Shotcrete atau Beton Semprot ( Spray Concrete )
Shotcrete atau gunite pertama kali ditemukan oleh Carl Ethan Akeley (1864-1926) pada 1910. Arsitek Amerika ini telah terinspirasi untuk mewujudkan reproduksi yang nyata dari dinosaurus untuk sebuah taman wisata. Mengingat ukuran struktur, ia mempunyai ide untuk mengembangkan "semen gun" mesin yang memungkinkan penyemprotan dari cementitious mortar, maka dinamakanlah Shotcrete.
Pada tahun yang sama, Kaspar Winkler mendirikan Sika. Sejak saat itu Sika telah memberikan kontribusi sangat besar bagi perkembangan teknologi Shotcrete , baik bahan kimia dari material shotcrete itu sendiri ( Sigunit ) maupun peralatannya yang dikenal dengan nama Alliva Machine.
Bidang Aplikasi Shotcrete
Shotcrete terutama digunakan dalam proyek konstruksi bawah tanah sebagai perkuatan struktural awal ataupun permanen untuk bangunan struktur seperti jalan bawah tanah, terowongan kereta api, pembangkit listrik tenaga air (PLTA), , tambang bawah tanah, kereta bawah tanah, dll tempat penyimpananNamun shotcrete juga dapat digunakan untuk stabilisasi lereng mencegah supaya tidak longsor , kolam renang, saluran air, perbaikan beton, inner lining arsitektur dan struktur. Kira-kira 90% dari shotcrete akan diterapkan ke dalam proyek-proyek konstruksi bawah tanah. Total volume shotcrete yang diaplikasikankan di seluruh dunia adalah lebih dari 12 juta meter kubik per tahun.
Definisi Shotcrete
Menurut American Concrete Institute (ACI), shotcrete dapat didefinisikan sebagai mortar atau beton yang diberikan tekanan dengan kecepatan tinggi .
Komponennya campurannya terdiri atas semen, pasir, agregat, air, dan tambahan admixtures.
Perbedaan shotcrete dengan beton normal dapat dilihat dari 3 hal :
1.ukuran agregat maksimum yang digunakan.
2.Prosesnya
3.Campuran dari shotcrete bisa kering atau basah.
Mengenai terminologi kita dapat menjelaskan Gunite sebagai mortar yang disemprotkan sementara Shotcrete sebagai beton yang disemprotkan.Gunite adalah campuran antara semen dengan partikel/aggregat ukuran diameter yang dibatasi sampai 8mm.Sedangkan untuk Shotcrete penggunaan maksimum diameter aggregat adalah 16 mm. Namun, dalam 10 tahun terakhir ada kecenderungan untuk membatasi maksimal ukuran agregat sampai 10 mm.
Ada 2 cara proses shotcrete :
1. Proses kering
2. Proses basah
Selasa, 15 Desember 2009
Senin, 14 Desember 2009
Metode Perbaikan Beton
Kerusakan pada struktur beton baik kolom maupun balok seringkali kita jumpai baik pada bangunan gedung jalan , jembatan dsbnya.
Ada 4 (empat) metode perbaikan, yaitu patching (plester), grouting (menambah dengan bahan cement grout ), dan injection (suntik dengan bahan epoxy resin), shotcrete ( beton tembak/semprot ).
Pemilihan metode perbaikan sangat ditentukan oleh jenis kerusakan dapat berupa retak ( cracking), pengelupasan ( spalling ) , keropos ( honeycom )
I.Tulangan sudah luluh .Bila kondisi tulangan kolom sudah luluh dan tidak terlalu parah sehingga bangunan tidak miring, tulangan yang rusak tadi bisa dipotong dan diganti dengan tulangan yang baru. Periksalah apakah tulangan yang ada di kolom itu memang sudah cukup aman menahan beban atau belum. Bila sudah cukup kuat maka langkah perbaikannya adalah sebagai berikut. 1.Tahan beban kolom yang akan diperbaiki dengan menggunakan kayu atau besi. 2.Bongkarlah beton yang masih melekat di sekitar tulangan yang luluh. 3.Besi yang luluh dipotong lalu diganti dengan tulangan yang baru. Perhatikan sambungan tulangan baru dengan tulangan yang lama. Sebaiknya tulangan lama menindih tulangan baru (overlaping) dengan panjang sama dengan diameter x 40. Jika diameter tulangan 12 mm maka panjang overlaping tulangan sekitar 480 mm. Setelah itu, ikatlah sambungan dengan kawat atau bisa juga dengan las. 4.Pasanglah bekisting di sekitar kolom yang diperbaiki. Setelah itu, masukkan material beton dengan mutu beton sesuai dengan kualitas mutu beton yang lama dengan metode grouting dengan menggunakan material Sika Grout 215 atau bisa juga dengan metode shotcreting , beton yang disemprotkan pada dinding beton yang rusak tanpa menggunakan bekesting , daya rekatnya lebih baik dibandingkan dengan metode grouting.
