IN SITU DETERMINATION OF THE COMPRESSIVE STRENGTH OF MORTAR. Determination of the characteristic compressive strength in existing masonry (f k ) can

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1 IN SITU DETERMINATION OF THE COMPRESSIVE STRENGTH OF MORTAR JOINTS USING AN X-DRILL by P.D.V. CHRISTIANSEN 1 1 Danish Technological Institute, DK-8000 Aarhus, Denmark. PDC@dti.dk ABSTRACT Determination of the characteristic compressive strength in existing masonry (f k ) can be relevant in a number of situations, for example when constructions are exposed to additional loads, walls are removed, etc. Necessary parameters for determining f k are the compressive strength of the unit f b and the compressive strength of the mortar joints f m. From EN (3.2) [5] one gets for general purpose mortar: f k = K f 0,7 0,3 b f m where K is a parameter related to the group of units. For solid units (group 1) K is equal to While f b is easily found by testing samples of units, f m is more complicated to determine in situ. Taking out small specimens often fail and chemical analysis does not take the hardening process into account and is in general rather inaccurate. For this purpose the X-drill has been developed. The concept is to measure the maximum moment of torsion of the X-drill knocked into a predrilled small hole in a mortar joint. The moment of torsion per unit length (knocked into the joint) is linear proportional to f m and the method gives a slightly more accurate prediction of f k compared to the standardized method of measuring f m through EN [2].

2 NOTATION c p D o d i f b f d f k f m f yk h f K L i M v m v Proportionality coefficient between f m and m v,m Outer diameter of the X-drill Inner diameter of the X-drill Compressive strength of the unit Design compressive strength of masonry Characteristic compressive strength of masonry Compressive strength of the mortar Characteristic tension strength of the steel Height of the flange Parameter for determining f k (related to the group for the unit) Knocked in length of the X-drill Maximum moment of torsion Torsion moment per length unit m v,m p σ x Mean value of a series of m v Contact stresses between flanges and joint giving M v Normal forces in the masonry Width of the flange

3 1. INTRODUCTION The normal procedure for determining f k in existing masonry is to estimate the actual strength of the mortar joints through a chemical analysis where the content of lime/cement/aggregates is estimated. These chemical analyses gives rather uncertain results leading to very conservative parameters for f m and eventually in some cases causing steel constructions to be integrated in the existing masonry constructions not always leading to an aesthetic or an economic result. Developing a tool for on-site measuring of f m was thus rather important and the criteria for the tool were: accuracy, speed, economy and simplicity. Through numerous tests it was demonstrated that the X-drill fulfilled these criteria. The tool turned out to be more accurate than using the standardized method (EN ) when predicting f k and test results can be obtained in a few minutes. The cost of the X-drill (after the final design) and the corresponding torque-meter is quite moderate. The tool can operate without access to 220V electricity and is easily carried and operated in not very accessible areas. Measuring f m through EN [2] does not take into consideration the variation of the initial suction rate of the units (and thus the hardening process) and the actual quality of the workmanship (e.g. compressing and filling of the joints). When measuring f m with the X-drill these factors are automatically included since the test is performed on the actual masonry to be examined. Factors relevant for old structures such as wear and tear of the joints due to the climate and pollution are furthermore included in the results.

4 2. THE CONCEPT The X-drill The X-drill is shown in figure 1 and 2. Up till approximately a M5 mortar an X-drill made of stainless steel is used. For mortar stronger than M5 high-yield steel is used. Figure 1. X-drill. Photo Figure 2. X-drill. Sketch

5 The method To clarify the concept, the method of performance is described and commented. 1. A Φ6 hole of mm length is predrilled in the mortar joint. This hole, approximately equivalent to the inner diameter of the X-drill, will steer the X-drill and secure: a minimum of crushing when knocking the X-drill into the mortar joint and an X-drill only fixed through the flanges. 2. Afterwards a Φ 10 hole of 10 mm length is drilled. This minor hole will ensure a reduced influence on f m from any strong pointing mortar. 3. With a hammer the X-drill is knocked approximately mm into the hole measured from the point of resistance. This length is nominated L i. A specific value is not important since the relevant torque moment is per unit length. L i should be minimum 10 mm due to the accuracy though. L i should obviously be smaller than the shaft. For this prototype that is 50 mm (See figure 3). 4. A torque-meter with a trailing pointer is used to determine the maximum moment of torsion under fracture. This value is nominated M v. 5. The value m v is determined as: m v = M v /L i. 6. The value m v,m is determined as the mean value of a series of m v. 7. The number of measurements should at least be 5 depending on the variation of the results. 8. When the X-drill is pulled out the adherence dust is briefly examined. If part of the dust is yellow or red as the units the results are ignored (because the drilling has erroneously been performed partly in the unit).

