IN SITU MECHANICAL CHARACTERIZATION OF THE MORTAR IN MASONRY BUILDINGS WITH DRMS Emanuele Del Monte (1) and Andrea Vignoli (1) (1) DICeA, University of Florence, Italy Abstract In this paper the first results of a research project carried out to validate a Non Destructive Test (NDT) to determine the in situ mechanical characteristics of mortar in historical masonry buildings are presented. According to the compositions of historical mortars, the most commonly used in Tuscan buildings, 15 types of mortar with different mechanical characteristics were produced, obtained with 5 classes of lime (both hydraulic and hydrated) and river sand with 3 grading curves. Fifteen samples, one for each class of mortar, were obtained. Flexural and compression tests were performed on 6 of them, Drilling Resistance Measurement System (DRMS) test was applied on the other 6. DRMS is a drilling device that measures the resistance to perforation of the material being tested. During the test the resistance force to perforation is measured with continuity, maintaining rotational speed and penetration rate constant. The result is a curve that shows the drilling resistance profile. The research project is aimed at validating a correlation between the sizes measured with DRMS and the mechanical characteristics of mortar in masonry buildings. Keywords Non Destructive Test, existing buildings, masonry, mortar, DRMS 1. INTRODUCTION The mechanical characteristics of mortar in masonry buildings affect the load bearing capacity of the structure under both gravitational loads and horizontal actions, as the seismic action. All national [1] and international [2] Standards define the parameters of masonry, as compression and shear strength, depending on the mechanical properties of the elements: either bricks or stones and mortar. In new buildings this method is not particularly difficult to apply. In fact, it is possible to determine the mechanical characteristics of the components of masonry by simple laboratory tests carried out according to Standards [3] and [4]. Assessment of load bearing capacity of existing buildings is certainly more complex, since it is not 421
Transcript
IN SITU MECHANICAL CHARACTERIZATION OF THE MORTAR IN MASONRY BUILDINGS WITH DRMS
Emanuele Del Monte (1) and Andrea Vignoli (1)
(1) DICeA, University of Florence, Italy
AbstractIn this paper the first results of a research project carried out to validate a Non Destructive
Test (NDT) to determine the in situ mechanical characteristics of mortar in historical masonry
buildings are presented. According to the compositions of historical mortars, the most
commonly used in Tuscan buildings, 15 types of mortar with different mechanical
characteristics were produced, obtained with 5 classes of lime (both hydraulic and hydrated)
and river sand with 3 grading curves. Fifteen samples, one for each class of mortar, were
obtained. Flexural and compression tests were performed on 6 of them, Drilling Resistance
Measurement System (DRMS) test was applied on the other 6. DRMS is a drilling device that
measures the resistance to perforation of the material being tested. During the test the
resistance force to perforation is measured with continuity, maintaining rotational speed and
penetration rate constant. The result is a curve that shows the drilling resistance profile. The
research project is aimed at validating a correlation between the sizes measured with DRMS
and the mechanical characteristics of mortar in masonry buildings.
Keywords Non Destructive Test, existing buildings, masonry, mortar, DRMS
1. INTRODUCTION
The mechanical characteristics of mortar in masonry buildings affect the load bearing
capacity of the structure under both gravitational loads and horizontal actions, as the seismic
action. All national [1] and international [2] Standards define the parameters of masonry, as
compression and shear strength, depending on the mechanical properties of the elements:
either bricks or stones and mortar. In new buildings this method is not particularly difficult to
apply. In fact, it is possible to determine the mechanical characteristics of the components of
masonry by simple laboratory tests carried out according to Standards [3] and [4]. Assessment
of load bearing capacity of existing buildings is certainly more complex, since it is not
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possible to take samples of mortar to test mechanical strength. For that reason, in order to
assess the mechanical properties of mortar, in situ tests must be carried out, although Standard
test are still not available. Physical methods, as thermal analysis, x-ray diffraction analysis
and granulometric analysis, chemical methods and petrographic section analysis have been
codified in the Standards, but they only provide qualitative information on the composition of
mortar (binder and aggregate). However, moderately destructive methods to determine
mechanical properties of mortar are described in technical literature. These techniques are
penetration tests carried out on mortar joints and are divided into percussion methods and
rotation methods. The pendolum rebound hammer [5], the penetrometer of Liberatore and
others [6] and the penetrometer of Felicetti and Gattesco [7] are percussion tests correlating
the quality of mortar with its hardness. The pendolum rebound hammer [5] is based on
rebounding of a pendulum mass hitting the mortar surface with a small diameter head. The
other two methods, [6] and [7], estimate the properties of the material through the number of
hits needed to make a probe penetrate to a default depth. The rotation methods assess the
mechanical characteristics indirectly, evaluating parameters resulting from microdestructions
of material, although these instruments work as a traditional drill, whose drill bit rotate and
shifts at fixed speeds to penetrate into a mortar layer. Chagneau and Lavasseur [8] realized a
device for measuring the force required to drill a material, while Gucci [9] validated the
“PNT-G” technique based on the measurement of the energy required to drill a small cavity in
a mortar layer. The method presented in this paper is based on the principle that “lime mortar
is essentially made up of aggregated sand grains, therefore its mechanical strength depends on
the intensity of the links constituting it” [9]. Therefore, compressive strength of mortar is
correlated with the force required to reduce mortar to sand, similarly to the
“dinamostratigraphic test” of Chagneau e Lavasseur [8]. The device presented, called DRMS
(Drilling Resistance Measurement System), is based on a particular instrumented drill easily
measuring the drilling resistance of stones and mortars. It is a very simple technique for
estimating the compressive strength of mortar in situ. During the test the device measures
continuously the penetration force maintaining both rotational speed and penetration rate
constant. The test final result is a graph showing the variation of the force (DR) as a function
of the penetration depth. This system was originally planned to evaluate the consolidating
performance of conservative treatments applied to stones in monumental buildings [10].