II. Tulangan belum luluh .Kerusakan dengan kondisi tulangan belum luluh yang kerap terjadi adalah retak, beton hancur sebagian, dan selimut beton terlepas. 1.Retak Jika permukaan kolom atau balok retak maka langkah pertama yang dilakukan adalah mengecek apakah keretakan itu ada pada selimut beton ( non structural ) atau pada “daging” betonnya ( structural cracking ). Lebar dan kedalaman retak juga harus diperhatikan untuk menentukan metode perbaikan . Bila kedalaman kurang dari 2cm maka perbaikan bisa menggunakan metode patching saja atau bisa juga injection without pressure ( sistim gravity feeed) Keretakan pada selimut beton ( non structural cracking) bisa diatasi metode patching dengan menambal bagian yang rusak menggunakan bahan material perbaikan struktur berbahan dasar polymer ( Sikatop 121/122) atau injection without presssure , yaitu injeksi tanpa tekanan hanya dengan menuangkan material epoxy resin ( Sikadur 752 ) ke bagian yang retak. Bila ternyata keretakan ada pada “daging” betonnya, maka metode perbaikan adalah injection bertekanan . Proses pengerjaan metode injection adalah dengan membuat V curve sepanjang retakan yang terjadi dengan kedalaman kurang lebih 1 cm tuang material perbaikan struktur berbahan dasar semen pada celah retakan. Sedangkan proses metode injeksi dimulai dengan menutup permukaan retakan menggunakan material berbahan epoxy. Setelah itu, buatlah lubang di sepanjang retakan dengan jarak antarlubang sekitar 25 cm dan masukkan selang karet atau pipa alumium diameter 0,5 cm,kemudian tanam /seal dengan menggunakan Sikadur 31 CF Normal tunggu hingga mengeras kurang lebih 7-8 jam.Masukkan material perbaikan struktur berbahan dasar epoxy ( Sikadur 752 ) ke dalam selang /pipa. Jika material itu keluar pada salah satu selang karet maka segera tutup lubang selang itu. Masukkan kembali material epoxy ke lubang yang lain. Ulangi proses itu hingga semua selang/pipa tertutup. Jika semua sudah tertutup, ini berarti semua celah pada retakan sudah terisi material epoxy. Setelah selesai selang/pipa dipotong kemudian dirapikan dengan menggunaka gurinda.2. Beton hancur sebagian. Untuk memperbaikinya, metode yang digunakan adalah metode grouting atau shotcrete. Proses perbaikan untuk kerusakan kolom adalah sebagai berikut:- Tahan beban kolom dengan memberi kayu atau besi di sekeliling kolom. - Bersihkan beton yang masih menempel di sekeliling pecahan beton. Ketuk di sekeliling pecahan untuk memastikan sudah tidak ada lagi beton yang bisa terkelupas. - Buatlah bekisting di sekeliling kolom.
-Sebaiknya gunakan bonding agent (Sikabond NV )bahan sambungan beton lama dan baru untuk mendapatkan daya rekat yang baik juga supaya beton bisa monolith- Cor kembali bagian kolom yang terkelupas tadi dengan menggunakan metode grouting (Sika Grout 215 /214/FM) atau bisajuga dengan menggunakan Premix Concrete ( SIKACRETE 08 SCC )atau dengan menggunakan beton yang ditambahkan admixture Sikament LN atau Viscocrete 10 (bahan untuk Self Compacting Concrete)3. Selimut beton terkelupas. Kerusakan seperti ini tergolong kerusakan kosmetik. Maksudnya, hanya penampilannya saja yang rusak. Untuk memperbaikinya, metode yang tepat adalah dengan metode patching. Caranya adalah sebagai berikut: - Bersihkan atau lepas selimut beton yang masih menempel di sekitar selimut beton yang terkelupas. - Lapisi permukaan beton yang terkelupas dengan bonding agent ( Sikabond NV )- Berikan adukan plesteran khusus untuk perbaikan struktur pada permukaan beton yang terkelupas (Sikatop 121/122). Material plesteran ini terdiri dari dua komponen cairan putih dan bubuk semen.
Ada 4 (empat) metode perbaikan, yaitu patching (plester), grouting (menambah dengan bahan cement grout ), dan injection (suntik dengan bahan epoxy resin), shotcrete ( beton tembak/semprot ).
Pemilihan metode perbaikan sangat ditentukan oleh jenis kerusakan dapat berupa retak ( cracking), pengelupasan ( spalling ) , keropos ( honeycom )
I.Tulangan sudah luluh .Bila kondisi tulangan kolom sudah luluh dan tidak terlalu parah sehingga bangunan tidak miring, tulangan yang rusak tadi bisa dipotong dan diganti dengan tulangan yang baru. Periksalah apakah tulangan yang ada di kolom itu memang sudah cukup aman menahan beban atau belum. Bila sudah cukup kuat maka langkah perbaikannya adalah sebagai berikut. 1.Tahan beban kolom yang akan diperbaiki dengan menggunakan kayu atau besi. 2.Bongkarlah beton yang masih melekat di sekitar tulangan yang luluh. 3.Besi yang luluh dipotong lalu diganti dengan tulangan yang baru. Perhatikan sambungan tulangan baru dengan tulangan yang lama. Sebaiknya tulangan lama menindih tulangan baru (overlaping) dengan panjang sama dengan diameter x 40. Jika diameter tulangan 12 mm maka panjang overlaping tulangan sekitar 480 mm. Setelah itu, ikatlah sambungan dengan kawat atau bisa juga dengan las. 4.Pasanglah bekisting di sekitar kolom yang diperbaiki. Setelah itu, masukkan material beton dengan mutu beton sesuai dengan kualitas mutu beton yang lama dengan metode grouting dengan menggunakan material Sika Grout 215 atau bisa juga dengan metode shotcreting , beton yang disemprotkan pada dinding beton yang rusak tanpa menggunakan bekesting , daya rekatnya lebih baik dibandingkan dengan metode grouting.
