MATERIAL DEVELOPMENT OF CONCRETE

MATERIAL BETON

Concrete is a material that is very common in civilian buildings and the use and manufacture has been widely recognized in the community. The use of concrete is more commonly construed in reinforced concrete, which is concrete with reinforcing steel as reinforcement. Although the concrete itself is a mixture of portland cement or other hydraulic cement, fine aggregate, coarse aggregate and water, with or without additional ingredients to form a solid mass (SK SNI T-15-1991-03, page 2), in which water and cement as adhesive as well as sand and gravel as a filler material. In addition to these materials, it is sometimes given additional materials to obtain concrete with the quality and specific properties.
In terms of balance, the concrete has advantages - advantages such as easily molded according to the shape desired, fire resistant, durable / durable, the material easily available everywhere and relatively inexpensive, using unskill and semi-skill workers more than skill labor and equipment required is not too sophisticated. Although for a complex structure requires specification of labor and equipment-specific equipment. In addition to advantages, concrete also has its bad side is low voltage, low and volume duktibilitas unstable.
So rapidly knowledge about concrete, so that when this has been done a lot of research on concrete. Such as by using new materials, specific treatments or other supplemental materials. In addition to their constituent material, concrete quality is also determined by environmental conditions and workforce skills. Some of the things that contribute to the constituent material is a chemical compound content, material physical properties such as hardness, moisture, grain shape, gradation, porosity, etc., the content of impurities and cement fineness. As for the condition of the environment and labor skills, the role is moisture and weather temperature, method of implementation, precise work and the equipment used. The following is a brief description of the constituent materials of concrete.

1. PORTLAND CEMENT

Cement is a type of material that has adhesive and cohesive properties that allow the attachment of mineral fragments into a solid mass. Called portland cement because after hardens similar to Portland stone which is found in near Dorset, England (Wang & Salmon, 1992). The chemical composition of portland cement consists of a major component (main component) and a minor component (complement component). The major components, minor components and oxide-oxide boundary is (BS 4550, (1978)):

Component Of Portland Cement
Component	Senyawa	Formula	Simbol
Major	Tricalcium silicate b-Dicalcium silicate Tricalcium aluminate Tetracalcium aluminoferrite Gypsum Calcium oxide	3CaO. SiO2 2CaO. SiO2 3CaO. Al2O3 4CaO. Al2O3. Fe2O3 CaSO4. 2H2O CaO	C3S b-C2 S C3 A C4AF CSH2 C
Minor	Magnesia Potassium Oxide Sodium Oxide Titanium Oxide Manganese Oxide Phosporus Pentoxide	MgO K2O Na2O TiO2 Mn2O3 P2O5
Keterangan :C = CaO, S = SiO2, A = Al2O3, F = Fe2­O3, M = MgO,             = SO3 , K = K2O3      N = Na2O dan H = H2O

Limit of Oxide Composition
Oxide	Content ( % ) by Weight
C	60 – 70
S	17 – 25
A	3 – 8
F	0,5 – 6,0
M	0,1 - 4,0
K, N	0,2 – 1,3
	1 – 3

