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Reusing timber formwork: importance of workmen’s efficiency and attitude
Building and Environment 35 (2000) 135±143
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Reusing timber formwork: importance of workmen's eciency and attitude Y.Y. Ling*, K.C. Leo School of Building and Real Estate, National University of Singapore, 10 Kent Ridge Crescent, Singapore, 119260 Received 14 September 1998; received in revised form 23 November 1998; accepted 21 January 1999
Abstract A framework for identifying factors that may help in increasing the reuse of timber formwork was developed and 15 subfactors were identi®ed. A survey was conducted among formwork contractors to gauge whether these factors have signi®cant impact on the reuse of timber formwork. Data were collected via mailed standard questionnaire. It was found that the three sub-factors that had impact on the reuse of timber formwork are (a) working attitudes of workmen, (b) eciency of workmen, and (c) formwork stripping process. It is recommended that to increase the reuse of timber formwork, contractors should concentrate their eorts in improving the workmen. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Timber formwork is commonly used for the construction of cast-in-situ concrete structures because of its versatility and ease of handling. It may form 35± 60% of the total cost of constructing a concrete structure. Thus, the reusability of such formwork can bring about substantial cost savings to contractors. The ability to reuse timber formwork also means lesser demand on timber and its products, which leads to a reduction in logging and environmental degradation. Many construction projects in Singapore utilise site assembled timber formwork made of lumber and plywood. During the initial stages of concreting, many uses of formwork can be obtained and a reasonable ®nish of the concrete can be achieved. However, observations of construction practice revealed that formwork sets are only reused after substantial alterations and maintenance are carried out. It is therefore important to identify and recognise the various factors that are aecting the reuse of timber formwork. With better knowledge, solutions can be sought to maximise the numbers of reuse. The aim of this paper is there* Corresponding author.
fore to investigate the factors aecting the reuse of timber formwork in Singapore construction sites.
2. Factors aecting the reuse of timber formwork There are several factors aecting the reuse of timber formwork. These include the materials used to fabricate the formwork (H1), workmen who work with the formwork (H2), design of the completed structure (H3), design, fabrication, and stripping of the formwork (H4), and site management issues (H5). These are shown in the framework in Fig. 1. Under these ®ve factors, speci®c sub-factors are identi®ed and now discussed. 2.1. H1: Materials used to fabricate the formwork 2.1.1. Quality of components used to fabricate formwork Good quality formwork components should have the characteristics listed below so that they can be reused to obtain a satisfactory performance [1,2]. 1. It should be easy to work with by hand or machine and should not split when nailed. 2. It should be hard enough to withstand damage on
0360-1323/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 1 3 2 3 ( 9 9 ) 0 0 0 0 8 - 6
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Y.Y. Ling, K.C. Leo / Building and Environment 35 (2000) 135±143
Fig. 1. Factors aecting the reuse of timber formwork.
the contact surface under normal conditions of erecting and stripping of formwork, ®xing steel reinforcement bars and placing concrete. It should be light enough for workers to handle and carry to respective work areas. It should be sti so as to avoid undue de¯ection when loaded and strong enough to safely carry considerable loads and pressures that may be applied during concreting. It should be reasonably stable and not unduly liable to cast or warp when exposed to sun and rain or when unevenly wetted during rainy days or monsoon seasons. It should have the correct amount of moisture so that it will not warp and swell after concrete is placed. It should not have excessive formation of hemicelluloses (wood sugars) at the ®rst use after exposure to sunlight as it will retard curing of concrete.
2.2.1. Working attitudes of workers One of the main problems that contractors face is the contempt with which the general workers treat formwork items either in the way they ooad or handle the formwork or when they apply excessive forces during any handling or striking operations [5]. They are often unable to appreciate that a rather dirty or concrete-stained component, be it a piece of timber or plywood, can have some sort of value. These men usually do not have a feeling for the ®ner points of construction and are certainly unaware of the economics of reusing formwork. Crane operators' skills have a substantive impact on the number of uses obtained from formwork. The way in which they handle the panels, carry out the allied work of manoeuvring skips, reinforcement bars, cages of steel and other materials could possibly damage the formwork, sometimes beyond repair [5]. It is thus hypothesised that:
For the timber formwork to be reusable, the contractor must ensure that the formwork will produce a surface of concrete that is visually acceptable [2]. Generally, the surface must be smooth and uniformly coloured. To achieve this, the most important concerns are the surface quality of formwork, joints and the conduct of the entire formwork operation [3,4]. Hence, whether or not a set of used formwork can be reused, depends very much on the quality of components used to fabricate the formwork. Based on the above, it is hypothesised that:
H2.1ÐThe working attitudes of workmen will determine the number of times that timber formwork can be reused.
3. 4.
5.
6. 7.
H1.1ÐThe quality of components used to fabricate formwork will determine the numbers of reuse of timber formwork.
2.2. H2: Workmen who work with the formwork `Workmen' refers to the formworkers who fabricate and strip formwork and general workers who assist them.
2.2.2. Eciency of workmen Where maximum reuse of formwork is emphasised, every operation must be scheduled precisely to sustain eective continuity of employment of craftsmen and formworkers on site. This is to ensure that formworkers are able to meet the required output, make economic reuse of the formwork and be accurate in both erection and striking. True economy comes from a smooth daily repetition of the same operation [5]. If a reasonable schedule is planned and allowance made for familiarisation, each crew will become fast and ecient. In the long run, the job will be completed faster and at a lower cost as the reuse of the formwork is maximised. It is hypothesised that:
Y.Y. Ling, K.C. Leo / Building and Environment 35 (2000) 135±143
H2.2ÐThe eciency of workmen will determine the numbers of reuse of timber formwork.
2.3. H3: Design of the completed structure 2.3.1. Degree of structural design complexity and standardisation The reuse of timber formwork is very much dependent upon the design complexity of the structure [6]. When the structure is very complex, prefabricated timber formwork panels may not be possible. Modi®cations may be needed to deal with variations in the complex design, leading to expensive on-site time. Standardised and simple structural designs enable contractors to enjoy savings as there is a high level of timber formwork reuse and hence lesser costs are incurred in purchasing new lumber and plywood. Standardisation results in mobility and ¯exibility of formwork. This means that components transferred from one working area to another can be used satisfactorily with those already in existence [5]. Standardisation of components ensures reused formwork needs only minimum adjustment and refabrication. In this way, it is relatively simple to render formwork serviceable and reusable throughout a range of operations and thus provide economical formwork arrangement. An example is the standardisation of column sizes throughout the entire building. This results in a minimum number of fabrications of column formwork which can be reused consistently. Based on the above discussion, it is hypothesised that: H3.1ÐThe degree of structural design complexity will determine the numbers of reuse of timber formwork, and H3.2ÐThe degree of structural standardisation will determine the numbers of reuse of timber formwork.
