UNDERSTANDING BLOCKWORK LABOR PRODUCTIVITY: THE FACTOR MODEL APPROACH

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1 UNDERSTANDING BLOCKWORK LABOR PRODUCTIVITY: THE FACTOR MODEL APPROACH U.E.L. de Souza and F. Carraro Department of Civil Construction Engineering, Polytechnic School, University of São Paulo, São Paulo, Brazil Abstract Managing labor productivity in construction sites is a hard task. First of all, budgets sometimes over or underestimate the resource demands, making the site engineering activity of controlling workhours consumption more difficult. Moreover, nowadays cost control is becoming more and more important to ensure project success. The Factor Model approach provides an accurate and non expensive tool to help construction managers. Based on short time daily collection and analysis of data, the methodology can be applied to different jobs, in particular to study blockwork. This paper discusses blockwork labor productivity based on an extensive database, collected during 163 workdays, from four building construction sites in Brazil. The research includes daily observation of work contents and contexts. The information is analysed in order to define and quantify the factors inducing labor productivity variation. The research also defines the potential labor productivity index in each site. Keywords: blockwork, construction management, labor productivity. 1 Introduction Labor cost plays a very important role in defining building construction expenditures. In Brazil, it usually reaches 50% of total building production cost [1]. In this context, managing labor productivity is an important task to be accomplished by the site engineer. But this professional is frequently very busy, having to perform several other tasks. Therefore, it would be very desirable to have an accurate and easy to apply methodology to measure and analyse labor productivity. The Factor Model [2] approach provides a helpful tool for construction evaluation. It can be adapted to different labor consuming tasks ([3], [4], [5]). It has been used in Brazil to understand blockwork, formwork, rebar and concrete placement labor productivity lately. The University of São Paulo has conducted a research in these topics, sponsored by SECOVI-SP (a Brazilian institute representing building developers) and eight medium size construction companies. This paper discusses blockwork labor productivity, using the Factor Model approach. Data was collected from four different construction sites. 2 Definitions Labor productivity is considered to be the efficiency in transforming workhours (labor effort) into quantities of masonry. This paper adopts the unit rate (UR) to measure blockwork labor productivity. UR is defined as follows:

2 UR = WH / BQ (1) where WH (workhours) gathers all the hours worked by masons and laborers; and BQ (blockwork quantity) is the quantity of layed masonry (normally expressed in square meters). Small UR values mean good productivity. One can define different types of URs accordingly to the period of time associated to WH measurement. Daily unit rate (dur) means daily quantification. Cumulative unit rate (cur) demands cumulative quantification of WH and BQ. After two workdays, for example, evaluating cur implies quantifying the WH spent cumulatively in days 1 and 2 (the same applies to BQ calculation). Potential unit rate (pur) evolves a slightly different concept. The idea ([6]) is to observe dur referent to several days, and to define a UR level considered to be non exceptionally performable. Mathematically, pur is evaluated calculating the median of durs smaller than cur (values above cur denote labor productivity problems) at the end of the workdays collection. The Factor Model ([2]) states that many factors cause disturbances to crew performance. Understanding labor productivity means detect the factors influencing labor performance and quantify their effects on it. 3 Blockwork features and data collection Blockwork includes all the site activities necessary to produce masonry, from material transportation from site storage till completion of masonry. Masonry can be divided in two main parts (Figure 1): first layer (FL), which consists on defining the correct position of walls in relation to the floor and laying the first course of blocks; and the remaining layers (RL), which consists on laying all other blocks, as well as lintels and other accessories. In this research, WH and BQ were both measured separetedely for FL and RL. This procedure also allows masonry (M) production performance to be discussed, considering FL and RL together. a) b) Figure 1. Masonry parts: a) first layer; b) remaining layers. The crew responsible for masonry production can be classified into (Figure 2): direct labor (DL), that includes people effectively laying blocks and helpers nearby; support labor (SL), including foreman, helpers moving blocks and mortar from site storage to the floor in execution.

3 Total labor (TL) represents the summation of DL and SL. In this research, workhours were collected separatedely for DL and SL. a) b) Figure 2. Different roles being played by blockwork labor: a) direct labor-placing masonry units; b) support labor-producing steel reinforced concrete lintels. Table 1 presents different UR types, generated by different approaches in collecting WH and BQ, that will be used in this paper. Table 1. UR types defined by different approaches in measuring WH and BQ. WH BQ Time frame UR type Information provided DL FL cumulative DL/FL/cUR DL cumulative performance in producing FL DL M daily DL/M/dUR DL daily performance in producing M DL M cumulative DL/M/cUR DL cumulative performance in producing M TL M cumulative TL/M/cUR TL cumulative performance in producing M 4 Collecting blockwork productivity data 4.1 Case studies Four building construction sites were evaluated. Table 2 shows the main features of the buildings, while Table 3 characterizes the blockwork job. Table 2. Building constructions under evaluation. Site code Masonry area (m 2 /floor) Use Structure frame (1) # of repetitive floors SP office SRC 16 SP apartment SRC 13 SP apartment SM 12 SP apartment SRC 18 (1) SRC = Steel Reinforced Concrete; SM = Structural Masonry.