II. Tulangan belum luluh .Kerusakan dengan kondisi tulangan belum luluh yang kerap terjadi adalah retak, beton hancur sebagian, dan selimut beton terlepas. 1.Retak Jika permukaan kolom atau balok retak maka langkah pertama yang dilakukan adalah mengecek apakah keretakan itu ada pada selimut beton ( non structural ) atau pada “daging” betonnya ( structural cracking ). Lebar dan kedalaman retak juga harus diperhatikan untuk menentukan metode perbaikan . Bila kedalaman kurang dari 2cm maka perbaikan bisa menggunakan metode patching saja atau bisa juga injection without pressure ( sistim gravity feeed) Keretakan pada selimut beton ( non structural cracking) bisa diatasi metode patching dengan menambal bagian yang rusak menggunakan bahan material perbaikan struktur berbahan dasar polymer ( Sikatop 121/122) atau injection without presssure , yaitu injeksi tanpa tekanan hanya dengan menuangkan material epoxy resin ( Sikadur 752 ) ke bagian yang retak. Bila ternyata keretakan ada pada “daging” betonnya, maka metode perbaikan adalah injection bertekanan . Proses pengerjaan metode injection adalah dengan membuat V curve sepanjang retakan yang terjadi dengan kedalaman kurang lebih 1 cm tuang material perbaikan struktur berbahan dasar semen pada celah retakan. Sedangkan proses metode injeksi dimulai dengan menutup permukaan retakan menggunakan material berbahan epoxy. Setelah itu, buatlah lubang di sepanjang retakan dengan jarak antarlubang sekitar 25 cm dan masukkan selang karet atau pipa alumium diameter 0,5 cm,kemudian tanam /seal dengan menggunakan Sikadur 31 CF Normal tunggu hingga mengeras kurang lebih 7-8 jam.Masukkan material perbaikan struktur berbahan dasar epoxy ( Sikadur 752 ) ke dalam selang /pipa. Jika material itu keluar pada salah satu selang karet maka segera tutup lubang selang itu. Masukkan kembali material epoxy ke lubang yang lain. Ulangi proses itu hingga semua selang/pipa tertutup. Jika semua sudah tertutup, ini berarti semua celah pada retakan sudah terisi material epoxy. Setelah selesai selang/pipa dipotong kemudian dirapikan dengan menggunaka gurinda.2. Beton hancur sebagian. Untuk memperbaikinya, metode yang digunakan adalah metode grouting atau shotcrete. Proses perbaikan untuk kerusakan kolom adalah sebagai berikut:- Tahan beban kolom dengan memberi kayu atau besi di sekeliling kolom. - Bersihkan beton yang masih menempel di sekeliling pecahan beton. Ketuk di sekeliling pecahan untuk memastikan sudah tidak ada lagi beton yang bisa terkelupas. - Buatlah bekisting di sekeliling kolom.
-Sebaiknya gunakan bonding agent (Sikabond NV )bahan sambungan beton lama dan baru untuk mendapatkan daya rekat yang baik juga supaya beton bisa monolith- Cor kembali bagian kolom yang terkelupas tadi dengan menggunakan metode grouting (Sika Grout 215 /214/FM) atau bisajuga dengan menggunakan Premix Concrete ( SIKACRETE 08 SCC )atau dengan menggunakan beton yang ditambahkan admixture Sikament LN atau Viscocrete 10 (bahan untuk Self Compacting Concrete)3. Selimut beton terkelupas. Kerusakan seperti ini tergolong kerusakan kosmetik. Maksudnya, hanya penampilannya saja yang rusak. Untuk memperbaikinya, metode yang tepat adalah dengan metode patching. Caranya adalah sebagai berikut: - Bersihkan atau lepas selimut beton yang masih menempel di sekitar selimut beton yang terkelupas. - Lapisi permukaan beton yang terkelupas dengan bonding agent ( Sikabond NV )- Berikan adukan plesteran khusus untuk perbaikan struktur pada permukaan beton yang terkelupas (Sikatop 121/122). Material plesteran ini terdiri dari dua komponen cairan putih dan bubuk semen.
Polycarboxilate Superplasticizer
Concrete Admixture adalah salah satu bahan baku beton yang ditambahkan kedalam campuran beton sebelum atau selama pencampuran untuk mengubah sifat-sifat beton baik beton segar maupun beton yang telah mengeras untuk mencapai tujuan yang diinginkan atau tujuan dari campuran beton.Dan juga untuk tujuan ekonomi yang dapat memungkinkan pengurangan semen ,terutama digunakan dalam industri beton siap pakai ( readymix concrete) dan juga beton pracetak ( precast ).
Generasi terbaru dalam teknologi admixture adalah polycarboxilate . Polycarboxilate atau polimer adalah admixture beton termasuk dalam kategori (UHRWR) Ultra High Range Water Reducing . Dapat mengurangi penggunaan air yang cukup besar hingga 40% jika dibandingkan dengan beton standard , juga dapat membuat beton mengalir dengan sendirinya tanpa menggunakan alat pemadat/vibrator. Karena pengurangan airnya sangat tinggi dengan water cement ratio 0,25 maka dapat digunakan untuk mendesain beton mutu tinggi bahkan untuk mutu 100 Mpa. Juga dapat digunakan untuk beton dalam air ,under water concrete,anti-washout , untuk beton semprot /shotcrete .Bagaimana hal ini bisa terjadi dikarenakan adanya halangan steric juga berarti pemisahan partikel karena molekul polimer yang panjang.