2.4. H4: Design, fabrication and stripping of the formwork 2.4.1. Formwork erection process It must be ensured that the selected erection method meets all requirements of the speci®cations and also the chosen techniques can be carried out by using available labour. Where the supervisor has a ®rm idea of the techniques to be employed, it is possible to facilitate formwork construction and increase its reuse by adjusting design details [1]. In fabricating formwork, one must keep in mind the
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steps that can be taken to aid in removing them without injury to both the concrete and formwork, so as to preserve the integrity of the formwork. For example, inserts should be attached with duplex (double headed) nails driven from the outside of the forms wherever possible, because they can be pulled out easily, leaving embedded parts of the form temporarily in the concrete. It is thus hypothesised that: H4.1ÐThe formwork erection process will determine the numbers of reuse of timber formwork.
2.4.2. Formwork stripping (striking) process Proper stripping of formwork may increase its reuse. Proper stripping is determined by the stripping technique and parting agent used. Care is needed to ensure that no formwork is keyed into the concrete while stripping, particularly the end grain of timber [6]. It is always advisable to control the striking of formwork by removing one section at a time. Crash striking, where large areas of formwork are allowed to fall at one time, should not be permitted. It damages the formwork and is potentially dangerous to workers. It also stresses the structure by its sudden loading. Crush plates, key strips and wood wedges are useful in protecting surfaces from damage. Whenever possible, mechanical devices such as cables, ropes and wheeled supports should be employed. However, cranes and steel ropes should not be used to `tear away' formwork from the concrete surface [2]. The use of metal stripping bars, pries or nailbars to strip any timber formwork sets, is discouraged. It may damage or break the formwork and hence render it unsuitable for subsequent reuse. Protective sealant coatings and parting agents for plywood improves formwork lifespan and aid in stripping [2]. Regular plywood only requires a liberal amount of oil or parting agent a few days before the plywood is used. It should then be wiped o to leave a thin ®lm. Plywood-form coating lacquers, resin or plastic-base compounds and similar ®eld coatings sometimes are used to form a hard, dry waterproof ®lm on the plywood surface. It is therefore hypothesised that: H4.2ÐThe formwork stripping (striking) process will determine the reuse of timber formwork.
2.4.3. Amount of nailing to the formwork The use of nails is a dilemma faced by most contractors. In order to secure timber and plywood pieces together to hold liquid concrete, large amounts of nails
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are needed. On the other hand, nails will tear wood ®bres and further deteriorate the formwork. As far as it is permitted, few nails should be used in fabricating formwork and those that are used should be left with heads standing so that they can be easily removed, unless they are in the form face [7]. Nails must not be driven both ways in an internal angle. Duplex (double-headed) nails are useful and eective in such an operation but are expensive. Nailing should be done carefully using a hammer with smooth and well-dressed heads to prevent marring of the form. Slightly raised nail heads leave a depression in the concrete that is less conspicuous than `pimple' that results when nails are driven below the form surface [2]. If formwork is reused, nails should be bent but it is preferable to extract them [8]. In this way, the absence of nails imposes less danger to the workers and the integrity of the formwork will not be compromised. After nails are removed, the holes should be ®lled up with patching plaster, plasticwood, or other suitable materials. From the above, it is hypothesised that: H4.3ÐThe amount of nailing will determine the numbers of reuse of timber formwork.
2.4.4. Ease of maintaining formwork after stripping (striking) Timber formwork must be properly maintained so that it can be reused several times before it is rendered unsuitable for further uses. However, maintenance depends very much on the ease of carrying it out on site. Formwork should also be designed for easy maintenance and cleaning because if it is not carried out, it will not be in good working condition. The durability and maintenance requirements of the formwork are major considerations for contractors as they dictate the factor of reuse and hence the amount of savings enjoyed by the contractor [5]. Hence, it is hypothesised that: H4.4ÐThe ease of maintenance after stripping (striking) will determine the numbers of reuse of timber formwork.
2.5. H5: Site management issues 2.5.1. Supervision and planning of erection and stripping of formwork Supervisors are directly in charge of formwork construction and dismantling. They will be responsible for erection of formwork in accordance with design details and accepted tolerances, and with materials of the speci®ed quality. As direct line managers, they must
ensure that the erection and striking processes are carried out in the most appropriate manner to enable formwork to be reused in the subsequent concreting process [1]. It is thus hypothesised that: H5.1ÐThe supervision and planning of erection and stripping will determine the numbers of reuse of timber formwork.
2.5.2. Time availability In a lump sum ®xed price contract, the contractor is committed to the completion of the project within the contracted cost and time frame. Construction practice must be closely followed, faults must be corrected, yet the next critical step must not be delayed [9]. The consulting engineer usually does the design of permanent work while the design of formwork is by the contractor. A preliminary formwork design may be needed for estimating purpose and then the ®nal design may have to be completed in a great hurry when the contract has been awarded [10]. It is common to ®nd formwork as one of the critical activities in the construction schedule of any project [7]. While it may be economical to reuse formwork, it has been suggested that it is more productive to fabricate new formwork sets instead of sifting out suitable ones for reuse [11]. It is hypothesised that: H5.2ÐTime availability will determine the numbers of reuse of timber formwork.
2.5.3. Availability of space on-site to re-fabricate formwork for the next reuse Working space on the ground allows contractors to clean and oil formworks if they are to be reused in the next concreting stage. However, there may not be sucient space for fabrication or re-fabrication of formwork. Hence, reusability of formwork may be reduced since it is impractical to transport used formwork back to the fabrication yard to be re-fabricated before they are transported back to site [12]. Newly purchased timber and plywood pieces are therefore used for fabrication instead. Hence, it is hypothesised that: H5.3ÐThe availability of space on-site to re-fabricate for the next reuse will determine the numbers of reuse of timber formwork.
