COrv1PRESSIVE STRENGTH OF MORTAR IN MASONRY SIGNIFICANCE, INFLUENCES, TEST METHODS, REQUIREMENENTS

Peter Schubert1 and Gisbert Hoffmann2

1. ABSTRACT

The importance of a knowledge of mortar compressive strength in the joint is presented and justified The compressive strength of the masonry can be characterized much more accurately through the mortar compressive strength in the joint than through the existing standard compressive strength tested on mortar specimens made up in steel moulds. A knowledge of mortar compressive strength in the joint is also important for reinforced masonry, providing far more information than the standard compressive strength. A dose re1ationship has, for example, been established between mortar compressive strength in the joint and the bond strength between reinforcing steel and mortar.

Various test methods for determining mortar compressive strength in the joint are described and assessed.

The objective of current research is to quantify all significant influences on mortar compressive strength in the joint and to prepare a draft method capable of predicting compressive strength of mortar in the joint envisaged unit-mortar combinations. If necessary, this may be supported by simple short test methods. The first phase of the research project the influence of the water suction by the unit on mortar compressive strength in the joint has been investigated. Available results are presented and discussed.

Keywords: Masonry; Mortar; Compressive Strength; Influences; Test Methods.

lDr -Ing .. InstituI fur Bauforschung, RWTH Aachen, SchinkelstraJle 3, D-52056 Aachen, Germany 2Dipl.-Ing. , InstituI fur Bauforschung, RWTH Aachen, SchinkelstraJle 3, D-52056 Aachen, Germany

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2. IMPORTANCE OF MORTAR COMPRESSIVE STRENGTH IN THE JOINT

Masonry compressive strength fc ma may be determined by tests or by reference to evaluated test results from the' unit compressive strength fc un and the mortar compressive strength fc,mo. The following equation is used in Germany:

(1)

An equation of this nature is also included in the current edition of Eurocode 6 for masonry The informativeness of the equation is enhanced if the parameters are defined for each different type ofmasonry.

Mortar compressive strength has hitherto usually been determined on specimens made up in steel moulds and stored under defined conditions. Mortar in masonry joints usually hardens under significantIy different conditions. One decisive difference as compared to the standard test is the removal of a more or less substantial fraction of the mix water from the mortar through suction by the unit This fraction depends on the mortar composition, especially its water retaining capacity, and on the pore structure and moisture content of the units. In addition, there are a number of other influences on mortar compressive strength in masonry, as shown in Fig. 1.

mortar composition

L aggregate si J, J, binder I I add~ives I admirtures II water I type, content, ~eve curve, type, content, effect content

type, contenl fineness type, contenl article shape, othel components hardening fineness, effect

I • J. mortar in masonry 1

I I water retentivity compressive strength, ~ hardening ofthe mortar of the mortar time dependance ... ... I water suction I conditions in situ I

of the masonry units workmanship, hardening .,. .,. 1'" 1'" capillary pores moisture mortar laying temperature

size, content compaction humidity volume by laying joint thickness air movement

unit factors factors of workmanship

Fig. ] : Influences on the mortar compressive strength in masonry according to /1/

The standard compressive strength fc mo s and the compressive strength of mortar in the joint fc mo j were tested on "standara composition" mortars without adrnixtures (general purpose mortars) combined with various masonry units. The ratio amo = fc mo s/fc mo j ranged from 0.5 to 1.5. This difference between standard and mortar ' compressiv'e strength in the joint has no serious effect on masonry compressive strength, which is distinctIy less dependent on mortar compressive strength than on unit compressive strength.

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Nowadays, however, general purpose mortars are scarcely used . The overwhelming majority of masonry mortars are factory-made mortars. They usually contain various admixtures and additives designed to optimize specific mortar properties (workability, bonding with the unit etc.) . The variety of unit types and grades has also increased. Owing to the changed composition of the masonry mortar and the wide range of masonry units with often widely differing properties, greater differences arise between standard and mortar compressive strength in the joint. An additional aspect is presented by the ready to use mortar, in which hardening is delayed by up to 36 h in order to prolong workability time as much as possible. If a mortar of this type is worked at the beginning of the workability time, the unit may be able to remove mix water by suction rrom the mortar over a much longer period than with standard retarded mortars.