The fineness of the cement grains will affect the speed of reaction of cement with water and setting time of cement paste. The finer the grain of the vast permukannya cement will be greater. With the growing extent of the cement particle surface reaction with water will increase so that it will accelerate the hardening time. The level of grain fineness of cement is usually expressed in specific surface, ie the surface area of ​​the cement grains per one gram of weight. Specific surface is usually measured by the Blaine air permeability test or test turbidimneter Wagner (Portland Cemen Association, Principles of Quality Concrete (1975)).
In portland cement, a very important thing to note is binding and pengerasannya time (time of the set). This relates to such things as transportation, casting, compacting and several other conditions. Conditions of plastic concrete mix should be possible for some of these conditions to be easily done before compacting. This is because in case of dry cement containing large latent energy is energy which became active after being given water and the resulting mass into palstis so easy to do.
As a hydraulic binder, portland cement hardening and binding depends on the chemical reaction between water and cement. This reaction occurs in two different periods, namely binding and hardening period. Binding is the process of transition from the hard state to a plastic state. Moderate hardening is the process of adding strength after bonding. The process is initiated by the binding of the initial binding, the current state becomes stiffer as of the start of the cement after the cement was mixed with water. The next process is binding, ie the period between the beginning of the cement becomes stiffer (early binding) and the circumstances in which the cement paste becomes hard, although not strong enough. The current state of hardened is called late binding. Specifications for cement requires initial binding of cement shall not be less than one hour. Initial binding of cement is determined by several factors, namely age cement, temperature and amount of water used. Setting time is determined by the Vicat test is to penetrate 1cm in 30 seconds on the pasta that meets the normal consistency and is expressed in units of hours or minutes (Portland Cemen Association, Principles of Quality Concrete (1975)).

2. A G G R E A T
Aggregates are granular materials, such as sand, gravel, crushed stone and slag iron furnace, which is used together with a binder medium to form a hydraulic cement concrete or mortar (SK SNI T-15-1991-03). Its function is as a filler material and usually occupies about 75% of the total content of the concrete, therefore large influence on the properties and durability of concrete. For example, the concrete resistance to the effects of freeze-thaw, wet-dry, heating-cooling and abarasi-damage due to chemical reactions (Portland Cemen Association, Principles of Quality Concrete (1975)).
Given that the aggregate occupies a sizeable amount of the volume of concrete and affect the properties of concrete, it is essential to these materials are given more detailed attention. This material is relatively inexpensive, so it is advisable to use this material as much as possible to make it more economical. Besides, it can reduce shrinkage due to hardening of concrete and also affects the expansion coefficient due to heat. The selection of the type of aggregate that will be used depends on the quality of the aggregate, the availability at the location, price and type of construction that will use it.
Aggregates can be classified according to several criteria. Based on its size, coarse and fine aggregates known. In terms of its density, known lightweight aggregate (300 - 1800 kg/m3), normal (2400-3000 kg/m3) and the aggregate weight (> 4000 kg/m3). Based on the production process, known as natural aggregate (natural aggregates) and artificial aggregates (aggregates artificially). In addition it is also categorized based on mineral content, such as silica group minerals, carbonate minerals, iron sulphide minerals, clay minerals, micaceous minerals, sulfate minerals, ferromagnesian minerals and iron oxides (ASTM C 294, (1975)). Classification used in this paper is based on its size, ie fine aggregate (fine aggregates) and coarse aggregate (coarse aggregates).

2.1. FINE AGGREGATE
 

FINE AGGREGATE

Fine aggregate is an aggregate with a maximum grain size of 5.0 mm which can be either natural sand as a result of disintegration of rock that is naturally processed sand from stone crushing industry or a combination of both. Function of fine aggregate on the concrete is as filling material. Knowledge of the properties of fine aggregate is very important to be able to get the desired quality of the concrete in accordance with a more economical price. Some properties of fine aggregate are:
 amount retained on the next of a series of sieve sieve does not exceed 45% of the previous sieve.í
 smoothness modulus of 2.3 to 3.1.í
í To aggregate to the transport from the source, should not be changed modulusnya fineness greater than 0.2 of fineness modulus at the source. Fineness modulus changes may occur upon arrival at destination.
 As much as possible do not contain a substance impurities like mud, clay, free particles and organic substances are harmful.í Unless enclosed testing that the aggregate can be used.
í hardness test results five times, giving an average loss not greater than 10%, compared with the use of sodium sulfate or magnesium sulfate (ASTM C - 33, (1995)).