Afterward [11], it was used for estimating the mechanical properties of cement mortar with
compression strength between 14 and 45 MPa. In this paper the results of a laboratory
research project carried out on historical mortar are presented. Fifteen type of mortars, the
most commonly used in Tuscany building context, with low mechanical characteristics were
produced. The mortars were composed with 5 classes of lime (both hydraulic and hydrated)
and river sand with 3 grading curves. The test results with DRMS device aim at observing
experimentally the influence of parameters as the quality of the binder and the dimensions of
the aggregate, and at validating a correlation between the sizes measured by DRMS and the
mechanical in situ characteristics of mortar.
2. DRMS DEVICE
The system has been designed to measure the drilling resistance of natural stone as a
measure of its overall intrinsic qualities. The characteristics of the sample are assessed by
statistical analysis of all acquired data. The system consists of two main components: a
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drilling device and an electronic unit. For a correct position during the test the so called
Drilling Resistance Measurement System (DRMS) is equipped with a strong tripod with an
adjustable height from 800 up to 1600 mm and a head allowing 3D movements.
Figure 1: A – drilling device ; B,C – electronic unit
[DRMS manual hardware]
The mechanical device consists of two motors for positioning and drilling and a load cell for
measuring the force. The electronic unit is housed in an aluminum case and consists of two
actuators for the motors, a card conditioning and amplifying the load cell signal and an input
and output signal for acquisition and control.
The system control and data processing are performed via a PC equipped with management
software developed in LabView environment. The test can be performed with various drilling
parameters. The rotational speed can vary from 100 to 1200 rpm, while the penetration rate
can be set from 1 up to 40 mm/min. Both parameters remain constant throughout the test.
Particular attention should be given to the selection of the drill bits. Depending on the surface
tested, i.e. mortar joints of limited thickness, it was decided to use drill bits 5 mm in diameter.
Another important aspect to consider was the choice between drill bits available on the market
and those especially made for this specific method. Because of the great variability of results
obtained using drill bits of the first type and because of the importance of repeatability of the
test, special diamond drill bits were used.
3. TESTING
DRMS device as a method on assessment of mortars in the existing masonry buildings has
been validated through a research project on 15 types of mortars. The mortars were made by
combining 5 types of binders (both hydraulic and hydrated) conforming to [12] with river
sand of three different grain-size classifications conforming to [13].
In this particular case, three types of NHL 2, NHL 3.5, NHL 5 natural hydraulic lime
provided by the supplier (T.C.S.) were used. These are pure natural hydraulic limes of Saint-
Astier without addition of clay or any other material. The hydrated binder was made of putty
lime also mixed with “cocciopesto”. This type of binder was chosen to reproduce historical
mortars. All three different types of aggregate were river sand with maximum size variable
from 1 to 8 mm. Each mortar was composed of the following proportions by weight: 1 part
binder to three parts of aggregate, while the water amount varied from mixture to mixture
(Table 1). For each mortar a minimum of 12 samples were prepared: 6 underwent bending-
compression tests, the other 6 underwent DRMS tests. Preparation, seasoning and bending-
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compression tests of samples (sized 40x40x160 mm) were performed conforming to [4]. As
far as the DRMS tests were concerned, ten 30 mm deep holes were made alternatively to
avoid noise between each test. During the tests 20 records were acquired every 1 mm,
therefore 601 records were acquired for each hole, 601x10 = 6010 records were acquired for
each sample and 6010x6 = 36060 values were acquired for each mortar.