4 Table 3. Blockwork features. Site Block material Block face size Crew composition Labor source (1) (cm x cm) (masons:helpers) SP 08 ceramic 25x25 13:6 SUB SP 17 ceramic 19x39 13:7 SUB SP 21 concrete 19x39 5:4 CONTR SP 34 ceramic 25x25 7:4 CONTR (1) CONTR = prime contractor direct employed; SUB = subcontractor hired. 4.2 Collected data Tables 4 to 7 gather field collected data and calculated UR for each case studied. Table 4. Site SP08: UR calculation. Workday Direct Labor Total Labor DL/M DF/FL/cUR TLM/cUR workhours Quantity Workhours dur cur (WH/m) (WH/m2) FL RL M FL(m) RL(m2) M(m2) (WH/m2) (WH/m2) * * * * *Data representing more than one day (used just for cumulative analysis purpose).

5 Table 5. Site SP17: UR calculation. Workday Direct Labor Total Labor Quantity DL/M DL/FL/cUR TL/M/cUR workhours dur cur FL RL M FL(m) RL(m 2 ) M(m 2 ) (WH/m 2 (WH/m) )

6 Table 6. Site SP21: UR calculation. Workday Direct Labor Total Labor Quantity DL/M DL/FL/cUR TL/M/cUR workhours dur cur FL RL M FL(m) RL(m 2 ) M(m 2 ) (WH/m 2 (WH/m) ) Table 7. Site SP34: UR calculation. Workday Direct Labor Total Labor Quantity DL/M DL/FL/cUR TL/M/cUR workhours dur cur FL RL M FL(m) RL(m 2 ) M(m 2 ) (WH/m 2 (WH/m) ) ,

7 5 Data analysis 5.1 Labor daily performance in each site Daily Unit Rate (dur) observation helps explaining factors that make productivity vary within a certain site. Table 8 providing some explanation for days with higher DL/M/dUR. Table 8. Daily explanation for higher DL/M/dUR. Site Workday DL/M /dur Explanation lift maintenance operations SP " " " masons turnover " " " SP " " " " " window frame assemblage rain work in top course SP " window frame assemblage rain work in top course " rework laying lintels; piecemeal work SP laying lintels " From Table 8 one can conclude that site SP08 has problems related to the transportation system. The lift is used to transport blocks and mortar bags. Days when disruptions affect the equipment yield very high dur values. In the site SP17, labor turnover is a major problem. This fact had strong influence over the dur. In the site SP21, days 1 and 10 are influenced by window frame assemblage in the walls; days 2 and 13 were disrupted by rain; and in days 4, 5, 15 and 16, masons worked in the last layer, which is a more difficult job. In site SP34, days when lintels are fixed yield poor labor productivity indexes. In site SP34 it was possible to track WH and BQ separately for two different gangs working independently. It was noticed that one of them performed better than the other one in a daily basis. The site engineer justified this trend detecting that the second crew was often disrupted by lack of tools and by its inexperience when compared to the first one.

8 5.2 Comparisons among sites Table 9 enables sites comparison based on different performance indexes. Table 9. Sites performance measured by distinct URs. Site DL/M/pUR DL/FL/cUR (WH/m) DL/FL/cUR DL/M/cUR TL/M/cUR SP SP SP SP Some conclusions can be drawn from Table 9 analysis: pur seems to be similar for every site; site SP21 presents a smaller value but this happens because pur is associated, in this case, to an easier part of the job (the lower layers execution); DL/FL/cURs, expressed in terms of linear meter, are closer to each other than if they are expressed in terms of masonry square meters; this happens because the effort spent in locating walls is significant compared to the laying blocks task; DL/M/cURs, expressed in terms of masonry square meter, indicate this part of the job to be harder than the RL part (compare to the DL/M/cURs values) DL/M/cURs appear to show similar performances, except in site SP17, where turnover worsened labor productivity; site SP08 presented the worst performance, measured by TL/M/cUR ; this is explained by the adopted transportation system (lift), that demands more helpers in the operation. 6 Conclusions Factor Model approach proved to be helpful in understanding labor productivity. It can help controlling day-by-day work, for example showing opportunities to improve conditions adversely affecting productivity (in SP34, the site engineer could provide better tools to the gang presenting poor performance). It can also help construction planning process; if a crane were adopted in site SP08, this would represent US$ 4,032 economy per month (the contractor and subcontractor evaluated that they can advantageously share crane rental cost in a next similar job). The approach is also extremely important to improve budgeting process, providing more confident information about expected performance. 7 References 1. Revista Construção São Paulo. São Paulo-SP, Brazil, PINI, No.2641, Sep Thomas, H.R., Yakoumis, I. (1987) Factor model of construction productivity. Journal of Construction Engineering and Management, ASCE, Vol.113, No.4. pp Sanders, S.R., Thomas, H.R. (1991) Factors affecting masonry-labor productivity. Journal of Construction Engineering and Management, ASCE, Vol.117, No.4. pp Souza, U.E.L., Thomas, H.R. (1996) The use of conversion factors for the analysis of concrete formwork labor productivity. Managing the construction project and managing risk CIB W-65 The organization and management of construction: shaping theory and practice 8th International Symposium, E. & F.N. Spon, London, pp Carraro, F., Reis, P.F., Souza, U.E.L. Produtividade no serviço de concretagem (Productivity on concrete placement service). Paper to XXVIII Jornadas Sul-Americanas de Engenharia Estrutural, São Carlos-SP, Brasil, Vol.6, pp Souza, U.E.L.(1998) Produtividade e custos dos sistemas de vedação vertical. Tecnologia e gestão na produção de edifícios: vedações verticais. PCC-EPUSP, São Paulo, pp