 
2.2. AGGREGATE ROUGH

Coarse aggregate is aggregate that has a grain size of 5-40 mm. This material can be produced from the natural disintegration of rock in the form of crushed stone (Natural Aggregates) or from industrial rock breakers (artificially Aggregates). In general, the coarse aggregate may consist of natural gravel, broken natural gravel, broken stone, which had cooled blast furnace slag, or broken hydraulic cement concrete or a combination of these materials. Before use coarse aggregate properties should be adjusted to the requirements set forth in ASTM C-33.
Some things to consider in the selection of aggregate is:
Aggregate Size
Construction portion size should not be less than 4 times the maximum aggregate size and not larger than 1/5 the smallest distance between the side areas of reference. In addition to the maximum aggregate size should not be larger than ¾ times the minimum clearances between reinforcement and not greater than 1/3 times the slab thickness and concrete cover layer should be thicker than the maximum size of aggregate.
Pollutant
Aggregate shall not contain impurities which materials will eventually complicate the making and casting of concrete, which does not produce durable concrete and ugly surface and reduce the compressive strength. The materials that may contaminate the aggregate are:
Clay and silt falls
The effect is to cover the surface of the aggregate so that the bond between the cement paste and aggregate is reduced. High absorption properties will increase water demand, which in turn reduces the strength and durability of concrete as well as sensitive to shrinkage and expansion.
Charcoal stone, wood fragments and Gypsum
Stone and wood charcoal fragments will reduce the compressive strength of concrete and concrete surfaces become dirty and ugly. Moderate casts its existence can be coarse grains and refined. Coarse granular casts are not so harmful to concrete, but its fine grain would endanger the concrete as it reacts perfectly with cement and will eventually expand. Standard portland cement plaster use a maximum limit of 5%.
Organic matter and Organic Salt
Organic materials can be materials such as decomposed humus or soil that contains organic. The effect will be negative to the development of early compressive strength, but after a long period of time the strength of the concrete will increase again (recovered). Moderate organic salts can be sulfate salts. The effect does not affect the early development of the compressive strength of concrete but in old age.

Violence
Have sufficient hardness to resist wear, breakdown degradation (reduction in quality) and disintegration (decomposition) is currently experiencing harsh movements in the mixer as well as receiving friction during casting and solidification. Aggregate hardness tested using the Los Angeles Machine Test.

Smoothness
Aggregate seamless physically will not experience large volume changes due to heating and cooling or wetting and drying. Rock particles which are physically soft power absorption will have a great, easy to break and easy to shrink / swell due to the influence of water, so when the weather changes will bubbling surface which when broken will leave a hole in the concrete surface. Aggregate smoothness is affected by porosity, ie pore continuity and amount. Presence of the pore space will reduce the aggregate solid part. Consequently easily take in water and aggressive solutions, so that the compressive strength of concrete is reduced, easy to wear, reduced elastic modulus and large shrinkage occurs.

Form of granules
A series of experiments have proved that the coarse aggregate concrete with spherical air cavity will have fewer than the concrete of the angular coarse aggregate. Thus it takes a lot more mortar for concrete with aggregate beragregat angular than rounded. Known to some kind of form of granules, such as round, irregular, angular, flat, elongated and flattened and elongated.

3. WATER

Water is one of the important ingredients in the manufacture of concrete in order to function as a reactant chemical reaction between the cement with water and aggregate to wet mix to lubricate the process to be easy. Semen will not be able to function okay without reacting with the water. Because of its function as a reactant, then the quality of the water used must be completely controlled and in accordance with the standards set (Portland Cemen Association, Principles of Quality Concrete, (1975)).
Water containing impurities materials will cause serious effects on the setting time, the concrete surface discoloration, reduction in compressive strength and the effect of corrosion on the reinforcement of concrete (Portland Cemen Association, Principles of Quality Concrete, (1975)). Generally potable fresh water, whether treated by companies as well as non-treated water can be used for the manufacture of concrete. Freshwater medium that can not be taken not to be used for the manufacture of concrete, except when first tested. Water testing can be done in two ways, namely test with mortar test specimen and test chemical content. Water will meet the standards when the mortar compressive strength at the age of 7 and 28 days, at least 90% of the compressive strength of mortar using potable fresh water or distilled water (Portland Cemen Association, Principles of Quality Concrete, (1975)). Several compounds in natural water without treatment can adversely affect the quality and nature of the concrete, among others:

Salts Chloride and Sulfate
Its existence may damage the concrete and corrode the metal embedded in the concrete. Levels of sulfate in the water depends on the levels of sulfate in the aggregate and cement because that determines the amount of sulfate contained in concrete. Sulfate levels in the concrete should not exceed 4% of the weight of cement SO3.
Organic substances, inorganic salts, Carbonate and Bicarbonate
The effect will affect the cement binding the slow time (setting time) and lower compressive strength of concrete. The water is dark colored, foul odor or contain moss needs special attention and should be tested before use.

Dirt and industrial materials
Some of them are sugar, salt nitrate, some kind of acid, oil, dirt and combustion fibers. Its effect is to slow down the setting time and reduce the strength of concrete. These materials can be found in industrial waste water or water from a muddy or flooded areas. Before use, the water should be tested content terlabih feces and tested over time and the strength of the binding cement concrete.

Some Kind of Oil
Some type of oil is also sometimes terandung in water and generally will reduce the strength of concrete. Mineral oil (mining), if it is not mixed with oil derived from animal or plant sources had little influence on the addition of the power compared to other oil types. However, mineral oil in an amount greater than 2% by weight of cement will reduce the strength of concrete is greater than 20% (Portland Cemen Association, Principles of Quality Concrete (1975)).

4. Admixture (ADDITIVES)
In general it can be said that the history of the use of cement and concrete is started from the time of Egypt and Greece, although some claim it occurred since 9000 years ago, where the ancient cement and concrete using natural pozzolan (Molinowski & Garfinkel, (1988)). The development of cement and concrete knowledge next stained with widespread use of mineral additions such as fly ash (fly ash), silica fume, rice husk ash, and ground granulated blast furnace slag (blast furnace slag). In today's production of cement, mineral admixture is very interesting as contributing material to add to or change the concrete characteristics such as energy reduction and carbon dioxide. Especially is needed to increase the performance of concrete in terms of high strength, high durability and reduced heat of hydration. It could be argued that the use of an additive in concrete, is basically meant to improve the properties of concrete in accordance with the desired (Nagataki, (1992)).
Many people also use the term material aditive for admixture, but basically is the same sense, the additional material. The emphasis of the second term, ie aditive an ingredient that is added during the production process dipabrik cement, admixture was added at the time of implementation of field concrete (mixing). The use of additives should be preceded by tests in the laboratory. The additional ingredients may include:
1. Air entraining Agent (ASTM C 260)
That additional material whose function is to increase the air content in the concrete so that the concrete resistant to freezing and washing conditions, especially for areas with winter snow.
2. Chemical Admixture (ASTM C49)
Chemical admixture is a chemical produced by a particular industry such as chemicals whose function is to control the setting time either speed up or slow down, reduce water requirements, increase slump and others.
2. Mineral Admixture
ie additional minerals form solid materials by first smoothed. Its function is to improve the properties of concrete that is easy to work and to improve the strength and durability of concrete. Some of them are additive pozzolan, slag, fly ash (coal), rice husk ash and silica fume.
4. Other Supplementary Material (Miscellanous Admixture)
Are included in this category of materials are materials in addition to those mentioned above, additional types of materials such as polymers, fiber mash, corrosion resistance, additional materials which can be inflated as well as additional materials to improve bonding (adhesive).

Effectiveness of the use of admixtures depends on several things, such as the type and amount of cement, concrete slump, stirring time, and the water content of the air (Portland Cemen Association, Principles of Quality Concrete, (1975)). In general, the use of a chemical admixture is limited to 5% by weight of cement, mineral admixture being able to be used in a greater proportion. The additional material may be added to the concrete mix or mortar, before or after mixing (Nagazaki, (1992)).