2.5.4. Ease of handling reused formwork The availability of plant and equipment on site
Y.Y. Ling, K.C. Leo / Building and Environment 35 (2000) 135±143
determines the ease of handling timber formwork. For example, cranes are gauged to the main loads to be handled over a given radii and thus may not be the type best suited to formwork and concreting operations [13]. However, time schedule should be set for equipment demand and arranged for critical formwork handling operations to phase in with the other general lifting duties. One aspect of handling that often causes problems relates to the casting of concrete. Handling systems are considered during the design process but what is overlooked is the fact that the placing of concrete completely alters the handling situation. While it is easy to lower a form panel or even a complete form unit, it is not easy to remove it after casting when the fresh concrete curtails the headroom and blocks access for lifting and handling. This can be illustrated in the case of slab formwork where considerable attention is paid to lifting hooks and means of transferring forms from one bay to another. As a result, this compels formworkers to remove formwork piece by piece, and at times forcefully and hence damaging them, in order to expedite work. New sets of formwork have to be fabricated for other concreting purposes [12]. Based on the above discussion, it is hypothesised that: H5.4ÐThe ease of handling formwork will determine the numbers of reuse of timber formwork.
2.5.5. Co-operation of other trades The striking, maintenance and re-fabrication programme of timber formwork relies on other related trades completing their work. For instance, when drainpipes and wares are to be incorporated in or under the raft foundation, the operations of excavation, drainlaying and laying of reinforcement bars must be completed prior to the erection of formwork. The delay of any of these trades will inevitably hold back the striking of formwork, its maintenance and its subsequent numbers of reuse [1]. Hence, the speed and eciency of other trades are highly in¯uential on the reuse of formwork. Crews installing reinforcing steel will be used most eciently if their work can be scheduled to ®t the concreting sequence without causing lost time in either the concreting or bar setting schedules. Where possible, the form design should permit the reinforcement to be preassembled before installing, so that reinforcing bar crew can work under a more favourable condition and at their own pace without lost motion [14]. Their work can then be scheduled ahead of formwork setting, so that formwork can be set one day and concrete placed the next, thus expediting the schedule and possibly increasing the number of formwork reuses.
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It is therefore hypothesised that: H5.5ÐThe co-operation of other trades will determine the numbers of reuse of timber formwork.
2.5.6. Safety of workers during re-fabrication of formwork It is important to ensure safety on any construction site. Accidents result in extra time and cost and sometimes the loss of human lives. Overall safety on site is paramount for the welfare of workers. About 50±80% of formwork labour is material handling [15]. Material is being put up, taken down and moved horizontally. The areas to which material is being moved is not completely decked over nor is all the material immediately secured to what is already in place. Unless all safety features are available and the safety of the formworkers is ensured, no formwork should be re-fabricated on site. It is advised that formwork be fabricated o-site and transported to site instead. Safety in the fabrication yard is easier to control and enforce thus preventing the chances of unnecessary accidents. However, given these inconveniences, contractors may then decide not to reuse the timber formwork. Thus, it is hypothesised that: H5.6ÐThe safety of workers during re-fabrication of formwork will determine the numbers of reuse of timber formwork.
3. Method In order to explore the true impact of these factors on the reuse of timber formwork in construction sites, a questionnaire was prepared to test these hypotheses. The questionnaire survey was drafted in English and Chinese. Two reasons for providing the Chinese version are to assist the predominantly Chinese-educated timber formwork contractors in Singapore to understand the questionnaire, and to improve the response rate. In the questionnaire, the respondents were required to indicate on a ®ve-point scale the level of impact of a factor in in¯uencing formwork reuse, 1 being `no impact', 2 for `low impact', 3 for `moderate impact', 4 for `high impact' and 5 for `very high impact'. The questionnaire also invited respondents to suggest other factors that in¯uence formwork reuse. Data were collected by postal survey between July and August 1997. The target population was timber
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Table 1 Average annual turnover of company
Table 3 Average timber formwork sub-contract value
Average annual turnover (S$)
Percentage (%)
Average sub-contract value (S$)
Percentage (%)
Less than S$500,000 S$500,000±S$1,000,000 S$1,000,000±S$1,500,000 S$1,500,000±S$2,000,000 S$2,000,000±S$5,000,000 More than S$5,000,000
0 30 0 0 10 60
Less than S$500,000 S$500,000±S$1,000,000 S$1,000,000±S$1,500,000 S$1,500,000±S$2,000,000 S$2,000,000±S$5,000,000 More than S$5,000,000
50 30 0 0 0 20
formwork contractors. These contractors are listed in the Singapore Contractors Association Limited's Directory of Contractors 1997/98 and the Singapore Industrial Directory: the Greenbook 1996/97. A total of 40 relevant contractors were identi®ed. As the population was not large, no further sampling was done. Instead, the entire population of timber formwork contractors was surveyed. Each contractor was sent a survey form, together with a self-addressed and stamped envelope. The data obtained were edited, coded, checked and analysed using non-parametric inferential statistics.
4. Results After one reminder, 10 completed questionnaires were received by 31 August 1997. This represented a response rate of 25%. 4.1. Respondents' pro®le Six of the respondents have a turnover of more than S$5 million, and the rest have turnovers between S$500,000 and S$5 million (see Table 1). All the respondents, who are managing directors of their ®rms, had undertaken formwork subcontracts for residential projects (see Table 2). The high percentage of participation in residential projects may be due to the increasing demand for both public and private housing for the last three to four years. The low participation
Table 2 Types of project involved Types of project involved
Percentage (%)
Residential Institutional Commercial Industrial
100 40 20 40
rate in commercial projects could be due to the higher utilisation rate of proprietary or metal formwork systems in these projects. Eighty percent of the average value of the timber formwork sub-contracts was less than S$1 million. Only 20% of the sub-contracts cost more than S$5 million (see Table 3). 4.2. Hypothesis testing Wilcoxon signed rank tests were conducted at A=0.10 to identify factors which have impact on the reuse of timber formwork. This non-parametric statistical test was chosen because an inference about the value of the population median could be made from the test result, the assumption that the population had normal distribution was unnecessary, and the sample size was small. The null hypothesis, H was set up below. H: M M0 where, M denotes the true population median, and M0 is the median score, which is 3, (on a scale of 1±5). When a factor has an impact on the reuse of timber formwork, its true median score would be larger than M0. Therefore, the one-sided alternative hypothesis, A+ was set up below. A : M > M 0 When the probability of M M0 which was calculated using the Wilcoxon signed rank test was larger than or equal to 0.10, the null hypothesis was accepted. It was then concluded that the factor does not have a high impact on the reuse of timber formwork. When the probability was less than 0.10, the null hypothesis was rejected. The alternative hypothesis was accepted and it was concluded that the attribute was important at 90% con®dence level. One-tailed probabilities of all factors were calculated and shown in Table 4. Based on the Wilcoxon signed rank test, null hypotheses numbers H2.1, H2.2 and H4.2 were rejected. This means that the alternative hypothesesÐthat these
Y.Y. Ling, K.C. Leo / Building and Environment 35 (2000) 135±143
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Table 4 Factors determining the reuse of timber formwork No
Factors and sub-factors
H1 H1.1 H2 H2.1 H2.2 H3 H3.1 H3.2 H4 H4.1 H4.2 H4.3 H4.4 H5 H5.1 H5.2 H5.3 H5.4 H5.5 H5.6 a
Materials Quality of components used to fabricate formwork Workmen Working attitudes of workmen Eciency of workmen Design of the completed structure Degree of structural design complexity Degree of structural standardisation Design, fabrication and stripping of the formwork Formwork erection process Formwork stripping (striking) process Amount of nailing Ease of maintenance after stripping (striking) Site management issues Supervision and planning of erection and stripping Time availability Availability of space on-site to re-fabricate for the next reuse Ease of handling Co-operation of other trades Safety of workers during re-fabrication of formwork
Number of respondents
Probability (Wilcoxon signed rank test)
No impact
Low impact
Moderate impact
High impact
Very high impact
2
1
1
2
4
0.213
0 1
2 1
1 2
4 2
3 4
0.037a 0.098a
2 1
1 2
0 0
2 5
5 2
0.138 0.161
4 1 3 0
1 1 3 3
1 1 0 4
3 4 4 2
1 3 0 1
0.787 0.082a 0.862 0.422
1
2
2
3
2
0.273
2 0
1 4
3 3
1 2
3 1
0.344 0.531
0 2 1
3 2 3
3 1 2
3 4 2
1 1 2
0.289 0.500 0.422
Sub-factor is important at 90% signi®cance level.
