Sunday, May 24, 2020
الخرسانة المسلحة بواسطة الألياف الفولاذية
Steel Fiber Reinforced Concrete
ظهرت الحاجة لاستخدام الخرسانة المسلحة بالألياف الحديدية و تتميز بزيادة مقاومة الشد وتقليل الانكماش وبالتالي التقليل من التشققات و زيادة المرونة ومقاومة الصدمات و تستخدم هذه الخرسانة فى الأرضيات ذات المساحات الكبيرة و المعرضة الى احمال مثل المطارات و الطرق و المخازن و الخزانات و كذلك المنشآت ذات الارتفاعات العالية والمنشآت البحرية والمدنية العملاقة.
الخلطة الخرسانية المعتمدة في إعداد هذه الخرسانة تحتاج إلي إضافة بعض الألياف المختلفة وبالأخص الألياف المعدنية مثل ألياف الهاركس و ألياف الحديد الملساء وكذلك الألياف الغير معدنية مثل ألياف الصوف الزجاجي وألياف البولي بروبلين وذلك لتحويلها إلى مادة لها ممطولية معقولة و ذلك قصد الاستفادة من أقصى مقاومة لصلب التسليح في الخرسانة ذات المقاومة العالية و الحصول
على قوة تماسك كافية بين الحديد و الخرسانة.
و باختصار الخرسانة المسلحة بالألياف الحديدية تتميز بأنها:
- خرسانة مصنوعة من الأسمنت والركام والمحتوية على ألياف صلب وهي قطع من الصلب بطول 3 إلى 8cm وقطر من 0.5 إلى 0.8mm موزعة توزيعاً عشوائياً في جميع الاتجاهات خلال الكتلة الخرسانية.
- زيادة مقاومة الخرسانة للشد و التشققات و زيادة المرونة ومقاومة الصدمات و الاحتكاك و لهدا فهى تستخدم بصفة رئيسية فى الارضيات الواسعة و المعرضة الى احمال متحركة بدلا من حديد التسليح.
- تستخدم ايضا فى بعض انواع الخرسانات سابقة التجهيز مثل الانابيب الخرسانية و الانفاق و كدلك فى تصليحات الخرسانة .
- و من عيوبها أنها صعبة التشكيل لهدا يضاف اليها مخفض الماء او ملين.
Tuesday, May 12, 2020
Combined Electromagnetic Induction and Radar-Based Test for Quality Control of Steel Fiber Reinforced Concrete
Despite its brittleness and low capacity to bear tensile stress, ordinary concrete is the most frequently used building material in the world. Concrete is almost always reinforced by some kind of steel elements (e.g., bars, stirrups, meshes) when used for construction to counteract its brittleness. Over the last 50 years, steel fiber has become a more and more popular type reinforcement for concrete. Currently, steel fiber reinforced concrete (SFRC) is one of the main building materials. SFRC is characterized by higher tensile, shear and flexural strength, better performance when exposed to elevated temperatures and lower shrinkage than ordinary concrete of similar composition. Usually, steel fiber is arranged randomly but evenly in the SFRC volume. Nevertheless, in some circumstances, irregularity in the fiber distribution may occur. These irregularities in fiber spacing significantly affect its properties. The homogeneous spacing of randomly oriented fiber within a structural element is crucial for guaranteeing proper structural performance with a satisfactory degree of repeatability. Many attempts are being made to study fiber spacing using different destructive and non-destructive methods. The most successful methods are X-ray tomography and cross-section analysis. Both methods give very precise results but are not feasible in common in-situ test scenarios. The simplicity of a conducted test and affordability of used equipment are two main factors influencing the practicability and popularity of a testing method. In many cases, construction companies already have apparatuses dedicated for non-destructive testing (NDT) location of steel bars in existing concrete structures. Harnessing such apparatuses for the assessment of steel fiber volume and spacing would be the most practical and sustainable solution. Two existing NDT techniques, electromagnetic induction and radar-based techniques, have the materials potential for testing SFRC. The inductive technique was proven as a robust and simple non-destructive method to assess the content and the distribution of steel fiber. Nevertheless, there is still a necessity to define its accuracy. Multiple attempts have also been made to use radar-based technique for the similar assessment of SFRC. The fundamental differences between electromagnetic induction and radar technologies are the detectability of objects of different materials and the dependency on the properties of the base material. Using the induction technology, only ferrous materials can be detected.
Radar techniques allow the detection of objects of different materials, including ferrous and non-ferrous metals, water-filled pipes, voids, etc. Moreover, the radar-based technique can be applied not only to detect location of reinforcement, but also to assess moisture in reinforced concrete. To conduct a research programme focused on using both techniques simultaneously to assess fiber spacing in hardened SFRC. Combining two separate NDT methods proved to be very efficient in assessment of mechanical properties of concretes with no fiber reinforcement. The results achieved this way were much closer to real strength characteristics than the results based on only one method. It is feasible to harness devices which are commercially available (and commonly used to detect rebars) to instantly detect fiber and assess their volume and spacing.