GAME!Klick here!

INSTALASI BANGUNAN(Sistem Pemipaan Air Bersih dan Air Kotor)

1.      SISTEM PEMIPAAN AIR BERSIH

Sumber Air bersih didapat dari 2 sumber antara lain : dari PDAM dan dari air sumur (deep well), dimana air yang bersumber di keduanya ini akan masuk ke dalam ground water tank. didalam ground water tank terdapat 2 sekat bak penampungan air yaitu raw water tank dan clean water tank,
Secara detail bagian-bagian dari sistem air bersih ini adalah sebagai berikut:

• Deep Well

Sumber pengadaan air bersih berasal dari air sumur (deep well). sumur ini menyuplai seluruh kebutuhan air bersih, baik untuk kebutuhan air sehari-hari maupun untuk sistem pemadam kebakaran. Air dari kedua buah sumur tersebut disalurkan ke bak air Raw Water Tank menggunakan pipa GIP (Galvanized Iron Pipe). Deep Well akan mengisi air secara otomatis jika air pada Raw Water Tank kosong dan akan mati jika sudah penuh.

• Transfer Pump

Transfer Pump atau pompa transfer berfungsi untuk memindahkan air dari Ground Water Tank menuju ke roof water tank. Transfer Pump biasanya berjumlah dua unit dimana satu pompa bekerja dan pompa yang lain sebagai cadangan.

• Sand Filter

Sand Filter berfungsi untuk menyaring kotoran didalam air yang berasal dari bak air Raw Water Tank. Peralatan ini berjumlah dua buah unit dan dipasang secara paralel, dimana jika satu Sand Filter bekerja maka Sand Filter yang lain sebagai cadangan. dan untuk membersihkannya dioperasikan secara manual (manual back wash). Sand Filter ini dilengkapi dengan pressure gauge di bagian pipa masuk dan pipa keluar untuk mengukur tekanan air.

• Packaged Booster Pump

Berfungsi untuk mendistribusikan air bersih dari roof tank ke pengguna. Distribusi air bersih pada dua lantai teratas menggunakan packaged booster pump, sedangkan untuk lantai-lantai dibawahnya dialirkan secara gravitasi.
Perlengkapan dan aksesoris di dalam ruang pompa antara lain :
- Butterfly Valve : membuka atau menutup aliran air
- Gate Valve : membuka atau menutup aliran air
- Strainer : menyaring kotoran pada bagian hisap pompa (suction)
- Flexible Joint : menahan getaran pompa terhadap instalasi pipa
- Check Valve : menahan balik aliran air
- Pressure Tank : mengatur (setting) besarnya tekanan air
- WLC : Water Level Control, mengendalikan pengoperasian pompa berdasarkan pada tingkat ketinggian air di dalam bak air
-Floating Valve : membuka atau menutup aliran air secara otomatis berdasarkan ketinggian air di bak air
- Pressure Switch : mengontrol pengoperasian pompa berdasarkan pada besarnya tekanan air
- Pressure Gauge : alat untuk mengukur besarnya tekanan air
- Vent Cap : membuang udara yang terjebak di dalam bak/tangki
- AAV : membuang udara yang terjebak di dalam instalasi pipa secara otomatis
- Pressure Reducing Valve (PRV)
PRV digunakan untuk menurunkan tekanan air didalam instalasi air bersih supaya air yang keluar dari kran air bersih mempunyai tekanan yang cukup.

2.      SISTEM PEMIPAAN AIR KOTOR / BUANGAN

Sistem Pembuangan Air Buangan, merupakan sistem instalasi untuk mengalirkan air buangan yang berasal dari peralatan saniter maupun hasil buangan dapur.
Sistem Pembuangan Air Kotor dibedakan berdasarkan cara pembuangannya :

·         Sistem pembuangan air campuran, yaitu sistem pembuangan dimana air kotor dan air bekas dialirkan kedalam satu saluran / pipa.

