Comparative Performance of Reinforced and Prestressed Brickwork Pocket-Type Retaining WaUs in Shear

B.P. Sinha, BSc, DBS, PhD, FICE, FIStructE, CEng.

1. ABSTR\CT

The paper presents the result of a series of tests done on reinforced and prestressed pocket-type retaining walls . It compares the structural performance of reinforced and prestressed, and the shear strength of such walls. The variable considered was shear span/effective depth ratio, while the brick, mortar strength and % area of steel ; were alI kept constant. From the tests it appears that the moment carrying capacity is higher for prestressed walls compared to reinforced as a result of degradation of moment due to shear. The cracking moment increases due to prestress as a result the shear capacity of prestressed pocket-type brick walls is higher than reinforced brickwork having similar cross-sections and % area of steel.

2. INTRODUCTION

Shear strength is a vitally important criterion in defining the strength of reinforced and prestressed tlexural members. The shear strength of reinforced and prestressed brickwork beams depends on the compressive strength of brickwork, % area of reinforcement, shear arm leffective depth ratio and the beam size; i.e. intluence of the effective depth. The British Standard I 5620 Part 2 also recognises the enhancement of shear strength due to prestress, if the prestressing force is applied normal to the bed-

Keywords . Masonry, Reinforced, Prestressed, Shear Strength

Senior Lecrurer, Department of Civil Engineering University of Edinburgh, Scotland

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joint. The characteristic shear strength in such members is given by:

fv = 0.35 + 0.6813, (i)

where 813 is the design loadlarea due to the prestress at right angle to the bed- joint. Although, some shear test results are available for reinforced2,3,4 and prestressed sections, 5,6, 7,8 ,9 it is still difficult to compare the results due to the anistropic nature of briclcwork, non-identical sections and axialIy applied prestress. In many of the tests, the briclcwork sections contained horizontal lamination due to longitudinal bed-joint. It was, therefore, a comparative investigation which was undertaken on an idemical reinforced and prestressed beams, where the steel was provided in a specialIy created pocket. In such a situation, the prestressing force was applied normal to the bed-joint to confirm the enhancement alIowed for the prestress by the British Standard 1. In this standard, it is also suggested that the design shear should be calculated from the

equation v = ~ where b is the width ofthe section and d is the effective depth for the bd

reinforced briclcwork. In the case of the prestressed section d is replaced by de, which is the depth of masonry in compression. Once the prestress is fulIy neutralised, the section becomes similar to a reinforced section, hence in the author's view both cases, the effective depth should be taken to obtain the design shear. This view needed to be substantiated with the experimental results.

3. SCOPE OF THE INv"ESTIGATION

In this investigation the variable considered were: % area of longitudinal steel and the shear armleffective depth ratio. The compressive strength of briclcwork, the depth of section and the prestressing force were kept constam. From an earlier investigation 1 O it became cIear that the section will fail in shear, if the % of prestressing steel was chosen to be 0.55 , hence this was used for the prestressed beams. As the ultimate strength of prestressing steel was higher than the reinforcing steel ; an equivalem are a oflongitudinal steel giving the % area equal to 1.59 was used to reflect this difference .

4. CONSTRUCTIONAL DETAILS

The cross-sections of the beams are shown in Fig. l. All beams were constructed vertically as a wall. The beams were prestressed at the age of 21 days. After prestressing it was grouted.

552.5mm

lF-~~t~E~~~-i 56] I C J50 x 150 x JO

~ro~t Steel end plote Inflll

Cross - section of reinforced and

prestressed beams

Creu - section showing end plctes for prestressed

brickwork beom

Fig.l Showing the cross-section of reinforced and prestressed brickwork beams.

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Three hole - 84 N/mm2 engineering c1ass B bricks were used for the construction of both prestressed and reinforced brickwork beams. A mortar mix of 1 : 1/4: 3 (cement : lime : sand) by volume was used for the construction of the specimens. The average compressive strengths of the 100mm mortar cubes were 22.3 N/mm2 and 26.1 N/mm2 for reinforced and prestressed beams respectively. The average strength of 100mm grout cubes was 24.7 N/mm2 for reinforced and 26.0 N/mm2 for the prestressed brickwork beams.

Six - seven wire, stabilised prestresssing strands of 10.9mm nominal diameter having ultimate strength equal to 1727 N/mm2 were used for prestressed beams. Hot rolled, high yield deformed bars were used for reinforced sections. The average ultimate strength was 600 N/mm2.