For the above reasons, the range of amo ratios has widened (cf. Fig. 2), reducing the informativeness of the standard compressive strength parameter and increasing the importance of knowing the mortar compressive strength in the joint. For example, German studies indicated a mortar compressive strength in the joint in specific cases amounting to only 10 % of the standard compressive strength. If standard compressive strength continues to be assumed in such cases, masonry compressive strength will be greatly overestimated. In individual cases, this may lead to a significant deterioration in the safety leveI.

fc.mo j in N/mm2

NMI INMIT NMlTa NMIH NMIHa 30

• 25 •

20

• • • • I •

15 • • • • DI> • • • • v • • • v AAC 10 o + +

'" C I • + • '" LC

• + • clay 5 • o CS

+ • CS • + clay

O O 5 10 15 20 25 30

(mo" in N/mm2

Fig. 2: Relation between mortar compressive strength in the joint fc moj hardened between different kind ofunits and standard compressive strength fc,mo,s

To avoid such cases, the influence ofunit contact on mortar compressive strength in the joint needs to be taken into account. A test including ali units and checking mortar compressive strength in the joint is naturally impossible. In Germany, therefore, a simple method of ensuring a minimum mortar compressive strength in the joint was required.

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On the basis of present knowledge, the least favourable re1ationships, i.e. the lowest mortar compressive strength in the joint, were mostly to be expected in the case of calcium silicate units. A specific reference calcium silicate unit with the most constant possible long-term properties was therefore selected for a prelirninary test in which all types of masonry mortar (with the exception of prescribed general purpose mortar) have to demonstrate a specific mortar compressive strength in the joint (see Section 3 for test method) .

The objective of research now commencing is to quantify ali significant influences on mortar compressive strength in the joint and develop a draft method. Using this method, possible supported by simple short test procedures, it should be possible to predict the mortar compressive strength in the joint for planned unit-mortar combinations. In the first stage of research, the influence on mortar compressive strength in the joint of mortar rnix water suction by the unit was investigated (see Section 4) .

3. TEST METHODS FOR DETERMINING MORTAR COMPRESSIVE STRENGTH IN THE JOINT

3.1 Cube Compression and Plate Compression Methods

As noted in Section 2, it was regarded as necessary in Germany to demonstrate mortar compressive strength in the joint as part of the prelirninary test for masonry mortars. A provisional guideline has been prepared, and is now available in a revised and supplemented version dating from August 199212/.

A specific reference calcium silicate unit is used in the prelirninary test. In its original version, the guideline contained on1y the cube compression method, in which a tleece is first placed on the surface of the reference calcium silicate unit and a grid mould with a grid interval of 20 mm is then placed on the tleece. The fresh mortar is placed in the grid mould. After the mortar has been smoothed off and the upper unit has been positioned, a second fleece is placed on the mortar joint and the upper unit finally placed in position The two fleeces are intended to prevent direct bonding of the mortar with the unit and so ensure that the grid mould can be removed without difficulty. At an age of 28 d, the approximately cuboid mortar specimens (dimensions: 20 mm . 20 mm . joint thickness) are removed from the grid mould and tested for compressive strength.

Owing to the relatively small contact surface of 400 mm2, the test forces are very low, especially in the case oflower-strength mortars. This causes difficulties in works tests (at mortar manufacturers), since the test machines installed there are generally designed for larger test forces and the required test sensitivity is not achieved. The objective of the factory mortar industry has therefore been to agree a test method with a larger contact surface. It should be possible to conduct the test without the comparatively complex grid mould. The intention was to cut the mortar specimen out of the "bed joint plate". The cutting process may lead to marginal failure, depending on the mortar composition and strength. Compressive strength test values for compressive stressing over the full area may be influenced substantially. In the ibac test method /3/ already available at that time (see Section 3.2), this is avoided by applying a partial area loading. However, the pressure area used in the ibac test is still toa small. Extensive tests with partial area loading and contact surfaces of various sizes were therefore carried out. These showed that a specimen area of 80 mm . 80 mm and a loaded area of 40 mm . 40 mm yields useful test resuIts. The fourfold test forces met the development requirements. Owing to the plateshaped specimen, this method was termed the plate compression method. It is included in the present version of the provisional guide1ines as a method equivalent to the cube compression method.

