11 th INTf:RNA TIONAL BRlCKJBLOCK MASONRY CONFERENCE

TONGJI UNIVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997

.BOND AND WATER RESISTANCE OF MASONRY WALLS

de Vekey,R.C. 1, Russell, A.D.2, Skandamoorthy, 1.3, Ferguson, W. A. 3

l.ABSTRACf

Low- absorption brick walls can be relatively water - resistant if the joints are sealed with mortar using good workmanship since the water can only penetrate very slowly through either the brick or the mortar. If there are any open paths, however, penetration is rapid due to gravity driving the water through downward sloping paths. Bond strength testing has been used to show that despite good workmanship and well selected materials, which gave very high and uniformly well-bonded water-resistant bed joints, severe rain leakage occurred via perforated perpend joints. This weakness is attributed to plastic shrinkage of the mortar during the early setting and hardening phase or to subsequent drying shrinkage.

2. INTRODUCTION

Because of the normal climate - a marine climate with strong warm westerly winds and high winter rainfall, rain penetration is one of the commonest problems to affect masonry -walls in the UK. This occurs, albeit less frequently, despite the almost universal use of double skin (cavity) walls. Additionally, with more emphasis being placed on high leveIs of thermal insulation, cavities are being filled with insulant and this can reduce their rain resistance performance. Thus research, in progress at the BRE, is aimed at investigating the link between materials and workmanship factors and the effectiveness of filled masonry cavity walls as a rainscreen. Early work has been reported by Newman et al (1,2)

and this and other issues regarding cavity versus solid walls are reviewed by de Vekey (3).

1 Head of Masonry Structures Section, BRE, Watford, UK, WD2 7JR 2 Head of Masonry Durability Section, BRE, Watford, UK, WD2 7JR 3 Members of Masonry Structures Section, BRE, Watford, UK, WD2 7JR

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One of the key factors which determines the performance of the wall is whether significant amounts of free water can penetrate the outer rainscreen leaf and run down the cavity face and from there into the insulation layer. Traditionally there have been two methods of preventing this, termed lhe 'overcoat ' and the 'raincoat'. The overcoat technique requires the use of a high water-absorption brick where the back faces of the bricks have the capacity to absorb any leaking water. This technique works well over the normal c1imatic range , since the bricks can dry out between rain events, but tends to fail if subjected to unusually long periods of driving rain which saturates the bricks. The raincoat technique employs low-absorption bricks and well-filled mortar joints to make the whole wall face resistant to penetration. The obvious extension of this technique, plastering over the bricks with a rendering mortar, is done in parts of the UK but is not popular in the south because the house buyer prefers the aesthetic appearance of exposed brickwork. Waterproofing walls, utilising polymer materials can be effective as shown by Stupart (4) but has limited durability, adds to the cost and reduces the ability of the walls to dry out in dry conditions by transmitting moisture through the pore system.

Obviously the 'raincoat' technique fails if there are even minor cracks or fissures in the joints and penetration can be rapid due to gravity driving the water through downward sloping paths. Bowler et al (5) showed that the rate of leakage due to such paths in either bed or perpend joints can be in excess of 0.3kg/m2/minute for a water head of 75mm. Such a head is equivalent to the pressure exerted by a 90mph gale but can also result from a sloping fissure in the perpend joint of a standard UK brick unit.

In recent work on 'raincoat' style walls, two examples with good workmanship, ie well­filled horizontal and vertical joints with the surface tooled to give the best surface bonding gave just as much rain penetration as a matching pair of walls built with poor workmanship, ie with furrowed horizontal joints, poorly-filled vertical joints and a plain struck joint finish . In an attempt to gain a fuller explanation a bond test was proposed to investigate the state of the bed joints. It was argued that if the unexpected leakage was occurring via the bed joints this implied that there were fissures present which would reduce the bond strength. The BRENCH method (6) was chosen because it could be carried out in-situ on the walls as they stood and involved the least likelihood of error from transport of specimens.

3. SPECIMENS

In phase 1 a low/medium absorption c1ay brick was combined with a retarded ready-to­use, 1:1:6 cement:lime:sand plus air-entraining plasticiser, mortar to construct four walls 2.25m long by 3.4m high with a test height of approximately 2.6m. The units were well fired extruded 3 hole perforated bricks to the UK dimensions of 215mm long x 102mm deep x 65mm high. The 24 hour water absorption of the units was a mean of 5.35 and a coefficient of variation úf 17.6% for ten replicates. In phase 2 a single wall (no. 5) of the same dimensions was built using high-absorption semi-dry pressed deep-frogged c1ay bricks in limebond, 1: 1 :5Y2 cement:lime:s&Ild plus entrained air, mortar.

