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COMPARATIVE STUDY OF STANDARD TEST PROCEDURES FOR MORTARS

HENRIQUES, FERNANDO MA

NeW univel'SilY of Lisbon, Quinta da Torre, 2825 Monte da Gaparica, Portugal

cHAROLA. ELENA 8 earstow Road #76, Great Neck, NY 11021 , USA

SUMMARY The various standard procedures currently in use for testing mortars are based on completely diverse specifications. Consequently, the results obtained by these tests cannot be compared. The present study was developed with the aim to determine the effect of different specifications, such as mixing and storage conditions, upon physico-mechanical properties of the resulting mortar specimens. For that purpose, two sets of samples were prepared using a lime-sand and lime:pozzolan:sand mortars. In each set samples were mixed according to different specifications, BS 4551 and EN 1961 and cured in two different environments. The samples prepared were tested for their mechanical properties: compressive and flexural strength, modulus of elasticity, water absorption (according to RILEM and NORMAL specifications) and, for some of the pure lime mortar samples, water vapour permeability. The results obtained are discussed in view of developing standards that are appropriate to test lime mortar fonnulations for use in historic structures.

1. INTRODUCTION

The present situation where multiple standard procedures exist for the same test is the result of the parallel development in different countries across the wor1d. Historically the first recommended practice for mixing mortars can be traced to the ear1y 19th century [1]. The first building lime standards in the US were published by the American Society for Testing Materials (ASTM) in 1912 (2). By 1931 mortar "types" were established according to performance which defined different curing conditions for the various mixes. Part of the problem in the establishment of mortar types is their specification which can be by composition or by performance, but not both (3). The lime mortar standards were being developed at the time that Portland cement was being introduced as a key material in mortars. Hence, most of the curing conditions were established on the basis of the hydration requirements of the latter material. It is obvious that lime mortars cannot perform as well under these conditions. The loss of centuries of experience with this material resulted from a combination of their poor performance in laboratory tests with the ease of application as well as increased construction speed possible with cement mortars. · All of the above has had an important impact in the field of historic preservation. It is practically impossible to refer to studies and experiences in other countries, because the differences in the testing procedures make it impossible to compare results across national boundaries (4). Each country in the wor1d is still struggling on its own. Only recently has CEN, the European Committee for Standardization, started the attempt of standardizing tests across national boundaries within Europe. The present study aims at highlighting the differences in performance obtained by testing samples of the same mortar composition prepared under one specification but cured in different conditions, e.g. mixed according to BS recommendations and cured under BS or CSTB conditions, or viceversa. The performance of the mortars was assessed by standard mechanical tests and by other tests, such as capillary water absorption measured according to different standardized procedures, e.g. RILEM . or NORMAL.

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2. EXPERIMENTAL

2.1 Mortars Two types of mortar compositions were tested: one based on pure hydrated lime and the second including a hydraulic component. The mixes used were hydrated lime and sand (1 :4) for the first one, and hydrated lime, pozzolan and sand (1 :1 :4) for the second. For both compositions, half of the mortars were prepared according to British specifications, BS 4551 [5], and half following European specifications, EN 196-1 (6]. In tum, half of the samples prepared with a given specification were cured according to British standards (BS 4551) and the other half following French specifications, Cahiers du CSTB [7, 8]. Consequently, from each mortar two sets of 6 samples were prepared, from which 3 were cured according to British standards and the other 3 according with French specifications. The differences in the preparation and curing conditions for the two specifications samples are summarised in tables 1 and 2.

Table 1 - Preparation of samples

BS 4551 EN 196-1

- solids 30 s mixing - liquids and binder 30 s mixing - liquids 30 s mixing with speed 1 - mix during 60 s - sand 30 s mixing with speed 1 - cleaning of the sides ( 15 s) - mix 30 s with speed 2 - stand for 10 min. - stop 1,30 min. -mix60 s - mix 60 s with speed 2

For the mixing process BS uses a mixer with just one speed, axially 120 r.p.m. planetary 60 r.p.m., while EN requires a mixer with two speeds (speed 1: axially 140 r.p.m., planetary 62 r.p.m.; speed 2: axially 285 r.p.m., planetary 125 r.p.m.).