factors had impact on the reuse of timber formworkÐ were accepted.
5. Discussion The statistical tests showed that the three sub-factors that aect the reuse of timber formwork signi®cantly are as follows: . Working attitudes of workers (H2.1) . Eciency of workers (H2.2) . Formwork stripping (striking) process (H4.2) 5.1. Working attitudes of workers (H2.1) Contractors may face a problem of poor attitudes of workmen when they ooad and handle formwork sets. While timber formwork could be laid on the ground carefully and lightly after stripping without being damaged, ignorant workers would usually toss it
onto the ground. Such an action would result in the splitting of timber ®bre both on the surface and at the edges which are usually the weakest parts of a formwork. Such damage to the formwork could incur more resources in repair before they could be reused, or could render them non-reusable at all in the subsequent concreting process. Therefore, working attitude is a crucial determinant to the reuse of timber formwork.
5.2. Eciency of workers (H2.2) Good co-ordination determines the eciency of formworkers. Uninterrupted work ensures that formwork is erected and stripped in accordance with scheduled timings with little delays in between. The numbers of reuse of the formwork sets could then be maximised throughout the project since lag times are minimised. Formworkers could only be ecient and eective when they are familiar with their work.
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Y.Y. Ling, K.C. Leo / Building and Environment 35 (2000) 135±143
5.3. Formwork stripping (striking) process (H4.2) Proper stripping of formwork depends on the releasing agent used and the skill of the workmen. Release (parting) agents that are applied onto the formwork forms a thin layer separating itself from the curing concrete. As there is no direct contact between the two components, little or no liquid concrete will cure in between wood grains. This enables the formworker to pry open the formwork with the minimum eort and at the same time preserves the formwork integrity. The removal techniques and processes adopted for the removal would aect the integrity of formwork. If suitable tools are not used and care is not exercised during stripping, timber and plywood pieces could be easily torn beyond repair. Whenever possible, air should be introduced between the contact surface of formwork and concrete ®rst so that the formwork are removed with the least diculties and without much damage. The importance of formwork stripping process in determining formwork reuse is also related to workmen having the correct attitude (H2.1). Workmen who have the right attitude are more likely to strip the formwork properly.
6. Conclusion This paper explored 15 sub-factors that have an impact on the reusability of traditional formwork. The ®eldwork showed that only three sub-factors have signi®cantly high impact on the reusability. The ®rst two sub-factors, `working attitudes of workers' and `crew eciency' relate to the factor of `workmen' who handle the formwork. The third sub-factor, `formwork stripping (striking) process', is also related to the workmen who handle the formwork. The conclusion is therefore that the workmen are the common denominator that determines the number of times formwork can be reused. The workmen need to have the correct working attitude and be able to work eciently, to ensure that the formwork has a high reuse. The formwork stripping process is also related to the working attitude and eciency of the workmen. Based on this ®nding, it may be possible to increase the numbers of reuse of timber formwork by motivating the workmen, instead of concentrating on the design or other factors. It is recommended that monetary reward be used to improve work attitudes and crew eciency. With a higher monetary compensation, workers have more incentive to execute their work correctly at the ®rst attempt. Eciency of erection and stripping of formwork is further enhanced if such rewards are directly associated with the workmen's performance.
From this study, it appears that other factors such as materials used to fabricate the formwork, design of the completed structure, and site management issues have little impact on the reusability of timber formwork. The limitation of this research is that the analysis is based on a small number of responses. The response rate is low, despite the mailing of reminder letters and telephone calls made to the ®rms. In addition, respondents may have dierent perceptions of the language used to describe the impact of each factor. For example, a `moderate impact' rating given by one respondent may not have the same degree of impact as another respondent's `moderate impact' rating. The ®ndings have two major implications. The ®rst is that it is possible to increase the number of times timber formwork is reused, leading to a reduction in trees felled which is advantageous to the environment. Contractors may also enjoy cost savings. The second implication is that contractors should concentrate on managing workmen better to increase the numbers of reuse of timber formwork, instead of trying to handle multiple factors that may also aect timber formwork reuse, but to a lesser extent.
