATI4), Figg (water)@) and CAT@) methods are based on the rate of water absorption, which is a combination of the rate of penetration and the effec- tive porosity. The two parameters cannot be separated because the volume of concrete saturated during the test is not known. Ho et al.(7) measured the penetration depth at various times directly by split-

*Blue Circle Industries PLC, BCC Technical Services Division, ting requiring many to 305 London Road, Greenhithe, Kent DA9 SJQ, UK. give reliable data. Methods for measuring the gas

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A water absorption test for concrete

permeability in unsaturated concrete and water permeability in saturated concrete are well established (e.g. Reference 9).

This paper presents a simple analysis of absorption and describes a method which enables the porosity and sorptivity to be measured.

Theoretical model The flow of a fluid through a porous medium

usually follows Darcys law:

Q = KAa(dp/dx)/V . . . . . . . . . . . (1) where Q = mass flow rate of the fluid

K = intrinsic permeability of the medium A = cross-sectional area of flow

dp/dx = pressure gradient 6, rl = density and viscosity of the fluid

This expression is valid for laminar flow and where the free path of the fluid molecules is small compared with the pore size in the medium. Consider a cylinder of concrete, cross-sectional area A , length l , sealed on its curved surface, in contact with water at one end and open to the air at the other (Figure 1). At a time t after coming into contact with the concrete the water will have penetrated to some depth x(t) . To find the rate at which x increases it is necessary to know the pressure gradients within the sample. Let P, be the pressure -of the water in contact with the concrete surface and P, that of the air at the other surface. At the air/water interface within the sample the pressure in the air (P, , ) and water (P, , ) will be different because of capillary forces.

In a capillary of radius r the pressure difference across the interface is given by:

P = 2s cos 9/r . . . . . . . . . . . . . (2)

where S = surface tension I3 = angle of contact

To estimate the magnitude of this effect for an air/water interface in concrete we take the radius of a large capillary pore, r, to be about 1 giving

P = MO x 1 0 3 ~ a

(S = 70 x 10-3N/m, cos 0 = 1)

This is equivalent to a head of about 14m of water. Thus in many practical situations the capillary forces will be the dominant driving forces in the absorption of water by concrete. Hydrostatic pressure differences will therefore be ignored in the analysis.

With reference to Figure 1 the pressures in the air and water at the interface will be taken to differ by a constant amount Pcapr the capillary pressure. It is assumed that the concrete is sufficiently homogeneous for PClp to be independent of x .

Water can be taken as an incompressible fluid and its volume rate of flow, V,, is thus given by

A = n? 2 r

Figure 1: Cross-section of concrete cylinder considered in analysis.

v, = Q , / ~ , = [KA/V],I [(Pw - Plw)/xI.. . (3) This must be equal to the volume rate of flow of the

air at the interface, which is given by

K = y (=)(F). (4) KA P,, - P, P,, -l P, . . . . . where the term (P,, + Pa)/2pI, is associated with the compressibility of the air. Equating these flows and using ea - P,, = Pcap (constant) and P, = P, (ignoring hydrostatic forces) leads to an expression relating P,, and x . Because of the large difference between the viscosities of air and water P,, is found to be significantly different from P, only for x less than 1/10. For such small values of x the assumption that equilibrium flow conditions exist is probably invalid. The high values of P,, predicted by the expression for small values of x will not be observed because of the compressibility of the air. We will therefore make the approximation that P,, = P,. Equation (3) for the rate of flow of water becomes

V, = K A P C a p / x ~ , . . . . . . . . . . . . ( 5 ) The volume of water required to saturate an element of thickness dx is given by

dV = d x A z . . . . . . . . . . . . . . . (6) where z is the effective porosity of the concrete, i.e. the volume of water that unit volume of the concrete will absorb in the given conditions. Thus the rate of increase of x is given by

dxldt = V,/Az = KPcap/xztl, . . . . . . . (7) This may be integrated to give

x(t) = (2KPc,pt/zq,)~2 . . . . . . . . . . ( 8 ) The depth of penetration of water into unsaturated

concrete is thus proportional to the square root of time and the constant is known as the sorptivity, S.