Table 2 - Curing conditions

BS 4551 CSTB

- moulds placed in plastic bags at - 20 °C and 50 % RH 20 °C for 1 to 3 days

- saturated chamber at 20 °C

The mortars prepared were identified by a two letter code, the first letter indicating the preparation method (Q for BS and~ for EN), and the second, the curing procedure (Q for BS and £for CSTB). The type of mortar is identified by using lower case letters for the pure lime mortars, and upper case for the lime-pozzolan mixes. The list of identification codes and the description of the corresponding mortar mix is given in the following list:

- BB: lime:pozzolan:sand mortar, prepared and cured according to BS - bb: same as previous, but referring to a lime:sand mortar - BC: lime:pozzolan:sand mortar, prepared according to BS and cured under CSTB conditions - be: same as previous, but referring to a lime:sand mortar - EB: lime:pozzolan:sand mortar, prepared according to EN and cured under BS conditions - eb: same as previous, but referring to a lime:sand mortar - EC: lime:pozzolan:sand mortar, prepared according to EN and cured under CSTB conditions - ec: same as previous, but referring to a lime:sand mortar

The samples were prepared using classic prismatic steel moulds of 40 x 40 x 160 mm. For the water vapour permeability test, due to the limited availability of moulds (100 mm diameter and 10 mm thickness), only samples of the lime:sand mortars mixed according to BS (bb and be) were prepared. It was consi9ered critical to use the same batch preparation for all samples to avoid any differences which might occur from batch to batch.

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2.2 Tests To characterise the mortars described previously, the following determinations were carried out: compressive and flexural strengths, dynamic modulus of elasticity (according with the French standard NF e 10-511 [9D, capillary water absorption (according with RILEM 11.6 [10) and NORMAL 11/85 [11D and water vapour permeability (according with CSTB [7) and NORMAL 21 /85 [12)). The samples were kept in their respective curing conditions until the tests were performed. It should be noted that the lime:sand samples cured in BS conditions could only be removed from the moulds after 7 days, since at the specified 3 days they did not have the required consistency for removal.

2.3 Results of the tests Since the aim of this study was to emphasize large differences in performance of the same mortar mix prepared and cured under different conditions, rather than accurate determination of the values measured, the minimum number of samples was used to obtain representative results. The precision of the measurements is indicated by the mean value of the standard deviation selected from the range obtained for each set of experimental values. The ranges were fair1y narrow within each type of mortar while differing widely between types for some of the tests.

2.3.1 Compressive and flexural strengths The results are presented in table 3. The standard deviation for the compressive strength of pure lime mortars is :t 0,03 MPa and :t 0.14 MPa for the mortars with pozzolan. For the flexural strength the values are :t 0.01 MPa and :t 0.09 Mpa.

Table 3- Compressive and flexural strengths

Samples Compression Flexion (MPa) (MPa)

BB 4.17 2.01

BC 4.03 1.70

EB 2.83 1.23

EC 1.38 0.32

bb 0.19 0.11

be 0.60 0.29

eb 0.21 0.12

ec 0.60 0.29

2.3.2 Dynamic modulus of elasticity

The results obtained for the dynamic modulus of elasticity (according with the French standard NF B 10-511) are presented in table 4. The standard deviation for the elasticity modulus is of :t 50 MPa for pure lime

mortars and of :t 300 MPa for those with pozzolan.

