absorption and sorptivity tests

TN 001 ABSORPTION & SORPTIVITY.DOC

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1. INTRODUCTION Tests that measure concretes propensity to suck in water are generally termed absorption or sorptivity tests. The difference between the two terms is generally considered to be that absorption is the capacity of a sample to hold water while sorptivity is the rate at which the sample fills the sample.

2. PRINCIPLE There are two ways of measuring absorption/sorptivity. In one the concrete is immersed under a small head of water (20-30mm) while in the other less common method the sample is held in contact but above the water. The difference is that gravity adds to the driving force in the immersion method and detracts from the driving force in the suspension method. The gravitational component has been found to be insignificant compared to the suction force and in essence the two methods measure the same thing i.e. water drawn in under capillary suction. Research has shown: a) the penetration is dependent on initial water

content b) drying to a constant rate of moisture loss provides

more accurate results than drying for a set time as it provides a more consistent moisture content

c) the depth of penetration can be calculated from:

d= (r2PuT/4)where r = nominal pore radius Pu = atmospheric pressure = viscosity T = time

From this the nominal pore radius can be calculated for any depth of penetration if time and temperature are known The many tests fall into three categories: I) Sample is immersed for a set time that does not

lead to saturation. If immersion time and temperature(absorption) are constant the weight gain is a function of average pore diameter. The depth of penetration will vary and no estimate of voids van be made without this.

II) Sample is immersed such that it becomes saturated. The result measure the volume of

permeable voids (VPV). As the sample is saturated only voids can be measured.

III) Sample is immersed and measurements of height rise and/or weight gain are measured. In some tests measurements are made at various time increments (normally root time based) while in others the depth is measured at a standard time. If depth in a given time is measured the result relates only to nominal pore radius. If depth and weight gain are measured then voids and nominal pore radius can be estimated assuming that capillaries are all full. Measurements at various times improve the accuracy of the method.

While there are many tests around the world for measuring absorption/sorptivity only those commonly recognised in Australia are discussed here.

3. TESTS METHODS

3.1 AS4058 The principle concrete quality test for pipes in AS 4058 is the absorption test undertaken in accordance with AS4058 Appendix F. This requires that cores of approximately 120mm diameter be taken through the pipe thickness for pipes 14-28days after casting. Cores are kept damp until tested. They are then oven dried, weighed, immersed, boiled, cooled and reweighed. Absorption is calculated as the weight of water absorbed during immersion, boiling and cooling divided by the weight of dry sample. It can be assumed that the procedure will saturate all voids. Although not mentioned in the standard the absorption can be converted to voids percentage by multiplying the absorption by the specific gravity of the concrete, i.e. approximately 2.3. AS4058 requirements are shown in Table 1

Table 1 - AS 4058 Requirements for Absorption

Abs

orpt

ion

mus

t not

ex

ceed

Equi

vale

nt

void

s

Vic

Road

s VP

V C

lass

pressure, sewage or marine or other aggressive environments

6.5% 15% 2. Good

for drainage pipes 8.0% 18.4% 4. Marginal

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In considering revision to AS4058 one must consider whether the absorption test prescribed continues to be valid in light of developments since the last standard was published in regards test methods, acceptance criteria and durability criteria.

3.2 RTA T362 The NSW RTA T362 sorptivity test dries sample slowly to avoid thermal shock. After drying at a RH of 50% for the prescribed drying time the samples are immersed at 23C for different times. The sorptivity is then measured as the depth of penetration except that the depth is doubled for class A and B1 concretes. The method is specified in RTA B80 specification as a test for curing. Criteria and testing intervals specified are shown in Table 2 together with the mix requirements for each exposure category. The test method has been specifically developed by RTA to provide a check on the adequacy of curing for different grades of concrete. In developing the criteria a number of variables are introduced between different grades eg drying time, immersion time and result calculation method. This means that there may be no relationship between RTA sorptivity and different grades of concrete. The test requires that samples are stored at constant 50% humidity and are broken in a beam test rig. These requirements for special facilities limits seriously limits where the test can be undertaken. The breaking of the sample to obtain a result means a result can only be obtained at one time interval for each sample.

