Overview of Constructability Improvement through Innovations in Construction Methods during Construction Phase

What is constructability?

Construction Industry Institute (CII) defines 'constructability' is the "optimum utilization of knowledge and construction experience into the planning, design, procurement and construction phase to achieve overall project objectives". CII developed the concept constructability  in 1983, the Business Roundtable's Cicé conduct studies within 4 years to improve the quality, efficiency, productivity and compliance costs in the construction industry. The results are summarized in a report given the title "Integrating" (TCMC, 1991). Since that the CII at the University of Texas in the United States continued the research and establish local and regional groups to raise awareness about the benefits to be gained through constructability programs, such as improving the quality, reliability and cost-effective and time of the project.

CONSEPTUAL-> DESIGN->PROCUREMENT->CONSTRUCTION->START UP
Start----------------------------------------------------------------------------------------Complete

Fig.1 Project Life

Constructability concepts
In order to implement the concept of constructability CII (Buildability, UK) was made constructability program based of the seventeen concepts. Implementation starting from early stage is conceptual planning then design and procurement, and implementation on site.

The seventeen of the CII constructability concepts are as follows:

Conceptual Planning Phase:

1.      A formal constructability program is made an integral part of the project execution plans.

2.      Early project planning actively involves construction knowledge and experience.

3.      Construction personnel are involve in developing the project contracting strategy.

4.      Project schedules are sensitive to construction requirements.

5.      Basic design approaches Consider major construction methods Such as modularization or preassembly.

6.      Site layout promotes efficient construction

7.      Project Team Participants Responsible for constructability are Identified early in the project.

8.      Such advanced information technologies as 3D computer modeling or field notebook computers are applied.

Design and Procurement Phase:

9.     Design and construction procurement schedules are driven.

10.  Design are configured to enable efficient construction considering issues like simplicity, flexibility, sequencing of installation, and labor skills and availability.

11.  Design elements are standardized

12.  Construction efficiency is Considered in specification development.

13.  Preassembly modular design and are prepared to facilitate fabrication, transportation and installation.

14.  Construction design promotes accessibility of personnel. Materials, and equipment.

15.  Design facilitate fabrication, transportation, and installation.

16.  Design and construction sequencing system facilitates the turn over and star-up

Construction Phase:

17.  Innovative construction methods are use-Such as innovative sequencing of field tasks, or use of temporary construction system, or innovative use of construction equipment.

Radtke et al. (1993) Project Level Model proposes to streamline the implementation of constructability programs. Implementation programs divided into corporate and project programs. According to Radtke at level project program, first; compile constructability capabilities, second; planning for the implementation of constructability and third; implementation of constructability

Innovation in The Construction Methods
A single concept of constructability in the construction phase on site is:
'Constructability increased when performed  innovation in the construction methods '
Some sense of the term used for the purposes of this research that the term 'construction methods' is broad in scope and usually refers to the technical manner in which various construction resources are located. While the 'innovative construction methods' refers to the extraordinary  way that is unusually carried out to completion of construction and general creative undertaken to address challenges in the field. 'Innovation' can be discerning subjective in this term, an example adaptation to the developing of non-construction aspect for development purposes may also be said to be innovative (O'Connor and Davis, 1988).

O'Connor and Davis (1988) describe a method of Construction Innovation include activities at the implementation stage as the following:

1.      Innovation in determination of sequence work of project

2.      Innovations in the use of materials / temporary construction systems

3.      Innovations in carpentry tools (hand tools)

4.      Innovations in the equipment use for construction

5.      Constructor-optional preassembly

6.      Innovation in temporary facilities

7.      Options 'post-bid constructor' relationship with the layout, design and selection of permanent material.

Implementation Constructability Program in Indonesia
The Contributions construction industry in Indonesia to increase the welfare of society is very significant. Amount of labor which absorbed in the construction industries sector increased from 2.8 million in 1990 to 3.5 million in years 2000. World Bank Report (2001) released approximately 1.400 contractors is a member of the Intellectual Property Rights (Association of Construction Experts Indonesia) and 43.000 small and medium sized contractors are members of the Construction Association of Indonesia (GAPENSI). Therefore, efficiency is a relevant issue in efforts to increase the performance of the Indonesia’s contractors for survives.