Bisa berbentuk cair maupun powder , biasanya yang mensuplai dalam bentuk cair adalah perusahan kimia konstruksi yang sudah memodifikasi menjadi produk jadi yang sudah dipatentkan seperti Viscocrete ( Sika ), Glenium ( BASF ) , ADVA ( Grace ) , Structuro ( Fosroc ) dan masih banyak lagi dengan nama yang beraneka ragam. Spesific Gravity 1.02 – 1,07 ,tersedia dalam kemasan drum /200 ltr atau pail/20 ltr.
Sedangkan produk aslinya adalah dalam bentuk powder yang juga sudah banyak beredar di pasar dengan spesifikasi sebagai berikut :
Polycarboxylate Superplasticizer (Polycarboxylate High-range Water Reducer) ----Third generation of concrete admixtures, fine comprehensive performance and green environmental protective products. Modified polycarboxylate high range water reducer is powder which is made by spray drying process. Characteristics:It is especially suitable for high-performance powder construction material, used as dispersant. Its dispersiveness is good, water reducing rate is high and adaptability to various cements is fine. Fluidity of mixtures is excellent when used in concrete and mortar.Specification: Appearance: light grey powderWater content: 3% Max.Characteristics: Non-toxin, non-taste and non-inflammationUnit weight ( g./L ): 500-600PH value (20 deg. C., 20% solution): 6-8SO4 ion content: 2.6%Cl ion content: 0.01%Suggested mixed amount: 0.16-0.3% ( of the cementing material amount )Application:This product can be applied in the High strength concrete, High fluidity concrete, High duration concrete, Calcium aluminate refractory concrete, Low cement refractory concrete and Grouting materials, Steam curing concrete, High strength mortar, Gypsum product, Self-flowing ground, and Stuffing material etc.Packing: 20kgs/plastic sealed bag with liner.
Generasi terbaru dalam teknologi admixture adalah polycarboxilate . Polycarboxilate atau polimer adalah admixture beton termasuk dalam kategori (UHRWR) Ultra High Range Water Reducing . Dapat mengurangi penggunaan air yang cukup besar hingga 40% jika dibandingkan dengan beton standard , juga dapat membuat beton mengalir dengan sendirinya tanpa menggunakan alat pemadat/vibrator. Karena pengurangan airnya sangat tinggi dengan water cement ratio 0,25 maka dapat digunakan untuk mendesain beton mutu tinggi bahkan untuk mutu 100 Mpa. Juga dapat digunakan untuk beton dalam air ,under water concrete,anti-washout , untuk beton semprot /shotcrete .Bagaimana hal ini bisa terjadi dikarenakan adanya halangan steric juga berarti pemisahan partikel karena molekul polimer yang panjang.
Bisa berbentuk cair maupun powder , biasanya yang mensuplai dalam bentuk cair adalah perusahan kimia konstruksi yang sudah memodifikasi menjadi produk jadi yang sudah dipatentkan seperti Viscocrete ( Sika ), Glenium ( BASF ) , ADVA ( Grace ) , Structuro ( Fosroc ) dan masih banyak lagi dengan nama yang beraneka ragam. Spesific Gravity 1.02 – 1,07 ,tersedia dalam kemasan drum /200 ltr atau pail/20 ltr.
Sedangkan produk aslinya adalah dalam bentuk powder yang juga sudah banyak beredar di pasar dengan spesifikasi sebagai berikut :
Polycarboxylate Superplasticizer (Polycarboxylate High-range Water Reducer) ----Third generation of concrete admixtures, fine comprehensive performance and green environmental protective products. Modified polycarboxylate high range water reducer is powder which is made by spray drying process. Characteristics:It is especially suitable for high-performance powder construction material, used as dispersant. Its dispersiveness is good, water reducing rate is high and adaptability to various cements is fine. Fluidity of mixtures is excellent when used in concrete and mortar.Specification: Appearance: light grey powderWater content: 3% Max.Characteristics: Non-toxin, non-taste and non-inflammationUnit weight ( g./L ): 500-600PH value (20 deg. C., 20% solution): 6-8SO4 ion content: 2.6%Cl ion content: 0.01%Suggested mixed amount: 0.16-0.3% ( of the cementing material amount )Application:This product can be applied in the High strength concrete, High fluidity concrete, High duration concrete, Calcium aluminate refractory concrete, Low cement refractory concrete and Grouting materials, Steam curing concrete, High strength mortar, Gypsum product, Self-flowing ground, and Stuffing material etc.Packing: 20kgs/plastic sealed bag with liner.
Sabtu, 12 Desember 2009
Introduction of Concrete Admixtures ( Chemicals for Concrete )
Admixtures are materials other than cement, aggregate and water , in liquid or powder form that are added to fresh concrete either before or during its mixing to improve its properties, such as workability, curing temperature range, setting time , slump retention,etc. Some admixtures have been in use for a very long time, such as calcium chloride to provide a cold-weather setting concrete. Others are more recent and represent an area of expanding possibilities for increased performance. Not all admixtures are economical to employ on a particular project. Also, some characteristics of concrete, such as low absorption, can be achieved simply by consistently adhering to high quality concreting practices.
