Process and quality improvement using Six Sigma in construction industry/Proceso tobulinimas ir kokybes gerinimas statybu sektoriuje taikant "Sesiu Sigma" metoda.
Tchidi, Megan Florent ; He, Zhen ; Li, Yan Bo 等
1. Introduction
In construction industry, customer requirements are constantly
increasing. Clients are considered as the most important stakeholder in
every construction project (Idoro 2010).
Thus, an important subject in the refurbishment market is how to
create and transfer the real value required by the customers, and
promote SQ to win customer satisfaction. In the new economic times with
fast growing information transfer and emergence of new competitors, many
enterprises set business targets on "improving the quality of
service and seeking customer satisfaction" (Huang, Hsueh 2010).
The transfer of a maximum of activities from the building site
towards a factory becomes inevitable. Many researches (Alinaitwe et al.
2006; Blismas et al. 2006; Goodier, Gibb 2007; Tam et al. 2007a, b;
Jaillon, Poon 2009; Pasquire et al. 2005; Ko 2010; Polat 2010) have
shown the advantages of prefabrication in the construction. Arditi and
Mochtar (2000) have showed that the increase in the use of prefabricated
components in the building industry would contribute to higher
productivity and easier management in quality issues and project
control. The use of prefabrication and system formworks were not the
only means of reducing construction waste. For Jaillon and Poon (2008),
Jaillon et al. (2009), the adoption of waste management plans and the
trip ticket system were important.
In the past, the quality concept was based on construction drawings
and the need to satisfy the required standards. But now, satisfaction of
required standards is not enough for quality in the construction
industry. Also, it has to satisfy and even please the customer. Idoro
(2010) had established that clients consider project quality as the most
important factor in the award of contract. In short, quality is
unceasingly moving to a higher developmental manifestation. According to
Navon and Goldschmidt (2010), many opportunities to optimize resources
are missed due to the lack of real-time, up-to-date information. Labor,
in particular, is a very important resource and it is very difficult to
collect real-time data relating to its on-site performance. Measuring
labor performance manually is labor intensive, inaccurate and error
prone. This is why it is not done on a regular basis.
Banuelas et al. (2006) believed that the prioritization of projects
is determined by many criteria, such as a cost-benefit analysis or the
Pareto priority index. But more and more construction projects are
affected by poor technical quality and poor management teams.
Rehabilitation and strengthening of reinforced concrete structures is a
dynamically growing division of structural engineering. Strengthening of
the reinforced concrete structures is one of the most difficult and
important tasks of civil engineering. Individual approach to the problem
is a necessity since any ready-made solution can be applied. One of the
prime objectives is to provide detailed technical and cost-effective
analyses (Kamifiski, Trapko 2006).
It is therefore important to improve the performance quality which
is very necessary for the construction industry. The Construction
Industry Review Committee (2001) reported that although many quality
management systems and philosophies (quality control, cost of quality,
total quality management quality assurance) have been adopted and
implemented by many organizations, serious problems can still be found
on construction sites and the desired quality standard has not always
been reached. Love and Li (2000) found that without an effective quality
cost system in place, performance improvement can be very difficult to
identify and measure. The quality of products or services does not only
focus on their ultimate delivery, but also on the quality of the whole
business process. Tam et al. (2008) said that, to provide optimum
benefits, companies should apply the most reasonable and cost effective
method to achieve client satisfaction. According to Enshassi et al.
(2009), materials in project and availability of personnel with high
qualifications strongly affect the quality performance of a project.
Besides, the performance of the client is important because any decision
made will affect project success. Failure on the part of the client
might lead to stress factors causing significant problems in successive
stages of the project. The client plays an important role in the
procurement of construction activities and in the implementation of
construction projects (Egbu, Ilozor 2007; Alinaitwe 2008). Therefore,
the construction industry pursues a perfect management philosophy
application to have an internal requirement for Six Sigma. Six Sigma was
used in the manufacturing industry to reduce the wastes due to
manufacturing process deficiencies. It is now used by almost all
industries including the construction industry (Pheng, Hui 2004;
Stewart, Spencer 2006; Han et al. 2008). For Banuelas et al. (2005), Six
Sigma has been considered a powerful business strategy that employs a
well-structured continuous improvement methodology to reduce process
variability and drive out waste within the business process using
effective application of statistical tools and techniques. It is a
quality philosophy at the highest level, relating to all processes, and
a quality measure at the lowest level (Koch et al. 2004; Rajagopalan et
al. 2004). Its programs improve operational performance in order to
enhance customer satisfaction with a company's products and
services. Schonberger (2008) and Chakravorty (2009) have pointed out
that the objective of Six Sigma programs is to create a higher perceived
value of the company's products and services in the eyes of
customers. Six Sigma programs have performance metrics and measurements
based on cost, quality, and schedules (Keller 2004). Six Sigma
implementation uses a systematic procedure; a five-step DMAIC (Define,
Measure, Analyze, Improve, and Control) methodology. Its principles are
a problem-solving framework. In DMAIC each stage is based on a data
analysis research. A detailed description of DMAIC methodology can be
found in Pyzdek (2003) or Keller (2004). Project selection and
prioritization is an important element of Six Sigma programs. Many
studies in construction only focus on the DMAIC of the Six Sigma
methodology. They did not mention the Design for Six Sigma process to
design or to redesign process or quality. According to Harry and
Schroeder (1999), the only way to surpass the five sigma quality levels
is to redesign their products, processes and service by using the DFSS.