Table 2 - RTA B80 Sorptivity Requirements

RTA T362 Sorptivity Test

Expo

sure

C

lass

ifica

tion

Min

imum

Cem

ent

Con

tent

(kg/

m3 )

Max

imum

w/b

ratio

Dryi

ng Ti

me

at 2

3C

(day

s)

Imm

ersio

n Tim

e A

t 23

C (h

rs)

Max

Sor

p. G

P C

emen

t (m

m)

Max

Sor

p. b

lend

ed

Cem

ent (

mm

)

A 320 0.56 21 6 35 35 B1 320 0.5 21 6 25 25 B2 370 0.46 28 24 17 20 C 420 0.4 35 24 N/A 8 U Project Specific

This method is similar in many respect to the CSIRO test developed by Ho and Lewis but which has seldom been used commercially because of the large number of samples required due to their being broken open for each test.

3.3 AS 4056, ASTM C642 & AS1012.21 AS 4056, ASTM C642 & AS1012.21 tests require that samples are immersed and boiled such the permeable voids are saturated. Procedures are shown in Table 3.

Table 3 - Comparison of Absorption Test Methods

AS4056 AS1012.21 ASTM C642 Weigh sample Weigh sample Weigh sample Dry at 105oC to constant wt

Dry at 105oC for 24hrs

Dry at 105oC to constant wt

Cool sample Cool sample Cool sample Weigh sample in air, M1

Weigh sample in air, M1

Weigh sample, M1

Immerse sample for 48hrs

Immerse sample to constant wt



Immerse sample and boil for 5hrs

Immerse sample and boil for 5-6hrs

Immerse sample and boil for 5hrs

Cool sample in 2 hrs

Cool naturally Cool naturally




Weigh sample in water M4

Weigh sample in water M4

AS4056 gives the absorption as (M2-M1)/M1. Result is weight of water absorbed as a percentage of sample weight but can easily be calculated as VPV ASTM C642 & AS1012.21 gives the VPV as (M3-M1)/ (M3-M4). Result (VPV) is volume of continuous voids as a percentage of the sample volume. Concern is sometimes expressed about thermal shock causing cracking and the test methods discounted on that basis. Phaedonos(1) of Vic Roads reviews the ASTM C642 absorption test. This is similar to the absorption test in AS 4058 and AS1012.21 in that the samples are dried in the oven at 105C. The paper states results indicate that the oven drying temperature of 100-110C has a negligible effect on the pore system or microstructure of concrete. The excellent correlation between oven drying at 50C and 105C indicates there is very little mass loss when the temperature increases from 50-105C, thereby confirming that no combined water is lost. He goes on to discuss other results ie that oven drying temperature and the overall treatment of the sample during the test procedure have no detrimental effects on the microstructure. He provides reference to various authors that conclude the same. In view of this there is no concerned about microcracking when curing at 105C.



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Phaedonas also states Whereas the ASTM C642 method requires oven dried samples to cool naturally followed by a two day water immersion where 99% of the water absorption is achieved, the AS 4058 method requires oven dried samples to be cooled and rapidly heated to 100C for the 5 hour boiling period, The ASTM C642 method is therefore considered as subjecting test specimens to more sever testing due to an induced thermal shock and the possibility of macro cracking. The AS 4058 test produces 0.5% to 2% higher volume of permeable voids. The ASTM C642 test (with refinements) has been made an Australian Standard (AS1012.21-1999) and in a phone conversation with Phaedonos he recommended that the AS1012.21 test should be adopted in place of AS 4058 testing. Vic roads criteria for VPV using the ASTMC642 method are given (Table 4). These criteria could also apply to AS1012.21.

Table 4 - Criteria For VPV

Dura

bilit

y C

lass

ifica

tion

Volu

me

of

Perm

eabl

e Vo

ids

(VPV

) (%

by

volu

me

Imm

erse

d an

d Bo

iled

Abs

orpt

ion

(%

by w

t)

Sorp

tivity

(m

m/s

ec0.