In construction projects the use of knowledge and construction experience to all project activities will result in cost and time efficiencies, improved quality and safety (Jergeas and Put, 2001). The Construction Industry Cost Effectiveness (Cicé) Task Force on U.S. estimates that the increase constructability could be saved 10 to 20 times the cost of the program constructability (TCMC, 1991). Study of the Construction Industry Institute (CII) in "Constructability" report 1986 showed that the implementation of constructability programs for specific projects could be saved 6% to 10% of construction costs (TCMC, 1991). Furthermore a case study conducted by Russell et al. showed that the increase in constructability will decrease 5% to 10% of the entire project time (Trigunarsyah, 1999).

The Greatest influence on increasing the performance of project when apply constructability program in the conceptual planning stages, design and procurement (pre-construction). However, in the construction stage also can still efforts to increase constructability by performing “innovation of construction method", of course the contractor can still getting benefits by implemented constructability at this stage though not as much as in the planning, design and procurement stages.
Implement construction experience into the design is very low when the traditional approach used. Trigunarsyah (2002) concluded that the project delivery in Indonesia generally use the traditional approach in which contractors and consultants is taken by a separate contract, that system causing no any contractor involvement in providing inputs for planning and design concepts. Consequently the opportunity for cost savings is lost due to the possibility of a design that does not 'construct-able'. Of course this is an opportunity for contractors to conduct constructability improvement through innovation of construction methods in the construction phase of the project.

Contractors and consultants in Indonesia still has not been formally implementing constructability programs. As in Malaysia constructability implementation does not well integrated and understanding to the constructability still individualized, the study of West Port Highway project conducted by Nima et al. (2002) pointed this out. They may understand the importance of modularization,  pre-assembly and innovation of construction methods but it is not implemented in the integrated program. Many opportunities to improve the project performance are lost due to have not integration in constructability implementation.

The contractor may be doubt to performing innovation in construction methods for various reasons. Survey by Jergeas and Put (2001) described that some contractor doubt to make innovations in construction methods due to the three main constraints (perception): a) the fear of risk, b) fell it costly to do innovation, and c) it takes time to train site employees. But not least also some contractors who are motivated to perform an innovative  in the construction methods as some of them are The Catalic Delta Constructions (DCC) in innovation of materials/construction of a temporary system for project-Bridge Road in Columbia, Hazama Co. to innovation in use sliding form to build Silo and pre-assembly panel precast at cement plant project in Kalimantan.

From the owners perspective  may be implementation the constructability program during construction phase by conducting innovation of construction methods will contribute only a small benefit, not as much benefits as when if constructability implemented from pre-construction phase. From the perspective of the contractors applying the innovation of construction method is very important because at this stage the contractors role in to perform constructability improvement.

References: Anderson, Stuart D., “ Implementing Project Constructability: Design Problem  Civil Engineering”

Clough, Richard H., (1981), “Construction Contracting” John Wiley & Sons,Inc.
Jergeas, George, and John V. P., (2001), “Benefit of Constructability on Construction Project”, J. Constr. Mgmt., ASCE, 127
Nima, Mekdam A. et al., 2002, “Constructability Concept in West Port Highway in Malaysia”, J. Constr. Mgmt., ASCE, 128(4)
O’Connor, James T. and Victoria S. D., (1988), “Constructability Improvement During Field Operation” J. Constr. Mgmt., ASCE, 114(4), 549-563.
O’Connor, James T. and Tucker, R.L., (1986), “Industrial Project Constructability Improvement” J. Constr. Mgmt., ASCE, 112(1)
Radtke, M. W., and Russel, J. S., (1993), “Project Level Model Process for Imple-menting Constructability” J. Constr. Mgmt., ASCE
The Construction Management Committee, (1991), “Constructability and Construct- ability Programs: White Paper”, J. Constr. Mgmt., ASCE
Trigunarsyah, Bambang (1999), “ Implementing Constructability On Construction Industry In Indonesia”, Proposal Research, UI
Trigunarsyah, Bambang, (2002), “Constructability Practice Among Project Designers In Indonesia”, Journal Teknik Sipil Univ. Tarumanegara,
World Bank, (2001), “Indonesia Country Procurement Assesment Report Reforming the Public Procurement System” Report No.21 823.