The chemistry of concrete admixtures is a complex topic requiring in-depth knowledge and experience. A general understanding of the options available for concrete admixtures is necessary for acquiring the right product for the job, based on climatic conditions and job requirements. Based on their functions, admixtures can be classified into the following five major categories:
Retarding admixtures
Accelerating admixture
Super plasticizers
Water reducing admixtures
Air-entraining admixtures
Among other important admixtures that do not fit into these categories are admixtures whose functions include bonding, shrinkage reduction, damp proofing and coloring. The following paragraphs provides details on the above-mentioned categories of concrete admixtures.
Retarding admixtures
Retarding admixtures slow down the hydration of cement, lengthening set time. Retarders are beneficially used in hot weather conditions in order to overcome accelerating effects of higher temperatures and large masses of concrete on concrete setting time. Because most retarders also act as water reducers, they are frequently called water-reducing retarders. As per chemical admixture classification by ASTM-ASTM C 494, type B is simply a retarding admixture, while type D is both retarding and water reducing, resulting in concrete with greater compressive strength because of the lower water-cement ratio.
Retarding admixtures consists of both organic and inorganic agents. Organic retardants include unrefined calcium, sodium, NH4, salts of lignosulfonic acids, hydrocarboxylic acids, and carbohydrates. Inorganic retardants include oxides of lead and zinc, phosphates, magnesium salts, fluorates and borates. As an example of a retardant's effects on concrete properties, lignosulfate acids and hydroxylated carboxylic acids slow the initial setting time by at least an hour and no more than three hours when used at 65 to 100 degrees Fahrenheit. The concrete contractor, however, need not memorize these chemical-specific results. Given the specific job requirements and goals, the concrete supplier should offer appropriate admixtures and concrete mixes from which to choose.
Accelerating admixtures
Accelerators shorten the set time of concrete, allowing a cold-weather pour, early removal of forms, early surface finishing, and in some cases, early load application. Proper care must be taken while choosing the type and proportion of accelerators, as under most conditions, commonly used accelerators cause an increase in the drying shrinkage of concrete.
Calcium chloride is a common accelerator, used to accelerate the time of set and the rate of strength gain. It should meet the requirements of ASTM D 98. Excessive amounts of calcium chloride in concrete mix may result in rapid stiffening, increase in drying shrinkage and corrosion of reinforcement. In colder climates, calcium chloride should not be used as an anti-freeze. Large amount of calcium chloride is required to lower the freezing point of the concrete, which may ruin the concrete.
Superplasticizers
Super plasticizers, also known as plasticizers, include water-reducing admixtures. Compared to what is commonly referred to as a "water reducer" or "mid-range water reducer", super plasticizers are "high-range water reducers". High range water reducers are admixtures that allow large water reduction or greater flowability (as defined by the manufacturers, concrete suppliers and industry standards) without substantially slowing set time or increasing air entrainment.
Each type of super plasticizer has defined ranges for the required quantities of concrete mix ingredients, along with the corresponding effects. They can maintain a specific consistency and workability at a greatly reduced amount of water. Dosages needed vary by the particular concrete mix and type of super plasticizer used. They can also produce a high strength concrete. As with most types of admixtures, super plasticizers can affect other concrete properties as well. The specific effects, however, should be found from the manufacturer or concrete supplier.
Water reducing admixtures
Water reducing admixtures require less water to make a concrete of equal slump, or increase the slump of concrete at the same water content. They can have the side effect of changing initial set time. Water reducers are mostly used for hot weather concrete placing and to aid pumping. A water-reducer plasticizer, however, is a hygroscopic powder, which can entrain air into the concrete mix via its effect on water's surface tension, thereby also, obtaining some of the benefits of air-entrainment (see below).
Air-entraining admixtures
Air-entraining agents entrain small air bubbles in the concrete. The major benefit of this is enhanced durability in freeze-thaw cycles, especially relevant in cold climates. While some strength loss typically accompanies increased air in concrete, it generally can be overcome by reducing the water-cement ratio via improved workability (due to the air-entraining agent itself) or through the use of other appropriate admixtures. As always, admixtures should only be combined in a concrete mix by a competent professional because some of them can interact in undesirable ways.
Others :
Mineral admixtures , make mixtures more economical, reduce permeability, increase strength, and influence other concrete properties. Mineral admixtures affect the nature of the hardened concrete through hydraulic or pozzolanic activity. Pozzolans are cementitious materials and include natural pozzolans (such as the volcanic
ash used in Roman concrete), fly ash and silica fume. They can be used with Portland cement, or blended cement either individually or in combination as a cement substitutes. Cement substitutes also change concrete properties, but typically are not classified as admixtures.
Bonding admixtures, including addition of compounds and materials such as polyvinyl chlorides and acetates, acrylics and butadiene-styrene co-polymers, can be used to assist in bonding new / fresh concrete with old / set concrete.
Coloring agents , have become more commonly used, especially for patios and walkways. Most are surface applied and often have the additional effect of surface hardening. Such surface applied coloring admixtures generally should not be used on air-entrained concrete. Integrally colored concrete is also available.
Waterproofing and damp proofing admixtures, including soaps, butyl stearate, mineral oil and asphalt emulsions, are used to decrease the amount of water penetration into the larger pores of concrete.