The ou ter leaf walls were built off the laboratory floor and had a metre length retum at either end. In phase 1 two of the walls were built with 65mm wide cavities and the other

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two with 125mm cavltles. One of each cavity size was constructed using 'poor workmanship' and the remaining pair were built with 'good workmanship'. The two qualities of workmanship are listed in Table 1. Figure 1 is of the cavity face and one retum of wall 1 after bond tests had been carried out.

Table 1. Workmanship

FEATURE BAD WORKMANSHIP GOOD WORKMANSHlP

WALLS 1 (125 CAV) 3 (65 CAV) 2 (125 CAV) 4 (65 CAV)

Bed joints Furrowed Ful1y filled

Head (perpend) joints On1y front face filled Fully filled

Weather face joint finish Struck flush Bucket handle tooling

Cavity face joint finish Extruded snots Struck flush

Each wall was provided with an internal concrete blockwork inner leaf built into a steel frame to allow easy removal and inspection of the outer leaf. AlI the walls were tested initially with the cavity between the fixed outer leaf and removable inner leaf and subsequently they were tested filled with a pelleted fibreglass insulant installed by injection through holes in the brick leaf.

4. APPARATUS

Air perrneability was measured using the technique !

described by Newman and Whiteside (7).

Rain penetration tests were perforrned by spraying the walls from spray heads located at the top of the wall controlled from a servo- " , operated valve and programmer. The flow rate was varied between 15 and 30 litres per hour for 6 hours per day over four days. This represented the range between a long summer shower and persistant winter driven rain. Collection units were positioned at the dpc leveI on both sides of the outer leaf and on the cavity Figure} View of cavity face and return of wall No.}

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face of the inner leaf to allow the flow rate down the externaI face and the flow rate of any through leakage to be monitored. Any damp penetration through to the inner leaves was monitored by observing colour changes to the surface of the blockwork. Bond strength was measured using the BRE 'BRENCH' with a one metre long moment armo The apparatus and technique have been fully described elsewhere (6.8) and are based on the same engineering principIes as the bond wrench described in ASTM C952 (9) but optimised for use on site.

5. RESULTS AND DISCUSSION

The detailed individual bond test results are listed in Table 2 and the statistical analyses are given in Table 3.

As might be expected, the walls with poor workmanship leaked fairly freely through the rainscreen layer. Figure 2 shows that fissures ran through many of the poorly filled perpend joints. The surprising result, however, was that the walls with the good workmanship performed no better! and also appeared to be leaking via the perpend joints. The results of air permeability measurements confrrmed this result since all the walls except Wall 4 gave readings off the scale of the measuring equipment ie. in excess of 35Vminute. In view of the degree of care that had been taken to ensure that the vertical joints were absolutely fully filled, the initial hypothesis was that the leakage might be via poorly filled bed joints.

Additionally wall 5 in phase 2 also performed poorly in the air leakage test and visibly leaked water through both the bed and perpend joints.

Figure 2. Failure surface for a unit in walll (l25mm cavity, poor workmanship)

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Since poorly filled bed joints with fissures, which would allow water to fiow across the wall, imply a poor bond on bed it was suggested that the bondstrength test núght shed further light on the unexpected behaviour. Thus an initial set of five positions were tested covering the núd position of each of the four wall specimens, which had been wetted and dried several times in the course of the experiments, plus the side returns of wall NO.1 which had remained dry since drying out after construction. For each position a total of ten replicate unit bonds were tested divided into five with the externaI weather face in tension and the other five with the cavity face in tension. (Five specimens were taken from each return due to the smaller length) ..

The tests indicated that the bond on bed was very uniform and surprisingly good for alI four walls and that the failures were generally occurring within the mortar bed, ie there were no visible water paths through any of the bed joints. Figure (2) shows a typical failure plane from a unit in wall 1 showing the very uniform adherence of the mortar to both unit faces despite the obvious fissures through the poorly filled perpend joints. An analysis of variance for the four walls (Table 3) indicated that there was no significant effect of the workmanship factor (ie. between w'llls built with good and those built with poor workmanship nor of the pointing finish factor ie between the joints tested with the weather face in tension and those with the cavity face in tension. The 65mm wide cavity walls had slightly better bond (mean fxk = l.39N/mm2) than the 125mm wide cavity walls (mean fxk = 1.21N/mm1) which was just significant at 95% probability. This may have been a fortuitous difference and it is difficuIt to suggest a rational expIanation except, perhaps, differences in drying rate between the two walls at some stage.