9.2.1 Materials and Equipment
The tests were carried out on the industrial floor located inside a depot building in Koszalin, Poland . The tested floor (493 m2) was composed of two layers. A 100 mm-thick concrete undercoat and a 150 mm-thick main SFRC layer. Hooked steel fiber (dimensions of the fiber 50mm 1.0 mm, tensile strength 1115 MPa) at a volume of Vf = 0.3% constituted the reinforcement. The declared strength class of concrete was C20/25. The tests were carried out six years after the concrete was casted.
The tested industrial floor
Both NDT devices used in the research programme were developed for the assessment of rebar location in hardened concrete. The measuring device using electromagnetic induction technique was designed to scan a flat square area of 0.36 m2. The principle of the device’s operation is as follows. When alternating current runs through the probe coil of the device, an electromagnetic field appears around the coil. If there is a ferromagnetic material in the field, it brings about a change in the voltage of the coil, and the voltage change appears according to the diameter around the cover. The tested industrial floor.
Both NDT devices used in the research programme were developed for the assessment of rebar location in hardened concrete. The measuring device using electromagnetic induction technique was designed to scan a flat square area of 0.36 m2. The principle of the device’s operation is as follows. When alternating current runs through the probe coil of the device, an electromagnetic field appears around the coil. If there is a ferromagnetic material in the field, it brings about a change in the voltage of the coil, and the voltage change appears according to the diameter around the cover thickness of the rebar. The method dedicated to the rebar detection was adopted in the tests to localize steel fiber in the area. Using the electromagnetic induction device, 32 randomly chosen square areas of the floor were tested. The total tested area was equal to 11.5 m2, which represented 2.3% of the area of the whole floor. The measuring device allowed to conduct tests in depth up to 150 mm. During the tests, four measuring depths were scanned: 30, 60, 90 and 120 mm. The applied radar apparatus emitted radar pulses spread over the frequency range from 1.0 to 4.3 GHz. The lower the frequency, the deeper the subsurface is penetrated, while the higher the frequency, the smaller objects can be spotted.
When the radar device is moved over the surface, a measurement is taken every 5 mm. At one scanner position, a high number of pulses are emitted and recorded to determine the full reflection pattern of the objects under the surface. Multiple acquisitions are used to reduce the noise in the data, which leads to a clean image.
The signal acquired by the radar front-end is further conditioned by the following steps:
_ Correction of antenna sizes and positions;
_ Background removal with automatic foreground/background detection to mask uniform structures such as the surface and possible stratifications.
_ Automatic gaining to compensate the damping of the radar waves in the base material;
_ Time-zero estimation (automatic recognition of the surface position);
_ Temperature compensation to allow immediate and accurate measurements directly after start-up.
Test Results and Analysis
The exemplary images of the industrial floor tested using an electromagnetic induction technique . The obvious disadvantage of this scanning method is the “shadow” cast by the fiber present in the top layers. The created “shadow” increased blackening of the images in deeper layers. Therefore, in subsequent layers, the shaded areas should not be taken into account to assess fiber presence.
Exemplary images of a scanned area (600 mm · 600 mm) at various depths using the electromagnetic induction technique.
The advantage of the electromagnetic induction method is a possibility of instant assessment of fiber volume and spacing in the scanned area. Images of tested areas located at the depth of 30 mm, are examples of such possibility.
Images of detected fiber (using the electromagnetic induction technique) in different tested areas at the same depth of 30 mm
The relative percentage of detected fiber in the layers located every 30 mm is presented. In subsequent layers, the shaded areas were not taken into account to assess relative percentage of fiber content. The electromagnetic induction technique detected almost 50% of steel fiber in the second layer located at the depth of 30–60 mm. The other layers contained from 15.5% to 20.2% of steel fiber. The low fiber content in the top layer can be explained by the so-called “wall effect”. The results obtained for the 0–30mm and 30–60mm depths closely corresponded to the actual fiber content. The smaller amount of fiber detected at depth of 60–120mm did not harmonize with genuine fiber content. The analysis of specimens was obtained by coring confirmed high uniformity of fiber spacing across the thickness of the floor (apart from the top layer). The explanation for this phenomenon might be a low measure of sensitivity of the apparatus. The depth and “shadow” cast by the fiber present in the top layers play a key role in this phenomenon.
Relative percentage of the detected fiber at each scanned depth.
