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DEVELOPMENT OF A CONSTRUCTION QUALITY CONTROL

PROCEDURE USING UL TRASONIC TESTING

Brian HOBBS Department of Civil & Structural Engineering

University of Sheffield, England

ABSTRACT

This paper describes a research programme aimed at developing a non-destructive testing method for use in site quality control procedures for masonry construction. Recent developments in load bearing masonry design are reviewed and it is argued that these trends and likely future developments require increased confidence in the quality of the completed masonry. Non­destructive testing is proposed as a means of achieving this and the potential advantages of ultrasonic testing are outlined . An extensive experimental programme of ultrasonic testing covering a wide range of types of brickwork and blockwork is reviewed. Attention is concentrated on the results at early ages and it is shown that this type of testing is capable of providing useful data within 1 - 3 days after construction. Typical results are presented showing the correlation between changes in pulse transit time and some common workmanship defects . The application of the testing technique to reinforced brickwork and blockwork walls is also described. Finally , site procedures are proposed for the use of this testing method as part of the regular quality control processo

INTRODUCTION

Modem developments in loadbearing masonry design are leading to more and more efficient structures, in which the stress leveis are considerably higher than has been customary in traditional forms of masonry construction. For example, high rise, multi-storey loadbearing wall structures provide a cost-effective and efficient solution for buildings such as apartment blocks and hotels. The use of slender walls of only l00-200mm thickness is common, even for the lower storeys, with resulting high compressive stress leveis . Developments such as the application of reinforcing and, more recently, prestressing techniques to masonry to enhance its flexural resistance bring with them a requirement for high compression zone stress leveis. The aesthetic qualities and good durability record of masonry are leading to a resurgence in its use in building structures and retaining walls . Loadbearing masonry has recently re-entered the field of major civil engineering construction in the United Kingdom with the construction of prestressed brickwork abutments for two bridges [1] . Further applications of prestressing are likely, both in building and in civil engineering, as confidence in the technique is developed. Refinement and further development of prestressed masonry hold out the prospect of new and exciting applications for structural masonry . Curtin [2] has summarised the development of the technique and predicts its widespread use in the future.

As the stress leveis used in the design of any structural material increase, there is a consequent for improved quality control to ensure that the "as built" material is capable of resisting the imposed stresses . In addition to strength considerations, serviceability criteria such as weather resistance and durability may be adversely affected by inadequate leveis of quality control. Current quality control procedures for masonry may be divided into two phases:

(i) Materiais testing and (ii) Monitoring of construction workmanship standards

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Materiais quality control may be effected by suitable programmes of destructive testing . Samples of the bricks or blocks may be regularly tested for compressive strength, water absorption etc. by the manufacturer and, deemed necessary, additional sampling and testing of the actual site stocks may be specified. The suitability of the materiais used to produce the mortar and of the mix proportions may be assessed by pre-construction testing for sand grading, mortar cube strength etc . Regular sampling of the materiais during construction for similar tests may be used to check that the quality of the material incorporated into the structure meets the required standards. Compressive strength testing is typically conducted 7 or 28 days after construction, so the presence of defective material may only be detected after a considerable amount of additional construction has been completed. The costs of remedial measures may then be very high and this situation can lead to disputes on site.

The effective control of workmanship standards is more difficult than materiais quality control. Current procedures normally rely on continuai inspection of the construction work and are thus time consuming, expensive and prone to errors of omission and oversight. Many of the potential defects resulting from inadequate workmanship standards are difficult to detect after the masonry construction is complete. The reliability of masonry structures may be significantly affected by such defects and the difficulty of detecting their presence is a potential cause for concern . The perceived difficulties of ensuring adequate site quality control may even lead to a reluctance on the part of structural designers to make use of loadbearing masonry in significant structures . The particular problems in relation to reinforced and prestressed brickwork have been discussed by Bell and Bradshaw [3].