·          Sistem pembuangan air terpisah, yaitu sistem pembuangan dimana air kotor dan air bekas masing-masing dialirkan secara terpisah atau menggunakan pipa yang berlainan.

Sistem Pembuangan Air Buangan dibedakan berdasarkan perletakannya:

1.      Sistem pembuangan gedung, yaitu sistem pembuangan yang berada didalam gedung.

2.      Sistem pembuangan luar, yaitu sistem yang berada diluar gedung, disebut juga riol gedung.

Sebelum air buangan dari peralatan saniter maupun dari buangan dapur dibuang ke saluran umum / kota maka harus dilakukan pengolahan terlebih dahulu dengan Sewage Treatment Plant   (STP), sehingga memenuhi ambang baku yang dipersyaratkan.

Peralatan Utama dan Fungsi

a.       Pompa Submersible, berfungsi untuk menaikan level air kotor pada daerah level terendah ke instalasi pengolah yang levelnya lebih tinggi.

b.       Sewage Treatment Plant ( STP )
STP berfungsi sebagai pengolah air buangan sehingga memenuhi persyaratan sebagai air buangan rumah tangga ( domestic waste ), yaitu dengan ketentuan :
a. Kandungan zat tersuspensi rata-rata dalam waktu 24 jam adalah 20 mg / liter.
b. Kebutuhan biologi untuk oksigen ( BOD ) rata-rata dalam waktu 24 jam adalah 20 mg / liter dengan kapasitas maksimum yang diperbolehkan s/d 30 mg / liter.
     Demikianlah informasi mengenai Sistem Pemipaan Air Bersih dan Air Kotor. emoga bermanfaat. Terimakasih.

KONSEP DASAR DAN PENDEKATAN PERENCANAAN

Cara yang digunakan untuk memberikan gaya prategang adalah dengan menggunakan tendon baja yang diberi tegangan dengan cara di tarik. Ada dua metode pemberian tegangan prategang, yaitu Pretensioning (pra penarikan) dan Postensioning ( Pasca tarik atau Penarikan purna ). Pretensioning yaitu metode pelaksanaan yang memberikan gaya prategang sebelum beton tersebut dicetak (dicor). Sedang pos tensioning adalah metode pelaksanaan yang memberikan gaya prategang setelah beton dicetak.  

A. PRETENSIONING – PRA TARIK

Metode ini kebanyakan dilakukan di lapangan dan biasanya tempat mencetakan (castng yard) dilakukan dekat dengan tempat pemakaian. Prosedur utamanya adalah sebagai beriku :

1. Tendon dipasang memanjang diantara dua jangkar di tempat pengecoran. Tendon tersebut kemudian ditarik hingga mencapai nilai tegangan tarik f_si yang tidak lebih dari 85% kuat tariknya (f_pu) dan tidak lebih dari 94% kuat luluh (f_py). Hal ini sesuai dengan yang disyaratkan dalam SK SNI T-15-1991-03 pasal 3.11.5 ayat 1. Secara matematis dapat dituliskan f_si ≤ 85% f_pu dan f_si ≤ 94% f_py. Setelah tendon ditarik, tendon tersebut dijangkar sedemikian rupa pada kedua ujungnya agar gaya tarik (prategang) yang telah diberikan tidak hilang.
2. Setelah dilakukan penarikan tendon, maka acuan beton (bekisting) segera dipasang sesuai dengan bentuk yang diingingkan. Lihat gambar berikut. 

Kabel ditarik dengan tegangan f_si dan tegangan tersebut dipertahankan dengan jalan dijangkar pada kedua ujungnya dan bekisting dipasang. Siap untuk dilakukan pengecoran.