6 9. Extremely high values are ignored (while extremely low values are not). Extremely high values indicate that the X-drill was probably restrained in the units, while extremely low values indicate worn or washed joints. Figure 3. X-drill. Before measuring the maximum moment of torsion: Mv Figure 4. Determination of L i. Distance knocked in the pre-drilled hole

7 Figure 5. The X-drill is normally knocked mm into the predrilled hole with a suitable hammer

$Figure 6. Measuring the maximum moment of torsion in the moment of fracture 3.$

8 Figure 6. Measuring the maximum moment of torsion in the moment of fracture 3. DEVELOPMENT OF THE X-DRILL Geometry The outer diameter (D o ) The X-drill is developed for joints with a nominal thickness of 12 mm (for other thicknesses the approach is equivalent). Taking into consideration tolerances in-plane and out-of-plane an outer diameter of 10 mm is found appropriate. Making the test in the T-cross between vertical and horizontal joints in the lower part of the vertical joints gives a theoretical tolerance of 2.5 mm in all directions (the calculations are trivial and not repeated here). The lower part of the T-cross

9 gives the best representative test. In the upper part of the T-cross there is a possibility of sunken mortar not giving results representative. Figure 7. Φ10 X-drill in the lower part of a T-joint (12 mm) The inner diameter (d i ) Test with different inner diameters showed only little variation of the results consistent with the assumed line of fraction as shown in the figure below.

10 Figure 8. Line of fraction depending mainly on the outer diameter The only (theoretically) geometrically possible line of fracture is circular around the X- drill. This implies that the inner diameter of the X-drill has no or only little influence of M v. Optimizing the inner diameter is thus a matter of (steel) stresses in the shaft and flanges. The inner diameter was minimized though giving room for a larger grain size in the mortar than tested in this project. The flanges Geometry of the flanges is determined through the following criteria: Minimum dimensions to avoid excessive crushing of the mortar joint Optimized dimensions introducing yield stresses in the complete flange when yield stresses appear in the shaft (when maximum torque is applied on the X-drill)

11 The calculations are trivial and not repeated in this article. The following optimized values for h f and x were found: x d i h f = 1.7 mm = 6.5 mm = 1.75 mm Figure 9. Optimizing the dimensions of the X-drill

12 Material Stainless steel and black steel of high quality can be used. Stainless steel is normally only available in moderate strengths (f yk 220 N/mm 2 ). This quality can normally be used up to M5 mortar joints. For strong mortar joints (> M5) the use of high yield steel is necessary. Correlation to f m measured through EN [2] To establish the correlation between f m measured through the standardized method EN [2] and m v,m several tests in wall panels were performed. The characteristics of the mortar and units for the different wall panels are given in the following table.

13 Table 1- Characteristics of the mortar and units in the tested wall panels Wall Mortar Unit panel Type f m Type f b Initial suc- (EN ) (EN 772-1) tion rate (N/mm 2 ) (N/mm 2 ) (kg/m 2 ) A Design mortar 3,233 Yellow solid machine made B CL 40/60/850 Wet mix * 1,095 Yellow solid machine made 57 2,1 57 2,1 C L 100/1200 Wet mix 0,389 Yellow solid machine made 57 2,1 D CL 50/50/700 Dry mix ** 4,993 Yellow solid machine made E Design mortar 2,839 Yellow solid machine made 57 2,1 57 2,1 F CL 40/60/850 Wet mix 1,481 G CL 40/60/850 Wet mix 1,375 H L100/1200 Wet mix 0,338 I L100/1200 Wet mix 0,566 Red soft moulded Red soft moulded Red soft moulded Red soft moulded 16 2,6 16 2,6 16 2,6 16 2,6 J L100/1200 Yellow soft 51,8 *** 1,2 Dry mix 0,398 moulded