There is thus a clear need to develop methods of assessing the quality of workmanship after the construction has been completed . Any form of destructive test will entail subsequent reinstatement of the damage and a non-destructive testing approach is therefore preferable. The work described in this paper has been directed at the development of a suitable non-destructive testing procedure.

SITE WORKMANSHIP PROBLEMS

Research on workmanship factors and their intluence on masonry strength has been summarised by Hendry [4]. He covers a range of defects in the completed work. These may be broadly divided into two groups; those which are externally visible, and those which cannot readily be detected by external inspection. The first group includes overall geometrical defects, such as lack of verticality or straightness, and local errors, such as the use of excessively thick bed joints. These defects may be detected by simple inspection procedures and so present no real difficulty in the quality control processo Hendry deals with the following defects which fali into the second group:

(i) Incorrect proportioning and mixing of mortar (ii) Incorrect adjustment of suction rate of bricks (iii) Incomplete filling of mortar joints (iv) Disturbance of units after laying (v) Unfavourable curing conditions

Quantitative data on the possible effects of these faults on the properties of the masonry is very limited . Hendry's conclusions on the importance of the above defects, which generally relate only to compressive strength, may be summarised as follows:

(i) - small effect on masonry constructed using low or medium strength units but potentially much more significant when high strength bricks are used

(ii) - little effect on basic compressive strength, but in some circumstances the strength of slender walls built with low strength bricks may be affected

(iii) - incomplete filling of bed joints or "furrowing", may have a serious effect with potential reduction in wall compressive strength of 25 %, whereas incomplete filling of perpend, or head, joints has little effect on compressive strength

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(iv) - similar to (ii) (v) - inadequate protection during curing may reduce waJl strength by up to 10%

It is concIuded that these effects are additive and typical compressive strength reductions for unsupervised construction, which may be assumed to incorporate a range of defects in both groups, are quoted as 30 - 40%. An additional potential defect falling into the second group may be introduced in some forms of reinforced masonry. Many reinforced masonry construction techniques require the fiJling of relatively smaJl voids in the masonry with a grout or concrete mix in order to achieve bond between the reinforcement and the masonry . In forms such as grouted cavity or hoJlow block construction the infiJl is totaJly enclosed by the masonry and its quality and degree of compaction is very difficult to check. Defective areas may lead to rapid corrosion of reinforcement or, in extreme cases, structural distress due to lack of reinforcement bond.

In addition to their influence on compressive strength the defects listed may be expected to adversely affect the flexural strength and shear resistance, and to have potentiaJly serious effects on durability and resistance to environmental influences, such as the penetration of driving rain.

ASSESSMENT OF ULTRASONIC TESTING

An interest in the structural applications of high strength brickwork and, in particular, in prestressed brickwork [5]led the author to initiate research into the development of testing methods for possible application in improving site quality control procedures. The practical requirements of suitable test procedures are that they should:

cause no damage to the finished masonry have no effect on surface appearance (for fair-faced work) have no effect on bond with any plaster or render to be applied later use robust and reliable equipment suitable for routine use on a construction site

After considering a range of non-destructive testing methods, it was decided that ultrasonic testing using readily available equipment developed for concrete testing had the potential to fulfiJl these requirements. A feasibility study was undertaken to assess whether useful data could be obtained using this technique [6]. This programme of work incIuded basic pulse velocity tests on a wide range of brick and block types and on mortar cubes, together with tests on smaJl prisms of masonry constructed so as to incorporate a range of defects. Additional work was also carried out on two forms of reinforced masonry. Detailed resuIts have been presented elsewhere [6, 7, 8, 9] and only the main points will be reviewed here.