The various specimen sizes and different contact surfaces naturally have to be taken into account in deterrnining the rninimum mortar compressive strength in the joints to be

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achieved in the test. Comparative tests revealed a sufficiently elose correlation between the test values for the two test methods. A partial analysis for specific mortars is shown in Fig. 3. The test values according to the plate method were roughly twice those for the cube method. The minimum compressive strength requirements were specified accordingly. The minimum compressive strengths for the plate method correspond to the values required for standard compressive strength; the minimum values are halved for the cube method.

fc .mo .jC in N/mm2

8 .0 -r---------------------------------,

6 .0

4 .0

2 .0

/ •

/ . / /.

/ / .

• M'" •

,

./

/. /

/-. /

/ <,.>-----------, fc.rno.jC = 0,60 * fe .mo.IP

rxy = 0,97

0 .0 ~-----r-----_,_----_r----__r-----r_-------l

0 .0 2 .0 4 .0 6 .0 8 .0 10.0 1 2 .0

fe .mo.IP in N/mm2

Fig.3: Relation between mortar compressive strength in the joint tested by the cube method fc,mo,jC and plate method fc,mo,jP

Whereas the cube method is suitably solely for preliminary tests with fresh mortar, the plate method can also be used to deterrnined the mortar compressive strength in the joint of mortar in finished masonry. However, it will not always be possible to extract a specimen with the required size of 80 mrn . 80 mrn from the mortar bed joint.

3.2 ibac Method

The method is described in detail in /31. It is characterized by the use of cubic or cylindrical mortar discs (side length or diameter roughly 50 mm), cut from the mortar joint by means of a cut-off wheel or core drill and compression tested over a partial area of the surface. It is advisable to place a compensating layer between the steel test dies and the mortar specimen. Fig. 4 indicates the test method. As already noted in Section 3.1, partial are a loading largely eliminates the influence of removal-related side effects.

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Mortar specimen

Joint thickness t j

Felt layer

Platen De = 20 mm

Side view Topview

Load partial area

Mortar specimen 50mm· 50mm

FigA: Determination ofmortar compressive strength in masonry by the ibac method

Theoretical considerations and extensive tests indicated a test diameter of 20 mm and a specimen side length or diameter of 50 mm as the most favourable combination. For this case, a factor ofroughly 1.0 was determined for conversion to the standard test, meaning that the measured mortar compressive strength in the joint may be regarded as a justifiable approximation of the standard compressive strength in terms of the specimen shape. Differences between the standard and the mortar compressive strength in the joint are caused mainly by differences in consistency, by compaction and by hardening conditions.

The ibac test method can be used both for preliminary tests on masonry mortars and as a test of compressive strength in the joint on mortars from masonry. Owing to the comparatively small specimen dimensions, especially the small load contact surface, the method is particularly suitable for determining the compressive strength of mortar taken from masonry, especially for bed joint mortar from the web zone of perforated units.

4. INITrAL TESTS TO DETERMINE INFLUENCES ON COMPRESSIVE STRENGTH OF MORTAR IN THE JOINT

4.1 rnfluence ofJoint Thickness

/4/ investigates the influence of joint thicknesses of 10 and 20 mm respectively on the mortar compressive strength in the joint of a lime-cement mortar and a cement mortar combined with a solid calcium silicate unit and a vertical perforated clay brick. Other parameters which were varied were the moisture state ofthe units (air dry and prestored in water), the compaction ofthe mortar between the stones, the sand composition ofthe mortar and the test age. Two-unit specimens with a bed joint were prepared. Selected test results are shown in Fig. 5.