But this approach is difficult to apply due to lack of data to support
the claim and the absence of assumptions used to formulate it (Banuelas,
Antony 2004). It is difficult to evaluate whether their work is
applicable in other industry than manufacturing. Design for Six Sigma
(DFSS) is a systematic methodology based on Six Sigma for designing or
redesigning products, services, or processes to meet or exceed customer
requirements and expectations (Laudati 2007).While Six Sigma works to
improve existing processes, DFSS starts at the early stage of the
project, design, and development of products and services. The DFSS
methodology uses a "roadmap" to guide the progress through
each project. As written by Goffnett (2004), Schroeder et al. (2005), Zu
et al. (2008), although there have been numerous case studies,
comprehensive discussions, books and websites addressing Six Sigma, very
little scholarly research has been conducted on Six Sigma and its
influence on quality management theory and application. Especially in
the construction industry, project managers must often face unexpected
problems. The proposed solution usually cannot be followed as in the
production phase. The management flow is influenced by many factors. The
achievement of many works is very difficult to measure using explicit
data. When evaluating achievement, and starting an improvement activity,
the biggest obstacle is fuzzy data and materials. Therefore in the
project management field, using Six Sigma management method competently
is definitely difficult. The construction process is an outdoor activity
that is affected by external conditions and is less repetitive than the
manufacturing process. Simple and repetitive construction activities in
which productivity is immediately reflected by the process variation
factors helps to meet quantity for cost effectiveness (Han et al. 2008).
The members of the construction team (architect, engineer and
contractor) usually change with each project. In contrast to
manufacturing, the product of construction (i.e. the built facility) is
stationary, while the production facilities are mobile. The vast
majority of individual construction firms is small and designs or builds
are limited amount of facilities (Ilias, S0ren 2000). How do we apply
Six Sigma management methods in the construction industry to obtain a
suitable and reliable quality improvement, cost, resources, time
reduction and customer satisfaction? How to combine Six Sigma with
finite element method?
2. Methodology
The goal of this study is to demonstrate the potential of applying
Six Sigma to the construction industry. To attain this goal, a
systematic methodology was proposed in this study. The proposed approach
falls into three phases. The first step was to document the process,
productivity and quality with relevant data through interviews with the
company staff, Six Sigma black belt consultants, construction managers,
engineers, clients and architects. Training materials, minutes of
project meetings, management culture and presentations were collected
and analysed in order to supplement the interviewee response.
The second phase is about DMAIC process. The goal of this phase is
to improve and optimize construction process and quality based on five
steps. The research has defined and applied the five steps according to
the construction work process:
--Define: Organize goal clarity, state opportunity, form the
project team, analyse SIPOC, recognize the current process.
--Measure: Define the measure indicators, collect data, seek for
the variation source and determine the current process Sigma level.
--Analyse: Analyse process data, analyse the causes of potential
problems, the nature and its impact.
--Improve: Identify problem-solving and process improvement
programs, define new standards and assessment process.
--Control: Control method, review performance on a regular basis,
expand the quality of follow-up, and improve process standardization and
documentation.
After these five steps, the paper has conducted an evaluation to
determine the Six Sigma Quality Level. If Sigma level is inferior to 6,
then the DFSS procedure will be developed using the DCOV (Define,
Characterize, Optimize and Verify) approach. The third phase is the
DCOV. The goal of this phase is to avoid, to prevent defects since the
prefabrication of composite components (beams, floor, etc.) and to
redesign when the expected Sigma level is not reached. The four steps of
the DCOV are:
--Define: Describe project objectives, compose multi-functional
design project team, collect customer requirements and define project
objectives and processes.
--Characterize: Propose and screen concept for the technical
requirements in accordance with the concept of steel-concrete beams and
then establish preliminary design.
--Optimize: Conduct optimal design and implement process documents
and product life-cycle cost-optimization. Monte Carlo simulation is used
in this phase on a computer to simulate the production of steel-concrete
beams according to the initial design and simulation of different design
elements in the scope of their respective values. The risk analysis
software OptQuest of Crystal Ball tool is used to optimize
steel-concrete beams design. The finite element (ANSYS) model is then
established in order to verify the technical and scientific requirements
of proposed model and to be able to compare the results to the case
study one.
--Verify: Verify process and product quality, and then finish the
requirement document. The feasibility is validated and the scorecard
tools are used to record the process capability indicators of the pilot
production. The model provides a methodology for quantifying the changes
required in a design or redesign to meet the desired performance
outcome.
This paper focuses on the following objectives:
--investigate construction process and quality problems in
construction;
--review the theory of Six Sigma in construction industry;
--apply prefabricated composite structure combined with Six Sigma;
--redesign steel-concrete components using DCOV;
--show the effectiveness of Six Sigma in improving construction
process and quality.
In order to prove the validity of the developed model, a real case
study based on construction quality and process is presented to resolve
concrete cracking and slippage problems. The approach is then applied to
steel-concrete composite structure to improve construction process and
quality and to reduce construction materials. Three composite
steel-concrete beams were tested by investigating their strength and
crack limit by using finite element method. It combines composite
structure system with finite element method (ANSYS) based on Six Sigma
management in order to produce high quality construction projects and to
prevent construction failure. Comparisons between the finite element
results and experiment results have been conducted. An economical and
high quality prefabricated composite beam model is proposed.
The application of Six Sigma to improve construction process and
quality and to reduce the reworks of the main structure based on
statistical analysis during construction is explained.
3. Construction process management model
The construction industry is one of the most complex industries.