14 * Wet mix is factory made premixed lime and aggregates mixed with water. Normally cement and additional water is added on-site ** Dry mix is factory made premixed lime, (cement) and aggregates. On-site only water is added *** These units were taken from an existing construction and cleaned. Due to dust the initial suction rate may not be accurate For all 10 wall panels at least 5 measurement of M v was performed on each wall to determine m v,m. In the following table the mean value (m v,m ) is stated, together with the values for f m found through EN

15 Table 2. Values for m v,m using the X-drill Wall Mortar EN X-drill panel Type f m m v,m COV * Mean value Mean value (N/mm 2 ) (Nm) A Design mortar 3,233 0,996 0,128 B CL 40/60/850 1,095 0,347 0,106 C L 100/1200 0,389 0,153 0,534 D CL 50/50/700 4,993 1,134 0,152 E Design mortar 2,839 0,945 0,095 F CL 40/60/850 1,481 0,393 0,285 G CL 40/60/850 1,375 0,511 0,240 H L100/1200 0,338 0,096 0,454 I L100/1200 0,566 0,098 0,296 J L100/1200 0,398 0,175 0,421 * COV= sample variance/sample mean The values for the COV for the weak mortars (L100/1200) seem quite high. These mortars are probably more sensitive to local variations in the: material parameters (e.g. the suction rate and thus the hardening conditions) execution(e.g. compression of the units and thus the water content)

16 f m determined through EN (N/mm2) In the following graph the values for the X-drill (m v,m ) is plotted against f m. The coefficient of correlation is 0.93 which (for masonry) is quite extraordinary. The relations seem to be linear with a proportionality coefficient (c p ) of 3.62 (N/mm 2 /Nm) X - Drill versus EN R² = 0,9322 y = 3,6215x 0 0,2 0,4 0,6 0,8 1 1,2 m v,m determined from the X - Drill (Nm/mm) Figure 10. Graph of correlation between f m determined through EN and m v,m The preliminary conclusion is that a value for f m can be obtained using the formula: f m = c p m v,m The parameter f m is a component used when calculating f k so finding a direct link from m v,m to f m (found via EN ) is not a crucial issue, since it must be expected that the determination of the parameter f m and its relation to f k has variations (and errors). Therefore an examination of the compressive strength f k of the masonry itself versus f m and m v,m is valuable, in order to adjust c p if necessary.

17 Correlation to f k measured through EN [3] The walls were tested according to EN giving measured values for f k. The values are shown in figure 11 (nominated Test ) 0,70 0,3 Furthermore is f k calculated using formula (3.2) in EN : K f b f m where f m is determined from EN and via f m = c p m v,m (NB: In some countries there may be other expressions for f k for weak mortars. E.g. in Denmark the following expression is valid: f k = K f b 0,70 (½f m ) 0,3, for mortars with a high content of lime). c p is adjusted so the values for f k found via: f m = c p m v,m is smaller than f k determined via: test or f m obtained from EN In the following diagram the results are shown. f k f k measured and calculated Test Via EN Via X-drill A B C D E F G H I J Walls Figure 11. Diagram showing f k determined via test, EN and X-drill

18 The criterion is obtained using: c p = 3,2 (i.e. using c p = 3,2 the values for f k found via the X-drill is smaller than f k found via test or via EN ) As it can be seen this value for c p is slightly smaller than the value found via linear regression analysis shown in figure 10 but is regarded to be conservative and is used for practical design and determination of f k. The average deviation between f k determined by test compared to: EN is: 30,1 % the X-drill is: 28,4 % so the X-drill gives a slightly better prediction of f k. Correlation to normal stresses σ To determine any influence from normal stresses a series of tests with a variation of normal stresses were performed. The stresses applied on the specimens were mainly in the interval of N/mm 2 corresponding to a load up to 120 kn/m (on a wall with a thickness equivalent to 108 mm). To get a correct regression analysis a few tests were performed for loads corresponding to a normal stress equivalent to: MPa. The test was executed on wall panel J.