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fc,moj2Omm

NM II CS • o •

• [J O

NM 11 HLz O . O 3 O

2.5 O

2

45' O

O 2 2.5 3

16

O 45'

O 2 2.5 3 6

16 f..",ojlOmm

O

NM 111 CS • O 12 •

O

o· • O 12 NM 111 Hlz

10 • O • O

45' 45' °Ow~~~~--------~10---1-2 ----~16 °O~~------------~10---1-2----~16

o Dal compacted, prestnred in water • compacted, prestored in water O Dal compacted, airdry • compacted, airdry

NM ll: lime-cemenl mortar NM li: cemenl mortar CS: calcium silical units HLz: clay units

fc,mojlDmm

Fig. 5: Relation between mortar compressive strength in the joint with a joint thickness of20 mm fc,moj20mm and ajoint thickness of 10 mm fc,mojlOmm

The mortar compressive strength in the joint of the calcium silicate specimens was usually greater with the thicker joint than with the 10 mm joint. This is probab1y due to the fact that overall water loss from the mortar due to suction by the units is not so high at a larger as at a smaller joint thickness. The influence of joint thickness is not so c1ear in the tests with vertical perforated c1ay bricks. Prewetting the units and consolidating the mortar had no c1ear effect on mortar compressive strength in the joint.

4.2 Influence ofWater Suction by the Unit

In order to quantify the relationship between mortar compressive strength in the joint and removal of mortar water through unit suction, initial tests were made using reference caJcium silicate units and various natural stones.

The method used to determine the water content in the mortar as a function of contact time with the unit is described below. After the mortar had been rnixed, two-unit specimens with joint thicknesses of 10 to 12 mm were made up . At specified times - 10, 30, 60, 120 and 300 rninutes after fabrication of the specimens - the upper unit was removed from each two-unit specimen and a mortar specimen removed from the centre of the bed joint. This was weighed immediately after removal and after 24 h drying at 105 0e. This method was used to determine the water content of the mortar in the joint as a function of unit contact time by reference to the water content of the unused fresh mortar.

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In the first test series, 3 mortars of identical composition with differing water-binder (w/b) ratios were tested in combination with caIcium silicate reference units. The results are shown in Fig. 6.

water content in m.-% 20

mortar 1 18

16 mortar 2

14 , mortar 3

12

10 "

" 8 " "

6

4

2 - -- - -- _"":."':..._--:....-;- ....::.. ~ ------...... -::-.:_-- -- -:::: --- - - --

O

O 10 30 60 120 tin ..,Imin

300

Fig. 6: Water suction of calcium silicate reference units

The time curves for water content are fundamentally similar for ali 3 mortars. The water content of the mortar has changed very littIe after roughly one hour's contact time with the unit. Water absorption by the unit is completed when its maximum water absorption capacity has been reached or the physical and chemical water bonding cannot be overcome by capillary forces in the unit pores. Absorption of water by the caIcium silicate units is largely completed after 2 h. The residual water volume of some 2 M.% (corresponding to a w/b ratio of about 0.2) which still remains in the mortar is insufficient for complete hydration ofthe cement. The mortar compressive strength in the joint of 5 N/mrn2 deterrnined at an age of 28 d appears disproportionately high in reJation to this low w/b ratio. One possible explanation is that part ofthe water absorbed by the unit remigrates to the mortar after the initial water suction phase. This "internaI curing" may cause a corresponding increase in strength.

In a second test series, a mortar developed for pointing masonry was used. The binding agent was burned oil shale and hydrated lime. The mortar contained methyl cellulose to enhance its water retaining capacity. Mortar consistency was stiff to slightIy plastic. The mortar was used with 3 different sandstones - Obernkirchen, Ruhr and Schopper sandstone. Table 1 shows the principal characteristics ofthe sandstones.

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Schopper Ruhr Obernkirchen Sandstone (S) Sandstone (R) Sandstone(O)

Bulk density fkgldrn J ] 2,68 2,75 2,69 Capillary water suction [kgl(m2hO. 5)]

11,6 0,2 1,9

Total porosity [V 0/01 22,9 6,9 19,0 Pore diameter (median) 12,89 0,07 1,64 [11m] Water suction by 15 N/mm2

21,0 6,9 19,0

Saturation value 0,79 0,91 0,65

Table 1: Property values of sandstones

The water content ofthe joint mortar was determined as a function of contact time with the various sandstones, as described above. The results are shown in Fig. 7.

water content in M.-% 10 ,-________ ~----------~--------------------------_,

lo-.. ; '< ------~--:-----J.. " ", _ -- ;-------L---------, _________ ~

-----

8 .