S = ( 2 K P c , , / ~ ~ , ) 1 2 . . . . . . . . . . . (9) The mass of the concrete cylinder at any time M(t )

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Magazine of Concrete Research: Vol. 40, No. 143: June 1988

is equal to the initial mass M(0) plus the mass of absorbed water. Thus

M(t) = M(0) + Azo,x(t) = M(0) + AzowSt2.. . . . . . (10)

The sorptivity can thus be derived from the slope of a plot of mass vs square root of time if the porosity is known. If the absorption is allowed to continue until the whole cylinder is saturated the porosity can be obtained from the difference between the dry and saturated weights.

z = (M(sat) - M(O))/Ala,. . . . . . . (1 1)

Experimental method Figure 2 is a schematic drawing of an apparatus

designed to monitor the weight of a concrete cylinder as it absorbs water under conditions corresponding to those considered in the preceding analysis. The sample is in the form of a cylinder 150 mm diameter and 50 mm thick. The curved surface is sealed by painting with bitumen and then covering with waterproof adhesive tape. The top surface is covered by a perspex disc. The disc is sealed with grease to the edge of the top surface of the sample and to the tape and encloses an air gap of about 2 mm over the concrete surface. A metal capillary connects this air gap to atmosphere. The sample and cover are suspended in a sling from a 5 kg capacity load beam, the output of which can be recorded with a microcomputer-based data logger. At the start of the test, water is poured into the container surrounding the sample to a depth of 50 mm above the top surface of the sample. The capillary is 60 mm long and therefore protrudes above the surface of the water. The top surface of the sample thus remains dry and open to the atmosphere, allowing air to escape as water is absorbed through the lower surface of the sample. The weight of the sample is monitored until the rate of increase becomes very slow, which indicates that the sample is effectively saturated. The further

load beam

sample water

Figure 2: Experimental arrangement.

108

slow increase in weight is associated with movement of water into less accessible pores and the flow of water into the space between the sample and the cover driven by the small (about 50 mm) pressure head. The measured change in weight is actually the change in buoyancy of the concrete as the water replaces air within it. Since the initial weight of the sample in the water can only be obtained by extrapolating the measured results back to zero time, the weight of absorbed water was obtained from initial and final weighings in air. For homogeneous samples that give very good agreement with the f 1 2 dependance, initial and final weighings and two during the absorption are sufficient to calculate the sorptivity and porosity. However, with saturation times varying between 2 hours and several days (for well-cured concrete), specifying times for taking measurements would need care. Furthermore, the weight vs square root time plot gives a useful indication of the homogeneity of the concrete.

Experimental results Examples of the results obtained on three mixes are

presented in Figures 3 and 4. The concretes were based on a 1 : 2-5 : 3.5 (cement : sand : coarse aggregate) mix with the water content gauged to give a slump of 60mm. Mix 1 contained OPC. In mix 2, 25% of the OPC (w/w) was replaced by pulverized-fuel ash (pfa) and in mix 3, 60% (w/w) was replaced by ground granulated blastfurnace slag (ggbs). One batch of samples was de-moulded at 24 hours, cured in water for 27 days, oven-dried at 105C for 6 days and cooled in a desiccator for l day before testing. The results, shown in Figure 3, conform very closely to the square root of time dependence over the whole period of absorption and change rapidly to an almost constant value when saturation is complete. Figure 4 shows the results obtained for the second batch of samples from the same mixes. These were cured in air at 20C and 50% relative humidity for 27 days after de-moulding at 24 hours. These samples were oven-dried in the same way as the water-cured samples. The values derived from these plots are given in Table 1. The correlation coefficient for the least squares fit to the linear portion of the plot is generally better than 0.999 for the water-cured samples but somewhat lower, about 0.99, for the air-cured samples. This is due to the inhomogeneities caused by the variable degree of hydration through the sample associated with air curing.