The quality of the engineering process plays a pivotal role in the
production of a high-quality construction product. This issue is
increasingly gaining consensus. However construction engineering is a
very strong creative design labor result. The construction process
involves human input, methods, tools, management and many other aspects.
The other stage of the construction life cycle, like the design, code
and other links influence the existing factors of construction quality.
But when the construction bug is found late, the rectification price
will be high. Modern quality management puts emphasis on the quality
during the process, rather than testing the product processes. If we aim
to improve construction quality, design process quality must be
enhanced.
Therefore, improving only the quality of the project is not enough.
We must also make continuous improvement to the entire construction
development process, discover in time process quality problems, and
eliminate problems before they occur. Six Sigma management is just such
a system. It focuses on the process resulting in continuous improvement.
From the researches of Pheng and Hui (2004) and Schwalbe (2006), in Six
Sigma, the definition of quality is broadened to include economic value
and practical utility to both the company and the customer. In the
process of construction industry projects, Six Sigma is a quality
management theory.
The purpose of project quality management is to ensure that the
project meets its requirements by applying the theories of Six Sigma,
which does not consists not having defect, but pursues the perfection.
It has a fixed position, a production flow, various structures, various
quality requirements, various construction methods, high integrity and
long construction cycle. Harry and Schroeder (1999) have used the DMAIC
method to conduct the quantification of construction flow analysis and
found the most essential factor to improve.
Six Sigma management DMAIC flow utilizes statistics and
quantitative methods. It explores the process and identifies errors and
defects. It uses powerful analytical tools of quality management to
carry out the system analysis of problems and the exploration of the
causes of problems. Thus the DMAIC urges the organization to take
improvement measures promptly to enhance the entire process of quality.
Before the team starts to operate, the DMAIC project regulation
must first have the authorization of project initiators and other senior
leadership. As a benchmark achievement activity of the define phase
team, the project regulation has directly reflected the quality of the
project team activity, directly related to the project progress and
DMAIC process implementation.
[FIGURE 1 OMITTED]
The define phase must set up the implementation of the project team
of the DMAIC construction process flow.
Although the control stage is the final stage of DMAIC flow, the
project team must maintain the key stage of its construction process
improvement results so that after the construction optimization process,
it can maintain stability, predictability, and conform to the
client's requirement. The project team needs to establish the
document surveillance process, a formulation process management plan, to
consolidate the realized achievements. At the same time, the process
change must be paid attention to. The measurement and monitoring have to
be continually carried on. The process corresponding adjustment has to
be carried every time the customer needs a change. According to Zhao et
al. (2008), Six Sigma quality management tools and methods are important
supports. Application of these tools and methods can help organizations
to improve the efficiency of identifying problems and analysing the
causes. In order to holistically improve construction process quality, a
process modeling based on DMAIC and DCOV is established and shown as
following in Fig. 1. The model is divided in two main parts: the DMAIC
(Define, Measure, Analyse, Improve, Control) to improve the problem
identification and analysis performance, the prefabricated composite
components construction based on DFSS which has used DCOV process for
design optimization and prevention of errors.
4. Case study
The design of specimens (ZL1, ZL2 and ZL3) for the experimentation
is referenced to a prototype of steel reinforced concrete beams of
Xin-chun building in Tianjin (China). The test beams are in rectangular
section of 120^240 mm and a span of 3300 mm. Tianjin Xianyi Construction
Technology Co. Ltd., according to the specimen model design drawn,
produced concrete beams and steel beams which were sent to the
structures laboratory of Tianjin University. Six Sigma methodology has
been divided into five stages to realize project quality optimization
and to enhance customer satisfaction. In this project, Six Sigma quality
management theory analysis (FMEA) method has been applied. Various
quality problems have been analysed through the quality production
process. This research, after adopting the design and technical
correction measures, finds out the factors which influence the project
quality and the reliability of potential quality problems. It improves
the project product quality and resistance to various performance
characteristic abilities. In view of the project characteristic, it
carries on the control improvement of the key technique which affects
the composite structure quality to realize an excellent construction, in
order to enhance the satisfaction of customer. The construction shows
cracks in the steel reinforced concrete composite beams development
situation, and steel reinforced concrete composite beam failure modes.
During the experiment, the location of the supports and
cross-arrangement of dial indicator measuring specimen deflection were
at 650 mm from the end of beams. There is also the sample layout dial
gauge measuring the interface slip.
Furthermore, in this project, the paper applies the FMEA method of
Six Sigma quality management theory. The quality control process method
is often referred to as D-M-A-I-C.
4.1. Process for implementation of Six Sigma management method in
construction industry
Define
According to the quality survey standards of the produced concrete,
three specimens of steel concrete beams have been selected for research
analysis, using statistical methods. Quality problem tests were also
conducted and the summarized data are shown in Table 1.
Reinforced concrete structures are being used much more during
constructions. The entire construction quality, the structural security,
construction cost as well as customer satisfaction depend on the
concrete quality. Steel and concrete have been intensively and
extensively used as structural materials for bridges, buildings, and the
other structures, in civil and architectural engineering. The
theoretical analysis of the actual behavior of modern structures from
high strength concrete and steel are more frequently employed during
design, realization and the utilization of building structures. The
combination of steel-concrete materials gives rise to the occurrence of
special phenomena whose influence on structural reliability isn't
at present commonly implemented in design (Kala et al. 2010; Gailius,
Kinuthia 2009).