The percentage deviation of fiber volume for 32 tested areas is presented in Figure 9.6. The results were grouped by depth. The top layer was characterized by the highest percentage deviation of fiber volume. The proximity of the surface influenced the homogeneity of fiber distribution. The second layer, placed at the depth of 30–60 mm, was characterized by the lowest value of the percentage deviation of fiber volume. Along with the depth, the value increased. The second layer, due to the best reading parameters and lowest percentage deviation of fiber volume, was the most suitable for assessing volume and spacing of steel fiber volume. The proximity of the surface influenced the homogeneity of fiber distribution. The second layer, placed at the depth of 30–60 mm, was characterized by the lowest value of the percentage deviation of fiber volume.
Percentage deviation of fiber volume.
The results of fiber detection process of a square section (600 mm _ 600 mm) of the industrial floor, based on the radar technique. The red color indicates areas characterized by high concentration of fiber. The testing equipment was able to detect a high concentration of fiber only in the top layer, which can be observed in the bottom and right view of the image. The advantage of the method is the 3D image of the tested area, which can be rotated at any angle in the program supplied with the device.
Two-dimensional visualisation of data obtained by the radar-based technique.
Three-dimensional visualisation of data obtained by the radar-based technique.
Conclusions
The results obtained during the research programme allowed us to form the following conclusions:
The method based on the electromagnetic induction technique can be applied to estimate the approximate volume of steel fiber in a hardened SFRC. However, the method requires calibration to obtain good quality of results in deeper layers due to the “shadow” cast by the fiber present in the top layers. The method can be applied to detect steel fiber up to the 120 mm thickness of the tested element.
-The method based on the radar technique is suitable for instant detection of the areas with a clearly spaced fiber volume (too low or too high local fiber concentration). Theoretically, the method can be applied to detect steel fiber presence up to the 200 mm thickness of the tested element, yet only fiber present in upper layers is correctly detected. The testing equipment based on the radar technique used for fiber detection is able to recognize fiber concentration fields but not a single fiber.
-Both methods together can detect fiber concentration in SFRC volume but cannot detect a single fiber.
Sunday, May 10, 2020
CURING CONCRETE AND SAW CUTTING
·
Thoroughly and completely cure and seal
as specified. Fully protect freshly placed concrete from the elements including
drying by the sun and wind, and washed by rain.
·
Start initial curing as soon as free
water has disappeared from concrete surface after placing and finishing.
Weather permitting; keep continuously moist for not less than seven days.
·
Curing methods during cold weather
protect concrete from freezing and cured.
·
No curing and sealing compound or method
shall be used, which will impair bond or penetration of subsequently applied
finishes or materials. Coordinate with material manufacturers.
·
Prohibit pedestrian traffic from newly
placed slabs for a minimum of 24 hours. Prohibit vehicular and other
construction traffic from newly placed slabs for a minimum of 7 days.
·
Control joints at slab-on-grade shall be sawcut to 1/3 the
thickness of the slab saw at locations indicated on the Drawings, as soon as
slab surface will bear the sawcut equipment and operator without damage
(generally within 8 to 10 hours of pour). For areas to receive Polished
Concrete Process coordinate joint cutting and filling with trades performing
polishing, Project Manager and Owners representative for appropriate timing of
this task.
i.
After slab has cured for 7 days, fill
sawcuts with approved material. Do not fill saw cuts with sand when slab will
receive Polished Concrete Process.
ii.
In areas to receive Polished Concrete
Process fill joints with backer and sealant after polishing process is
completed as directed by Project Manager and Owners representative.
·
Saw cut construction joints to 1/3 the
thickness of the slab using wet diamond blades. Begin saw-cutting prior to time
initial shrinkage cracks develop but not more than 8 hours after placement.
Completely remove concrete and slurry resulting from saw-cutting operations
from groove and from slab surfaces. Repair raveling and spalling along joint as
directed by Architect/Engineer.
Thursday, May 7, 2020
STEEL FIBER CONCRETE MIX
Ready-mixed concrete
shall be in accordance with Contract requirements and meeting applicable
requirements.
The ready mix trucks shall be in good
condition, kept clean of hardened concrete and other foreign materials in
injurious amounts. Schedule delivery of the concrete to avoid delays in placing
after mixing or holding dry mixed materials too long in the mixer prior to
adding mix water at the location of the pour. The correct quantity of water
shall be contained in the mixer and add additional water at the pour location shall not be permitted.
Ready-mixed concrete will be moved from the mixer to the point of placing as quickly as dosable by methods that will maintain the needed workability and will prevent segregation.
Within 2 hours of the time of loading, the concrete shall be discharged from the truck-mixer. If reducing admixtures are used or in cool, humid weather or when chilled concrete is produced. The time of loading will start from adding the mixing water to the dry mix of cement and adding the cement to the wet aggregate whichever is applicable.