Table 4 - Modulus of elasticity E (MPa)

Samples E (MPa)

88 2700

BC 2600

EB 1800

EC 1700

bb 2530

be 2610

eb 2760

ec 2590

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2.3.3 Capillary water absorption This test was performed following two different test methods, RILEM 11.6 [7] and NORMAL 11/85 [8], allowing for comparison of the results since the specimens of a same batch were used. There are three main differences between the RILEM and NORMAL methods: test units, conditions and contact mode with

the water. RILEM uses kg/rn2.sy. as test units, while NORMAL uses g/crn2.sy.. Regarding the conditions, RILEM specificies that 4 of the 6 faces should be watertight while in NORMAL the samples are simply placed in the test tank. Another major difference is the contact mode with the water: immersion of the samples to a 2 mm height, for RILEM; and samples placed on filter paper saturated with water, in NORMAL. In this latter configuration the water absorption occurs through continuity of more complex capillarity phenomena between the water in the filter paper and the test material rather than by direct water absorption as is the

case for RILEM. In both methods, the idea is to create reproducible conditions of water absorption by controlling the evaporation rate in a closed environment which will saturate with water vapour at the test temperature. In the case of the RILEM test, the evaporation can only occur through the top face. The water absorption coefficients and the maximum water absorbed (asymptotic value as defined in NORMAL 11/85) were calculated and are presented in table 5. For comparison purposes the coefficients determined by the NORMAL standard were converted to kg/rn2.sy. (units used by RILEM). The standard deviation for the capillary water coefficient is of :t 0.04 kg/rn2.sy. and for the water absorbed of :t 1.3 kg/rn2

for pure lime mortars and :t 3.6 kg/rn2 for those of pozzolan.

2.3.4 Water vapour permeability The water vapour permeability test was performed only on the lime:sand mortars mixed according to BS (bb and be) using both the CSTB [7) and the NORMAL 21/85 [12) specifications. The CSTB uses the dry cup method provided by 50 g of CaC'2 inside the cup and an external ambience of 20°C and 90% RH while the NORMAL uses a wet cup method in a closed environment with silica gel. The actual temperature at

which these tests were run was 22.5°C. The 1 cm thick samples, with an active area of 38.48 cm2, were dried to constant mass and then inserted into the dry-cup holder for the CSTB test. The test was run until the differences in mass of the samples were lower then 5% (approximately 12 days). The samples were re-dried to constant mass, inserted into the wet-cup holder, and then subjected to the NORMAL test. This test was run for 1 O days when the required stability in the sample weight was obtained.

Samples

bb

eb

be

ec

BB

EB

BC

EC

Table 5 - Water absorption coefficients (kg/m2.s~ and maximum water absorbed (asymptotic values in kg/m2)

RILEM NORMAL

Coefficients Asymptotic Coefficients Asymptotic (kg/m2.s~) (kg/m2) (kg/m2.s~) (kg/m2)

0.26 19.3 0.14 18.9

0.24 19.9 0.13 19.6

0.23 17.4 0.07 15.9

0.23 17.4 0.09 17.4

0.40 23.4 0.33 23.1

0.40 26.6 0.30 26.6

0.38 21 .6 0.30 21 .7

0.30 19.4 0.24 19.2

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Test results are presented in table 6, expressed as water vapour transmission, WVT (g/m2.h), permeance (glm2.s.Pa) and permeability (g/m.s.Pa), following ASTM E 96-92 standard [13), as well as in the units specified by RILEM (s) and NORMAL (g/m2.24h corrected to 20°C) and calculating m, the ratio of the water vapour transmission coefficient in air to that in the material, as defined by the DIN standard [14]. The NORMAL units [g/m2.24h] although referred to as permeability are actually water vapour transmission 14,13). The RILEM units (s) are a mathematical simplification of the permeability expressed in s.I. units (kg/m.s.Pa) [4]. The standard deviation in the rate measurement is of :t 0.003 g/h.