References [1] Richardson JG. Formwork construction and practice. London: Viewpoint Publication, 1977. [2] Wilshere CJ. Formwork. London: Thomas Telford, 1989. [3] Gould JV. Designing with plywoodÐconsiderations for selecting and using plywood correctly for concrete forming. In: Panarese WC, editor. Proceedings of the Second International Conference on Forming Economical Concrete Buildings, American Concrete Institute, Detroit, 1986. p. 39±48. [4] Chew LT. Specifying Timber for Construction: The Malaysia Experience. Paper presented at the Seminar on the Use of Malaysian Timber in the Construction Industry, Singapore 1991. [5] Hurd MK. Formwork for concrete, 6th ed. Detroit: American Concrete Institute, 1995. [6] Richardson JG. Supervision of concrete construction. London: Viewpoint Publication, 1986. [7] Austin CK. Formwork to concrete, 3rd ed. London: George Godwin, 1978. [8] Construction Industry Research and Information Association. Environmental handbook for building and civil engineering projects: design and speci®cation. London: Thomas Telford, 1994. [9] Peurifoy RL. Formwork for concrete structure, 2nd ed. New York: McGraw-Hill, 1976. [10] Richardson JG. Practical formwork and mould construction. London: Viewpoint Publication, 1976. [11] Badzinki Jr S. Carpentry in commercial construction. Englewood Clis: Prentice Hall, 1980. [12] Brett P. Formwork and concrete practice. London: Heinemann Professional, 1988. [13] Health, Safety Executive. Final report of the advisory committee on falsework. London: HMSO, 1975. [14] Harrison JGH. A study of the formwork design for ElfAquitaine's world headquaters building at La Defense, Paris,
Y.Y. Ling, K.C. Leo / Building and Environment 35 (2000) 135±143 France. In: Panarese WC, editor. Proceedings of the Second International Conference on Forming Economical Concrete Buildings, American Concrete Institute, Detroit, 1986. p. 239± 61.
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Reusing timber formwork: importance of workmen's eciency and attitude Y.Y. Ling*, K.C. Leo School of Building and Real Estate, National University of Singapore, 10 Kent Ridge Crescent, Singapore, 119260 Received 14 September 1998; received in revised form 23 November 1998; accepted 21 January 1999
Abstract A framework for identifying factors that may help in increasing the reuse of timber formwork was developed and 15 subfactors were identi®ed. A survey was conducted among formwork contractors to gauge whether these factors have signi®cant impact on the reuse of timber formwork. Data were collected via mailed standard questionnaire. It was found that the three sub-factors that had impact on the reuse of timber formwork are (a) working attitudes of workmen, (b) eciency of workmen, and (c) formwork stripping process. It is recommended that to increase the reuse of timber formwork, contractors should concentrate their eorts in improving the workmen. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Timber formwork is commonly used for the construction of cast-in-situ concrete structures because of its versatility and ease of handling. It may form 35± 60% of the total cost of constructing a concrete structure. Thus, the reusability of such formwork can bring about substantial cost savings to contractors. The ability to reuse timber formwork also means lesser demand on timber and its products, which leads to a reduction in logging and environmental degradation. Many construction projects in Singapore utilise site assembled timber formwork made of lumber and plywood. During the initial stages of concreting, many uses of formwork can be obtained and a reasonable ®nish of the concrete can be achieved. However, observations of construction practice revealed that formwork sets are only reused after substantial alterations and maintenance are carried out. It is therefore important to identify and recognise the various factors that are aecting the reuse of timber formwork. With better knowledge, solutions can be sought to maximise the numbers of reuse. The aim of this paper is there* Corresponding author.
fore to investigate the factors aecting the reuse of timber formwork in Singapore construction sites.
2. Factors aecting the reuse of timber formwork There are several factors aecting the reuse of timber formwork. These include the materials used to fabricate the formwork (H1), workmen who work with the formwork (H2), design of the completed structure (H3), design, fabrication, and stripping of the formwork (H4), and site management issues (H5). These are shown in the framework in Fig. 1. Under these ®ve factors, speci®c sub-factors are identi®ed and now discussed. 2.1. H1: Materials used to fabricate the formwork 2.1.1. Quality of components used to fabricate formwork Good quality formwork components should have the characteristics listed below so that they can be reused to obtain a satisfactory performance [1,2]. 1. It should be easy to work with by hand or machine and should not split when nailed. 2. It should be hard enough to withstand damage on
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Fig. 1. Factors aecting the reuse of timber formwork.
the contact surface under normal conditions of erecting and stripping of formwork, ®xing steel reinforcement bars and placing concrete. It should be light enough for workers to handle and carry to respective work areas. It should be sti so as to avoid undue de¯ection when loaded and strong enough to safely carry considerable loads and pressures that may be applied during concreting. It should be reasonably stable and not unduly liable to cast or warp when exposed to sun and rain or when unevenly wetted during rainy days or monsoon seasons. It should have the correct amount of moisture so that it will not warp and swell after concrete is placed. It should not have excessive formation of hemicelluloses (wood sugars) at the ®rst use after exposure to sunlight as it will retard curing of concrete.
2.2.1. Working attitudes of workers One of the main problems that contractors face is the contempt with which the general workers treat formwork items either in the way they ooad or handle the formwork or when they apply excessive forces during any handling or striking operations [5]. They are often unable to appreciate that a rather dirty or concrete-stained component, be it a piece of timber or plywood, can have some sort of value. These men usually do not have a feeling for the ®ner points of construction and are certainly unaware of the economics of reusing formwork. Crane operators' skills have a substantive impact on the number of uses obtained from formwork. The way in which they handle the panels, carry out the allied work of manoeuvring skips, reinforcement bars, cages of steel and other materials could possibly damage the formwork, sometimes beyond repair [5]. It is thus hypothesised that:
For the timber formwork to be reusable, the contractor must ensure that the formwork will produce a surface of concrete that is visually acceptable [2]. Generally, the surface must be smooth and uniformly coloured. To achieve this, the most important concerns are the surface quality of formwork, joints and the conduct of the entire formwork operation [3,4]. Hence, whether or not a set of used formwork can be reused, depends very much on the quality of components used to fabricate the formwork. Based on the above, it is hypothesised that:
H2.1ÐThe working attitudes of workmen will determine the number of times that timber formwork can be reused.
3. 4.
5.
6. 7.
H1.1ÐThe quality of components used to fabricate formwork will determine the numbers of reuse of timber formwork.
2.2. H2: Workmen who work with the formwork `Workmen' refers to the formworkers who fabricate and strip formwork and general workers who assist them.
2.2.2. Eciency of workmen Where maximum reuse of formwork is emphasised, every operation must be scheduled precisely to sustain eective continuity of employment of craftsmen and formworkers on site. This is to ensure that formworkers are able to meet the required output, make economic reuse of the formwork and be accurate in both erection and striking. True economy comes from a smooth daily repetition of the same operation [5]. If a reasonable schedule is planned and allowance made for familiarisation, each crew will become fast and ecient. In the long run, the job will be completed faster and at a lower cost as the reuse of the formwork is maximised. It is hypothesised that:
Y.Y. Ling, K.C. Leo / Building and Environment 35 (2000) 135±143
H2.2ÐThe eciency of workmen will determine the numbers of reuse of timber formwork.