19 m v,m determined from the X - Drill (Nm/mm) m v,m determined from the X - Drill (Nm/mm) The results are shown in the following figures: 0,25 0,2 0,15 0,1 0,05 0 X-drill in compression y = 0,0138x + 0, ,2 0,4 0,6 0,8 1 1,2 Compression (MPa) Fig 12. Values for the X-drill for compressive stresses equal to MPa 0,25 0,2 0,15 0,1 0,05 0 X -drill in compression y = -0,0008x + 0, Compression (MPa) Fig 13 Values for the X-drill for compressive stresses equal to MPa As it can be seen in the figures the regression line for the interval: 0.0 1,2 MPa is slightly increasing while the regression line for the interval: MPa is slightly decreasing. The slope of the regression lines is very low and not significant and it is therefore concluded that the influence of any compression is negligible.

20 4. PRACTICAL DETERMINATION OF THE COMPRESSION STRENGTH OF THE JOINTS 1. The value for the torsion moment per length unit is nominated m v and is calculated as: m v = M v /L i 2. The mean value for all values of m v is found and nominated as: m v,m 3. f m is found via the formula: f m = 3,2 m v,m 4. The characteristic compression strength for the masonry f k is found through the expression given in EN (3.2): f k = K f b 0,70 f m 0,3, (or f k = K f b 0,70 (½f m ) 0,3 in some cases) where K = 0,55 for group 1 units 5. f b is easily found by taking out representative samples of the unit for test according to EN [1] 6. Finally the design value is found by: f d = f k /γ m The value for the safety factor γ m is discussed in the next chapter.

21 5. DISCUSSION The safety factor γ m The safety factor is normally found through the National Annexes and varies for different countries (e.g. in Denmark the value for γ m = 1.6 [6] and in UK it is γ m = 2.7 for 2nd class of execution control and units of category I [7]). One could argue that the safety factors are normally found through common declarations of units and mortars, compression test in laboratories, etc. These tests are found in situ and thus taking into consideration the actual state of the construction, the wear of the joints, the different conditions of hardening, etc. Furthermore a safety factor should represent the number of samples related to size of the constructions. In EN 1990 [4] and relevant National Annexes guidelines are given for determination of characteristic values and safety factors through evaluation of COV. These methods are not suitable for a compression strength of a combined material, where the coefficient of variation of f b and f m is normally different, and where f b and f m in the formula (3.2) in EN are mean values. A new statistical approach should be developed taking into consideration the specific issues. For now the usual safety factor is considered conservative, when taking 5-10 representative samples of the relevant construction.

22 The fracture Based on m v the maximal shear stress (τ max ) along the line of fraction can easily be determined. The value for τ max should not be compared with f vk0. The maximal shear stress τ max >> f vk0 due to the nature of the fraction. See the following figure. Figure 14. Illustration of stresses in the line of fracture When f vk0 is determined, dilatation of the specimen perpendicular to the line of fracture is possible. When shear stresses are introduced from the X-drill, dilatation along the line of fracture is not possible. The torsion will instead introduce tension stresses in the cross-section close to the test area keeping the specimens steady from any dilatation.

23 Furthermore, the assumptions of a line of fracture completely circular will require a homogeneous material. In reality the substance in the joint consists of binder and aggregate and part of the aggregate has strength far beyond the joint itself forcing the line of fracture to divert slightly from the circular shape. This is shown in the figure above where the line of fracture is indicated with a thick line.

24 6. LITERATURE 1. EUROPEAN STANDARD. EN Methods of test for masonry units part 1. Determination of compressive strength 2. EUROPEAN STANDARD. EN Methods for test for mortar for masonry part 11: Determination of flexural and compressive strength of hardened mortar 3. EUROPEAN STANDARD. EN Methods for test for masonry part 1: Determination of compressive strength 4. EUROPEAN STANDARD. EN Basis of structural design 5. EUROPEAN STANDARD. EN Design of masonry structures part 1 1: General rules for reinforced and unreinforced masonry structures 6. EN DK NA:2008. National Annex to Eurocode 6: Design of masonry structures - Part 1-1: General rules for reinforced and unreinforced masonry structures ( DK/ydelser/Standardisering/Fagomraader/ByggeriOgAnlaeg/Eurocodes/Nationale _Annekser/Documents/NA%20EUROCODES%20ENG/EN% %20DK%20NA.pdf) 7. EUROCODE FOR MASONRY. EN AND EN Guidance and Worked Examples, B.A. Haseltine, etc.

DS/EN DK NA:2013

DS/EN DK NA:2013 National Annex to Eurocode 6: Design of masonry structures - Part 1-1: General rules for reinforced and unreinforced masonry structures Foreword This national annex (NA) is a revision and compilation of