shear behavior and failure pattern of precast wall panel

Shear Behavior and Failure Pattern of Precast Wall Panel

Connections with Full and Semi

Blockouts (Slots)

Dr.K.Ramadevi1* and S. Parkavi2

1Professor, Civil Engineering Department, Kumaraguru College of Technology,

Coimbatore

2Assistant Professor, Civil Engineering Department, United Institute of Technology,

Coimbatore

1Email: [email protected] 2Email: [email protected]

Abstract

In the growing infrastructure, the amount of savings in labour and material, enhanced

product quality and workmanship and to improve rate of construction, precast industry serves

an important role. To improve the structural integrity and to provide continuity to the precast

elements, precast connections plays an important role. Earthquake loads or other disasters

contribute to the major structural damages to the connections. This study aims to propose four

different types of wall – to – wall connections – Loop connection with full and semi blockouts

(slots) and Toothed connection with full and semi blockouts in order to determine the shear

characteristic of the wall panels. The fibre reinforced concrete (FRC) wall panels and the base

slab with semi and full blockouts were casted in the precast plant and tested in the laboratory

until failure. All specimens were subjected to monotonic lateral loading to find the load

capacity and deflection at ultimate load due to the failure of the connection. The load-deflection

behaviour, mode of failure and maximum deflection of wall panels were determined for all the

four different types of connections.

Keywords: Precast concrete structures, FRC wall panels, Wall connection, Blockout,

Monotonic Loading, Energy absorption.

1. Introduction

Precast concrete is a substitute to cast-in-situ concrete. While cast-in-situ concrete is

cast in its site, precast concrete is cast at casting plant, either at the building site or in a factory,

and is then lifted to its final resting place and fixed securely. This means that unlike cast-in-

situ construction, which is monolithic or continuous, precast concrete buildings are made of

separate pieces that are connected together.

*Corresponding Author

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The main objective of the structural connection is to transfer forces among the precast

concrete elements in order to attain structural interface once the system is loaded. By the ability

to transfer forces, the connections should have the intended structural behaviour of the

superstructure and the precast subsystems that are combined in it. Therefore, the structural

connections should be regarded as an important part of the structural system and they should

be designed properly as the same for the precast concrete elements. To understand the structural

behaviour of fibre reinforced wall to wall connections on any lateral loading such as earthquake

or wind loading.

The wall connections are subjected to in-plane lateral ground movement. Cracks

occurred at the bottom and wall interface. The results can assert that the common connection

has also low efficiency in terms of capacity against lateral loading [1]. Connection between the

exterior and interior precast concrete walls by loops and gap between walls are filled by

concrete. Crushing and spalling was concentrated at the joint. The connection showed a ductile

behavior - few line cracks and large deflection occurred to give a warning before failure [2].

Studied behaviour of tension joint using direct overlapping loops, evaluating the influence of

the addition of steel fibres to the in-situ concrete [3].

Connection design is one of the most significant considerations for a successful

construction of precast reinforced concrete structures in terms of the structural behaviour [4].

Proposed a new connection in order to recover rotational loading capacity and develop a finite

element model of precast wall with connections by considering all details of different parts for

a contemporary connection, as well as the proposed connection [5].

Investigation on flexural behaviour of precast concrete (PC) wall – steel shoe composite

assemblies with various dry connection details at mid-span is done [6]. An innovative

horizontal wall-to-wall connection using FE model for precast shear wall systems was

developed and compared with cast-in-situ wall system [7]. The use of shear key is to connect

two separate precast components to improve the shear resistivity of the joint surfaces. The

proposed shear key shape in this study comprises of triangular, composite rectangular, semi-

circular and trapezoidal [8].

The evaluation of the out-of-plane response of confined masonry walls with toothed

connections when damaged due to in-plane forces was studied. The confined masonry walls

with or without toothing enhanced the interaction between masonry walls and RC confining

elements and were able to postpone the failure by controlling out-of-plane deflections even

after an in-plane drift cycle of 1.75% [9].

The enhancement in the strength of the M35 grade concrete mix by the addition of

polypropylene fibres on different percentages was observed [10]. By providing fibres in the

critical zones, it is possible to improve the performance of the frames against lateral loading

[11]. Behaviour of cast in-situ hybrid fibre-reinforced concrete frames [12] & [13] with infills

against lateral reversed loads were investigated.