2.3. H3: Design of the completed structure 2.3.1. Degree of structural design complexity and standardisation The reuse of timber formwork is very much dependent upon the design complexity of the structure [6]. When the structure is very complex, prefabricated timber formwork panels may not be possible. Modi®cations may be needed to deal with variations in the complex design, leading to expensive on-site time. Standardised and simple structural designs enable contractors to enjoy savings as there is a high level of timber formwork reuse and hence lesser costs are incurred in purchasing new lumber and plywood. Standardisation results in mobility and ¯exibility of formwork. This means that components transferred from one working area to another can be used satisfactorily with those already in existence [5]. Standardisation of components ensures reused formwork needs only minimum adjustment and refabrication. In this way, it is relatively simple to render formwork serviceable and reusable throughout a range of operations and thus provide economical formwork arrangement. An example is the standardisation of column sizes throughout the entire building. This results in a minimum number of fabrications of column formwork which can be reused consistently. Based on the above discussion, it is hypothesised that: H3.1ÐThe degree of structural design complexity will determine the numbers of reuse of timber formwork, and H3.2ÐThe degree of structural standardisation will determine the numbers of reuse of timber formwork.
2.4. H4: Design, fabrication and stripping of the formwork 2.4.1. Formwork erection process It must be ensured that the selected erection method meets all requirements of the speci®cations and also the chosen techniques can be carried out by using available labour. Where the supervisor has a ®rm idea of the techniques to be employed, it is possible to facilitate formwork construction and increase its reuse by adjusting design details [1]. In fabricating formwork, one must keep in mind the
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steps that can be taken to aid in removing them without injury to both the concrete and formwork, so as to preserve the integrity of the formwork. For example, inserts should be attached with duplex (double headed) nails driven from the outside of the forms wherever possible, because they can be pulled out easily, leaving embedded parts of the form temporarily in the concrete. It is thus hypothesised that: H4.1ÐThe formwork erection process will determine the numbers of reuse of timber formwork.
2.4.2. Formwork stripping (striking) process Proper stripping of formwork may increase its reuse. Proper stripping is determined by the stripping technique and parting agent used. Care is needed to ensure that no formwork is keyed into the concrete while stripping, particularly the end grain of timber [6]. It is always advisable to control the striking of formwork by removing one section at a time. Crash striking, where large areas of formwork are allowed to fall at one time, should not be permitted. It damages the formwork and is potentially dangerous to workers. It also stresses the structure by its sudden loading. Crush plates, key strips and wood wedges are useful in protecting surfaces from damage. Whenever possible, mechanical devices such as cables, ropes and wheeled supports should be employed. However, cranes and steel ropes should not be used to `tear away' formwork from the concrete surface [2]. The use of metal stripping bars, pries or nailbars to strip any timber formwork sets, is discouraged. It may damage or break the formwork and hence render it unsuitable for subsequent reuse. Protective sealant coatings and parting agents for plywood improves formwork lifespan and aid in stripping [2]. Regular plywood only requires a liberal amount of oil or parting agent a few days before the plywood is used. It should then be wiped o to leave a thin ®lm. Plywood-form coating lacquers, resin or plastic-base compounds and similar ®eld coatings sometimes are used to form a hard, dry waterproof ®lm on the plywood surface. It is therefore hypothesised that: H4.2ÐThe formwork stripping (striking) process will determine the reuse of timber formwork.
2.4.3. Amount of nailing to the formwork The use of nails is a dilemma faced by most contractors. In order to secure timber and plywood pieces together to hold liquid concrete, large amounts of nails
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are needed. On the other hand, nails will tear wood ®bres and further deteriorate the formwork. As far as it is permitted, few nails should be used in fabricating formwork and those that are used should be left with heads standing so that they can be easily removed, unless they are in the form face [7]. Nails must not be driven both ways in an internal angle. Duplex (double-headed) nails are useful and eective in such an operation but are expensive. Nailing should be done carefully using a hammer with smooth and well-dressed heads to prevent marring of the form. Slightly raised nail heads leave a depression in the concrete that is less conspicuous than `pimple' that results when nails are driven below the form surface [2]. If formwork is reused, nails should be bent but it is preferable to extract them [8]. In this way, the absence of nails imposes less danger to the workers and the integrity of the formwork will not be compromised. After nails are removed, the holes should be ®lled up with patching plaster, plasticwood, or other suitable materials. From the above, it is hypothesised that: H4.3ÐThe amount of nailing will determine the numbers of reuse of timber formwork.
2.4.4. Ease of maintaining formwork after stripping (striking) Timber formwork must be properly maintained so that it can be reused several times before it is rendered unsuitable for further uses. However, maintenance depends very much on the ease of carrying it out on site. Formwork should also be designed for easy maintenance and cleaning because if it is not carried out, it will not be in good working condition. The durability and maintenance requirements of the formwork are major considerations for contractors as they dictate the factor of reuse and hence the amount of savings enjoyed by the contractor [5]. Hence, it is hypothesised that: H4.4ÐThe ease of maintenance after stripping (striking) will determine the numbers of reuse of timber formwork.
2.5. H5: Site management issues 2.5.1. Supervision and planning of erection and stripping of formwork Supervisors are directly in charge of formwork construction and dismantling. They will be responsible for erection of formwork in accordance with design details and accepted tolerances, and with materials of the speci®ed quality. As direct line managers, they must
ensure that the erection and striking processes are carried out in the most appropriate manner to enable formwork to be reused in the subsequent concreting process [1]. It is thus hypothesised that: H5.1ÐThe supervision and planning of erection and stripping will determine the numbers of reuse of timber formwork.
2.5.2. Time availability In a lump sum ®xed price contract, the contractor is committed to the completion of the project within the contracted cost and time frame. Construction practice must be closely followed, faults must be corrected, yet the next critical step must not be delayed [9]. The consulting engineer usually does the design of permanent work while the design of formwork is by the contractor. A preliminary formwork design may be needed for estimating purpose and then the ®nal design may have to be completed in a great hurry when the contract has been awarded [10]. It is common to ®nd formwork as one of the critical activities in the construction schedule of any project [7]. While it may be economical to reuse formwork, it has been suggested that it is more productive to fabricate new formwork sets instead of sifting out suitable ones for reuse [11]. It is hypothesised that: H5.2ÐTime availability will determine the numbers of reuse of timber formwork.