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Brief History of Readymix Industry in Indonesia

Bagaimana sejatinya sejarah perkembangan 'beton siap pakai' atau readymix dimulai? Sejak ditemukannya cement pozzolan perkembangan penggunaan beton untuk struktur bangunan semakin pesat. Kebutuhan akan ketepatan pengiriman dan konsistensi mutu beton menjadi penting. Readymix adalah pilihannya.

Definisi Beton Siap Pakai atau Readymix Concrete menurut SNI 03-4433-1997/ASTM C94-96 adalah
Beton yang dicampur dalam mesin pengaduk stasioner atau truk mixer dan dikirim ke pemesan dalam keadaan plastis/belum mengeras.

SEJARAH

Pada Tahun 1930 beton siap pakai dalam skala industri pertama kali dibangun di Inggris. Kemudian pada tahun 1960-an permintaan Readymix berkembang pesat di Inggris dan juga negara-negara di Eropa  untuk pembangunan infrastruktur.

Di Indonesia penggunaan beton siap pakai (Ready-Mix Concrete) dalam skala bisnis pertama kali dimulai oleh PT. Jaya Readymix pada tahun 1972 di Jakarta untuk melayani proyek-proyek konstruksi yang mulai berkembang. Seiring dengan meningkatnya pertumbuhan konstruksi diawal tahun 1980-an bermunculan perusahan-perusahan beton siap pakai (RMC) lainnya, demikian juga di Jawa Timur khususnya di Surabaya.

Permintaan RMC menurun setelah terjadinya krisis moneter pada tahun 1998 karena proyek-proyek banyak menghentikan kegiatannya. Kegiatan pembangunan mulai meningkat kembali sejak tahun 2000. Sampai tahun 2006 permintaan RMC di Jawa Timur tumbuh rata-rata 15% per tahun dan pada tahun 2005 total volume kebutuhan beton di Jawa Timur mencapai 1.125.000 m3 dengan produksi rata-rata perbulan lebih kurang sebesar 94.000 m3. Bersambung....

*) Materi ini pernah penulis sampaikan di Kuliah Tamu di Universitas Kristen Petra Surabaya
dan Institute Teknologi Nasional Malang.

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Factors Affecting to High Cement Content in Concrete Mix Design

 

Introduction

Generally concrete ingredients consist cement, fine aggregate, coarse aggregate and solvent water. Some additional additive or other materials for increase the concrete performance.

In The project of Construction Road Sindhuli-Nepalthok Section 3 NEPAL the source of material aggregate is explored from the riverbed of Sunkhosi river from borrow pit sta. 1+000, sta 18+000, 29+500, 30+800 and others.

As known, Aggregate quality the thing important to produce of the economical concrete. It will influence cement content in the concrete.

Following a case study of factors affected to high cement content in mix design.

• Quality of Concrete Ingredients

Water : Well water located at side of Sunkoshi river. This water is acceptable for water mixing for concrete.

Cement : Pxxx Cement Grade-43

Mortar strength test results: for period April 2010 up to Dec 2010 the mortar stress tendency lower than specification refer to (Indian Standard) IS-8112.

In this case, necessary to increase cement into the mix proportion due to low quality of cement.

Fine Aggregate

The quality and properties of aggregate most important to get durability of concrete.

The Sunkoshi fine aggregate two matters to be concerned that the grading related to fineness modulus, and Mica.

1. Grading and Fineness Modulus (FM)

Fineness Modulus (FM) is the sum of cumulative percentages retained on sieves starting from size 0.15mm to maximum size divided by 100. It is note that a fine grading has a lower FM.

Variation for FM of Sunkoshi fine aggregate (washing screen plant) is too many variation. In Similar borrow pit has minimum value of FM is 1.91 and maximum value is 2.97, with standard deviation 0.29.