5 )

1. Excellent 0.20

A major shortcoming of these tests is that they waste the opportunity to plot weight gain with time. This is particularly true of the AS1012.21 test where the sample is immersed for 48hrs without boiling in any event. Given a large enough sample the flow will approximate to uniaxial

3.4 ASTM C1585 SORPTIVITY TEST In this test an oven dried sample is suspended above water and the height rise and weight gain measured at time intervals. The sorptivity is calculated as the weight gain per unit cross section against the square root of time. It provides a relative measure that combines pore size diameter and number of pores. The test was originally developed for use on bricks and introduced to Taywood Engineering in the early 1980s by Ken Baker of Halpern Glick as a potential method of measuring concrete pore characteristics. Taywood used the test extensively and were one of the first companies to use Sorptivity as a performance requirement. Only recently has the test become and ASTM method.

Oven drying is not seen as an issue for the reasons discussed in section 3.3. The test method plots weight gain (absorption) at several time increments for the same sample thereby increasing accuracy by enabling initial surface effect to be discarded and results to be based on the statistical fit of a number of measurements. Hence, it gives more reliable absorptions than simple weight gain at a set interval. The test can also be set up to measure absorption by placing the surface of interest in touch with the water. The performance of the exposed (surface can be assessed and the effect of distance form the surface can also be determined. The height rise measurement is comparable to the RTA test method but unlike the RTA test the ASTM test is simple to undertake. Converting the height rise and weight gain to a pore volume does not provide a reliable VPV in the same fashion as the boiled absorption type tests. However the test method lends itself to establishing this at the end of the sorptivity test as an additional procedure. The test method is not unique and was originally developed by Fagerlund(6) . Similar tests are commonly used for absorption tests on bricks eg Reda(5) and it is the brick test, with an extended measurement time, that has become the ASTM C Sorptivity test method.

4. PREVIOUS RESEARCH

4.1 VPV VS OTHER TEST METHODS & W/B RATIO Whiting compared VPV results with results from water permeability and air permeability on mixes with different w/c ratio and a mix with silica fume (Table 5). The results show that VPV differentiated better than air permeability between the concretes at w/c ratios less than O.4 but neither method differentiated between concretes at w/c ratios over 0.4. By comparison water permeability differentiates the high w/c ratio concretes but measurements at low w/c ratios could not be made. Phaedonos(3) reports on the relationship between VPV and various concrete properties as follows: Relationship to strength is poor reflecting that

strength is a poor indicator of durability VPV detect the improved performance of slag, fly

ash and silica fume in a similar fashion to other durability tests

Hydrophobic admixtures reduce VPV very significantly while water-proofers did not reduce VPV significantly

4.2 SORPTIVITY V.Sirivivatnanon(4) tested fly ash, slag and silica fume concrete using the RTA sorptivity test and ASTM VPV



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test. The results for GP, FA and slag cement systems indicated that the two tests gave similar results in the w/c range 0.34-0.6 except for one anomaly with the slag cement at a w/c ratio of 0.43. For silica fume one high w/c mix (0.87) indicate VPV was less sensitive than RTA sorptivity in this high w/c range.

Table 5 - Results of VPV and Other Penetrability Tests

w/b SCF (wt %

binder)

VPV (%)

Air Permeability

(Darcy) Water

Permeability (Darcy)

0.26 12% 6.2 29 * 0.29 - 8.0 33 * 0.40 - 12.2 120 0.03 0.50 - 12.7 170 0.20 0.60 - 12.5 150 0.23 0.75 - 13.3 150 0.86

0

20

40

60

80

100

0.3 0.4 0.5 0.6 0.7 0.8 0.9

w/c ratio

RTA

S (s

olid

line

)

12

13

14

15

16

17

VPV

% (d

otte

d lin

e)

GP - RTASFA - RTASSF - RTASHS - RTASGP - VPVFA - VPVSF - VPVHS - VPV

Figure 1 - Sirivivatnanon Test Results for VPV and RTA

4.3 SORPTIVITY TESTING VS VPV As noted above VPV is a measure of voids and sorptivity (whether my mass gain or depth of penetration) is a measure of nominal pore radius and number of pores. While there may be some relationship between these two parameters its not likely that the relationship would be constant across all concrete types. Reda(5) measured total absorption and sorptivity on a range of samples. The results for each tests method gave a low standard deviation however a plot of total voids vs sorptivity gave a very poor correlation suggesting that the two properties are not related.