The results for the water-cured samples demon- strate the beneficial effects of pfa and ggbs hydration on the transport properties of concrete, despite the reduction in strength. The time taken for saturation to a depth of 50mm is at least twice that for the OPC concrete. The results for the air-cured samples show the increased sensitivity of the concretes containing

A water absorption test for concrete

pfa or ggbs to poor curing. For the OPC concrete the time taken to saturate the air-cured sample was half that for the water-cured sample but for the concrete containing ggbs the saturation time was reduced by an order of magnitude when the curing conditions were changed. These results were obtained on single discs from each mix and curing condition but similar results were obtained for equivalent mixes based on 12 other OPCS.

Application The technique can be used, for example, to study

the effects of curing conditions, mix design and replacement of different levels of cement by secondary materials, such as pulverized-fuel ash (pfa) and ground granulated blastfurnace slag (ggbs), on effec- tive porosity and sorptivity. An assessment can be made of the quality of the surface layer of different

1.20

o

+

0

0

+

0

+

+ +

0 0

o o 0

l I 0 1 2 3 4 5 6 7

TIME1/ - h/2 I I 0 1 4 9 16 25 36 49

TIME - h Figure 4: Experimental resuIts--air-cured concrete.

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Magazine of Concrete Research: Vol. 40, No. 143: June 1988

T A B L E 1 : Experimental results.

Mix 1 Mix 3 Mix 2 I I I

Blend (w/w) I OPc I 7 5 z z z a a + I 60% ggbs 40% OPC + Water/cement ratio

38.6 39.2 44.0 28 day strength (MPa)

Air Water Air Water Air Water Curing

0.57 0.56 0.59

18.9 38.2 31.4 Effective porosity (YO) 12.1 12.9 12.3

3.1 43.3 4.1 25.5 6.2 12.2 Time to saturate 50mm (h) 28.4 7.6 24.6 9.9 20.0 14.3 Sorptivity (mm/h) 13.6 13.1 12.8

concretes designed to give the same 28 day water- cured strength. Although all the present work has been carried out on cast cylinders of 150 mm diameter, similar results would be expected using specimens of different shapes and sizes. Slices of cylindrical cores taken from concrete structures should be suitable subject to re-conditioning following the wet culling operations.

While oven-drying is a quick and reproducible method of conditioning concrete, it is not suitable for providing data directly applicable to concrete in the field. Some tests have been carried out on samples cured at 20C and 50% relative humidity for 180 days. The results were similar to those obtained after oven- drying of air-cured samples at 28 days, although there was even greater curvature on the t* plots. Obviously the variation in hydration through the sample is com- pounded in this case by the variable degree of water retention.

Conclusion A simple analysis of the absorption of water into

unsaturated concrete has been presented. The depth of penetration is found to be proportional to the square root of time and a method for determining the con- stant of proportionality, termed the sorptivity, has been described. The experimental method includes the measurement of the effective porosity of the sample. The analysis shows that the square of the sorptivity is proportional to the intrinsic permeability of the sample, the capillary forces and the inverse of the effective porosity. The available experimental results closely follow the form expected from the analysis and the derived values for the sorptivity show significant differences for concretes containing different cements or subject to different curing regimes. The method may therefore prove useful in the study of the quality of concrete surfaces subject to wet/dry cycling. The sorptivity would seem particularly pertinent to rein- forced concrete where the water might carry chloride or other harmful ions into the concrete.

R E F E R E N C E S 1.

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7.

8.

9.

10.

DHIR, R. K., CHAN, Y. N. and HEWLETT, P. c. Near surface characteristics and durability of concrete: an initial appraisal. Magazine of Concrete Research. Vol. 38, No. 134, March

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893-901.

Contributions discussing the above paper should be in the hands of the Editor not later than 31 December 1988.

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