The grading is very important such that a reasonable workability and minimum segregation are obtained. A workability mixture can produce a strong and economical concrete.

However, a change in FM of 0.2 in fine aggregate may lead to a significant change of water demand for the same workability of concrete mixture. That is also will lead increased cement demand for keep strength (w/c).

2. Mica
Almost Fine aggregate from Sunkoshi River contains ‘mica’ with various content of mica. Some standard specification not clear yet how much mica content minimum will allow in fine aggregate. Some rumors says in German not allowed mica content in fine aggregate. So far, although not many research about effect mica in fine aggregate but of course mica in fine aggregate will effect to strength of concrete, that will contributed to cement demand in concrete.

The others parameter such as soundness, alkali reactivity are still in tolerance. In addition, sand equivalent, clay lump, washing loss, organic impurities may in tolerances because this fine aggregate is processed trough washing plant, except it not wash properly.

Coarse Aggregate
Table-2 below shown results of abrasion and elongation test for Sunkoshi coarse aggregate.

Table-2 Abrsion and Elongation of Coarse Agg. Sunkoshi
Kind of    Agg.       |    Surface    |   Abrasion        |   Elongation Index  |
Gravel                    |                   |    TS.40% max |       BS.25% max   |
Max. 40 mm          |   Smooth     |     36- 38%      |          30.0%          |
Max. 25 mm          |   Smooth     |     36 -40%      |          27.1%          |

Refer to table-2, elongation index of aggregates is more than 25 % and the surface aggregates is smooth. A note, elongation index of gravel40mm higher than gravel 25mm. Higher elongation will guide to increase sand aggregate ratio (S/a).

The stress at which the breaks developed depend largely on properties of the coarse aggregate; smooth gravel leads to breaking at lower stresses than rough and angular crushed rock, because mechanical bond is influenced by the surface properties and certain degree, by the shape (elongated) of the coarse aggregate. (Neville, properties of concrete 4th, 1995, p288)

Mix Proportion

The mixproportion of concrete for Nepalthok site are as follows;

Class Concrete      Targer Strength     W/c      S/A      Cement/m3
D1=40mm max            21.2 Mpa        55        40           327
D2=25mm max            21.2 Mpa        56        40           300

In General, the larger maximum size of aggregate used then the smaller value of sand aggregate ratio (s/a), it will smaller water demand in the same workability. That means, the larger maximum size of aggregate used is more economically (lower cement demand) than smaller size used.

But in this case, relation between aggregate maximum and S/A is conversely(see Table 3 and 4 above). For example: mix-proportion in Table-3, S/A for D1 (agg.40mm under) is equal D2 (agg.25mm under), the same water content 180 kg/cm3 at the same slump. In the same target strength cement content for D1 is bigger than D2. This reality probably cause by shape or elongated of aggregate.

• Concrete Production (Process)

During production of concrete, many factors may influent to strength of harden concrete.
Condition of mixer-pan or Truck Mixer for concrete mixing, variation of quality of aggregate, quality of cement and water, technician, making sample, curing etc., all of them will effect to variation quality of concrete.

• Conclusion

According to data above, mainly factors affecting high cement demand on concrete (mix-design) as following;

1. Quality of sand specially variation in grading, FM and Mica Content in fine aggregate.
2. High elongation and smooth surface of coarse aggregate
3. When low quality of Cement used, alone or together with quality of aggregate will contributed to concrete stress, and leads to increase cement demand to reach the target strength.

Nepal, Road Nepalthok-Shinduli Sec-III, 2010

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Managing Temperature in Mass Concrete Pouring



 Scope
The primary characteristic that makes mass concrete different from other concrete work is thermal behavior. Cement hydration process is exothermic by nature, in large mass concrete where the heat is not quickly dissipated, temperature rise can be quite high. Tensile stress may develop from the volume change due to the increase and decrease of temperature within the mass. A structure or portion there of may be considered mass concrete if measure are necessary to control thermal behavior to alleviate cracking. Prof Lee in his paper has already investigated that the minimum dimension of 1.50 meter is considered as mass concrete.