3. Setelah bekisting terpasang dengan baik, segera dilakukan pengecoran beton yang berisi tendon dalam keadaan tertarik. Kemudian dilanjutkan dengan tahap perawatan pengerasan beton. Pada proses pengecoran ini, pelaksanaannya harus disertai dengan pengendalian keamanan dan kualitas pekerjaan mengingat resiko kecelakaan dan yang lebih penting yaitu beton yang akan dihasilkan adalah beton bermutu tinggi serta antara beton dan tendon harus saling melekat dengan baik.  Lihat gambar berikut. 

Keadaan setelah dilakukan pengecoran. Pada saat ini tendon belum diputuskan/anker belum dilepas. Sehingga tegangan dalam tendon masih berupa tegangan aksial tarik.

4. Apabila beton telah mencapai kekerasan dan kekuatan tertentu ( f_ci’ ), maka tendon dipotong atau jangkar dilepas pada kedua ujungnya sehingga terjadi pelimpahan tegangan ( T_o ) kepada seluruh penampang efektif (transformasi) beton. Pada saat ini gaya tarik yang bekerja pada tendon dilimpahkan kepada beton, tetapi sudah dalam keadaan tekan (desak). Jadi gaya tarik pada tendon berubah menjadi gaya tekan sesaat setelah anker/jangkar dilepas. Tegangan-tegangan yang timbul sesaat setelah tendon dipotong/dilepas dari jangkarnya disebut tegangan pada saat transfer (pelimpahan tegangan). Dengan diputusnya tendon dan berlangsung pelimpahan tegangan, maka beban mati (berat sendiri) sudah diperhitungkan bekerja serentak dengan gaya prategangan. Lihat gambar. 

Keadaan tegangan dalam tendon setelah tendon diputuskan/anker dilepas.  Tegangan dalam tendon sudah berupa tegangan aksial tekan.

Kombinasi beban mati (berat sendiri) dengan prategangan (belum terjadi kehilangan gaya prategangan).  ……… beton belum dibebani beban layan

Salah satu fenomena yang tidak bisa dipungkiri yaitu akan terjadi kehilangan gaya pratekan (akan dibahas khusus) saat mulai bekerja penuh, maka pada saat dibebani dengan beban sebenarnya (beban mati dan hidup)  keadaan tegangan akan seperti pada gambar berikut :

 
Keadaan setelah dibebani beban layan

Pada keadaan ini (belum dibebani beban layan), batasan harga tegangan tarik pada sisi atas balok tidak melebihi 0.25 akar f'c(sekitar 40% kuat tarik) dan tegangan tekan pada sisi bawah tidak lebih dari 0.6 f_ci’. Apabila tegangan tarik terhitung melampaui nilai tersebut, maka harus dipasang tulangan tambahan (tulangan biasa) di daerah tarik tersebut untuk memikul gaya tarik total dalam beton yang dihitung dengan penampang utuh. Sehubungan dengan pemotongan tendon, kekuatan yang disyaratkan untuk melakukan pemotongan tendon adalah beton pada saat itu sudah mencapai kekuatan 27.5 Mpa sampai 30 Mpa (waktu pengerasan ± 24 jam).   

5. Setelah beton sudah cukup kuat dan memenuhi persyaratan, maka komponen prategang dapat dilepas dan diangkat dari cetakannya dan dipindahkan ke temnpat penyimpanan/pemasangan, sehingga tempat pencetakan dapat digunakan lagi untuk pencetakan berikutnya.

Beton yang sudah dicetak dapat diangkut ke lokasi pembangunan dalam keadaan siap tinggal dipasang pada tempatnya. Apabila keadaan memungkinkan baik dari segi fisik maupun ekonomi, maka lahan pencetakan disiapkan menjadi satu dengan lokasi pembangunan sehingga dapat mengurangi biaya transportasi. Setelah digunakan beton akan menerima beban layan seperti beban mati, hidup atau suatu kombinasi beban, maka kehilangan gaya pratekan akan terjadi dan beton akan bekerja seperti biasa yaitu pada sisi atas akan terjadi tekan dan sisi bawah akan terjadi tarik. SK SNI T-15-1991-03 memberikan batasan :