2.5.3. Availability of space on-site to re-fabricate formwork for the next reuse Working space on the ground allows contractors to clean and oil formworks if they are to be reused in the next concreting stage. However, there may not be sucient space for fabrication or re-fabrication of formwork. Hence, reusability of formwork may be reduced since it is impractical to transport used formwork back to the fabrication yard to be re-fabricated before they are transported back to site [12]. Newly purchased timber and plywood pieces are therefore used for fabrication instead. Hence, it is hypothesised that: H5.3ÐThe availability of space on-site to re-fabricate for the next reuse will determine the numbers of reuse of timber formwork.
2.5.4. Ease of handling reused formwork The availability of plant and equipment on site
Y.Y. Ling, K.C. Leo / Building and Environment 35 (2000) 135±143
determines the ease of handling timber formwork. For example, cranes are gauged to the main loads to be handled over a given radii and thus may not be the type best suited to formwork and concreting operations [13]. However, time schedule should be set for equipment demand and arranged for critical formwork handling operations to phase in with the other general lifting duties. One aspect of handling that often causes problems relates to the casting of concrete. Handling systems are considered during the design process but what is overlooked is the fact that the placing of concrete completely alters the handling situation. While it is easy to lower a form panel or even a complete form unit, it is not easy to remove it after casting when the fresh concrete curtails the headroom and blocks access for lifting and handling. This can be illustrated in the case of slab formwork where considerable attention is paid to lifting hooks and means of transferring forms from one bay to another. As a result, this compels formworkers to remove formwork piece by piece, and at times forcefully and hence damaging them, in order to expedite work. New sets of formwork have to be fabricated for other concreting purposes [12]. Based on the above discussion, it is hypothesised that: H5.4ÐThe ease of handling formwork will determine the numbers of reuse of timber formwork.
2.5.5. Co-operation of other trades The striking, maintenance and re-fabrication programme of timber formwork relies on other related trades completing their work. For instance, when drainpipes and wares are to be incorporated in or under the raft foundation, the operations of excavation, drainlaying and laying of reinforcement bars must be completed prior to the erection of formwork. The delay of any of these trades will inevitably hold back the striking of formwork, its maintenance and its subsequent numbers of reuse [1]. Hence, the speed and eciency of other trades are highly in¯uential on the reuse of formwork. Crews installing reinforcing steel will be used most eciently if their work can be scheduled to ®t the concreting sequence without causing lost time in either the concreting or bar setting schedules. Where possible, the form design should permit the reinforcement to be preassembled before installing, so that reinforcing bar crew can work under a more favourable condition and at their own pace without lost motion [14]. Their work can then be scheduled ahead of formwork setting, so that formwork can be set one day and concrete placed the next, thus expediting the schedule and possibly increasing the number of formwork reuses.
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It is therefore hypothesised that: H5.5ÐThe co-operation of other trades will determine the numbers of reuse of timber formwork.
2.5.6. Safety of workers during re-fabrication of formwork It is important to ensure safety on any construction site. Accidents result in extra time and cost and sometimes the loss of human lives. Overall safety on site is paramount for the welfare of workers. About 50±80% of formwork labour is material handling [15]. Material is being put up, taken down and moved horizontally. The areas to which material is being moved is not completely decked over nor is all the material immediately secured to what is already in place. Unless all safety features are available and the safety of the formworkers is ensured, no formwork should be re-fabricated on site. It is advised that formwork be fabricated o-site and transported to site instead. Safety in the fabrication yard is easier to control and enforce thus preventing the chances of unnecessary accidents. However, given these inconveniences, contractors may then decide not to reuse the timber formwork. Thus, it is hypothesised that: H5.6ÐThe safety of workers during re-fabrication of formwork will determine the numbers of reuse of timber formwork.
3. Method In order to explore the true impact of these factors on the reuse of timber formwork in construction sites, a questionnaire was prepared to test these hypotheses. The questionnaire survey was drafted in English and Chinese. Two reasons for providing the Chinese version are to assist the predominantly Chinese-educated timber formwork contractors in Singapore to understand the questionnaire, and to improve the response rate. In the questionnaire, the respondents were required to indicate on a ®ve-point scale the level of impact of a factor in in¯uencing formwork reuse, 1 being `no impact', 2 for `low impact', 3 for `moderate impact', 4 for `high impact' and 5 for `very high impact'. The questionnaire also invited respondents to suggest other factors that in¯uence formwork reuse. Data were collected by postal survey between July and August 1997. The target population was timber
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Table 1 Average annual turnover of company
Table 3 Average timber formwork sub-contract value
Average annual turnover (S$)
Percentage (%)
Average sub-contract value (S$)
Percentage (%)
Less than S$500,000 S$500,000±S$1,000,000 S$1,000,000±S$1,500,000 S$1,500,000±S$2,000,000 S$2,000,000±S$5,000,000 More than S$5,000,000
0 30 0 0 10 60
Less than S$500,000 S$500,000±S$1,000,000 S$1,000,000±S$1,500,000 S$1,500,000±S$2,000,000 S$2,000,000±S$5,000,000 More than S$5,000,000
50 30 0 0 0 20
formwork contractors. These contractors are listed in the Singapore Contractors Association Limited's Directory of Contractors 1997/98 and the Singapore Industrial Directory: the Greenbook 1996/97. A total of 40 relevant contractors were identi®ed. As the population was not large, no further sampling was done. Instead, the entire population of timber formwork contractors was surveyed. Each contractor was sent a survey form, together with a self-addressed and stamped envelope. The data obtained were edited, coded, checked and analysed using non-parametric inferential statistics.