Table 1: composition of mortars and compressive strength
amount [g] mortar binder
aggregate max. size [mm] binder aggregate
Rc [MPa]
N5/01 NHL 5 01 2300 6900 4.15
N5/04 NHL 5 04 2600 7800 4.20
N5/08 NHL 5 08 2600 7800 6.82
N3.5/01 NHL 3.5 01 2600 7800 3.92
N3.5/04 NHL 3.5 04 2600 7800 5.90
N3.5/08 NHL 3.5 08 2600 7800 5.05
N2/01 NHL 2 01 3500 10500 4.99
N2/04 NHL 2 04 3000 9000 7.32
N2/08 NHL 2 08 3000 9000 7.54
G/01 Putty lime 01 6000 9000 1.39
G/04 Putty lime 04 6000 9000 1.42
G/08 Putty lime 08 6000 9000 1.19
Putty lime 01 6000 GC/01
“cocciopesto” 04 6000
3000 1.79
Putty lime 04 6000 GC/04
“cocciopesto” 04 6000
3000 1.98
Putty lime 08 6000 GC/08
“cocciopesto” 04 6000
3000 1.55
Mortar mixtures with NHL 5 binder were used to determine the most suitable drilling
parameters to perform the tests. The choice was a fair compromise between a good readability
of results and a minimum gap compared to the maximum value recordable by the load cell
(100 N). In DRMS test the drilling force is directly proportional to the penetration rate (v) and
inversely proportional to the rotational speed ( ). The wearing state of the drill bit was
assessed at short regular intervals to obtain homogenous and comparable results. For this
reason, all drill bits were previously tested and subsequently their working state was regularly
checked every 10 holes. ARS (Artificial Reference Stone), a ceramic material, was used for
the test. Each mortar was tested with a different drill bit which never needed replacing, since
with the drilling parameters used wearing of the drill bit was almost negligible, probably
because of the low mechanical properties of the mortars. In fact, the phenomenon of wearing
is regarded as mainly due to the nature of silica sand and to the rotational drilling component.
Consequently, in these mortars, because of the weak linkages formed between the aggregates
and low rotational speed in relation to penetration rate, the drill tip tends to remove a sand
grain instead of drilling it. The results of the compression tests show that the mortars reached
a resistance between 1 and 8 MPa. Obviously, the mortars composed with air binder reached
lower values which were achieved with a seasoning time longer than that of mortars
composed with hydraulic binder. Test results as the average values obtained from each sample
are shown in Table 1. For compression tests in particular the average value is obtained from
12 samples. The DRMS test result shows the drilling resistance profile (x = hole depth in
millimeters; y = Drilling Resistance in Newton).
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0
10
20
30
40
50
60
70
80
90
100
0 5
10
15
20
25
30
DR
[N
]
depth [mm]
Figure 2: typical hole in the N35/01 mortar
0
10
20
30
40
50
60
70
80
90
100
0 5
10
15
20
25
30
DR
[N
]
depth [mm]
Figure 3: typical hole in the N35/04 mortar
0
10
20
30
40
50
60
70
80
90
100
0 5
10
15
20
25
30
DR
[N
]
depth [mm]
Figure 4 : typical hole in the N35/08 mortar
In Figures 2,3 and 4 three typical results of tests performed on mortars with aggregate of
maximum sizes 01, 04 and 08 mm are shown. The diagraphs demonstrate that the maximum
size of aggregate affects significantly the result. In fact, in the type 01 mortar the
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characteristic value of the binder is evident, while in the type 04 and 08 mortars the presence
of sand of bigger size causes drilling resistance increments that cannot be attributable to the
binder. Therefore, the data average taken as characteristics value of the mortar can lead to
errors. For this reason it was necessary to perform a statistical treatment of data in order to
highlight the experimental value attributable to the binder and the one attributable to other
phenomena like the presence of voids or aggregate of significant size.
4. STATISTICAL TREATMENT OF DATA
In order to validate the DRMS diagnostic method it was necessary to perform a careful
statistical analysis of data. The aim is to validate correlation curves between the drilling
resistance and the compressive strength of the drilling parameters considered. Therefore two
correlation curves for the drilling parameters v = 40 mm/min = 100 rpm and v = 40
mm/min = 300 rpm were validated, while for v = 40 mm/min = 1000 rpm validation was
not possible because the data were not sufficient to perform a least square regression. The
mortars tested with the latter parameters were those with hydraulic binder (NHL2, NHL3.5)
and aggregate of bigger size (08), and were used to observe the system response to good
quality mortar and aggregate of bigger size. The regression curves were obtained comparing
the average value of compressive strength tests and the average value of the test with DRMS
for each mortar. Since increasing the size of the aggregate the results showed a high
variability, this last value was carefully studied. Statistical analysis to outliers were conducted
in order to highlight the value of the binder. The box blots on the data sample of 36060 values
were calculated for each mortar and thus the outliers were determined (Figures 5 - 10).