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https://link.springer.com/article/10.1007/s13369-014-1273-6


2. Objectives The main objectives of this research work are listed as under.

• To propose four different types of connections in order to improve shear

characteristic of the pre cast wall panels connected to base slab with semi and full

blockouts.

• To determine ultimate strength of the wall panels tested for lateral monotonic

loading with four different types of connections.

• To find the ultimate deflection of the wall panels.

• To investigate the failure behaviour of wall panels with different types of

connections

3. Properties of Materials used in this Research work

Both the wall panel specimens were casted with ready-mixed concrete of grade of M40. Recron

3S fibres were used which acts like tributary reinforcement in concrete which seizures cracks,

increases resistance to impact/abrasion and significantly improves quality of construction in

walls, it also improves resistance to plastic & drying shrinkage/cracking. The properties of

fibres used in this research work are shown in Table 1.

Table 1. Properties of fibres used

S.No. Properties of Recron 3S Fibre Values

1 Effective Diameter 20 - 40 μ

2 Specific gravity 1.34 - 1.39

3 Melting point 250 - 265 Deg.C

4 Elongation 20 - 60%

5 Alkaline stability Very good

6 Acid resistance Excellent

7 Soundness by Le Chatlier test (mm) 10

8 Tensile strength (N/mm2) 28007

9 Percentage elongation (%) 29.8

10 Denier 64.285

In this research work, concrete mix design for M40 grade of concrete with

polypropylene fibre (Recron 3s) is done, as per IS 10262:2009. The mix proportion arrived for

M40 grade concrete adopted is shown in Table 2.

Table 2. Mix proportion arrived for M40 grade concrete

Cement

Kg/m3

Fine

aggregate

Kg/m3

Coarse

aggregate

(12mm) kg/m3

Coarse

aggregate

(16mm) kg/m3

Water

content

Kg/m3

Polypropylene

fibre

g/m3

400 750 400 800 140 600

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The materials used for grouting the wall-to-wall connection are 1 kg of non-shrink grout

(CONBEXTRA GP 2) and 500 g of stone chips with 100 ml of chemical bond (Emaco SBR)

with required amount of water.

4. Tests on Companion Specimens

The compressive strength, split tensile strength and flexural strength of concrete were

determined by casting cubes of size 150×150×150 mm, cylinders of size 300 × 150 mm and

prisms of size 500 mm × 100 mm × 100 mm and allowed for 14, 21, 28 days curing. The results

of compressive strength, split tensile strength and flexural strength with Polypropylene fibres

(Recron 3s) are shown in Table 3.

Table 3 Test results on companion specimens

S. No No. of

days

Compressive

strength

(MPa)

Split tensile

strength

(MPa)

Flexural strength

(MPa)

1 14 26 2.06 1.725

2 21 32.38 2.67 2.29

3 28 42 3.19 3.34

5. Experimental Work

5.1 Specimen detailing

Four different types of connections are adopted for interconnecting two walls

horizontally by Looped Connection with semi and full blockouts and by Toothed Connections

with semi and full blockouts in base slab. The height, width and thickness of walls were 1300

mm, 1000 mm and 75 mm respectively. The base slab is of 2860 mm length and 860 mm with

and 150 mm in thickness. Thickness dimensions corresponded to a prototype scale of the

precast panel typically used for medium rise construction. The detailing for wall to wall

connection is shown in Figures. 1 to 4. Along 1300 mm height of wall, six looping bars of 8

mm diameter were placed at a spacing of 200 mm centre to centre. A longitudinal bar was then

inserted in the loop between the two walls. This area created a gap of 100 mm width which

then was filled with non-shrink grout to produce moment resisting connection between the two

walls. Tooth connection consists of three numbers of tooth of dimension shown in figures 3

and 4. Reinforcement provided for wall panels are 8 mm diameter rod with 300 mm c/c and for

base slab 8 mm diameter rod with 360 mm c/c.