4. Results After one reminder, 10 completed questionnaires were received by 31 August 1997. This represented a response rate of 25%. 4.1. Respondents' pro®le Six of the respondents have a turnover of more than S$5 million, and the rest have turnovers between S$500,000 and S$5 million (see Table 1). All the respondents, who are managing directors of their ®rms, had undertaken formwork subcontracts for residential projects (see Table 2). The high percentage of participation in residential projects may be due to the increasing demand for both public and private housing for the last three to four years. The low participation
Table 2 Types of project involved Types of project involved
Percentage (%)
Residential Institutional Commercial Industrial
100 40 20 40
rate in commercial projects could be due to the higher utilisation rate of proprietary or metal formwork systems in these projects. Eighty percent of the average value of the timber formwork sub-contracts was less than S$1 million. Only 20% of the sub-contracts cost more than S$5 million (see Table 3). 4.2. Hypothesis testing Wilcoxon signed rank tests were conducted at A=0.10 to identify factors which have impact on the reuse of timber formwork. This non-parametric statistical test was chosen because an inference about the value of the population median could be made from the test result, the assumption that the population had normal distribution was unnecessary, and the sample size was small. The null hypothesis, H was set up below. H: M M0 where, M denotes the true population median, and M0 is the median score, which is 3, (on a scale of 1±5). When a factor has an impact on the reuse of timber formwork, its true median score would be larger than M0. Therefore, the one-sided alternative hypothesis, A+ was set up below. A : M > M 0 When the probability of M M0 which was calculated using the Wilcoxon signed rank test was larger than or equal to 0.10, the null hypothesis was accepted. It was then concluded that the factor does not have a high impact on the reuse of timber formwork. When the probability was less than 0.10, the null hypothesis was rejected. The alternative hypothesis was accepted and it was concluded that the attribute was important at 90% con®dence level. One-tailed probabilities of all factors were calculated and shown in Table 4. Based on the Wilcoxon signed rank test, null hypotheses numbers H2.1, H2.2 and H4.2 were rejected. This means that the alternative hypothesesÐthat these
Y.Y. Ling, K.C. Leo / Building and Environment 35 (2000) 135±143
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Table 4 Factors determining the reuse of timber formwork No
Factors and sub-factors
H1 H1.1 H2 H2.1 H2.2 H3 H3.1 H3.2 H4 H4.1 H4.2 H4.3 H4.4 H5 H5.1 H5.2 H5.3 H5.4 H5.5 H5.6 a
Materials Quality of components used to fabricate formwork Workmen Working attitudes of workmen Eciency of workmen Design of the completed structure Degree of structural design complexity Degree of structural standardisation Design, fabrication and stripping of the formwork Formwork erection process Formwork stripping (striking) process Amount of nailing Ease of maintenance after stripping (striking) Site management issues Supervision and planning of erection and stripping Time availability Availability of space on-site to re-fabricate for the next reuse Ease of handling Co-operation of other trades Safety of workers during re-fabrication of formwork
Number of respondents
Probability (Wilcoxon signed rank test)
No impact
Low impact
Moderate impact
High impact
Very high impact
2
1
1
2
4
0.213
0 1
2 1
1 2
4 2
3 4
0.037a 0.098a
2 1
1 2
0 0
2 5
5 2
0.138 0.161
4 1 3 0
1 1 3 3
1 1 0 4
3 4 4 2
1 3 0 1
0.787 0.082a 0.862 0.422
1
2
2
3
2
0.273
2 0
1 4
3 3
1 2
3 1
0.344 0.531
0 2 1
3 2 3
3 1 2
3 4 2
1 1 2
0.289 0.500 0.422
Sub-factor is important at 90% signi®cance level.
factors had impact on the reuse of timber formworkÐ were accepted.
5. Discussion The statistical tests showed that the three sub-factors that aect the reuse of timber formwork signi®cantly are as follows: . Working attitudes of workers (H2.1) . Eciency of workers (H2.2) . Formwork stripping (striking) process (H4.2) 5.1. Working attitudes of workers (H2.1) Contractors may face a problem of poor attitudes of workmen when they ooad and handle formwork sets. While timber formwork could be laid on the ground carefully and lightly after stripping without being damaged, ignorant workers would usually toss it
onto the ground. Such an action would result in the splitting of timber ®bre both on the surface and at the edges which are usually the weakest parts of a formwork. Such damage to the formwork could incur more resources in repair before they could be reused, or could render them non-reusable at all in the subsequent concreting process. Therefore, working attitude is a crucial determinant to the reuse of timber formwork.
5.2. Eciency of workers (H2.2) Good co-ordination determines the eciency of formworkers. Uninterrupted work ensures that formwork is erected and stripped in accordance with scheduled timings with little delays in between. The numbers of reuse of the formwork sets could then be maximised throughout the project since lag times are minimised. Formworkers could only be ecient and eective when they are familiar with their work.
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5.3. Formwork stripping (striking) process (H4.2) Proper stripping of formwork depends on the releasing agent used and the skill of the workmen. Release (parting) agents that are applied onto the formwork forms a thin layer separating itself from the curing concrete. As there is no direct contact between the two components, little or no liquid concrete will cure in between wood grains. This enables the formworker to pry open the formwork with the minimum eort and at the same time preserves the formwork integrity. The removal techniques and processes adopted for the removal would aect the integrity of formwork. If suitable tools are not used and care is not exercised during stripping, timber and plywood pieces could be easily torn beyond repair. Whenever possible, air should be introduced between the contact surface of formwork and concrete ®rst so that the formwork are removed with the least diculties and without much damage. The importance of formwork stripping process in determining formwork reuse is also related to workmen having the correct attitude (H2.1). Workmen who have the right attitude are more likely to strip the formwork properly.
6. Conclusion This paper explored 15 sub-factors that have an impact on the reusability of traditional formwork. The ®eldwork showed that only three sub-factors have signi®cantly high impact on the reusability. The ®rst two sub-factors, `working attitudes of workers' and `crew eciency' relate to the factor of `workmen' who handle the formwork. The third sub-factor, `formwork stripping (striking) process', is also related to the workmen who handle the formwork. The conclusion is therefore that the workmen are the common denominator that determines the number of times formwork can be reused. The workmen need to have the correct working attitude and be able to work eciently, to ensure that the formwork has a high reuse. The formwork stripping process is also related to the working attitude and eciency of the workmen. Based on this ®nding, it may be possible to increase the numbers of reuse of timber formwork by motivating the workmen, instead of concentrating on the design or other factors. It is recommended that monetary reward be used to improve work attitudes and crew eciency. With a higher monetary compensation, workers have more incentive to execute their work correctly at the ®rst attempt. Eciency of erection and stripping of formwork is further enhanced if such rewards are directly associated with the workmen's performance.
From this study, it appears that other factors such as materials used to fabricate the formwork, design of the completed structure, and site management issues have little impact on the reusability of timber formwork. The limitation of this research is that the analysis is based on a small number of responses. The response rate is low, despite the mailing of reminder letters and telephone calls made to the ®rms. In addition, respondents may have dierent perceptions of the language used to describe the impact of each factor. For example, a `moderate impact' rating given by one respondent may not have the same degree of impact as another respondent's `moderate impact' rating. The ®ndings have two major implications. The ®rst is that it is possible to increase the number of times timber formwork is reused, leading to a reduction in trees felled which is advantageous to the environment. Contractors may also enjoy cost savings. The second implication is that contractors should concentrate on managing workmen better to increase the numbers of reuse of timber formwork, instead of trying to handle multiple factors that may also aect timber formwork reuse, but to a lesser extent.
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