1. André Lubeck; Dr. Gihad Mohamad; Dr. Fonseca, Fernando Soares; PhD.
COMPRESSIVE STRENGTH AND FAILURE MODE OF AXIALLY LOADED HOLLOW CONCRETE BLOCK MASONRY
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André Lubeck; Dr.
Gihad Mohamad; Dr.
Fonseca, Fernando Soares; PhD.

2. Introduction
Masonry is a complex structural material that behaves differently from the blocks and mortar.
Especially, mortar changes the behavior in function of stress level and confinement applied.
Previous studies have shown that mortar has a great influence on the failure mode of masonry.
13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS

3. Introduction
While idealizing the masonry system and deciding what materials to used, the performance of each component cannot be evaluated separately, given the physical and mechanical interaction that occurs between them.
In this context, it is necessary to evaluate the mortar strength influence on the masonry failure mode and to determine in what circumstances mortar joint crushing occurs.
This research was conducted to evaluate the mortar strength influence on the prism strength/block strength efficiency ratio and on the failure mode of hollow concrete block masonry under axial compression.
13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS

4. Methodology Two-blocks high prisms were constructed.
Only one type of concrete block were used and two types of mortar having different compressive strengths were tested.
For each prism group, six samples were constructed. The thickness of the mortar joints was 1±0.3 cm.
A high-speed camera, capable of shooting 24-frames-per- second, was used to record the test.
13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS

5. Methodology Concrete block geometry.
Shooting of a concrete block prism during compressive test.
Prism image during test.
13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS

6. Block properties 13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS

7. Mortar properties
The amount of water added was adjusted to obtain the required consistence of 230 ±10mm.
The compressive and bending tensile strengths were determined using prismatic samples of 4 x 4 x 16cm according to Brazilian standard NBR (2010).
Indirect tensile strength was obtained using cylindrical samples of 5 cm x 10 cm (diameter x height), according to NBR 7222 (2011).
The mortar strength-block strength ratio was 0.14 for the weaker mortar and 0.98 for the stronger mortar.
13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS

8. Results and discussion
13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS

9. Results and discussion
The stress at the onset of failure using net area, σpi,net, was the stress when either joint crushing or the first crack in blocks was observed.
The σpi/fp ratio was be used as a measure of prism ductility. The lower this ratio, the more ductile was the failure.
In some cases, the stress at the onset of failure could not be determined because failure was abrupt.
The prism strength/block strength ratio (fp/fb) is a measure of the efficiency of the prism.
13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS

10. Stress-strain curves Axial stress versus strain curve for the prisms.
13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS

11. Results and discussion
Prisms constructed with weaker mortar experienced larger deformation than that of the prisms with stronger mortar.
For the prisms with the stronger mortar fp,net was 15.2 MPa and the efficiency ratio In turn, for prisms with the weaker mortar fp,net was 14.0 MPa and the efficiency factor of 0.67.
The failure mode was completely different for the two types of prisms.
Prisms with stronger mortar had conical-break post-failure cracking. In turn, prisms with weaker mortar experienced joint crushing at stress levels between 30 and 50% of the prism strength.
13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS

12. Failure mode – Strong Mortar Prisms
Cracking pattern of prisms S5 and S6 (strong mortar)
13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS

13. Failure mode – Weak Mortar Prisms
Failure sequence of the prism W1 (weak mortar)
The rupture started with the mortar joint crushing and after that, vertical cracks appeared on the blocks.
The mortar crushing started at a stress of 2.8 MPa (s/fp = 0,26).
13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS

14. Failure mode – Weak Mortar Prisms
Failure sequence of the prism W6 (weak mortar)
The mortar crushing started at a stress of 2.7 MPa (s/fp = 0,23).
13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS

15. Failure mode – Weak Mortar Prisms
Aspect of the mortar joint of prism W6 after the failure
The block-mortar bond was completely broken and the mortar porous structure completely destroyed in prisms with weaker mortar.
13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS

16. Conclusions
The difference between the strength of prisms constructed with the stronger mortar and that of the prisms constructed with the weaker mortar prisms was less than 8%.
Mortar controls the prisms failure mode. Prisms with stronger mortar behaved as a homogeneous material, with a conical-break cracking pattern. In contrast, prisms with weaker mortar experienced mortar crushing and complete degradation of the bond between the mortar and blocks. This behavior was observed at stresses close to 50% of the strength.
The measurement of the compressive strength of prism is not a reliable parameter without analysis of the failure mode; the compressive strength of prisms built with strong and weak mortars were similar. However, the failure mode of the prisms with weak mortar start by crushing at the half of the final strength observed at machine test.
13th Canadian Masonry Symposium
June 4th - June 7th, 2017 – Halifax, NS