History
Prior to 1994, mass concretes are done without any mineral additives. In 1994, fly ash was started to be produced and marketed. Since that time, most mass concrete has been done using 30% fly ash content in mix proportion to reduce heat that is developed during the hydration process.



Temperature
a. Temperature of Normal (Hot Weather) Concrete is 30 'C to 37 'C
To achieve a lower peak temperature of interior mass concrete during hydration period, pre‑cooling concrete materials prior to mixing is practiced. Aggregates in the stockpile are sprayed with water or shaded. Cement in silo was kept at least one night before, and water in tanks can be chilled with ice. For several projects where the initial fresh concrete temperature is very low (max 32oC), crushed ice was introduced into the mixing water. Furthermore, the use of ice in mass concrete would lead to higher production cost and extra supervision should be applied to achieve the most economic result.

b. Calculation of Initial Fresh Concrete Temperature
For example, calculation of fresh concrete temperature:
Concrete Grade K400 (cube,kg/cm2): Mass of cement = 400kg ( Tc=89 'C), Aggregate =1797kg (Ta=29,7 'C), Water=138kg (Tw=29.5 'C), Water in Aggregate=67kg (Twa=29.2 'C).

According to American Concrete Institute, calculation of initial concrete temperature as below formula;

Tf = [0.22 (Ta.Wa + Tc.Wc) + Tw.Ww + Twa.Wwa] / [0.22 (Wa + Wc) + Ww + Wwa]
Tf = 37.3 'C

Where Tf : Fresh Concrete Temperature
Ta : Aggregate Temp ( 29.7 'C)
Tc : Cement Temp ( 89 'C)
Tw : Water Temp ( 29.5 'C )
Twa : Water in Agg Temp ( 29.2 'C )
Wa : Mass of Aggregate ( 1797 Kg )
Wc : Mass of Cement ( 400 Kg)
Ww : Mass of Water ( 138 Kg )
Wwa : Mass of Water in Agg ( 67 Kg)

Calculation of Peak Concrete Temperature :

Tp = {(W xC)+t} /100
TP= 82.6 'C

Where: Tp : Peak Concrete Temperature
W : Mass of Cement ( 400 Kg )
C : Coefficient of Temp Rise 12 'C
t : Ambient Temp ( 34.6 'C)

1. Effect of Temperature on Fresh Concrete
The effects of temperature on fresh concrete are mainly due to its accelerating influence on rate of cement hydration. This results in :
a. Reduction in initial degree of workability for constant mix proportioning about 2 cm slump per 10 'C. Alternatively the water content for equal workability has to be increased about 7 kg per 10 'C. To maintain the W/C ratio for equal strength, cement content is also increased about 10 to 20 kg.
b. Faster rate of workability loss as indicated by shorter setting time of cement or concrete. In addition, the increased rate of evaporation of water produces further stiffening of the mix. Set retarding admixture is needed to extend the setting time and water reducing admixture (plasticizers) for restoring workability. Re-tempering of concrete with water may lead to increase in effective W/C ratio and loss of strength. Re-tempering with admixture or addition of admixture is the recommended practice where necessary.
c. Rapid water evaporation from concrete before placing promotes faster stiffening. After finishing, the loss of bleeding water from large exposed surfaces due to a combination of climatic factors; high temperature, low relative humidity, high wind velocity and high solar radiation of unprotected surface, may lead to potential plastic shrinkage cracking. A rate evaporation exceeding 1kg/m'/hr is considered to be critical.

2. Effect of Temperature on Hardened Concrete
High temperature increases the rate of cement hydration, this produces:
a. Two opposing effects on hardened concrete strength are produced : a faster rate of hydration after setting leads to a higher strength at early ages under hot humid conditions; a high initial rate of reaction during the setting stage of cement produces a non‑uniform paste structure.
b. Potential differential thermal cracking may occur as the heat of hydration developed within the interior of thick section is prevented from dissipating rapidly to the exterior as the conductivity of concrete is low. A differential temperature is created between faster cooling surface and the slow cooling interior. For a minimum dimension of a member in excess of 1.5 to 2.0 meters, the rise in temperature is of the order of 12'C per 100 kg/cum in cement content. A differential temperature of 20'C is often taken as critical value for which the tensile strain capacity of concrete is reached.
Two possible situation for such occurrence are :
• The differential occurring before the peak temperature e.g. no or insufficient surface insulation, for which the subsequent eventual cooling of the interior may produce internal cracking
• The differential occurring after the peak temperature, e.g. when the surface is suddenly cooled by premature removal of insulation, for which external cracking may occur.