3. DISCUSSION OF RESULTS

The results obtained can be analysed in several ways. In the first place the two different mortar mixes, lime:sand (henceforth lime mortar) and lime:pozzolan:sand (henceforth hydraulic mortar), can be compared to evaluate the influence of the addition of an hydraulic component to the pure lime mortar. Then the type of mixing used in the preparation of the mortars (BS and EN) as well as the curing conditions (BS and CSTB) can be evaluated.

Table 6 - Water vapour permeability test

Units CSTB NORMAL

bb be bb be

WIT g/m2.h 16.46 16.09 12.09 8.16

Permeance g/m2.s.Pa 1.81x10.a 1.83x10.a 1.23x10.a 0.832x10.a

Permeability g/m.s.Pa 1.81x10.a 1.83x10.a 1.23x10.a 0.832x10.a

RILEM s 1.81x10·11 1.83x10·11 1.23x1ff11 0.832x1ff11

NORMAL g/m2.24h 328 331 249 168

DINm - 10.6 10.8 15.7 23.3

3.1 Effect of preparation procedure The effect of the mixing procedure used during the preparation of the mortars can best be appreciated in figure 1 which compares the results obtained from the mechanical tests for pairs of samples that only differ by their preparation conditions. Curing conditions were constant for each pair. Figure 1 shows that variations in the mixing procedure practically have no effect on the mechanical properties of pure lime mortars, whereas those for hydraulic mortars are significantly affected. The compressive and flexural strengths and the modulus of elasticity are higher when mortars with hydraulic properties are mixed according to the British standard. This could be attributed to the longer mixing time, including the 10-min standing, prescribed in this standard. This time would allow for the hydration reaction

of the hydraulic components to begin.

3.2 Effect of curing conditions The effect of the curing conditions used for the mortars can best be appreciated in figure 2 which compares the results obtained from the mechanical tests for pairs of samples that only differ in their curing. The main difference in the curing conditions is that the British standard calls for a moisture saturated chamber and the CSTB only requires 50% RH. These conditions affect both the lime and the hydraulic mortars, but whereas the lime mortars improve in mechanical resistance curing at 50% RH, the hydraulic mortars decrease slightly. These results can be explained considering the reactions that are taking place.during curing.

5

4 . - !~ -- -,-----, . I ' ' I 1 i I ! ;

I ! : !-

bb/eb bc/ec BB/EB BC/EC

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2.5

2 ;- - -

,.. a. ::;;; ;; 1,5 . c;, c ~ VI

~ 1 r- - - - - - -~ i u::

bbleb bclec BB/EB BC/EC

3.000 r-------~

bbleb br./ee 88/EB BC/EC

Fig. 1 - Effect of the preparation procedure on the results of mechanical tests

5

4

,.. a. 5 ~3 - ------c ~ VI (l) > ·;;; :3 2 -a. E 0 u

0 bb/bc eb/ec BB/BC EB/EC

2,5 ; - ---- ---------·--

2 -

~ 1 5 L _____ _ ~ . I :ij I i;; I

~ d------ - ' x (l)

u::

0 bblbc eblec BBIBC EB/EC

Fig. 2 - Effect of the curing conditions on the results of mechanical tests

3.000

2.500

,.. ~ 2.000 z. '()

"" VI ~ 1.500 0 VI :> :; -g 1.000 ::;;;

500

bb'bc eb'ec BBISC EB/EC

In the case of hydraulic mortars moisture is required for the continued hydration of the hydraulic components and for the subsequent growth of the resulting crystalline phases. The presence of high humidity will favour these reactions which affect the ear1y strength of the mortar. On the other hand, lime mortars require carbon dioxide for the carbonation reaction. Although the presence of moisture will facilitate the carbonation reaction of the lime and crystallization of the resulting calcite crystals, too much moisture, as under the BS conditions, will slow down the reaction. This can be explained by considering that all the exposed surfaces of the lime mortar are covered with a layer of liquid water and that the C02 has to diffuse through it before it can reach the lime surface. It is interesting to note that both lime mortars and the hydraulic mortars prepared according to the BS have similar values of the elasticity modulus. This is significantly smaller for the hydraulic mortars prepared with the European standard.