The research programme comprised tests designed to:-

(i) characterise the component materiais i.e. bricks, blocks and mortars (ii) establish basic data for weJl constructed laboratory specimens, ranging from smaJl stack­

bonded prisms up to storey height wall panels (iii) determine the effects of various simulated workmanship defects

The work to date has shown that it is possible to obtain consistent and reliable pulse velocity data for a wide range of bricks and blocks and for mortar cubes . The variability of the data was within the limits normally anticipated in conventional destructive testing of masonry. The characteristic pulse velocity of a brick or block type is strongly dependant upon the nature of the material and the manufacturing process, varying from around 2 km/sec for low strength units up to over 5 km/sec for very strong engineering bricks. With some types of extruded, wire-cut clay bricks the velocity is also strongly dependent upon the direction of testing, the velocity parallel to the extrusion direction being of the order of 25% lower than in the perpendicular direction . In extruded bricks, from another manufacturer, however, there was virtuaJly no difference between the different directions. Typical results obtained from tests on mortar cubes at various ages after

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casting are shown in Figure 1. The tests relate to the four standard designations in BS5628 Part 1 [10] and a designation (iv) mix from which the cement was deliberately omitted cured in the laboratory, together with additional data for designation (i) mortar cured in a mist room at 20°C and 98%RH. There is a clear and distinct relationship between UPV and age for each set of tests, although the results for the two strongest mixes, (i) and (ii), are quite close together. The properties of mortar in a joint between two bricks or blocks are, of course, likely to be different from mortar cast in cubes . The cube results do indicate, however, that UPV results reflect the rei ative strengths and rates of strength gain of different mortar mixes .

The influence of curing mortar joints between clay bricks has been investigated by separating the 10mm thick mortar joint from the bricks and measuring the direct pulse transit times on a grid of points. This data was used to generate pulse velocity contours over the 220mm x 100mm plate of mortar. Typical results are shown in Figure 2. Although there is some irregularity around the edges of the joint, there is a general trend of increasing velocity towards the centre ofthejoint, the difference between the edge and the centre being around 10 - 15%.

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Age (days)

FIGURE 1: Pulse Velocity Results for Mortar Cubes.

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U.P.v. = 2.80 lan/s

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FIGURE 2: Pulse Velocity Contours for Mortar Joint.

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Various configurations of small masonry specimen have been used in the research programme. TypicaI results for 300mm transducer separation on small panels, 5 courses high x 2 bricks long, constructed using solid Class B engineering bricks and designation (i) mortar are presented in Figure 3. This figure illustrates that reliable UPV data can be obtained within 1 day after construction, that rapid changes take place during the first 3 days or so, and that the results tend to stabilize thereafter for this brick/mortar combination.

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TRANSDUCER POSITIONS

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FIGURE 3: Typical Nominal Pulse Velocity Results for 5 x 2 Prism with Solid Class B Engineering Bricks .

Mean results for 3 course, stack bonded prisms built with the mortar mixes given in Figure 1 are shown in Figure 4. The general form of the results is similar, giving some confidence in the use of small test specimens for this type of testing, and the separation between the results for different mortars at early ages is apparent. Similar specimens were used to investigate the effects of some construction defects . Results for various widths of bed joint furrow in prisms constructed using class B engineering bricks and designation (i) mortar are presented in Figure 5. The significant effect of furrowing on the measured transit time and hence the calculated nominal pulse velocity, is evident. Results indicating that laying bricks with an excessive initial rate of suction may lead to an increase in pulse transit time of around 10% have also been obtained [7] and Noland et aI have reported the ability to detect delaminated bed joints and defective vertical collar joints [10]. Tests on reinforced masonry have been carried out using the through-thickness, direct reading technique [8]. The results indicate that voids and porous areas in the infill concrete in grouted cavity and hollow block walls can be detected .

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In order to make a preliminary assessment of the application of this testing technique under site conditions, Wright (9) carried out a programme of tests on 140mm thick hoIlow concrete blockwork walls constructed under unsupervised site conditions and compared the data with similar walls carefuIly constructed in a laboratory. The nominal indirect pulse velocity measured with a vertical transducer separation of 900mm for the laboratory waIls was 1.54km/sec with a coefficient ofvariation of3% whereas the equivalent figures for the site waIls were O.96km/sec and 6% . It was also noted that the attenuation of the signal amplitude was greater in the site waIls , making it more difficult to obtain readings over the longer path lengths . The time taken to record the data was also noted and it was reported that, an average, one vertical path reading took 2 minutes, including the application and subsequent removal of the paste couplant.