Construction process also influences time and cost. Therefore
during construction enough attention must be paid to concrete quality
and to the construction process. Due to the fact that in construction
customer requirements are variables, the Quality Function Deployment
(QFD) method has been applied to transform the customer request into the
technical and quality control request. The deployment method has been
taken according to the quality function of the customer requirements. To
preserve construction integrity, concrete quality objectives will be to
guarantee that cracks and slippage won't appear. To avoid damp and
to preserve the building integrity, concrete quality objectives are to
guarantee that no leakage will appear. Currently, the construction
process in this industry is still traditional, with labor intensive in
sittu construction techniques. That implies a lot of material (cement,
sand, gravel, etc.) waste. This kind of condition increases the economic
cost of the enterprise and affects the level of confidence of the
enterprise. Quality management and control is not rigorous, construction
quality is poor with attendant high costs. Strengthening various stages
of control of the construction process will guarantee that all the
components satisfy the prerequisite conditions as design demands. That
will control the quality efficiency; and improve the processes which are
essential to the development of the enterprise. Therefore a fundamental
analysis must be done to determine and to control the factors which
affect concrete crack, slippage between concrete and steel.
The main Six Sigma team is composed of: team members, client, Black
Belts, Green Belts and other members: organizations which improve
implementation measures and data collection.
[FIGURE 2 OMITTED]
Measure
The measure stage goal involves the choice of quality
characteristics to determine the critical to quality (CTQ), to define
the standard results and to measure the system analysis.
According to the site construction investigation and based on
statistical method, some specimens have been selected to conduct
analytical studies by using statistical method. The statistical method
adopts the cause and effect diagram (CE) in Fig. 2. This paper carries
out construction process and quality improvement analysis using
prefabricated composite structure based on Six Sigma.
The process monitoring method is defined, the data are collected
and all the problems are found. The statistical crack damage frequency
and slippage damage are found. This paper:
--analyses prefabricated composite beams quality problems;
--investigates the current raw material control method through site
control and related person interview;
--conducts the statistics and analysis of composite concrete cracks
and slippage;
--analyses the reasons why the mix ratio test is insufficient;
--utilizes finite element method (FEM) to propose an appropriate
model.
Analyse
During the prefabricated composite component production process,
the component appearance quality is not stabilized. There are more
shrinkage cracks, slippage and other problems as enumerated in Table 2.
Through investigative analysis and based on Table 2, the
prefabricated composite component shrinkage cracks, slippage, and
smoothness occupy a larger part of quality problems, followed by
interface connection and comprehensive reflection for appearance quality
problems.
The analysis stage includes the establishment of process capability
and definition of the end results. Through the five aspects for which
the measures have been utilised, the application of brainstorming
conducts the analytical study of the reasons and degree which affect
concrete quality (see Table 3).
According to the key prefabricated component appearance quality
problems and through the system analysis diagram (Fig. 2), the main
causes of quality problems have been found.
From the analysis of the quality control of prefabricated composite
components main reasons are as follows:
--insufficiency of technical training;
--insufficiency of steel stiffness;
--some problems with laboratory test;
--some problems with the entering field material control;
--the establishment of work shed maintenance;
--unreasonable concrete mix;
--weather and temperature influence.
Through the concrete quality problems and the slippage, we conduct
an analysis (FMEA), measurement and find that the key problems which
affect the concrete crack are the cement variety, the mix proportion of
concrete, the admixture, the construction technique, disposable
formation as well as the maintenance conditions.
Improve
In view of the above analysis of concrete crack and slippage
problems, through reasonable measures and methods, the following
measures have been adopted.
When selecting concrete raw material, the coarse and suitable
aggregate must adopt a continuous grading; the fine aggregate adopts the
medium sand suitability. The concrete joins the retardant, the water
reducer, the slag powder and so on, to reduce the cement volume dosage.
The hydration heat used on the cement should be as low as possible and
adopt long setting time cement. During construction, the paper either
discharges leakage or facilitates mixing of dry and hardened concrete
where leakage occurs. Construction continues when this operation is
complete.
The prefabrication system uses equipment of proportioning and
mixture controlled by computer. Additives are added in order to obtain
specific mechanical performances. The placement and compaction of
concrete are performed with suitable equipment. Protection is provided
when climatic conditions are unfavorable. The quantity of mixing water
is reduced to its strict minimum and the compaction as well as the
curing of the concrete is carried out using controlled systems. The
resistance of the concrete can thus be adjusted with precision to the
specific applications for which it is intended.
If the workers are inexperienced, machinery is used for
prefabrication. The skills of the technical director and the production
chief are not at the required level. So training and site visit will
enhance the technical operation experience. The provision for training
and education among staff and involving employee participation are more
effective approaches in implementing waste management (Shen, Tam 2002;
Lingard et al. 2000).
The use in exchange of manganese steel to replace the ordinary
carbon steel and the manufacture extrusion steel is changed into a rigid
mold.The research improves the laboratory control at the work site and
analyses the reasons why the mix ratio test is inadequate. The best mix
proportions are presented in Table 4 and help to guarantee that the mix
ratio of the prefabricated concrete is satisfactory.
The moisture and water contents of the sand and gravel are
controlled to avoid reduction of concrete cohesive force. The paper also
works to avoid the water content which can influence the mixture
water-cement ratio and controls the mud impurity contained in the sand
and gravels.
Ventilation and conservation of the environment of the
prefabricated elements have been improved. The paper allows an extension
of work shed, meets the required conditions for prefabricated product
stack, and avoids a high temperature exposure that can create cracks.
The quality of prefabricated products is not influenced by environmental
factors. Concrete prefabrication offers many more answers to the
requirements of suitable construction than the other methods of
construction.
The study of Arditi and Gunaydin (1997) and Tam and Le (2007) have
shown that quantities of resources are wasted each year because of the
inefficient or non-existent quality management procedures.
Control
The goal of the control stage is to guarantee the process
improvement and not let the situation revert to the previous situation.
The quality control of prefabricated composite components is based
on a system of self-checking under the surveillance of a third party.
The control of the production in factory includes regular procedures,
instructions, inspections, tests and the use of the results for
equipment control, raw materials and other materials, production process
and products. The results of the inspections are included in registers
put at the disposal of customers: (a) the concrete compressive strength
is determined based on the 28-day compressive strength prior to any
evaluation. The approved concrete strength is achieved using
mathematical statistics method and must be conformed to qualified
requirements; (b) the components have to be smooth, dense and solid,
with no voids and warped appearance; (c) the exterior size of
construction is within allowable design specifications.
After the implementation of all measures, the quality testing
investigation indicated: the implementation improvement measure, the
extrusion forming component in various cracks, distinct improvement of
the smoothness and the component size aspects, achievement of the
expected effects (Table 5).
After strengthening the control goal, consolidating the
achievement, we guarantee that the improvement will be maintained and
the slippage will not be high, the concrete crack will not exceed 0.2
mm. This helps to improve the project quality and enhances customer
satisfaction.
4.2. Process implementation of redesigning steel-concrete
components using DCOV
After adopting Six Sigma approach, we found that the Six Sigma
levels were not reached. Then the paper decides to redesign processes
and the steel-concrete beams. In order to produce high quality
construction through continuous improvement implementation, products,
processes and services were redesigned by applying DFSS to surpass the
five sigma quality levels. It studies Design For Six Sigma model,
combines steel-concrete beams materials design and production
characteristics and proposes a unique design model of steel-concrete
beams.
DFSS, a non-defect product and process design method, is based on
concurrent engineering ideas. It takes full account of customer needs
from the earlier stage, using system approach to solve problems, to
realize product/process design robustness. Then it improves quality,
develops speed and reduces product life-cycle costs. It helps to solve
product and process design issues. DFSS common models include: DMADV,
IDDOV, ICOV, etc. Although these models are varied, each has its
features, but the basic idea is similar. They take design process as the
main thread. Each step uses system approach to optimize, evaluate, and
reduce design defects and changes. According to the design
characteristics of steel-concrete components, we summed up suitable
design for DFSS model using the DCOV which is Define, Characterize,
Optimize and Validate.
Define
The main task of the Define phase is to describe project
objectives, compose multi-functional design project team, collect
customer requirements and define project objectives and processes. It
determines the scope of the project and starts the project. Analyse
important points and difficulties of product and process design of
steel-concrete components.
Project objectives
The objective of this project is to develop prefabricated
steel-concrete beams which satisfy customer requirements in a
competitive market under existing production equipments conditions
(specific price, parameters, etc.). The research designed and
constructed reinforced concrete beams. In the laboratory, chemical
anchors and bonded steel technology were used to link the concrete and
steel beam to form a composite beam. Since then, the laboratory has
completed a follow-up of paste work using strain gauges. The sectional
form of the reinforced beams is shown in Fig. 4.
Customer requirements analysis
Starting from project quality requirements, this paper first
identifies functional requirements of customers. Then it links up the
functional requirements with products or service characteristics and
further identifies the project product or service technical parameters
according to functional requirements and product characteristics
relation matrix, as well as the correlation between product
characteristics matrix. Through quality planning, the technical
parameters of the design specifications are identified.
[FIGURE 3 OMITTED]
The data of prefabricated component problems are collected during
12 months. The statistical results of the Sigma level are shown in Fig.
3.
Determine project scope
SIPOC tools from a macro point of view describe and examine the
organization's processes and guide the team members to improve the
attention focused on the scope limitation in order to avoid unnecessary
expanded project study scope. SIPOC tools application can determine the
project study scope of steel-concrete beams development as new product
development process, namely, product design and production process.
Characterize
The main purpose of characterize phase is to propose and screen
concept for the technical requirements in accordance with the concept of
steel-concrete beams and then establish preliminary design.
Conceptual design
The performance of steel products is determined by its internal
microstructure decision to meet the required yield strength. There are
three possible structural designs. We select the composite beam design
using concrete, reinforced bar and steel beam.
Material performance test provides the theoretical analysis of
necessary material parameters. The follow-up analysis was important. The
test material is divided into three parts, sheet steel beams, steel bars
and concrete.
Proposed design parameters
According to the conceptual design conclusions, we determine the
ideal steel-concrete structure, while the structure of steel products
(reinforced bars and H steel beam) depends on the ingredients and
process decisions. We use brainstorming tools to determine the
composition design and technological design of prefabricated
steel-concrete beams.
Proposed process parameters
Product design parameters are decided by the process control
parameters. According to the design parameters of steel-concrete beams,
we use brainstorming tools to determine the steel-concrete beams process
parameters.
Proposed preliminary design
According to the conceptual design of steel-concrete beams, we
select composite products with same strengthening mechanism, but the
performance indicators requirement is below to the one required to
conduct the regression analysis of historical data. Based on the
principle that P < 0.05, the paper obtains the performance transfer
function.
[FIGURE 4 OMITTED]
This in order to obtain required performance indicators of
steel-concrete beams, namely, to increase the yield strength,
compressive and tensile strength without reducing elongation index.
According to mature product design, we propose preliminary design
steel-concrete beams (see Fig. 4).
Optimize
The main objective of optimization stage is based on the determined
transfer function of the previous stage to conduct optimal design and
implement process documents and product life-cycle cost-optimization.
Monte Carlo simulation and pilot production
Monte Carlo simulation is used on a computer to simulate the
production of steel-concrete beams according to the initial design and
simulation of different design elements in the scope of their respective
values. When the critical performance factors changes, Monte Carlo
simulation verifies the feasibility of the initial design. The most
important is that it reduces the risk of industrial pilot production.
The initial design can satisfy the requirements. However, the
performance indicators of process capability still need to be improved.
According to the preliminary design of small-batch pilot production, we
obtained steel-concrete data, and revise the performance transfer
function, to obtain steel concrete beams performance transfer function.
Optimal design
The risk analysis software Opt Quest of Crystal ball tool is used
to optimize steel-concrete beams design, this principle is based on the
minimum of whole scale of mixing elements principle (lowest cost). The
software finds a reasonable combination of different composite elements,
and implements performance indicators to meet the design requirements.
At the same time, it gives the target value and tolerance of design
elements. Finally, we obtain steel-concrete final design, performance
indicators for process capability scorecard.
According to finite element model mesh features, the finite element
analysis model for ZHL1-ZHL2 is selected at 600 mm from the end of the
anchor bolts data, the model ZHL3 is selected at 550 mm. Reinforced
concrete and H Steel Beam respectively uses SOLID65 and SOLID45 entities
unit to establish an independent separate model, as shown in Fig. 5.
The boundary conditions constraints and loading beam model status
of finite element analysis process are shown in Fig. 6.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Verify
According to the optimized design, the research has conducted
construction pilot production to validate its feasibility and use the
scorecard tools to record the process capability indicators of the pilot
production. The results showed the optimized steel-concrete beams
performance stability and the process capability sufficiency. This study
identifies potential needs of customers such as welding performance and
fatigue performance which are very regulatory test items. It uses
commission inspection qualification of random sampling of testing
detection of third-party.
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
During the early stages of specimen ZL1-ZL3 model experiment
process, measure of interface slip of H-beam-concrete composite is
conducted, and recorded during loading process, the location and the
interface slip value. In the finite element (ANSYS) model of beam
post-processing, the interface is selected through the spring unit and
view during the entire loading process unit slip value under arbitrary
load. We define the appropriate unit to extract the corresponding data
table, draw at any desired location the load-slip curve, the interface
of the relative slip along the beam cross-distribution curve, in order
to analyse slip characteristics. Then we draw the load-slip transfer
comparison curve as shown in Figs 7-9.
5. Discussion of the results
The specimens ZL1 and ZL3 failure under positive moment began to
receive the crushed concrete compression. This time the plasticity
development of the steel beam is not yet obvious, and the specimen
failure has certain suddenness.
The concrete of specimens ZL2 under negative bending is in tensile
deformation, cracks appeared earlier. With the increase of load, crack
develops, the concrete gradually withdraw work, and the specimen bending
moment is mainly bear by the reinforced bars and concrete.
Because the point is more dispersed, the phenomenon is not obvious,
but then we can see that the reinforced bars strain in the middle of
concrete slab is bigger than the reinforced bars strain in the concrete
slab edge. It is visible that steel beams and concrete slab work
together, but because of shear lag effect, strain of reinforced bars in
the concrete slab varies with position. But although close to the
ultimate load, the longitudinal reinforced bars in slab edge work well.
It can be seen that the strain distribution showed changes in wave
shape (ZL2). The reason is because concrete slab surface cracks, span
multi cracks, caused by the randomness of cracks. Although close to the
limit load, the edge of the plate concrete nominally participates and
work well. Although being involved in the whole cross-section flexural.
The comparison of specimen bearing capacity before and after
strengthening based on Six Sigma shows that the shear carrying capacity
after strengthening the reinforced concrete composite beam specimen
based on Six Sigma compared to the one before (without Six Sigma) has
substantially improved due to the concrete part quality. But not the
whole cross-section steel beam web yield. Therefore, the shear bearing
capacity of reinforced concrete beams before strengthening and the one
after strengthening when cracking occurs have been compared, as shown in
Table 6.
Steel reinforced concrete composite beam specimen load-bearing
capacity has greatly increased than before strengthening concrete beams.
The comparison of flexural bearing capacity before and after
strengthening is shown in Table 7. In the table, [M.sub.u] is based on
the actual strength of materials to be substituted into the standard to
obtain concrete beam flexural capacity.
Test data shows concrete beams through the shear connector with
steel beams combination. The composite beam shear and flexural capacity
have increased compared to the original concrete beams. And the flexural
bearing capacity has increased more than the shear bearing capacity.
Therefore, the composed beams after strengthening may appear
"strong bending weak shear" condition. Analyse steel
reinforced concrete composite beam shear properties has great
significance.
A three-dimensional finite element model can reflect reinforced
concrete slab of the beam, stud shear connectors and steel beam geometry
properties and material nonlinear work situation. In order to show that
the nonlinear analysis of composite beams bring all degree of the shear
connection number, the action of concrete slab when analysing reduce the
space between analysing and bending and shear strength has been taken
into account.
The maximum ultimate shear strength provided by the finite element
analysis results is 786 KN. The 1996 AS2327 standard specifications
maximum ultimate shear strength is 439 KN. The maximum shear strength of
composite concrete slab has increased of 80%. The vertical shear
strength of composite beam increases as soon as the degree of shear
connection increases. Based on experiments and nonlinear finite element
analyses, the concrete slab combined with composite action has great
impact on the vertical shear strength of composite beam. Liang et al.
(2005) also came to a similar conclusion on their study.
All different shear force combinations obtained from the finite
element analysis influent next ultimate shear strength of the composite
beam. We can see that the vertical shear strength follows shear force
combination increase and decrease, which also confirmed by the research
experimental findings of Donahey and Darwin (1988). This indicates that
the composite beam arranges sufficient shear connector.
We can see that the vertical shear strength of the composite beams
for arranged shear connector is higher than the one which does not have
composite effect. From three groups of test beams and the corresponding
finite element model beam curve comparison, the following features are
obtained:
--in the initial loading, interface slip and load showed similar
straight line. Sliding and the load growth were slow. When the load
reached their limit after more than 70%. Non-linear characteristics
began to appear and the slippage suddenly becomes large. The growth
speed significantly is faster than the load. After reaching the limit
load of 90%, the sliding rate growth achieves the maximum of amplitude
as shown in Figs 7-9;
--because of site test conditions characteristics, the measurement
of both right and left side of simple beam sliding data is different.
The finite element model of beam entity model and the loading conditions
are strict symmetrical in order to have the same result for cross-axis
on both sides. It can be seen that through the curves comparison,
slippage curve of the finite element and all experimental measured
results curve results trend are similar (Figs 7-9). The slippage in the
interface H steels-concrete increases with the load but remains low
throughout the test, mainly because of the high density of studs:
0.1-0.8 mm maximum in the elastic and the plastic range. To ensure
sufficient joint action between H plate steel and concrete, shear
connectors must be installed in areas of introduction of concentrated
loads. The key problems which affect the interface sliding are: used
glue, chemical anchor, shear connectors, concrete smoothness and
construction techniques.
Based on the finite element model of beam ZHL1-ZHL3, the location
of the selected sliding, the interfacial slip was measured. Through
curves comparison, under the same load, finite element slippage is
greater than the amount of the experimentation value but smaller than
the one with Six Sigma. This basically is conformed to the theory: the
more the distance between the beam extremity and the position where load
is applied is closer, high is the slippage. The gap slippage on both
sides of the cracks increases throughout the test. This is shown in Figs
7-9.
The Design brings full mobilization of shear steel beams and
concrete flange in the middle of the steel-concrete composite beam can
enhance the shear strength compared with steel beams. Composite beam
with shear span ratio is 2.0 or less, more fragile occurred shear
failure. Ignores concrete shear Distribution Conservative approach of
testing, confirmed that the current specification underestimated the
shear strength of composite beams, due to this fact. The model provides
a good estimated value for shear strength of concrete slab. Only when
under the positive moment, the effective shear zone of composite cross
section only has a small amount of longitudinal reinforced bars. It is
expresses by connecting the composite beam with regular plate stud pull
out strength force.
The proposed model design strength and results with finite element
analysis results and under Six Sigma effect are compared. We can see a
difference between the experimental value of design model and finite
element analysis results.
The results showed that the steel-concrete beams fully meet
user's requirements and ensure that the production volume is
controlled according to the optimized design. From the comparison
between the estimated data of the model and the obtained data of the
experimentation, the benefit gained from the model has been discussed. A
certain extent reflects that the finite element model of this paper can
be referential, can be used as a pilot based on an effective complement
to theoretical analysis.
From the case study, it clearly illustrated that a huge amount of
wastage can be reduced after adopting prefabricated composite structure
based on Six Sigma. The application of prefabrication based on Six Sigma
provides enormous advantages, such as improved quality control,
reduction of construction time (26.2%), construction waste (67%),
contribution to a clean environment. A huge amount of wastage can be
reduced after adopting prefabrication. Up to 84.7% materials waste can
be saved. But a redesign of process and product is necessary to obtain a
high Sigma level.
6. Conclusions
Prefabricated composite structure design based on actual
engineering prototype and related test theories is combined to the
notion of six sigma management in order to obtain better application
results.
Six Sigma centers on improving project by improving process,
analysing and selecting project as well as to compose the project team.
It also request strictly to implement DMAIC process model, to guarantee
improved results, to demonstrate its validity through each project
result. Six Sigma objectives depend on continuous improvement of the
project through data so as to realize the management effects.
Construction enterprises should realize the feasibility and importance
of this theory and method. Generally, the application of Six Sigma
principles to establish a quantitative and qualitative construction
engineering quality system may increase the cost. But from the
perspective of long-term benefit, applying Six Sigma management
increases quality management.
The composite structure prefabrication offers many more answers to
the requirements of suitable construction than the other methods of
construction. The application of Six Sigma help to reduce the
consumption of energy during construction, pollution, noise pollutions,
waste and to develop new ideas and strategies of durability during the
complete life cycle of a building in the context of sustainable
development. The results show that this approach is feasible in
construction quality management and has certainly significance to the
construction company's implementation.
doi: 10.3846/13923730.2012.657411
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Megan Florent Tchidi (1), Zhen He (2), Yan Bo Li (3)
(1) Management Sciences and Engineering, Tianjin University, 92
Weijin Road, Nankai District, Tianjin, China
(2) College of Management and Economics Tianjin University, 92
Weijin roads, Nankai District, Tianjin, China
(3) School of Civil Engineering, Tianjin University, 92 Weijin
road, Nankai District, Tianjin, China
E-mails: (1) ftchidi@yahoo.fr (corresponding author); (2)
zhhe0321@163.com; (3) liyanbo_tj@sina.com
Received 08 Jul. 2010; accepted 17 Jan. 2011
Florent Megan TCHIDI. Research assistant, PhD Candidate at Tianjin
University in the Department of Management Sciences and Engineering. He
has a Master of Sciences in Structural Engineering. Author and co-author
of several scientific papers. Member of ASCE (American society of Civil
Engineers), ASME (American Society of Mechanical Engineers) and (IIIS)
International Institute of Informatics and Systemics. His main research
areas include Steel-Concrete Structures, Prefabricated Structures,
Structures Optimization, Six Sigma, Lean Construction, Productivity and
Quality Management in construction.
Zhen HE. PhD, Professor, Supervisor and Vice Dean of College of
Management and Economics of Tianjin University. He is author and
co-author of more than 100 scientific and special papers, lecture notes
and textbooks. Active member of China Quality Association, he is Black
Belt expert. He has carrying out research on, Quality, Productivity and
Performance Improvement in the Manufacturing Industry. Research
interests: Six Sigma, Quality Management, Industrial Engineering,
Statistic process.
Yan Bo LI. Assoc. Prof., Supervisor. Dept of Civil Engineering,
Tianjin University. He is author and co-author of more than 50
scientific and special papers. Research interests: Structural Stability,
Structural Reliability, Steel-Concrete Structures, Seismic Design and
Experimental Research.
Table 1. Analysis of quality problems
Cracks quantity
No Defect [less than [greater than Accumulated
or equal to] or equal to] cracks
0.1 mm 0.1 mm
1 Cracks produced by 4 11 15 73%
bad mix proportion
2 Surface shrinkage 2 4 6 80%
cracks
3 Dehydration cracks 2 3 5 86%
4 Improper strength 4 4 8 88%
concrete poured,
durability
5 Improper maintenance 3 2 5 95%
techniques
6 Other reasons 3 6 9 100%
Total 18 30
Table 2. Analysis of quality problems
Inspection project Sampling Rejection Unqualified
inspection number frequency
quantity (%)
Shrinkage cracks 1250 48 41.8
Smoothness 1250 31 23.6
Components size 1250 22 18.6
([+ or -] 4 mm)
Component strength 1250 11 6.4
Slippage effects 1250 36 25.7
Interface connection 1250 25 19.89
Total 1250 173 100
Inspection project Cumulative
frequency
(%)
Shrinkage cracks 42.8
Smoothness 76.4
Components size 91.0
([+ or -] 4 mm)
Component strength 95.4
Slippage effects 97.3
Interface connection 98.1
Total 100
Table 3. Reasons and degree which affect concrete quality
No Reasons Impact on Impact on
concrete composite
beams
1 Technical training high high
insufficiency, inadequate
conservation, waiting
time
2 Transport and pumping low low
equipment with low
exposure, insufficiency
of steel mould stiffness
3 Unreasonable concrete mix is, high high
large amount of cement per
unit volume, sand-stone mud
and water content
4 High temperature of entering medium medium
mould concrete
5 Unreasonable pouring process, high high
inaccurate measurement
6 Bad mix proportion, laboratory high high
test distribution problems,
inadequate cooling step
7 Weather influence, high pouring medium medium
temperature, insufficiency of
site maintenance, weak shed
installation maintenance
8 Interface connection, low high
application of chemical
anchors
Table 4. The best mix proportion
Sunny times Rainy times
Mix (sand-water content (sand-water content
proportion 4%: Stone powder 6.5%: Stone powder
water content: 3.5%) water content: 6%)
C15 1:1. 976:3. 209:0.186 1:2. 024:3. 286:0.061
C20 1:1. 726:2. 939:0.194 1:1. 768:3. 010:0.082
C25 1:1. 539:2. 608:0.203 1:1. 576:2. 671:0.103
Table 5. Analysis of the investigation results
Inspection Production Qualified Rejection Qualified
project quantity quantity number rate (%)
Shrinkage 1250 1230 20 98.6
cracks
smoothness 1250 1244 6 99
slippage 1250 1246 4 99
strength 1250 1244 6 99
Table 6. Comparison of specimen shear capacity before and
after strengthening
Shear bearing capacity (kN.m)
Specimen Before After [V.sup.s.sub.u]
strengthening strengthening - [V.sub.u]/
[V.sub.u] [V.sup.s.sub.u] [V.sup.s.sub.u]
ZL1 66.3 165 1.49
ZL2 50.3 150 1.98
ZL3 46 120 1.60
Note: [V.sub.u] is shear bearing capacity of concrete beams
before and after strengthening; [V.sup.s.sub.u] expresses
the cross-section shear bear when steel reinforced concrete
composite beam is broken.
Table 7. Comparison of flexural bearing capacity of specimen
before and after strengthening contrast
Flexural bearing capacity (kN.m)
Specimen Before After [M.sup.s.sub.u]
reinforcement reinforcement - [M.sub.u]/
[M.sub.u] [M.sup.s.sub.u] [M.sub.u]
ZL1 14.8 100 5.76
ZL2 14.8 135 8.12
ZL3 13.6 112 6.42
Note: [M.sub.u] in Table 7 is concrete beam flexural bearing
capacity before and after strengthening; [M.sup.s.sub.u] is
the biggest moment of the section when strengthening concrete
composite beam is broken.