v  Tegangan tarik pada sisi bawah tidak boleh melebihi  ½ akar f'c, nilai ini diambil lebih kecil dari nilai modulus runtuh beton normal f_r akar f'c= 0.7 dengan alasan bahwa kemungkinan bahaya retak atau tekuk secara tiba-tiba pada daerah tersebut boleh dikatakan kecil umumnya posisi tendon berada di dekat sisi bawah. Pembatasan tegangan tarik ijin ini sudah memperhitungkan penyediaan tebal selimut beton dimana untuk daerah korosif tebalnya ditambah di atas syarat minimum dalam SK SNI T-15-1991-03.   

v  Tegangan tekan pada sisi atas tidak boleh melebihi  0.45 f’c    
B. POST TENSIONING – PASCA TARIK

Seperti dikatakan di depan bahwa post tensioning adalah suatu cara pemberian tegangan pada tendon dimana tendon baru ditarik setelah betonnya dicetak terlebih dahulu dan telah cukup keras untuk menaahan tegangan yang akan diberikan. Prosedur pelaksanannya adalah :

1.      Bekisting dipasang sesuai dengan penampang dan rencana letak komponen tersebut. Bekisting tersebut sudah dilengkapi dengan pipa selongsong lentur yang dibuat dari plastik atau bahan metal yang akan menyelubungi tendon. Pipa selongsong ini letaknya diatur dan ditahan untuk membentuk pola tertentu sesuai dengan momen perlawanan yang direncanakan.

2.      Bekisting kemudian di cor dengan adukan beton, tetapi tetap dijaga agar pipa selongsong tendon tetap kokoh pada tempatnya serat tidak kemasukan adukan. Setelah itu dilakukan proses perawatan perawatan beton sampai mencapai kekuatan tertentu.

3.      Selanjutnya tendon dimasukkan kedalam pipa selongsong yang sudah menyatu dengan beton. Namun terkadang juga pipa selongong sudah diisi tendon (belum ditarik) kemudian diletakkan di dalam bekisting. 

4.      Tendon kemudian ditarik dengan dongkrak pada salah satu ujungnya dan jangkar mati atau plat penahan pada sisi lainnya. Kadang jangkar mati atau plat penahan sudah dipasang tertanam pada ujung komponen. Pengikatan tendon oleh jngkar diatur sedemikian rupa agar tendon tidak slip (tergelincir) agar tegangan tarik tetap berada dalam tendon. Pada sistim postensioning ini, sudah terjadi kehilangan gaya pratekan pada saat penarikan yang disebabkan oleh perpendekan elastis, kehilangan tegangan akibat gesekan dan sebagian momen akibat beban sendiri sudah bekarja. Oleh sebab itu gaya dongkrak harus diperhitungkan terhadap kehilangan gaya tersebut. 

5.      Pipa selongsong yang berisi tendon yang sudah ditarik, bisa diisi dengan pasta semen atau juga bisa dikosongkan saja. Apabila digunakan tendon bonded, yaitu antara pipa selongsong dan tendon terjadi lekatan, maka kedalam pipa disuntik (grouting) dengan pasta semen. Hal ini dilakukan dengan pertimbangan lingkungan yang kemungkinan korosif. Pada sistim bonded ini, tegangan akibat beban layan dihitung berdasarkan penampang transformasi seperti ada metode Pra Tarik. Tetapi bila digunakan tendon Unbonded, yaitu tendon dan pipa tidak saling melekat, maka tidak perlu di grouting. Hal ini dilakukan mungkin dengan perhitungan ekonomis dan tergantung pada kebutuhan perlindungan. Namun untuk keadaan ini gaya prategang hanya diperhitungkan bekerja terhadap penampang betonnya saja (bukan penampang transformasi).

6.      Jangkar pada kedua ujung dikunci mati dan kemudian ditutupi atau dilindungi dengan lapis pelindung secara permanen.