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Figure 1. Loop with Semi Blockouts Figure 2. Loop with Full Blockouts

Figure 3. Tooth with Semi Blockouts

Casting, transportation and erection of required precast elements were fully sponsored

by M/S Velan Concast, Avinashi Road, Neelambur, Coimbatore. The precast factory has

special plant for the manufacture and maintenance of moulds. Casting and curing of specimens

was done in the plant. The mould was cleaned and shuttering was done to keep the mould in

position. The reinforced cage was positioned in the partly assembled mould, then the remaining

mould section is completed. Before pouring the concrete, the mould was checked. From ready

mix concrete (RMC) unit, the fibres of dosage 0.6% by volume were added to the concrete.,

The specimens were compacted using high-frequency external needle vibrators of diameter 35

mm with 1300 rpm. The specimens were transported from the plant to Structural Technology

Centre of Kumaraguru College of Technology after28 days of curing for testing. The specimens

were unloaded using 12 T capacity crane and erected in the test floor in the lab. The wall panels

were fitted to the base slab by grouting using FOSROC CONBEXTRA GB2 and

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MASTEREMACO SBR 2 as shown in Figure 5. After grouting the joint, the strength of the

grout and the wall panels were checked by Rebound Hammer in order to verify whether both

the wall panel and the grout have the equal amount strength.

Figure 4. Tooth with Full Blockouts

Figure 5. Grouting the joints of wall panels

6. Test set-up

In order to determine the shear stress at the interface of the connection, two fibre reinforced

walls were connected together and the connected wall panels are fixed to the base slab. The

shear loading was applied horizontally to the face of the wall panel. Wall panels are denoted

as wall 1 and wall 2 respectively. The base slab is fixed to the test floor of capacity 200 tones.

To prevent any movement, wall 1 (unloaded wall) was restrained by a fixed support with mild

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steel jockey while wall 2 (loaded wall) was set free. The test setup and the location of LVDT

and DG are shown in Figures 6, 7 and 8.

Figure 6. LVDT and DG position

Figure 7. LVDT and DG position (SIDE VIEW)

A load cell of capacity 50 Tones was positioned at the distance of 115 mm from the top

of the wall panel in order to apply the shear load on the sample. The load cell was bolted to the

load frame of capacity 100 Tones. A common console controlled all the jacks. Data acquisition

system with 10 channels was used to measure the load and deflections at various positions on

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the specimen. Lateral monotonic loading was applied at the wall panel using hydraulic jack of

capacity 100 T. LVDTs (Linear Variable Differential Transducer) of least count 0.01mm were

used for measuring deflections at the wall panels at various locations. A total number of three

(LVDT) and three Dial Gauges (DG) were placed on both the wall panels and base slab in order

to record the displacement when the load was applied to the specimen.. LVDTs 1, 2 and 3 and

DG 1,2 and 3 were used to determine the displacement of specimen once load was applied. In

this test, the load was progressively increased up to the failure of the specimens when no further

load could be sustained. At each load interval, the crack patterns, width and length of cracks

were noticeable. Damage crack pattern was observed during testing for all types of connections.

The entire set up with data acquisition is shown in Figure 8.

Figure 8. Test set up with data acquisition

7. Experimental results and discussions

7.1 Load Vs lateral displacement

The lateral load was applied to the wall panel by means of hydraulic jack. The

deflections of each load value are measured by means of LVDT till ultimate load. The load-

displacement relationship of tooth connection with full blockouts and semi blockouts and loop

connection with full blockouts are shown in Figures 9 to 11. All specimens have shown

consistent displacements at the free end as well as near the connection. However, less

displacement was measured on fixed end as compared to the free end, which allowed more

movement. The lateral displacement of the wall was measured by four LVDTs. Tooth

connection with full slotted connection was shown with no displacement measured until 4 kN.

The maximum displacement occurred in wall 1 at position LVDT 3 is 19.5 mm. In Tooth

connection with semi slotted connection, the maximum displacement occurred in wall 2

(loaded wall) at position LVDT 3 is 1.85 mm. In loop connection with full blockouts the

maximum displacement occurred in wall 2(loaded wall) at position LVDT 3 is 7.11 mm.

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Figure 9. Load Vs Displacement of tooth connection with full

blockouts

Figure 10. Load Vs Displacement of tooth connection with semi blockouts

Figure 11 Load Vs Displacement Loop connection with full blockouts

0

5

10

15

20

25

30

0 5 10 15 20 25 30

Load

(kN

)

Deflection (mm)

LVDT 1 (mm) LVDT 2 (mm) LVDT 3 (mm)

D.G 1 (mm) D.G 2 (mm)

0123456789

10111213

0 1 2 3 4 5 6 7

Load

(K

n)

Deflection ( mm)

LVDT 1 LVDT 2

0

1

2

3

4

5

6

7

0 2 4 6 8

Load

(kN

)

Deflection(mm)

LVDT 1 LVDT 2 LVDT 3D.G 1 D.G 2 D.G 3

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7.2 Failure Pattern - Observations

The post-test observations showed that the shear force from one precast element to the

other was transferred by the friction along the interface between precast wall and Grout joint.