Most type admixtures are affected by cement type and brand, water-cement ratio, aggregate grading, and temperature. Anyone who is working with admixtures must follow the instruction of products .In some cases, if exacting directions are not followed, including addition of supplemental materials to balance the negative or undesirable side effects of an admixture, the resulting concrete mix may be compromised. For example, by adding an unexact dosage of retarding admixtures generally have a possibility of rapid concrete stiffening, resulting in difficulty in concrete placement and finishing. Therefore, an in-depth knowledge of the potentially complex interrelated effects, besides specifications, is required to successfully use a number of admixtures. This is even more critical when a number of parties are involved in the manufacture of the concrete, for example the producer, the placing contractor and the builder, where the finished concrete is a combined result of a number of individual decisions. Choosing an appropriate admixture for a specific job should be the responsibility of an experienced expert.
The chemistry of concrete admixtures is a complex topic requiring in-depth knowledge and experience. A general understanding of the options available for concrete admixtures is necessary for acquiring the right product for the job, based on climatic conditions and job requirements. Based on their functions, admixtures can be classified into the following five major categories:
Retarding admixtures
Accelerating admixture
Super plasticizers
Water reducing admixtures
Air-entraining admixtures
Among other important admixtures that do not fit into these categories are admixtures whose functions include bonding, shrinkage reduction, damp proofing and coloring. The following paragraphs provides details on the above-mentioned categories of concrete admixtures.
Retarding admixtures
Retarding admixtures slow down the hydration of cement, lengthening set time. Retarders are beneficially used in hot weather conditions in order to overcome accelerating effects of higher temperatures and large masses of concrete on concrete setting time. Because most retarders also act as water reducers, they are frequently called water-reducing retarders. As per chemical admixture classification by ASTM-ASTM C 494, type B is simply a retarding admixture, while type D is both retarding and water reducing, resulting in concrete with greater compressive strength because of the lower water-cement ratio.
Retarding admixtures consists of both organic and inorganic agents. Organic retardants include unrefined calcium, sodium, NH4, salts of lignosulfonic acids, hydrocarboxylic acids, and carbohydrates. Inorganic retardants include oxides of lead and zinc, phosphates, magnesium salts, fluorates and borates. As an example of a retardant's effects on concrete properties, lignosulfate acids and hydroxylated carboxylic acids slow the initial setting time by at least an hour and no more than three hours when used at 65 to 100 degrees Fahrenheit. The concrete contractor, however, need not memorize these chemical-specific results. Given the specific job requirements and goals, the concrete supplier should offer appropriate admixtures and concrete mixes from which to choose.
Accelerating admixtures
Accelerators shorten the set time of concrete, allowing a cold-weather pour, early removal of forms, early surface finishing, and in some cases, early load application. Proper care must be taken while choosing the type and proportion of accelerators, as under most conditions, commonly used accelerators cause an increase in the drying shrinkage of concrete.
Calcium chloride is a common accelerator, used to accelerate the time of set and the rate of strength gain. It should meet the requirements of ASTM D 98. Excessive amounts of calcium chloride in concrete mix may result in rapid stiffening, increase in drying shrinkage and corrosion of reinforcement. In colder climates, calcium chloride should not be used as an anti-freeze. Large amount of calcium chloride is required to lower the freezing point of the concrete, which may ruin the concrete.
Superplasticizers
Super plasticizers, also known as plasticizers, include water-reducing admixtures. Compared to what is commonly referred to as a "water reducer" or "mid-range water reducer", super plasticizers are "high-range water reducers". High range water reducers are admixtures that allow large water reduction or greater flowability (as defined by the manufacturers, concrete suppliers and industry standards) without substantially slowing set time or increasing air entrainment.
Each type of super plasticizer has defined ranges for the required quantities of concrete mix ingredients, along with the corresponding effects. They can maintain a specific consistency and workability at a greatly reduced amount of water. Dosages needed vary by the particular concrete mix and type of super plasticizer used. They can also produce a high strength concrete. As with most types of admixtures, super plasticizers can affect other concrete properties as well. The specific effects, however, should be found from the manufacturer or concrete supplier.
Water reducing admixtures
Water reducing admixtures require less water to make a concrete of equal slump, or increase the slump of concrete at the same water content. They can have the side effect of changing initial set time. Water reducers are mostly used for hot weather concrete placing and to aid pumping. A water-reducer plasticizer, however, is a hygroscopic powder, which can entrain air into the concrete mix via its effect on water's surface tension, thereby also, obtaining some of the benefits of air-entrainment (see below).
Air-entraining admixtures
Air-entraining agents entrain small air bubbles in the concrete. The major benefit of this is enhanced durability in freeze-thaw cycles, especially relevant in cold climates. While some strength loss typically accompanies increased air in concrete, it generally can be overcome by reducing the water-cement ratio via improved workability (due to the air-entraining agent itself) or through the use of other appropriate admixtures. As always, admixtures should only be combined in a concrete mix by a competent professional because some of them can interact in undesirable ways.
Others :
Mineral admixtures , make mixtures more economical, reduce permeability, increase strength, and influence other concrete properties. Mineral admixtures affect the nature of the hardened concrete through hydraulic or pozzolanic activity. Pozzolans are cementitious materials and include natural pozzolans (such as the volcanic
ash used in Roman concrete), fly ash and silica fume. They can be used with Portland cement, or blended cement either individually or in combination as a cement substitutes. Cement substitutes also change concrete properties, but typically are not classified as admixtures.
Bonding admixtures, including addition of compounds and materials such as polyvinyl chlorides and acetates, acrylics and butadiene-styrene co-polymers, can be used to assist in bonding new / fresh concrete with old / set concrete.