5. TEST ARRANGEMENT

The beams were tested in a four point loading frame (Fig.2).

r;o IBI 18i 181 1~IIBIIBI [81 181 IH IH IH ~

Elevolion

Fig.2 : Test arrangement.

The supports for the beams consisted of one roUer and one pino Steel bearing plates were positioned, bedded on a layer of mortar, at supports and load application points to spread the load and to avoid stress concentration. The line loads, applied at small increments until failure, were measured and monitored at each jacking point by the load cells connected to a digital voltmeter. Strains in the strands and reinforcement were measured by the electrical strain gauges. Strains in brickwork were measured using 'demec' gauges. The detlection of the beams were measured by dial gauges reading to an accuracy of 0.0 lmm at the centre and to the accuracy of 0.02mm at the supports to detect any settlement. Measurements of strains and detlection were taken at each increment ofthe load.

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6 . RESUL TS AND DISCUSSIONS

6~ 1 Shear Strength

The shear strength and failure moments for both the reinforced and prestressed beams are shown in Tables 1 and 2. Also the theoretical predicted moments are shown for each individual beams. The theoretical moments are calculated fro~ an interactive computer program which takes into account the non-linear material behaviour 7 of brickwork and steel. The average compressive strength of the brickwork was obtained from testing the 6 - course prisms. All reinforced beams failed in shear and did not achieve the full flexural capacities. The prestressed brickwork beams also failed in shear, but for two beams all reached the flexural capacities at the same time, hence there was no degradation of moment. In both cases the shear strength increased with the decrease of shear spanleffective depth ratio (Tables 1 and 2) as can be seen in Fig.3 .

N

E ~ 3 z ., ., "

x

o - 1.09 ~ 2

~ ,," =)84~~)< _ .,

5 .. .c C/l 1 ~ Prestressed

(.0) - 1.05 ____ --=_ Reinforced " = 4 .;6 \d

o ~--~----~----~----~--~----~ o 2 3 5

(~) rotio

Fig.3 Showing the relationship between shear strength and shear arm / effective depth ratio.

The correlation coefficients for prestressed and reinforced brickwork beams were 97.5 and 99.5%. Within the range ofthe test, the relationships between the shear strength (v) and shear spanleffective depth (ald) can be represented by:

v = 6.84.(ald)" 1.09, forprestress beams and v = 4.l6(ald)"l.OS, for reinforced beams

(ii)

(iii)

The nominal shear strength has been calculated by dividing the shear force at failure with the effective thickness in both cases. The theoretical predicted failure moment for both types of beam are the saroe, because of the similar area of steel used.

From Tables 1 and 2 it can also be seen that the prestressing increases the failure moment and thus the load carrying capacity of the beam compared to reinforced brickwork bearos.

426

I

Table 1

% area of steel

0.55

Table 2

% area of steel

l.59

I

Shear strength of prestressed bricl-work beam

Effective Prestress in kt"I

Shear span Shear Failure Ipredicted !Mep

Effective depth Strength Moment I Moment M pre lN/mm2 kNm(Mexp) kNm(Mpre)

193.59 249.67

272.0

281.70 274.4

220.8 235.2

2

2.5

3

4.92

3.23 3.49

2.84

1.89 1.61

1.25 1.29

72.63 79.90

80.25

64.55 55.12

I

I 69 .14 71.22

64.50 67 .25

68 .72

69.30 67.30

66.08 66.79

Shear Strength ofReinforced Brickwork Beams

Shear arm I Shear stress Failure Predicted Effective depth N/mm2 Moment Moment

kNm(Mexp) k..Nm(Mpre)

2 2 . 17 50.57 67.30

1.89 44.53 I 67.30

3.3 1.15 42.82 67.30

1.09 41.73 67.30

064 44.48 67.30

6.07 0.64 44.10 67.30

1.13 1.19

1.17

0 .93 0.82

l

i 1.05 1.07

Mex M pre

0.75 0 .66 0 .64 0.62 0 .66 0 .66

6.2 Deflection

The typical deflection of prestressed and reinforced beams (a/d = 2) has been compared in FigA. The deflections in bOI h cases were measured from lhe same dalum, when the beams were placed on the lesting frame . Even in uncracked conditions, the preslressed beam seems stiffer than the reinforced brickwork. This can only be possible due to closing of micro shrinkage crack as a result of prestressing. As lhe preslressed beam cracks at a higher load than the reinforced beam it remains stiffer until failure.

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