When shear loading was applied, slip developed along the joint interface and caused joint

separation at the interface. Figures 12 and 13 shows the failure of connections in the

specimens.

i)Tooth Connection with Full Blockouts

First crack appeared at 8.7 kN. Crack at the wall and the base slab connection appeared

at the load of 14 kN. The ultimate load at the load reversal took place at 19.5 kN.

ii)Tooth Connection with Semi Blockouts

First crack appeared at 6.5 kN. Spalling of concrete at the connection of base slab to

wall panel was observed. Crack at the wall and the base slab connection appeared. Cracking of

bottom base slab grout occurred and the tooth connection separated at 10 kN.

iii)Loop Connection with Full Blockouts

First Vertical crack at grout appeared at 3 kN and propagated up to middle portion and

widened Mild cracks at wall slab joints back side were observed. Horizontal cracks at loaded

face of the panel appeared and propagated to the lateral side of the wall. Spalling of concrete

was observed in the wall and the ultimate load was 9 kN.

iv)Loop Connection with Semi Blockouts

Loop connection with semi blockouts is tested a lateral loading. Due to the uplift of

wall from the base slab through semi slot long the loading side, crack patterns and deflections

were not able to be are measured. Hence it was decided to change the loading pattern. Lateral

load was applied on the face of the wall panel (115 mm from top) instead of applying lateral

load along the thickness of wall panel.

Figure 12. Failure of tooth connection with full blockouts

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Figure 13. Failure pattern of loop connection with full blockouts

8. Conclusions

This experimental study was conducted to study about the shear characteristics of four set of

wall panels with different type of connections between two adjacent walls. The following

conclusions are drawn from present investigation.

• The wall-to-wall connection was subjected to shear loading (Lateral monotonic

loading) ie., one wall panel keeping the other wall panel arrested from lateral

displacement. The load was applied up to failure.

• The behaviour of wall panels resting on full blockouts is found to be better than that

resting on semi blockouts.

• Tooth connection with full blockouts withstood an average lateral load of 19.5 kN and

a maximum sway of 24.12 mm. Initial stiffness and ultimate stiffness observed are

157.895kN/mm and 808.46 N/mm respectively. Energy absorption in this type of

connection is 346.168 kN-mm. This type of connection showed a very good resistance

against uplift from base slab along the loading side which increased the shear resistance

of the connection.

• Tooth connection with semi blockouts withstood an average lateral load of 6.5 kN and

a maximum sway of 1.85 mm. Initial stiffness and ultimate stiffness observed are

1000.00 kN/mm and 35.1351 N/mm respectively. Energy absorption in this type of

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connection is 9.89 kN-mm. The uplift of wall panel from the base slab along the loading

side reduced the shear resistance of wall in this type of connection.

• Loop connection with full blockouts withstood a maximum lateral load of 10 kN and a

maximum sway of 7.11 mm. Energy absorption in this type of connection is 31.268

kN-mm. This loop bar connection is categorised as brittle connection.

• It was also noted that in the loop connection the uplift of wall panel from base slab

along the loading side was similar to that of tooth connection with full blockouts.

8. Scope for future work

• To increase the shear behaviour of the connection, the ratio of the transverse bar,

overlapping length and grout joint strength should be increased.

• The wall portion inside the slot shall be strengthened and reinforced with sufficient bars

connecting both the wall panels and base slab to avoid uplift.

• The wall panels with different fibres can be tested and the results may be compared to

determine the optimum fibre.

• Behaviour of tooth connection by introducing rebars at Transverse direction through

wall panels can be studied.

Acknowledgments

The authors would like to express their sincere gratitude to Er.K.M.Velumani, Proprietor,

M/S Velan Concast, Precast Concrete Manufacturers, Neelambur, Coimbatore, India. The

processes like Casting, transportation, and erection of required precast elements were fully

sponsored by them. The precast factory has special plant for the manufacture and maintenance

of moulds which enabled us to complete this research successfully.

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shear behavior and failure pattern of precast wall panel

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