Coloring agents , have become more commonly used, especially for patios and walkways. Most are surface applied and often have the additional effect of surface hardening. Such surface applied coloring admixtures generally should not be used on air-entrained concrete. Integrally colored concrete is also available.
Waterproofing and damp proofing admixtures, including soaps, butyl stearate, mineral oil and asphalt emulsions, are used to decrease the amount of water penetration into the larger pores of concrete.
Most type admixtures are affected by cement type and brand, water-cement ratio, aggregate grading, and temperature. Anyone who is working with admixtures must follow the instruction of products .In some cases, if exacting directions are not followed, including addition of supplemental materials to balance the negative or undesirable side effects of an admixture, the resulting concrete mix may be compromised. For example, by adding an unexact dosage of retarding admixtures generally have a possibility of rapid concrete stiffening, resulting in difficulty in concrete placement and finishing. Therefore, an in-depth knowledge of the potentially complex interrelated effects, besides specifications, is required to successfully use a number of admixtures. This is even more critical when a number of parties are involved in the manufacture of the concrete, for example the producer, the placing contractor and the builder, where the finished concrete is a combined result of a number of individual decisions. Choosing an appropriate admixture for a specific job should be the responsibility of an experienced expert.
Kamis, 10 Desember 2009
Power of Giving
There is tremendous power that comes with the act of giving - and I'll explain how this power is created and how it works.
The power of giving comes from a selfless act - where you simply give from your heart. This act of giving is not tied to any special event, holiday or celebration - it is merely a time when you give from the heart because you want to share what you have, show your appreciation and give because you truly care. There should not be any other motive behind it. You don't give so that you can get something in return.
When you give simply from the heart you are saying to the universe, your higher powers and you are also sending out energy that others pick up - this message that you communicate by the selfless act of giving is that you truly care and want to share what you have. You also say that you don't care if you don't get anything in return, you don't care if no one notices or if no one thanks you. After all that's not why you are doing it.
When you do this - the response that you get can be quite overwhelming. It will be as though a thousand hands came to help you even when you didn't ask for help. Things start to fall into place - because those who give from the heart - will get a thousand times the amount that they give.
Remember - I said that you give from the heart. You don't give with the expectation of something in return. You can give with the thought that you are sharing - that you are giving because you have so much. You are giving because you know and trust that you will always have what you want so you can afford to give something even before you receive what you seek.
This mere act of giving from the heart and with the full knowledge that you have plenty and with the trust that you will always have more than enough - creates a bond between you and your higher power, spirit, universe, God or whatever you want to call that part of us that we feel is connected to a higher entity. That bond is a trust - in which you are saying "I know I will always have more than I want so I am now giving something because I want to - fully trusting that I will always have more than what I want - and therefore I can share because I want to."
When you do this the universe or the higher power responds by saying - "ah-ha - ___ believes they have so much that they are already willing to share - let's give him/her some more so that they can continue to share." It really does work that way.
But too often we get caught up in our wants and needs, we get selfish - only focusing on ourselves and our own little problems - which when looked at in terms of the bigger picture are really not that severe. Sure there are some of us who have some serious setbacks that we want to overcome - and I'm not trivializing that or any of your own personal concerns. However, I do think that we need to look at the needs of those of those around us as well - and then ask ourselves what can we do to help make things better. After all - it's our world - if we don't share how can we help each other and how can we overcome our own setbacks. If you don't help your neighbor - and if everybody didn't help their neighbor - how would we ever get things done? There would be no progress because we would all be consumed with our own little world and not take the time to help each other.
Giving is a part of human nature. That's why when we do it we enjoy the process so much. We enjoy seeing the other person smile; we enjoy helping someone else in some small way.
I know that not all of you are in a position to give a great deal, or to buy the ideal present or to spend a lot of money. But you can give your time. You can give something small, you can make a small gesture to the universe and your higher power - that you believe you will always have what you want and more - you believe so much that you're willing to give and share that even before you receive it. This is something I teach in my Creating Power system. The Creating Power course spends a good deal of time teaching you how to generate the Power of Giving.
I have an uncle who has done quite well for himself and at age 72 he still runs his company and factory. I remember talking to him recently and he told me about the tough times. He said when he started out he didn't know how things would turn out. But he was always involved in charities and always trying to help those around him. He didn't have a lot of money to give - but he had some time - so he got involved with helping out people in need. Sometimes by getting old clothes from family members to donate, sometimes by collecting tins of food, anything he could do. As he started making some money he would donate a portion to his religious community - he never stopped donating and to this day is still involved in charities. His son who is a very successful doctor - will visit Africa once or twice a year to perform free surgery on those who are need. Both my uncle and his son said they gave because they believed they had so much - and they just wanted to share what they had. There was no other motive. They have been rewarded ten times over - they both live very comfortable lives. I have countless stories that I can share with you about how people who give always get more in return - but the key is that they give without expecting anything - they simply give - knowing and trusting that they will always have more than what they need.
Now we all can't perform free surgery - but we can give a little of our time - since we all have they same amount - surely you can spare an hour or two on the weekends. If you go shopping - buy something small for someone - with the belief that you will always have more than you need. You can even buy something for yourself with that same belief. You can give a small amount to charity - with the complete belief that you will always have more than you need.