Analysis of variance for the wetted face versus the dry returns indicated no significant effect of wetting and drying on the bond performance but in this case the weather face (tooled) joint was stronger (mean fxk = 1.23N/mm1) than the cavity face (either exuded or struck) joint (mean fxk = l.07N/mm1) . Again the significance was marginal at 95% probability but fo11ows the normally accepted wisdom that tooled joints should perform better than uncompressed joints.

Since both the visual evidence and the bond data indicated that the bed joints were alI very we11 filled and had no obvious water paths through them the obvious conclusion was that most of the Ieakage was occurring via the perpend joints even in the walls with good workmanship. Thus the unusual step was taken of cutting bond test specimens for the perpend joints of the good-workmanship walls. The technique adopted was to support a strip of the wall on a timber batten heId up with the jacks used to precompress the wall during cutting. The upper joint of the supported strip was then cut, followed by a vertical cut either end. -The batten was then lowered complete with a strip of units bonded only by their vertical joints. Sufficient masonry was removed to provide 10 intact specimens and there was surprisingIy little breakage. These were tested' by gripping the lower unit in a vice and appIying the Brench to the upper unit. For consistency with the main experiment, five samples were tested with the externaI weather face in tension (marked W) and the other five with the cavity face in tension (marked C). Figure (3) is of two of the cut Iengths after bond tests had been carried out spayed with phenolphthalein to show the depth of carbonation of the bedding mortar.

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Figure 3. Perpend joint test specimens after bond testing (reassembled)

Figure 4. Perpend joint test specimen after bond testing showing unbonded water path

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Analysis of variance for this new data versus the on bed data showed that there was no significant difference between the two joint finishes but that there was a very highly significant difference between the perpend joint (mean fxk = O.67N/mm2) and the bed joint of the same wall (mean fxk = 1.48N/mm2) at a probability leveI of 99.9%. To underline the statistical conclusion the broken, bond-tested joints were sprayed with phenolphthalein solution. This revealed that the mortar beds, although apparently completely filling the joints, had a proportion of the area which was not bonded to the units due to the presence of fissures. Where the fissures had been the mortar had been exposed to the carbon dioxide in the air, had carbonated and gave no colour change, whereas the bonded are as behaved as freshly broken mortar and caused a colour change to bright crimson. Figures 4 shows the end face of one of the units immediately after the bond test with a clear water path remaining between bonded areas.

The data from the phase 2 bond tests is given in Table 2b. Due to time constraints and experimental problems only four perpend specimens were tested. The result of a one way analysis of variance, given in table 3, indicated no difference between the bond strength of the two joint types. Taking a parallel with phase 1 this implies that both the bed and the perpend joints were partially debonded since they had a similar performance to the perpend joints of the phase 1 units. A possible explanation for this behavior is given by Figure 5 from a previous investigation using the same units. This shows that the differential shrinkage between the larger volume of mortar in the frog and the thin lOmm layer around the perimeter of the unit causes delamination and even total separation of the central mortar.

Figure 5. Bed joint test specimen showing separation of the central mortar from the edge mortar

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6. CONCLUSIONS

The bond of mortar to units in the bed joints of low absorption day units laid in a mortar of the correct plasticity is very high and consistent and can exceed the flexural strength of the mortar itself.

In such well bonded bed joints there is no reason to condude that any significant leakage of rainwater occurs through the bed since there are no fissures present.

Rainwater can leak via fissures and holes in poorly filled perpend (head) joints due to low quality workmanship.

Rainwater can also leak via fissures and holes in apparently well-filled perpend joints which result from plastic shrinkage of the mortar during the early setting and hardening phase or to subsequent drying shrinkage. While plane bed joints can shrink freely under gravity to compensate for this, the perpend joints are locked in position by the rigid end retums and the bond to the foundation and thus partial debonding occurs even in fully filled joints to relieve the resulting tension forces.

The differential shrinkage between the large central volume of mortar in deep­frogged units and the thin peripheral band of mortar can result in partial delamination of the bed joint resulting in possible water leakage paths and reducing the potential bond performance.

7. REFERENCES

(1) J.Newman,A.J. , Whiteside,D., Kloss,P.B. and Willis,W., Full-scale water penetration tests on twelve cavity fills-Part I. Nine retrofit fills, Building and Environment, V17, 3, pp175-191, 1982.