3. Methods of Temperature Control There are two stages of temperature control. The first is to control the initial temperature at the time of mixing as estimated from the above equation. The second is to control the rise in temperature due to heat of hydration, i.e. the peak temperature in the hardened concrete.
a. Reduction in initial concrete temperature - Precooling Method ·
. Preventing direct solar radiation on to aggregates with shading and painting cement silos in light color.
· Cooling of fine and/or coarse aggregates with chilled water, liquid nitrogen or dry ice (solidified carbon dioxide).
· Cooling of mixing water until below 10'C, and using ice block as part or total mixing water (+/- 2 ice blocks/m3)
· Sprinkling water on coarse aggregate to induce evaporative cooling of surface moisture.
· Use of liquid nitrogen to produce chilled water (Water Chiller), or slush (loose ice) or direct injection into fresh concrete.
b. Reduction in peak temperature ‑ Cooling Method and Material Selection.
· Reduction of initial concrete temperature by precooling methods
· Use water reduction admixture to reduce cement content
· Use of Low Heat Portland Cement (Type IV), Moderate Heat Portland Cement (Type II), or Sulphate Resisting Portland Cement (Type V)
· Use partial replacement of cement with pozzolans e.g fly ash to reduce cement content.
· POSTCOOLING by circulating cooling water or other liquid through thin‑walled pipes embedded in the concrete.
· Construction Management: Effort are made to protect structure from excessive temperature differentials by knowledgeable employment of concrete handling, construction scheduling and construction procedure.


Planning
The successful completion of a mass pour is largely determined by continuity of concrete supply, placement and compaction. Care must therefore be taken to ensure that :
a. The concrete supplier is able to meet the demand and that alternative sources are available in the event of breakdown.
b. The placing equipment has sufficient capacity and back up equipment is available.
c. The labor resources can handle the rate of concrete delivery.
It is also important to ensure compatibility between concrete production, transportation, placing rate, compaction rate and finishing. A deficiency in any one of these processes can lead to unacceptable delays.
Furthermore, the casting sequence and rate of supply must be such that a live working face is always maintained with the avoidance of Cold Joint.InsulationIf the most suitable concreting material are not available and it is expected that the temperature differential will be excessive and cause cracking, insulation can be applied. This prevents rapid heat loss from the surface and hence minimizes the temperature differential between the surface and the core.
If only modest insulation is needed, tenting may be sufficient. In its simple form, this will consist of polythene sheeting laid on the surface and fixed in such a way as to prevent evaporative cooling. To increase the insulating value, the sheeting can be raised on timbers, but care must be taken to ensure that the system is windproof.
For more effective insulation, quilts or foam mats, or soft board or sand laid on polythene sheets are methods which have been employed. Quilts or foam mats are probably the easiest to apply and remove and allow greatest flexibility.
The insulation should remain in place until the center of the pour has cooled to a temperature level which is low enough to avoid limiting differential being exceeded even if the surface should cool to ambient (below 20'C ). This period will vary with the size of pour (dimensions) and mix used. In usual, period of insulation + 14 to 21 days.
Plywood formwork is also good insulator and if left in place will allow the surface temperature to increase by 20'C or more thereby decreasing temperature differentials, if this system is adopted, however, care should be taken not to remove the formwork when the pour is at its peak temperature after 48 hours to increase or so, this could lead to rapid surface cooling and cracking. If the formwork has to be removed, it is best to loosen the shutters initially, but keep them in place for a period of say, 24 hours. This allows the surface to cool slowly, resulting in less severe thermal gradient.