If you want to receive love and affection - then give love and affection to those you care about. If you want to make more money - then give a small amount to charity. Give with the belief that you already have so much that you are now willing to share what you are about to receive - don't expect anything in return - just trust that you will get what you seek. This is a very powerful technique and there's no better time to give than during this holiday season. With that I wish you all the best for the christmas season. I hope you have a terrific time whatever you decide to do. Another year is almost over and new year 2010 is about to begin. Start being the best person you can be - you'll be rewarded in ways you never thought possible
With Creating Power you take charge of your life, learn to believe in yourself and more while creating the success and happiness you want and deserve. Creating Power teaches you how to eliminate the negative thoughts and take control of your mind in the very first week -- you'll be amazed at how much power you have in less than 7-days! You'll be on your way to creating a positive thinking pattern that lasts a lifetime.
The power of giving comes from a selfless act - where you simply give from your heart. This act of giving is not tied to any special event, holiday or celebration - it is merely a time when you give from the heart because you want to share what you have, show your appreciation and give because you truly care. There should not be any other motive behind it. You don't give so that you can get something in return.
When you give simply from the heart you are saying to the universe, your higher powers and you are also sending out energy that others pick up - this message that you communicate by the selfless act of giving is that you truly care and want to share what you have. You also say that you don't care if you don't get anything in return, you don't care if no one notices or if no one thanks you. After all that's not why you are doing it.
When you do this - the response that you get can be quite overwhelming. It will be as though a thousand hands came to help you even when you didn't ask for help. Things start to fall into place - because those who give from the heart - will get a thousand times the amount that they give.
Remember - I said that you give from the heart. You don't give with the expectation of something in return. You can give with the thought that you are sharing - that you are giving because you have so much. You are giving because you know and trust that you will always have what you want so you can afford to give something even before you receive what you seek.
This mere act of giving from the heart and with the full knowledge that you have plenty and with the trust that you will always have more than enough - creates a bond between you and your higher power, spirit, universe, God or whatever you want to call that part of us that we feel is connected to a higher entity. That bond is a trust - in which you are saying "I know I will always have more than I want so I am now giving something because I want to - fully trusting that I will always have more than what I want - and therefore I can share because I want to."
When you do this the universe or the higher power responds by saying - "ah-ha - ___ believes they have so much that they are already willing to share - let's give him/her some more so that they can continue to share." It really does work that way.
But too often we get caught up in our wants and needs, we get selfish - only focusing on ourselves and our own little problems - which when looked at in terms of the bigger picture are really not that severe. Sure there are some of us who have some serious setbacks that we want to overcome - and I'm not trivializing that or any of your own personal concerns. However, I do think that we need to look at the needs of those of those around us as well - and then ask ourselves what can we do to help make things better. After all - it's our world - if we don't share how can we help each other and how can we overcome our own setbacks. If you don't help your neighbor - and if everybody didn't help their neighbor - how would we ever get things done? There would be no progress because we would all be consumed with our own little world and not take the time to help each other.
Giving is a part of human nature. That's why when we do it we enjoy the process so much. We enjoy seeing the other person smile; we enjoy helping someone else in some small way.
I know that not all of you are in a position to give a great deal, or to buy the ideal present or to spend a lot of money. But you can give your time. You can give something small, you can make a small gesture to the universe and your higher power - that you believe you will always have what you want and more - you believe so much that you're willing to give and share that even before you receive it. This is something I teach in my Creating Power system. The Creating Power course spends a good deal of time teaching you how to generate the Power of Giving.
I have an uncle who has done quite well for himself and at age 72 he still runs his company and factory. I remember talking to him recently and he told me about the tough times. He said when he started out he didn't know how things would turn out. But he was always involved in charities and always trying to help those around him. He didn't have a lot of money to give - but he had some time - so he got involved with helping out people in need. Sometimes by getting old clothes from family members to donate, sometimes by collecting tins of food, anything he could do. As he started making some money he would donate a portion to his religious community - he never stopped donating and to this day is still involved in charities. His son who is a very successful doctor - will visit Africa once or twice a year to perform free surgery on those who are need. Both my uncle and his son said they gave because they believed they had so much - and they just wanted to share what they had. There was no other motive. They have been rewarded ten times over - they both live very comfortable lives. I have countless stories that I can share with you about how people who give always get more in return - but the key is that they give without expecting anything - they simply give - knowing and trusting that they will always have more than what they need.
Now we all can't perform free surgery - but we can give a little of our time - since we all have they same amount - surely you can spare an hour or two on the weekends. If you go shopping - buy something small for someone - with the belief that you will always have more than you need. You can even buy something for yourself with that same belief. You can give a small amount to charity - with the complete belief that you will always have more than you need.
If you want to receive love and affection - then give love and affection to those you care about. If you want to make more money - then give a small amount to charity. Give with the belief that you already have so much that you are now willing to share what you are about to receive - don't expect anything in return - just trust that you will get what you seek. This is a very powerful technique and there's no better time to give than during this holiday season. With that I wish you all the best for the christmas season. I hope you have a terrific time whatever you decide to do. Another year is almost over and new year 2010 is about to begin. Start being the best person you can be - you'll be rewarded in ways you never thought possible
With Creating Power you take charge of your life, learn to believe in yourself and more while creating the success and happiness you want and deserve. Creating Power teaches you how to eliminate the negative thoughts and take control of your mind in the very first week -- you'll be amazed at how much power you have in less than 7-days! You'll be on your way to creating a positive thinking pattern that lasts a lifetime.
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