(2) J.Newman,A.J., Whiteside,D. and Kloss,P.B. , Full-scale water penetration tests on twelve cavity ftlls-Part lI. Three Built-in fills, Building and Environment, V17, 3, pp 193-207, 1982.

(3) de Vekey,R.C. , Cavity walls - Still a good solution, BMS Proc. M(5), p35, 1993

(4) Stupart, AW., Testing of water repellent treatments for masonry, Masonry Intemational, VlO, I , pp 26-29, 1996

(5) Bowler, G.K., Jackson,P.J., Monk, M.G., The role of mortar workability (cohesivity) in the rain penetration of masonry, Masonry Intemational, V 10, I, pp24-25, 1996

(6) BRE Digest 360, Testing bond strength of masonry, Apri11991. (7) Newman,AJ. and Whiteside,D., Water and air penetration through brick walls - A

theoretical and experimental study, Trans.Brit.Ceram.Soc., V80, p27, 1981. (8) de Vekey,R.C., In-situ tests for masonry, Proc. 9th Intl. Brick and block Mas. Conf.,

VI, p621-627, 1991

(9) ASTM C952/76, Test of bond strength of mortar to masonry units, American Society for Testing and Materials. 1976.

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Table 2. Bond strength values as stress N/mml with % coefficients of variation

• WALL NlIMl:lhK 1 1 L j 4 4 , , JOINT TESTED BED BED/RET BED BED BED PERPEND BED PERPEND

1.39 1.45 1.56 1.43 1.44 0.39 0.66 0.76 1.16 1.02 0.95 1.20 1.10 0.49 0.75 0.89 1.21 1.42 1.15 1.16 1.43 0.8 0.76 0.75 1.32 1.06 1.87 1.53 1.15 0.66 0.6 0.55 0.98 1.33 1.18 1.41 1.74 0.74 0.73 -

MEAN 1.21 1.26 1.34 1.35 1.37 0.62 0.70 0. 74 STD.DEVN. 0.16 0.20 0.37 0.16 0.26 0.17 0.07 0.14 CV% 12.83 16.2 27.4 11.67 18.73 28.16 9.74 19.03

1.16 1.12 1.73 1.22 1.52 0.67 0.39 -1.05 0.98 L11 1.2 1.76 0.66 0.58 -1.07 .. 94 0.81 1.33 1.77 0.56 0.62 -1.28 1.09 1.18 1.34 1.59 0.69 0.67 -1.05 .97 1.12 1.26 1.31 1.07 0.21 -

MEAN 1.12 1.02 1.19 1.27 1.59 0.73 0.49 -STD.DEVN. 0.10 0.08 0.34 0.06 0.19 0.20 0.19 -CV% 8.92 7.62 28.18 5.09 11.92 26.82 38.64 -

MEANOF 10 1.17 1.14 1.27 1.31 1.48 0.67 0.60 0.74 STD.DEVN. of 10 0.13 0.19 0.34 0.12 0.24 0.18 0.17 0.14 CV%OF 10 11.41 16.79 26.96 9.24 16.39 27.36 29.03 19.03

Table 3. Results from the analyses of variance

PARAMETERS I DOF VARIANCE RATIO SIGNlFICANCE

3 way analysis of variance of central positions of four walls:

MAINVARIABLES Good v Poor workrnanship (BLOCK) 1/32 3.59 Not Significant

Weather joint v Cavity joint (ROW) 1/32 0.11 Not Significant

65 v 125rnrn cavity (COLUMN) 1/32 6.11 Significant@95%

INTERACTIONS

Block v Row 1/32 0.65 Not Significant

Block v Column 1/32 0.27 Not Significant

Rowv Colwnn 1/32 1.75 Not Significant

3 way interaction 1/32 1.55 Not Significant

2 way analysis of variance of (wetted) central positions of wa1l4 versus (dry) return:

Weather joint v Cavity joint (ROW) 1/16 6.44 Significant @95% Central face v Retum face (COLUMN) 1/16 0.20 Not Significant

2 way interaction (Row v Column) 1/16 1.36 Not Significant

2 way analysis of variance cf bed joints of wall 1 versus perpend joints of wall 1:

Weather joint v cavity joint (ROW) 1/16 3.23 Not Significant

Bed joints v Perpend joints (COLUMN) 1/16 76.95 Significant @99.9%

2 way interaction (Row v Column) 1/16 0.33 Not Significant

1 way analysis of variance of bed joints of wall 5 versus perpend joints of wa1l5:

Bed joints v Perpend joints 1/12 2.05 Not Significant

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