Temperature Measurement

To determine when insulation or formwork can be removed, it is advisable to in thermocouples to measure directly the in situ temperature. This is relatively cheap and simple, and gives a direct measurement of temperature differentials. The thermocouple should be located at the center and at the surface to measure the temperature extremes and hence the maximum differential. Monitoring can be either manual or automatic.
The measurement can be done on 2 directions, i.e. horizontal and vertical. Horizontal line is purposed to get how concrete differs from I point to other point horizontally, in usually the distance between the points is about 10 meters. Vertical line is purposed to get the picture how concrete differs among the bottom, center and top layers.
Vertically, concrete will be observed at 3 points, where the exact locations are :
o Top layer is at 15 to 25 cm below the upper rebars
o Center point is in between top and bottom points.
o Bottom layer is at 15 to 25 cm above the lower rebars.
Top layer will contact direct to weather, and assumed there will be heat leaks from concrete to air. Bottom layer will contact to ground lean concrete, and assumed there will be small heat leaks from concrete to ground.
The thermocouple is located at the position and tied up with wire to make sure it will not move during concreting. 3 cables at the same position horizontally will be collected and inserted into a PVC pipe (V2 inch diameter ) to protect any cable jointing from concrete fall. The PVC pipe is installed vertical and supported by a steel bar. The steel bar is tied up properly and pipe is tied up to the bar.

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Main Priority Factors Causing Nonconforming Product (NCP) On Readymix Concrete Product

American Concrete Institute (ACI) Manual of Concrete Practice Part-1 defines ready-mix concrete (RMC) as concrete made for customer or purchaser in plastic and unhardened state. Many complaints from customers in relation with nonconforming products (NCP) requires RMC producers to improve the quality of RMC. One of its efforts is to find out the main problems of nonconforming products.

An investigation was carried out main problems on five dry-mix plants of a RMC company. This research was identified factors causing fourteen types of NCP using cause-effect analysis. In order to determine the scores of importance and cause effects of the fourteen NCP types, a preliminary questionnaire was carried out. The result of this questionnaire indicated that only five out of fourteen types of NCP have significant financial effects on the company, namely; unspecified compressive strength, late product delivery , unspecified slump, less volume than ordered, and too fast or too late setting time. Further investigation was carried out by conducting pairwise comparison between two factors of the many factors causing each NCP. Matrix analysis of pairwise comparison was used to obtain priority scores of the factors causing the five NCP types.
It was found in this research, that the main priority factors causing the five types of NCP on the entire plant, were non-standard quality control equipment, low quality of fine aggregate, quality of chemical admixtorure, skill of quality control technician, inaccurate weighting by operators, skill of scheduling and delivering operators, accuracy of weighting tool, communications system between plant and site, accuracy of aggregate moisture content, fetch of delivery and traffic problems and influence of hot weather. There was no difference of main priority factors causing each NCP between the five plants; the difference occurred only on the sequence of the priorities.

Key Words: readymix concrete, nonconforming product, pairwise comparison.

For management topic http://www.holikbisnis.com

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How to Handling Volume Complains from Customers

“Pak volume beton kok kurang, hitungan saya sekian kubik padahal volume tulangan sudah saya abaikan, gimana nich!” komplain diatas sering terlontar dari customer. Memang banyak faktor sehubungan dengan kekurangan volume beton yang sudah terkirim. Kekurangan barangkali akibat kesalahan hitung pencampuran, misalnya seharusnya 4.5 m3 di mixing hanya 4 m3, meskipun hal ini jarang terjadi.

Mengapa masalah komplain volume kadang terjadi? Dalam AS 1379 dikatakan bahwa volume yang telah diukur dilapangan bisa berbeda dengan volume dari beton segar (fresh concrete) yang telah dicor, ini sebagai hasil dari pemadatan dan pengaruh pengerasan beton, perubahan temperature, penurunan bekisting dan fartor lain yang relevan.

Deviasi volume beton yang dihasilkan dengan volume terkirim dapat disebabkan oleh dua faktor yaitu faktor internal (supplied/readymix) dan faktor eksternal (dari konsumen dan lingkungan). Pengaruh yang disebabkan oleh faktor eksternal sepenuhnya tidak bisa dikontrol oleh pihak readymix. Sebaliknya bila deviasi volume disebabkan oleh proses produksi dari readymix maka hanya dapat diperbaiki oleh pihak readymix itu sendiri. Hal ini dapat ditelurusi pertama dengan mencocokkan volume yang tertulis didocket dengan jumlah batch yang telah dilakukan oleh operator. Kedua dengan melihat yield betonnya. Bila masalah diakibatkan oleh hal kedua, maka segera dilakukan koreksi terhadap perubahan kandungan proporsi campuran dan sebagainya.