Ohernkirchen Sand~tone 7 •

Ruhr Sandstone

Schopper Sandstone

o 10 30 60 120 t in .,Jmin

300

Fig. 7: Water suction of sandstone units

In addition, the mortar compressive strength in the joint and standard compressive strength ofthe mortar were tested. Results are given in Table 2 .

fc,mo,s28d fc,mo,sI4d fc,mojl4d

Sandstone Obernkirchen 1 Ruhr 1 Sch~er

N/mm" 13 ,5 9,5 5,7 j 6,4 1 6,6

Table 2: Mortar compressive strength Standard compressive strength at an age of28 d fc mo s28d and 14 d f mo s14d and compressive strength in the joint fc,mojl4d in contact to three diiferent sandstones

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As will be apparent from Table 2, the standard compressive strength is significant1y higher than the mortar compressive strength in the joint. Although there are no great differences in mortar compressive strength in the joint with the different sandstones, the lowest compressive strength is found for the combination with Obernkirchen sandstone, which according to Fig. 7 withdraws most water from the mortar.

The varying water loss of the mortar with the three different sandstones is basically explicable through their differing pore structure. The pore diameters relevant for capillary suction pressure lie within a range from about 0.1 to 100 !lm. The capillary suction pressure ofthe pores increases in proportion to 1/r. As indicated by Table 1, the sandstones differ considerably in terms of their capillary water absorption, overall porosity and median pore radius. The Schopper sandstone has a very high capillary water absorption and a high median pore radius. It may be assumed that it absorbs a large volume of water quickly as a resuIt of its relatively large pores, but that water absorption is restricted to a short period owing to the lower suction capacity of these large pores. The capillary water absorption, overall porosity and median pore radius of Ruhr sandstone are ali very low. lt may be concluded that the capillary suction pressure is high due to the small pores, but that the suction process takes place very slowly and that the quantity of water absorbed is relatively small. The Obernkirchen sandstone has a significantly higher capillary water absorption, its overall porosity is rough1y identical with that of the Schopper sandstone and the median pore radius is much larger than for the Ruhr sandstone. The Obernkirchen sandstone can accordingly absorb a large quantity of water relatively quickly. These differences in water absorption behaviour correspond generally with the test results in Fig. 7. The differences between water suction and water absorption may be explained through the presence of ultrafine particles from the fresh mortars.

5. CONCLUSION

The results of initial tests on the influences affecting mortar compressive strength in the joint and quantification of these influences confirm that the "water balance" in the joint zone, i. e the suction of mortar water by the unit and possible partial restitution of water to the mortar may have a decisive effect on mortar compressive strength in the joint. Subsequent tests will concentrate on the task of quantifYing these influences in terms of pore structure parameters and mortar composition.

6. REFERENCES

/1/ Schubert, P .: Zur Festigkeit des Mbrtels im Mauerwerk; Prüfung, Beurteilung. Berlin: Ernst & Sohn - in Mauerwerk-Kalender 13 (1988), S. 459-471

/2/ Deutsche Gesellschaft fur Mauerwerksbau: Vorlaufige Richtlinie zur Erganzung der Eignungsprüfung von Mauermbrtel. Bonn: August 1992

/3/ Schubert, P.; Schrnidt, St.: Bestimmung der Druckfestigkeit im Mauerwerk. Aachen: Institut fur Bauforschung, 1990.-Forschungsbericht Nr. F 304. In ibac-Kurzberichte 3 (1990), Nr. 28, ibac-RWTH Aachen (Selbstvgl.)

/4/ Asenbaum, K.: Einflüsse auf die Druckfestigkeit des Mauermbrtels und Errnittlung dieser Druckfestigkeit im Mauerwerk. Aachen: Technische Hochschule, Fachbereich 3, Diplarb, 1979

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