3.3 Other observations The capillary water absorption and water vapour transmission tests were carried out five months after the mechanical tests due to unrelated laboratory problems. During these months the samples were kept under their curing conditions. Since the lime mortars are particular1y slow in their setting, this difference in time could possible affect their setting and a second set of compression tests were carried out. The results are given in table 7.

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Table 7 - Compression strength for the lime:sand mortar mixtures; initial values and after 5 months storing under curing conditions

Initial (MPa) After 5 months (MPa)

bb 0.19 1.06

be 0.60 1.68

eb 0.21 1.09

ec 0.60 1.60

It can be seen that the compression strength increases significantly over this time. It falls close to the weakest hydraulic mortar (EC with a compression strength of 1.38 MPa) resulting from poorer preparation conditions (European standard) and curing at 50% RH (CSTB specifications). The capillary water absorption test mainly shows differences between the lime and the hydraulic mortars, regardless of their preparation procedure or curing conditions. It can be observed in table 5 that the hydraulic mortars absorb a significantly higher amount of water and that their water absorption coefficient is higher regardless of the testing method used. This could be attributed to a larger amount of smaller pores present resulting from the hydration reactions of the hydraulic components. It should also be noted that although the maximum water absorbed is similar for both methods the capillary water absorption coefficients differ, though showing the same trend. The larger RILEM coefficients can be explained by the test conditions: water is absorbed at a faster rate because the sample is immersed up to 2 mm in it and evaporation can only take place from the top surface. For the case of the lime mortars, there is a slight difference to be observed between those cured according to the CSTB and BS conditions. Those with higher mechanical strength show a lower capillary water absorption, as would be expected from a denser material. It is interesting to note that this effect is more evident with the NORMAL procedure than with the RILEM test (see table 5). The results obtained in the water vapour permeability tests can only be considered as indicative as more samples would have been required to obtain statistically significant data. However, the trends observed can

be regarded as valid. The first observation that can be made is that the data from the CSTB dry-cup method result in higher permeability values than those obtained with the NORMAL wet-cup method. This could presumably be explained by the fact that this latter test was run about one moth after the CSTB test and considering that the samples are lime mortars whose nature is changing significantly over time as discussed above (see

table 7). The second observation is that the NORMAL method showed differences between the sets of samples cured at different conditions, i.e., bb and be, which were not evident in the CSTB method. The samples (be) cured at 50% RH showed a lower water vapour transmission falling in line with the results from the mechanical tests and the capillary water absorption data (see table 5). More tests should be carried out to

confirm these hypothesis.

4. CONCLUSIONS

The previous discussion has shown the significant influence preparation and curing conditions can have on the performance of a mortar. Though this is obvious, the various choices of preparation and curing conditions appear as somewhat arbitrarily selected. However, most standards develop from a long history of laboratory testing, and the conditions established reflect that history though not explaining it. It is significant that standards for mortars developed at the time Portland cement was introduced requiring standard tests to assure its quality control. These tests were then adapted, or not, to test lime mortars. Since the design of the testing procedures considered the best conditions required for the setting of Portland cement, i.e., mechanical testing at 28 days, it is obvious that lime mortars would fail under these conditions. The poor laboratory performance and the lengthier application procedures required by traditional

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mortars lead to a decline in their use. This resulted in the loss of the practical knowledge of their preparation in the field which only recently has been regained in part through lengthy studies and tests. It is important that the correct use of lime mortars for the preservation of historic structures be assured through testing procedures. These should be developed specifically taking into account the nature of the material in question so as to provide a meaningful evaluation. As has been pointed out, this means that adequate and comparable procedures should be used [15). As clear1y demonstrated by this study, current standard procedures are not comparable. Although the call for international standardization has been made repeatedly over the past [16,17,18) and more recently during the ICCROM International Colloquium of Methods of Evaluating Product for the Conservation of Porous Building Materials in Monuments, Rome 1995, and the Dahlem Workshop, Ber1in 1996 [19), only the recent CEN effort promises progress. It is to be hoped that this will serve to inspire other nations wor1d-wide to join the international standardization effort.