All these test results indicate that ultrasonic testing has the potential to be developed into a useful technique for use in site quality control procedures. The ability to obtain results within I - 3 days after construction is particularly valuable, since the early discovery of any significant defects considerably reduces the cost of remedial measures.

PROPOSALS FOR SITE QUALITY CONTROL TESTING

Because ultrasonic pulse velocities are strongly dependent upon the exact type of brick or block and mortar mix being used, it will be necessary to carry out initial calibration tests at the start of a project. Sample wall panels are routinely constructed before the main construction starts so that the architect can check that the intended appearance will be obtained. Such panels can also be utilized to calibrate the ultrasonic testing method to be adopted.

It is likely that in most cases the primary concern will be with the quality of the horizontal bed joints. In this case it is proposed that indirect testing should be used with the transmitting and receiving transducers placed on a vertical line at mid-height of the bricks or blocks, separated by a distance equal to a number of masonry courses . This distance is regarded as the path length for the calculation of nominal pulse velocities from the recorded transit times . The calibration procedure will need to establish the following parameters for the quality control programme:

(i) Age of masonry at testing. Data can be obtained less than one day after construction, but results are changing quite rapidly with time at these very low ages and may thus be sensitive to variations in the actual time of testing achieved on site. The rate of change reduces considerably after 2 - 3 days , so site testing at this stage are likely to be less critically dependant upon the exact timing of the testo

(ii) Suitable transducer spacing. It will generally be desirable to test the greatest possible height of masonry at one time in order to minimise the number of initial measurements . It is suggested, however, that this should be limited to the height of one day's construction work if early age testing is to be employed. Greater heights would mean that areas of masonry with significant1y different rei ative ages could be included within one path length, leading to possibly erroneous results . Using standard equipment with 50mm diameter, 54kHz transducers it has been found that path lengths of up to 900mm are achievable with a wide range of masonry materiais using a simple water soluble paste couplant between the transducers and the faces of the bricks or blocks .

(iii) Standard transit time bando A number of readings at different positions along the sample wall panels will be needed to take account of the inherent material variability and establish the range of readings to be expected for good quality construction at the chosen testing age.

(iv) Maximum acceptable transit time. Initially this limit will need to be established by testing sample panels containing various known defects to establish the transit time corresponding to the maximum acceptable defect. It is anticipated, however, that if the test method

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FIGURE 4: Nominal Pulse Velocity Results for 3 x 1 Prisms with Perforated Class B Engineering Bricks.

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Prism age (days)

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50mmfurrow

FIGURE 5: Nominal Pulse Velocity Results for 3 x 1 Prisms with Bed-Joint Furrows.

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becomes accepted and more widely used, so that experience is gained with a wider range of materiais, then it may be possible to provide general guidelines on the relationship of these upper limits to the standard transit times for various classes of masonry construction.

The site control testing could then be carried out as a sequence of scans of the work as follows : Scan 1. Carry out tests on each day's construction using the standard transducer

spacing at intervals of, say, approximately 1m along the structure. Compare transit times with the standard and maximum acceptable times . Identify and mark as suspect any areas where the times approach or exceed the maximum acceptable value.

Repeat tests over the suspect areas at closer spacings, say on the centres of every brick or block in a course. Identify and mark as potentially defective any areas where the transit time exceeds the maximum acceptable value.

In the potentially defective areas, move the top transducer down towards the lower transducer by one course at a time. Plot transit time against transducer separation. If the problem is due to a uniform defect in ali courses then the plot will be reasonably linear. If it relates to specific defects in one or two individual courses then discontinuities in the plot will give an indication of the relevant leveis .