Apa sih yield beton itu! dan bagaimana mengetahuinya. Yield beton adalah volume beton yang dihasilkan dari campuran component material beton yang diketahui beratnya (kuantitas). Untuk mengetahuinya dilakukan perhitungan yield yang dijelaskan dalam ASTM C 138-92 yaitu volume beton segar yang dihasilkan adalah ditentukan dengan membagi berat total material tertimbang (batch) dengan berat rata-rata beton per meter-kubik, bila nilainya sama dengan satu (1) maka volumenya tepat. Toleransi yang diberikan oleh AS 1379 adalah maksimum 2% akibat dari variasi-variasi dari penimbangan, moisture content, temperature dan slump.

Umumnya masalah perbedaan volume tidak banyak disebabkan oleh pihak readymix tapi oleh faktor eksternal. Tampaknya perbedaan besar yield terjadi ketika beton segar tidak cukup mengisi bekisting yang telah dihitung volumenya.
Berikut dijelaskan beberapa faktor eksternal yang hanya bisa diatasi oleh pihak kontraktor sendiri (kecuali pengaruh lingkungan).

• Ketidaktepatan level, akurasi atau kesalahan perhitungan volume atau hitungan ketebalan rata-rata slab adalah contohnya. Variasi 5mm saja sudah menghasilkan 5 % berbedaan volume dari slab tebal 10 cm.
• Lendutan atau pergeseran bekisting yang diakibatkan oleh tekanan beton plastis.
• Over penggalian, permukaan tidak rata dari subgrade (dasar tanah), penurunan dasar tanah juga potesial menyebabkan masalah volume.
• Beton sisa dari pekerjaan. Sejumlah kecil beton sisa tiap kali pengecoran, sisa beton di hopper dan pipa concrete pump, beton untuk benda uji dan sebagainya patut diperhitungkan volumenya.

Untuk mencegah atau memi-nimalkan pertentangan yield yang diakibatkan oleh faktor eksternal adalah pihak konsumen atau kontraktor itu sendiri yang bisa melakukan dan mengatasi untuk bertindak, antara lain sebagai berikut:

• Pengukuran bekisting yang akurat. Perhitungan kebutuhan beton bukan didasarkan gambar rencana. Juga perhitungan ketebalan rata-rata harus realistik. Hati-hati dalam pengecoran volume banyak untuk memperhatikan truk terakhir sehingga kebutuhan beton cukup.
• Perkiraan beton terbuang. Harus diperhitungkan terhadap total kebutuhan beton adalah beton sisa di pipa dan hopper concrete pump, beton untuk buat benda uji, pemadatan, pengaruh lingkungan seperti temperature, loss air content dan sebagainya.
• Bekisting aman dan kuat, sehingga saat pengecoran mampu mendukung beton plastis dan alat-alat dan orang yang bekerja diatasnya.
• Permukaan subgrade harus baik, khususnya untuk struktur slab on grade harus rata dan dipadatkan pada level yang tepat.

Jika hal diatas sudah dilakukan oleh kontraktor dan masih terjadi kekurangan volume, maka harus dilakukan yied test seperti yang dijelaskan dalam ASTM 138, AS 1379 atau SNI 03.2002.

Itulah sekilas uraian tentang penyebab perbedaan antara volume aktual dan yang diorder. Semoga bisa jadi dasar pengetahuan dan lebih konfiden dalam menghadapi komplain volume dari customer.

Referensi:
ASTM Standards, Annual Book (2000), “Concrete and Aggregates”, Vol.04.02, American Society for Testing and Materials, Philadelphia.
Australian Premixed Concrete Association (2000), “Assessing Concrete Volume Requirements” Technical Bulletin, Astralia.

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