5. MECHANISMS OF DETERIORATION Pipes deep below the ground water table will be attacked by substances (acids, sulphate, chl;oride) dissolved in water penetrating by permeability. In

pipes just below the water table and above it the attacking substances will be taken in by water penetrating by sorptivity. Classical formulae for permeability and sorptivity suggest that the rate of penetration to a given depth is controlled by the pore radius while the volume of water delivered is related to the pore volume. For diffusion pore volume may be more significant.

Figure 2 - Six Notional Concretes

1. 5% voids, no capillaries

4. 2% voids, few large capillaries

3. 3% voids, few fine capillaries

5. 1% voids, few fine capillaries 6. 2% voids, many fine capillaries

2. 10% voids, few fine capillaries1. 5% voids, no capillaries

4. 2% voids, few large capillaries


5. 1% voids, few fine capillaries 6. 2% voids, many fine capillaries


Figure 2 depicts six concretes. The variable are discrete void percentage, capillary void number and capillary void size. The discrete voids have little effect on performance. For example the performance of concrete 1 would be little different to a concrete with 0% voids. Similarly the effect of discrete void percentage in concrete 2 and 3 makes little difference to performance. The difference in concretes 4,5 and 6 however would be marked. The early depth of penetration will be highest in concrete 4 although ultimately the depth of penetration in concrete 5 and 6 is likely to catch up, particularly at the low depths of significance in most deterioration mechanisms. The objective of a sorptivity/absorption test is to be able to differentiate these concrete in respect to durability. In making concrete pipe voids, as denoted in Figure 2, would most likely be caused by poor compaction while the capillaries are most likely to be a function of the concrete mix and curing. It would seem that discrete voids are not so significant and one issue with just measuring VPV is that such discrete voids could make a durable concrete appear of poor quality. Conversely a test that only measures pore radius (eg RTA sorptivity) gives no idea of the volume of flow, a key durability factor. Table 6 shows the likely grade testing the concretes by the various absorption./sorptivity methods would provide. This is a broad assessment where grade 1 is excellent and grade 5 is very poor.



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Table 6 - Test Methods Result (Grade) for Six Notional Concretes

Type 1 as Figure 2 1 2 3 4 5 6 Test method

Measurement function of

Grade Expected Using Test Method Shown

AS 1012.21 VPV Voids 1 5 3 2 1 2

AS 4056 Absorption Voids 1 5 3 2 1 2

TE Sorptivity No of pores and radius 1 1 1 5 1 3

RTA Sorptivity Pore radius 1 2 2 5 2 2

6. RECOMMENDATIONS Measurement of the rate of absorption with root time up to the point of saturation is gaining acceptance as a test method and ASTM and Canadian standards are being developed. The accuracy, measurement of nominal pore radius and number of pores, ability to measure VPV simplicity of tests all make it a clear leader in terms of durability assessment. The Taywood Sorptivity Tests is not well accepted and is unlikely to find favour at this stage as and Australian Standard tests are too far removed. The RTA sorptivity test carries some authority but it is considered far too limited, has too many variables and requires too much specialist equipment to be considered as it is for general use in quality. Of the boiled absorption tests the AS1012.12 is the most suitable for adoption. It currently allows for a 48hr immersion time without boiling. During this period the weight gain with time can be measured to establish a sorptivity. The Papworth Modified AS1012.21 includes an allowance for measurements of the sorptivity rate and is able to be undertaken as a standard test at BRC.

INTRODUCTIONPRINCIPLETESTS METHODSAS4058RTA T362AS 4056, ASTM C642 & AS1012.21ASTM C1585 Sorptivity Test

PREVIOUS RESEARCHVPV vs Other Test Methods & w/b RatioSorptivitySorptivity Testing vs VPV

MECHANISMS OF DETERIORATIONRECOMMENDATIONS

absorption and sorptivity tests

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