42363024181260 20 .818.215 .613.010 .47.85 .22.6
Figure 5: N35/01 mortar data Figure 6: N35/01 mortar data without outliers
917865523926130 16.814 .412 .09.67.24 .82 .40.0
Figure 7: N35/04 mortar data Figure 8: N35/04 mortar data without outliers
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917865523926130 19.616.814.011 .28.45.62.80.0
Figure 9: N35/08 mortar data Figure 10: N35/08 mortar data without outliers
The results of these calculations are shown in figures 11-16 that represent the graphical result
of the DRMS test considering all data for each mortar and show that the variability of the data
always decreases after removing the outliers. Also in this case the analysis to outliers lead to
far more homogeneous results. Obviously, this is more evident in mortars with aggregate of
bigger size. After performing all the statistical elaborations it was possible to find the x1-x2
range in order to calculate the characteristic average for carrying out the regression analysis
(Table 2). The x1-x2 range varies depending on the drilling parameters; the choice of
calculating the average of x1-x2 range was made in order to avoid interference caused by an
initial transient of drill bit, to limit the influence of a possible accumulation of material at the
bottom of the hole and, in any case, to make the mortar quality as homogeneous as possible
avoiding the surface of the sample that may have had different maturation degrees.
0
10
20
30
40
50
60
70
80
90
100
0 5
10
15
20
25
30
DR [N
]
depth [mm]
average
max
min
0
10
20
30
40
50
60
70
80
90
100
0 5
10
15
20
25
30
DR [N
]
depth [mm]
average
max
min
Figure 11: mortar N35/01 Figure 12: mortar N35/01 without outliers
0
10
20
30
40
50
60
70
80
90
100
0 5
10
15
20
25
30
DR [N
]
depth [mm]
average
max
min
0
10
20
30
40
50
60
70
80
90
100
0 5
10
15
20
25
30
DR [N
]
depth [mm]
average
max
min
Figure 13: mortar N35/04 Figure 14: mortar N35/04 without outliers
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0
10
20
30
40
50
60
70
80
90
100
0 5
10
15
20
25
30
DR [N
]
depth [mm]
average
max
min
0
10
20
30
40
50
60
70
80
90
100
0 5
10
15
20
25
30
DR [N
]
depth [mm]
average
max
min
Figure 15: mortar N35/08 Figure 16: mortar N35/08 without outliers
The validated regression curves are:
Rc = - 1.107 + 0.483 DR (1)
Rc = - 0.580 + 0.864 DR (2)
where Rc is the compressive strength estimated by DR (Drilling Resistance). Eq. (1) refers to
drilling parameters v = 40 mm/min and = 100 rpm, while Eq. (2) is valid for v = 40
mm/min and = 300 rpm. The results of the regression analysis (Table 3) and Rc vs DR
(Figure 17) and Rc(DR) vs Rc (Figure 18) graphs show that the validated curves fit well to data
and the errors on the predicted results about compressive strength are modest.
Table 2: DRMS tests data without outliers and compression tests data
mortar v
[mm/min] [rpm]
DR [N] aver.
0-30 mm
DR [N] aver.
x1-x2 mm
DR[N] max.
x1-x2 mm
DR[N] min.
x1-x2 mm
Rc
[MPa]
N3.5/01 40 100 11.33 11.05 20.46 2.02 3.92
N2/01 40 100 11.83 11.79 21.13 3.15 4.99
G/01 40 100 4.95 4.55 9.70 0.13 1.39
GC/01 40 100 7.52 6.79 16.17 0.01 1.79
N3.5/04 40 300 8.10 8.14 17.84 0.44 5.90
N2/04 40 300 8.49 8.52 18.97 0.05 7.32
G/04 40 300 2.00 1.99 5.04 0.01 1.42
GC/04 40 300 2.74 2.73 7.04 0.01 1.98
G/08 40 300 2.25 2.26 6.63 0.01 1.19
GC/08 40 300 2.79 2.80 7.88 0.01 1.55
N3.5/08 40 1000 7.08 7.13 19.84 0.01 5.05
N2/08 40 1000 11.09 11.15 31.44 0.03 7.54
The results are even more interesting if used to obtain a more qualitative estimate of the
mechanical characteristics of mortars. In fact, identifying 4 intervals, the stime is accurate:
