Improving the quality of autoclaved aerated concrete panel based on QFD and DOE theory

. Since the invention of autoclaved aerated concrete (AAC) used for blocks in Sweden in the 1920s, its applications have been expanded to many other areas, including wall panels, floor and roof panels, cladding panels and lintels. AAC production in Europe has slowed down considerably, while the industry is growing rapidly in Asia due to the strong demand in housing and commercial space. China is one of the largest markets in AAC manufacturing and consumption. However, the quality of AAC is unstable and most AAC producers have a low yield procedure in China. In this paper, an integrated methodology combining Quality Function Deployment (QFD) and Design of Experiment (DoE) was proposed to improve the quality of AAC panels. QFD is a useful tool for managing cross-functional design teams and exploring the customer needs. It achieves the maximum custom satisfaction by translating customer requirements to the engineering world. DoE was widely used for the design of multi-factor experiments. It provides adequate information based on limited time and cost. Customer requirements were collected through interviews and grouped by Affinity Diagram. Through construction of the 1st phase and the 2nd phase of QFD matrices, customer requirements were transferred to technical attributes and process procedures. Raw material composition, steam curing method and appropriate arrangement of reinforced steels were identified as the critical processes which affect the quality of AAC panel the most. Design of Experiment (DoE) was adopted to perform a series of experiments to obtain the best combination of raw material and steam curing method. Through application of the proposed methodology to AAC panel manufacturing, the product qualification ratio of AAC panel increased from 50% to 92.8%, higher than the expected 85%.


Introduction
Autoclaved aerated concrete is a lightweight construction material and composed of millions of microscopic cells generated during the manufacturing process.It is approximately 1/5 the weight of ordinary concrete with a dry bulk density ranging from 0.4 to 0.8 g/cm3 and a compressive strength ranging from 2 to 7 MPa [1].It has been successfully used around the world for over 80 years.Advantages of AAC include low density, ease of use, environmental friendly, high compressive strength, good thermal insulation, increased fire resistance and good sound insulation with a high quality reputation comparing with other types of concrete [2][3].Although chemical compositions of AAC in different places are different due to the variety of mixing proportions, the basic raw materials are common -sand, lime, cement, gypsum, water, aluminum powder and fly-ash in some manufacturers [2,4].The aluminum powder was added into the mixing ingredients as a poreforming agent, which liberates hydrogen gas and forms bubbles.The process of AAC manufacturing is illustrated in Figure 1.
The AAC manufacturing process starts when the sand is ground to the required fineness in a ball mill.Raw materials are then automatically weighed and measured in the mixer along with water and aluminum powder.At the same time, structurally welded, rust treated and dried steel bars are set in the metal mold properly.After mixing, the slurry is poured into metal molds in which the expanding agent reacts with other elements.The mixing results in a chemical reaction that expands the mixture to form small, finely-dispersed air spaces.The molds are sent to a pre-curing room for several hours.Then the semi-solid material is transported to the cutting machine.The cutting machine cuts the molds into the size required for the building elements by using steel wires.The final phase in the production process is steam curing in the autoclave, and then it is packaged and sent for storage.
Figure 1 Manufacturing process of AAC In China, AAC is identified as the only one type of wall materials to meet 50% the requirement of energy saving without adding other affiliated thermal insulation materials.AAC has a broad application prospect in China as the government is promoting building energy saving and carbon emission reduction [5].However, it is observed that the product qualification ratio of 5 cm AAC panels was extremely low, which is only 50%.Therefore, it is necessary to investigate the bad performance and improve the quality of 5 cm AAC panels.

QFD and DoE methodologies
QFD has for a long time been used as a successful design methodology to improve products' quality.American Supplier Institute defined QFD as 'A system for translating consumer requirements into appropriate company requirements at each stage from research and product development to engineering and manufacturing to marketing/sales and distribution'.QFD helps designers to utilize the voice of the customers (VOC), or customer requirements, to determine which engineering element or product specification is the most essential.This prioritization helps designers to know which part of the product or process is the most beneficial to focus on during design, resulting in products that better meet customer requirements and generate increased commercial success [6].It is often combined with other quality tools to improve existing products' quality or to develop new products.Some recent studies on QFD (mainly published in 2014) are selectively summarized.Moldovan [7] successfully applied QFD and knowledge management to understand customer needs and identify critical technical attributes to guide a new product (soft drink) design in a mineral water company.Shad et al. [8] proposed a combination of Analytic Hierarchy Process (AHP), QFD and linear Physical Programming (LPP) for supplier selection under uncertainty conditions to maximize overall customer satisfaction.In this paper, a set of triangular fuzzy numbers were used to address vague linguistic expressions.Rahmana et al. [9] presented an integrative approach of SERVQUAL, Kano model and QFD to improve the quality of simulation-based training on project management.Zaim et al. [10] proposed a combination of Analytic Network Process (ANP) and fuzzy logic to better rank technical attributes during QFD application.In this paper, new equipment which can be used to squeeze the polyethylene pipes to stop the gas flow without damaging the pipes was developed.Iqbal et al. [11] suggested an interesting and important research object.This paper introduced a statistical testing procedure to determine whether the final weight of technical attributes is significantly different and to investigate the robustness of different rating scale.John et al. [12] investigated the awareness of QFD in construction industry in Nigeria and they tried to promote its potential effectiveness in design and building industries in the future.Zhang et al. [13] applied QFD/TRIZ (Theory of Inventive Problem Solving) to improve the design of elastomeric hydraulic reciprocating sealing.In this paper, QFD was used to identify the key elements that affect the sealing efficiency-reinforcement.TRIZ was used to resolve the conflicts of key elements and give the efficiency-reinforcement design solutions.
The DoE methodology was developed originally by Ronald A. Fisher in 1925.However, classical parameter design is too complex to use.Especially, a large number of experiments have to be carried out when the number of the process parameters increases.To solve this task, the Taguchi method uses a special design of orthogonal arrays to study the entire parameter space only with a small number of experiments.Orthogonal array was selected because there was no interaction among factors.The arrays were created using an algorithm developed by Taguchi, which can allow each variable and setting to be tested equally.The selection of a suitable orthogonal array depends on the number of factors and their levels.Taguchi method took the design of experiments from the exclusive world of the statistician and brought it into the world of manufacturing.This method provides a simple, efficient and systematic approach to determine optimum conditions [14].The method is valuable when the design parameters are qualitative and discrete.It has been widely used in the industry for product development and to process quality improvement.The recent applications on Taguchi DoE (also referred to Taguchi method) are selectively summarized.Mohan and Reddy [15] used Taguchi DoE to evaluate the influence and specific function of eight important factors to enhance the mixed culture bioplastics production.Zirehpour et al. [14] applied Taguchi DoE to investigate the most significant factors in ultra filtration membrane to optimize the treatment of olive oil wastewater.Taguchi DoE was employed to evaluate the effect of aggregate content, sodium hydroxide concentration, curing time and curing temperature on compressive strength of class C ash-based geopolymers [16].Sivasakthivel et al. [17] proposed Taguchi DoE to optimize the performance of a ground source heat pump system for space heating and cooling application.Rahim et al. [18] applied Taguchi DoE to optimize the residual compressive strength of heated high performance concrete.Yeh and Tsai [19] presented an application of the grey-fuzzy Taguchi method to derive the optimum parameters for the fine-pitch copper (Cu) wire bonding process with multiple quality characteristics.
It is important to note that DoE and QFD are two complementary quality tools.Whereas QFD attempts to identify key product or process variables, DoE can help minimize the number of critical process parameters and optimize them.[20] A great number of studies which explore how to combine QFD with DoE are summarized.Ross [21] proposed the use of DoE in the establishment of relationship matrix, but he did not provide the detailed integration method and process.Sheng et al. [22] presented three integration ways of DoE and QFD, including establishment of relationship matrix, setting of targets and taking customer responses into account during DoE process optimization.Bouchereau and Rowlands [23] suggested Taguchi robust design is particularly useful for establishing the best operating conditions for manufacturing and can thus be integrated in the 3rd QFD phase, the process planning phase.He also suggested that the quality loss function curve can help determine the exact target values in the house of quality.Shi and She [24] studied the problem solving methods including TRIZ and Taguchi methods to overcome traditional QFD's weakness of not being a problem solving tool.Zheng and Chin [25] suggested QFD-based process quality planning should be closely coupled with design of experiment to clearly identify the linear relationships and correlations among elements of HoQ, to improve the effectiveness of process quality planning.An integrated design process combining QFD, conjoint analysis and Taguchi method was introduced to link marketing and R&D in the case of new product development.Jeang et al. combined QFD and Taguchi method to analyze the produced quality characteristics and to optimize the process parameters for hot-bar soldering process.

QFD application
Distributors' visits and customer interviews were conducted and 26 requirements were obtained and grouped by Affinity Diagram, as shown in Table 1.Based on the existing design parameters and discussion with design engineers, 35 corresponding technical attributes were listed in Table 2 High cost-performance ratio A26 The importance questionnaire for customer requirements was handed out to 20 customers.Numerical values 1-5 were used to indicate the intensity of importance.Number 5 indicates the highest level of importance while number 1 indicates the lowest level.Mean value of importance intensity with regard to each customer requirement was rounded to an integer value.The rounded mean value was input as importance rating in the second column in Table 3.Then, the two competitors, which also produce AAC products, were selected.The assessment to our products and competitors' products was conducted with regard to each customer requirement.Assessment values were entered in Table 3. Target value was set by multi-functional QFD team and improvement ratio was calculated by C6 = C5 / C2.Sales point denotes the attractiveness of each customer requirements in the market and carries three values of 1, 1.With technical attributes compared with process parameters, the 2nd phase of QFD was constructed and demonstrated in Table 6.The strength values of relationship were entered and multiplied by total score for each technical attribute.Critical process parameters such as "Raw material mixing" "Arrangement of reinforced steels" "Autoclave curing" were obtained as guidance for improvement.
Table 6 The 2nd phase of QFD

DoE application
Now the causes of bad performance of 5 cm AAC panels are identified through transferring customers' perspective to process parameters.Taguchi DoE is proposed to optimize these process parameters with limited time and resources.Raw material composition is affected by six indicators including cementitious material, waste slurry, sand slurry, water-to-cement ratio, C/S and aluminum powder.As cementitious material, waste slurry, sand slurry are mixed by a certain proportion, once the proportion of cementitious material and waste slurry are determined, the amount of sand slurry can be fixed accordingly.Therefore, sand slurry was not taken as a factor in experiment design.C/S ratio is constant and not considered as a factor either.In total, this experiment has 4 factors and 5 levels, as shown in Table 7.

Table 7 Design of Experiment for raw material mixing: Factors and Levels
Based on orthogonal array experimental design proposed by Taguchi, L25(54) has 25 combinations of experiment.Each combination was run 3 times and the mean value of the responses was input as product qualification ratio in Table 8.The 16th combination obtained the best result and it was considered as the optimized combination.
Steel bars in AAC panels can reinforce the structure.Load bearing capacity can be strengthened by adding more steel bars or increase the diameters of main steel bars.As 5 cm panel is thin and easy to break at loading and unloading, we need not only add steel bars, but also change the arrangement of reinforced steels to increase the load bearing capacity.Original arrangement of reinforced steels 4*Ф5.0 was changed to 5*Ф4.2 as shown in Figure 2. The experiment results show that the load bearing capacity with the current arrangement has been improved by 8% and the bond strength for steel bars increased from 0.8MPa to 1.3MPa.
Table 8 Design of Experiment for raw material mixing: Combinations and Responses Figure 2 The arrangement of reinforced steels Autoclave curing directly affects compressive strength, thereby indirectly affects the product qualification ratio.There are 5 factors in autoclave curing, including the highest pressure, vacuum degree, time of temperature increase, time of constant temperature and time of temperature decrease.Among these factors, the highest pressure is constant and controlled at 1.2 MPa.The other 4 factors have 3 levels, as shown in Table 9.
Table 9 Design of Experiment for autoclave curing: Factors and Levels Based on orthogonal array experimental design proposed by Taguchi, L9(34) has 9 combinations of experiment.Each combination was run 3 times and the mean value of the responses was input as compressive strength as shown in Table 10.The 7th combination obtained the best result and it was considered as the optimized set.Through product performance testing, compressive strength reached to 3.8 MPa, which solved the problem that product could not be steamed thoroughly.The total energy consumption did not increase.After the above improvements on the 3 process parameters, i.e., raw material mixing, arrangement of reinforced steels and autoclave curing, the final product qualification ratio reached up to 92.8%.
Table 10 Design of Experiment for autoclave curing: combinations and responses

Discussion
The critical quality indicators, including product qualification ratio, compressive strength, bond strength of reinforced bars and unit cost of 5 cm AAC panels were compared before and after the application of QFD and DoE.These indicators were illustrated in pareto chart in Figure 3.We can see that the product qualification ratio reached up to 92.8%, which is much higher than the original 50%.Compressive strength was raised from 3 MPa to 3.8 MPa.Bond strength for reinforced steels was raised up to 1.3 MPa.The decreased unit cost was mainly due to the reduction of raw material consumption, man power cost and electricity consumption after the production qualification ratio was improved.
Figure 3 Critical quality indicators before and after QFD-DoE application Based on the unit cost reduction from 820 RMB/m3 to 415 RMB/m3, assume monthly production of AAC panels is 1000 m3, the profit increase for one year is 1000 m3/month ´12 months ´ (820-415) RMB/ m3 = 4.86 million RMB/ year.Much less waste materials are produced and less waste processing is needed.At the same time, gypsum and tailings are successfully used in 5 cm AAC panels, which make extra contribution to the cost reduction.Significant energy saving of AAC products was also observed.AAC panel is only 1/3 energy consumption of clay bricks with a density about 50% of that of concrete hollow block as shown in Table 11.Therefore, the manufacturing of AAC products has great advantages on the energy consumption per unit weight.Meanwhile, it has very good thermal insulation properties as shown in Table 12.Normally, every 10000 AAC wall panels can save 0.62 tons of standard coal.Assume 1 m3 AAC can be divided to 680 wall panels, the amount of the saved standard coal is 1000 m3 ´ 680 wall panels/m3 ´ 12 months ´0.62 tons standard coal/10000 standard blocks = 505.92ton /year.Table 11 Three main wall material energy consumption rate Table 12 Thermal conductivity of some common wall materials The proposed manufacturing process parameters were summarized as operation guidance.Inspection standards for raw material, semi-finished products and finished products are amended.Training was provided to relevant employees to make sure that new standards and regulations are effectively complied to.

Conclusion
At the beginning, AAC products are briefly introduced and recent publications on QFD and DoE are briefly reviewed.At the same time, customer interviews were conducted to obtain customer requirements.Two houses of quality were built where Affinity Diagram was used to group customer requirements, technical attributes and process parameters.In order to improve product qualification ratio, compressive strength, load bearing capacity and bond stress for steels bars, Taguchi orthogonal array experimental design was adopted to optimize raw material composition and autoclave curing.Reinforced steels were redesigned.Optimal combinations were tested and adjusted.The proposed integral methodology of QFD and Taguchi DoE to improve AAC products' quality is very effective based on the results we have achieved.This combination can be further applied to other building material industries.It is worth to notice that based on the features of AAC products, only the 1st phase and the 3rd phase of QFD, product design and process parameter design, were chosen to interpret the voice of customer.Taguchi DoE was used to find the optimized operation conditions for three critical processes during the AAC manufacturing.Taguchi DoE integrated in the 3rd QFD phase was firstly proposed by Bouchereau and Rowlands [23] in 2000, however, no real case study was reported since then.This is the first trial to see how this match works.The factor and level selections for each experiment design are extremely critical and directly affect the success of experiment, so extra experience and skills are required during this process.

2 and 1 . 5 .
Absolute importance wasobtained by C8 = C1 × C6 × C7.Relative importance was calculated by C9 importance value reflects the voice of customer, benchmarking with similar products and market attractiveness. .

Table 2
Arrangement of technical attributes

Table 3
Quality planning table

Table 4
The 1st phase of QFD

Table 5
Arrangement of process parameters