Allowing for a reading rate of 20 per hour under site conditions , compared with the rate of 30 per hour achieved by Wright [91, and using 900mm path lengths at 900mm centres along the construction, Scan I could cover 16sq.m. of masonry per hour of testing. There is, however, considerable potential to develop the transducers to facilitate ease of handling and even automatic application of a suitable couplant. Such developments would great1y speed up the testing processo

A similar procedure could be developed for testing in a horizontal direction if defects in the vertical joints were of concern. Quality control testing for reinforced masonry could follow a similar pattern for the leaves of masonry, supplemented by through thickness direct readings on paths positioned so as to travei through the infill grant or concrete.

CONCLUSIONS

Ultrasonic testing using simple robust equipment can yield potentially useful information relating to the quality of masonry construction. Its adoption as an aid to site quality control is advocated and the procedures proposed represent a simple and realistic testing regime. Good quality construction will , in the end, always depend upon the skill, commitment and education of the workforce. In traditional construction, quality has tended to have been judged by the surface appearance of the finished work, with relatively little attention being paid to potential hidden defects . With low stress leveis and high factors of safety this approach still leads to safe structures but it is not acceptable for modern high stress structures . The adoption of a site quality control regime which includes checking of the finished work for the presence of internai defects will emphasise to the workforce the importance being placed upon the achievement of high quality throughout the work. Hopefully this will lead to better standards of construction with resulting greater confidence in the reliability of masonry structures . Such greater confidence may then lead to reductions in the high values of partial safety factor on material strength used in current design procedures, so leading to more efficient and economic masonry structures .

ACKNOWLEDGEMENTS

The financial support of the Science and Engineering Research Council , the support of the Brick Developrnent Association and Messrs Armitage Brick Ltd, in the form of technical advice

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and the supply of materiais, and the cooperation of the British Ceramic Research Association, the Cement and Concrete Association (now the British Cement Association) and the Building Research Establishment in allowing ultrasonic tests to be conducted on their test walls , are ali gratefully acknowledged.

REFERENCES

1. BRICK DEVELOPMENT ASSOCIATION "Structural Brickwork Awards 1990", BDA, Windsor, 1990 pp. 5-6.

2. CURTIN, W.G. "The Structural Challenge" , New Builder Supplement, 19th Oct. 1989, p.7 .

3. BELL, S.E. and BRADSHAW, R.E. "Construction ofReinforced Brickwork" , Proceedings of Institution of Structural Engineers Symposium on Reinforced and Prestressed Brickwork, Edinburgh, 1982.

4. HENDRY, A.W. "Structural Masonry", MacMillan, Basingstoke 1990, pp. 32-40. 5. HOBBS, B. and DAOU, Y. "Post-Tensioned T-section Brickwork Retaining Walls",

Proceedings 8th IBMAC, Dublin, Sept. 1988, paper no . 62 pp. 11. 6. HOBBS, B. "A Feasibility Study of the Use of Non-Destructive Testing Methods for

Structural Masonry" Report to Science and Engineering Research Council, March, 1986. 7. HOBBS, B. and WRIGHT, S.J . "An Assessment ofUltrasonic Testing for Structural

Masonry" Proceedings of British Masonry Society, V2, London, 1988, pp. 42-45. 8. HOBBS, B. and WRIGHT, S.J . "Ultrasonic Testing for Fault Detection in Brickwork and

Blockwork", Proceedings of Conference on Structural Faults and Repair, London, July 1987, pp. 6.

9. WRIGHT, S.J . "A Feasibility Study ofthe Use ofthe Ultrasonic Testing Method for Structural Masonry", MPhil Thesis, University of Sheffield, 1989. pp 265.

10. NOLAND, 1., ATKINSON, R.H. and BAUR, J. "An Investigation into Methods ofNon­Destructive Evaluation of Masonry Structures" Proceedings 7th IBMAC, BDRF, Melbourne, 1985.