No water from the truck water system or elsewhere shall be added after the initial introduction of mixing water for the batch except when on arrival at the job site the slump of the concrete is less than specified. If additional water is added, the drum shall be turned an additional 30 revolutions, minimum.
The adding of water reducing agents shall be performed as required and with the control and record data as specified. Reduction in cement content shall not be allowed because of higher strengths attained due to admixtures.
After checking slump, "Steel Fibers" at equal temperature to mixed concrete or super-plasticizer must be placed into the mix at job site, with one additional adjustment to mix, before any other quality assurance tests are performed.
If these are not usable, the fibers can be added to the batching plant using conveyor belts or' blast ' machines similar to those used to attach the fibers on site. It is important to remember that the
fibers land freely in the concrete when using conveyor belts and are then mixed
in the concrete using appropriate mixing procedures. Blast Machines have the
advantage of spreading the fibers in the concrete mix at a consistently high
pace so that the steel fibers are evenly distributed in each concrete load. If
steel fibers are applied without the use of blast machines at the batching
plant or on the job site, fibers can be "balanced" in clumps. This is
the product of unequal fiber distribution in the concrete and requires extra
mixing time to correct it.
In the
concrete, there are two common types of fiber balls that may form.
·
Dry balls most often occur when the steel fibers are dumped
in clumps into the ready-mix truck resulting in balling due to the fibers not
being allowed to integrate into the concrete mix properly.
·
Wet balls or steel fibers clumps also occur when the
concrete is mixed too long or too quickly. Wet balls can also occur when high
dosage levels are used for a small diameter steel fiber.
·
Before the first truck mixer arrives, blast machines and
conveyor belts should always be mounted and set up for service.
Wednesday, May 6, 2020
BEHAVIOUR OF FIBERS IN CONCRETE
Fibers contribute towards reducing the bleeding in fresh concrete and renders concrete more impermeable in the hardened stage. Contribution of certain percentage of fibers in concrete towards flexural strength is smaller compared to the strength given by the rebars. Most importantly fiber restricts the growth of crack under load thereby arresting ultimate cracking. Nonmetallic fibers like alkali resistant glass fiber and synthetic fibers provide resistance against chemicals.
Reinforcing capacity of fiber is based on length of fiber, diameter of fiber, the percentage of fiber and condition of mixing, orientation of fibers and aspect ratio. Aspect ratio is ratio of length of fiber to its diameter which plays an important role in the process of reinforcement.
STEEL FIBER
GLASS FIBER
Generally, the fibers used to reinforce concrete can be characterised as discontinuous, discrete fibers with length less than 50mm and diameter no more than 500mm. The actual purpose of incorporating fibers in the concrete matrix was the development of a composite with improved strength, both compressive and tensile. By analysing the results of the earliest developments in this field it can be observed that neither the compressive nor the tensile strength were increased by any appreciable amount.
CARBON FIBER
NATURAL FIBER
The actual benefits of fiber reinforcement were difficult to highlight by the researchers at that time. Later on, during the modern development of FRC in late 1970’s and early 1980’s, when the testing equipment and analysis procedures became more quantitative and better qualitatively the concept of energy absorption (or fracture toughness) was introduced. This concept enabled the toughness measurement of materials. It was then that the major advantage of FRC was discovered and it was not other than the outstanding property of absorbing large amounts of energy compared to Ordinary Portland Cement Concrete. Even today, after more than three decades of research in this field it can be said that the principal benefit of FRC is the high fracture toughness. However, further research with different types of fibers and admixtures targets the development of a composite with increased tensile and compressive strengths, besides the fracture toughness. These FRC composites are now known as the high performance fiber reinforced concrete (HPFRC). The production of a cement based material having high tensile and compressive strengths, remarkable energy absorption capacity and which will be homogeneous and isotropic (almost similar to cast iron) is no longer an utopia any more. The incessant research in the field of FRC has led to the production of HPFRC, which shows a combination of amazing properties compared to other cementitious composites.
POLYPROPYLENE FIBER
Fibers contribute towards reducing the bleeding in fresh concrete and renders concrete more impermeable in the hardened stage. Contribution of certain percentage of fibers in concrete towards flexural strength is smaller compared to the strength given by the rebars. Most importantly fiber restricts the growth of crack under load thereby arresting ultimate cracking. Nonmetallic fibers like alkali resistant glass fiber and synthetic fibers provide resistance against chemicals. Reinforcing capacity of fiber is based on length of fiber, diameter of fiber, the percentage of fiber and condition of mixing, orientation of fibers and aspect ratio. Aspect ratio is ratio of length of fiber to its diameter which plays an important role in the process of reinforcement.
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