ACKNOWLEDGEMENTS

The experiments described in this study were carried out at the Laborat6rio Nacional de Engenharia Civil (LNEC), in Lisbon.

REFERENCES

(1) Nicholson, P. (1823): The New Practical Builder. London, Thomas Kelly. (2) Speweik, J.P. (1995): The History of Masonry Mortar in America 1720-1995. National Lime Association, Arlington, VA. (3) Davison, J.I. (1975): Masonry: Past and Present ASTM STP 589, ASTM, Philadelphia, PA. (4) Henriques, F.A. (1996): Test Methods for the Evaluation of New Mortars for Old Buildings. Science & Technology for Cultural Heritage (in press). (5) British Standards Institution BSI (1980): Methods of testing mortars, screeds and plasters. BSI, London. BS 4551 :1980. (6) Co mite Euro peen de Normalisation CEN ( 1987): Methods of testing cement; determination of strength. CEN, Brussels. EN 1961 . (7) Centre Scientifique et Technique du Batiment CSTB (1982): Modalit0s d'essais des enduits exterieurs d'impermeabilisation demur a base de liants hydrauliques. Cahiers du CSTB, Paris, (230), cahier 1779. (8) Centre Scientifique et Technique du Batiment CSTB (1993): Certffication CSTB des enduits monocouches d'impermeabilisation. Cahiers du CSTB, Paris, (341 ), cahier 2669-4. (9) Association Franc;:aise de Normalisation AFNOR (1975): Pierres calcaires: mesure du module d'elasficite dynamique. AFNOR, Paris. NF B 10-511 . (10) RILEM Commission 25-PEM (1980): Recommended tests to measure the deterioration of stone and to assess the effectiveness of treatment methods. 11.6 Water absorption (capillarity) . In Materiaux et Constructions, vol. 13, n° 75. (11) NORMAL (1985): Assorbimento d'acqua per capillarifa. Coeficiente di assorbimento capillare. CNR - ICR, Rome, 11 /85. [12) NORMAL (1985): Permeabilita al vapor d'acqua. CNR - ICR, Rome, 21/85. [13) ASTM (1992): Standard Test Method for Water Vapor Transmission of Materials. ASTM E96-92. (14) DIN (1987): Bestimmung der Wasserdampfdurchlassigkeit von Bau- and Dammstoffen. DIN 52 615. [15) KnOfel, D. and Schubert, P. (1993) M6rlel und Steinerganzungstoffe in der Denkmalpflege, Ernst & Sohn, Berlin, p. 13.

(16) Report of the Committe on Conservation of Historic Stone Buildings and Monuments (1982) in Conservation of Historic Stone Buildings and Monuments, National Academy Press, Washington, DC. p. 8. (17) Recommendations of the Working Group for the Study of Mortars and Grouts for the Conservation of Ancient Masonry (1981) in Mortars, Cements and Grouts used in the Conservation of Historic Buildings, ICCROM, Rome, p. 413.

(18) Proposal for a Pilot Study on the Conservation of Historic Brick Structures (1987) in NATO-CCMS Pilot Study "Conservation of Historic Brick Structures'', Umweltbundesamt, Berlin 1990, p. 6. (19) Charola, A.E., De Witte, E. and Laurenzi Tabasso, M. (n.d.): Establishing Standards for the Quality Control of Conservation Materials and for Qualifying Practitioners Applying Them in Saving Our Architectural Heritage: The Conservaiton of Historic Stone Structures, Report of the 79th Dahlem Workshop, N.S.Baer and R.Snethlage, Eds, Dahlem Konferenzen, Berlin (in press).