Curious Civil Engineer

  Prestressing strands is high tensile strength steel. The tensile strength of prestressing strands can reach up to 1860 Mpa. the process of manufacturing prestressing steel differs significantly from the conventional reinforcement.  Figure 1 The process of manufacturing strands will begin by de-scaling the raw material (wire rod). The purpose of de-scaling is to remove the iron oxides (mill scale) from the surface of the wire rod. after removing the mill scale, the wire rod's surface is coated with a textured coating to enhance the adherence of lubricants to the raw material surface. The most commonly used coating is zinc phosphate.  Figure 2 The wire rod then passed through a series of dies. The process of reducing the wire rod diameter is known as cold-drawing. In this process, the wire rod is drawn through a die with the required diameter. To prevent the wearing of dies, the wire rod will be drawn through a lubricant box. the lubricant will coat the wire with a thin layer that

  Prestressing bridge concrete beams contributes to resisting the tensile stresses caused by the traffic loads and the beam's weight. Stressing concrete has various benefits, such as increasing concrete members' capacity, reducing the concrete member section, and increasing the span lengths for structures such as bridges and buildings. However,  The stressing of bridge beam ends can result in unwanted cracks. Therefore AASHTO calling for debonding a specific portion of strands at the beam end to minimize the stresses and reduce cracks. Figure 1 Debonding is achieved by wrapping the strands at the end of the beam with plastic. Wrapping the strand with plastic wrapping will prevent the strands from bonding with concrete, thus avoid transferring unwanted stress to the beam end. As a result, the unwanted cracks will be prevented by reducing the stress levels at the end of the beam.  

 Ductility is an important property of steel reinforcement. Ductility is the ability of the material to undergo plastic deformation before failure. The ductility of reinforcement is related to the elongation property. The ductility of reinforcement ensures safer and durable structures. Material that undergoes little or no plastic deformation is known as a brittle material. Brittle reinforcement can cause sudden structure failure because it doesn't undergo any plastic deformation before failure.  Figure 1 The ductility factor for reinforcement can be computed using the following equation: µ=ϵu/ϵy where, µ is the ductility factor ϵu is the ultimate strain ϵy is the yield strain In figure no:1-a, we can see the stress-strain curve for mild steel. The mild steel has well-defined yield stress and strain, as shown in the stress-strain curve. The reinforcing bar can recover all the elongation if the applied stress is lesser than the yield stress. This portion of the curve is known as the

 Construction management: Road construction course This course is designed to expand your practical side of engineering knowledge. This course is concentrating on the construction of roads. Here you will learn how the road is constructed. The construction stages such as protection of existing services, clearing the area, future utilities, preparing the road subgrade, then placing different pavement layers such as sub-base, base course, and asphaltic courses. We will also discuss the types of asphaltic course and the common defects of asphaltic pavement, such as permanent deformation, fatigue cracking, and low-temperature cracking. Also, I will show a typical cross-section of road and road profiles and many other subjects. Enroll from here  Pile foundation design This course will discuss the design of pile foundations. We will learn the methods of calculating the pile bearing capacity. The bearing capacity for a pile consists of skin and end bearing resistance. Here you will understand

Rebound hammer is a non-destructive test used to examine the hardened concrete. The principle of the rebound test is based on the rebound of a plunger. The plunger of the rebound hammer is pressed against a concrete surface. The hammer mass will be pushed back and secured to a pin. The impact spring is attached to the hammer mass, so it will be extended with the hammer mass. Then the hammer mass is released.finally, the mass will rebound against the concrete surface, as shown in figure no:1. The surface hardness and compressive strength will be correlated with the rebound of the spring controlled mass.  Figure 1 Figure 2 A rebound hammer can be used to evaluate the compressive strength of a concrete member, quality of concrete, and uniformity of concrete. The approximate impact energy required for the rebound of the hammer for different applications is shown in table no:1. Table 1 The procedure of conducting the rebound test will begin by the calibration

Acidic materials are the material with Ph lower than 7. material with Ph higher than 7 classified as basic material. Neutral material has a Ph of 7. concrete has a Ph higher than 7, so it is a basic material. Concrete structures that are constructed in the vicinity of the acidic environment, such as mines, will be vulnerable to damage. Drainage water from existing mines has a ph value of about 1. this low ph value will deteriorate and severely damage the concrete quickly. However, the exposure of concrete to a high ph-value of 5 to 6 will also damage the concrete but at a lower rate. Figure 1 Acidic material will react with concrete cement paste. The cement paste then transferred to calcium salt. The calcium slat will be washed out by the water, and the aggregates will be exposed. The acid damage is similar to abrasion damage, except the aggregates don't have the polished surface. The concrete is a basic material. Therefore it will neutralize the effects of acid at

  Soundness test conducted in cement to verify that cement will not undergo a destructive expansion after setting. The expansion of restrained concrete members will result in severe tensile stresses that can result in cracking and spalling of casted concrete. The expansion could occur due to the reaction of free lime, magnesia, and calcium sulfate.   Free lime found in the clinker. Free lime inter-crystallized with other compounds, and it tends to expand during the hydration process. The expansion of free lime is more than calcium oxide. Magnesia reacting with water similarly as calcium oxide does. But the crystalline formed due to the reaction of magnesia is deleteriously reactive, so it is unsound. Calcium sulfate can cause destructive expansion. The reaction of calcium sulfate will produce calcium sulfoaluminate (ettringite), which can result in the harmful expansion of cement.  Figure 1 A soundness test will be conducted using La Chatelier's apparatus, shown in

Sodium magnesium and calcium sulfates are salts that can react with hardened concrete and cause deterioration for concrete structures. Sodium magnesium and calcium sulfates react with hydrated lime and hydrated aluminate in cement. This reaction will produce calcium sulfate and calcium sulfoaluminate. The volume of these products is greater than the volume of the cement paste, which undergoes the reaction with sulfates salts. This reaction will cause a destructive volume change. The increase of existing concrete will generate stress, which may result in concrete cracking and spalling. The using of sulfate resistance cement is recommended where the sulfate concentration is high. Sulfate resistance cement or cement type V will resist the sulfate reaction. Type V cement has a low calcium aluminate content, which enables this cement type to provide a good resist the sulfate deterioration. Therefore it should be specified and used when it is recognized that concrete must be exposed to

Cooling of concrete is very important to prevent the thermal cracks. The rapid concrete hydration will generate a high amount of heat. This heat will cause high tensile stresses higher than fresh concrete strength. Concrete in hot weather and mass concrete are susceptible   to thermal cracking. In general concrete, the temperature is reduced to prevent thermal cracking. In addition to thermal cracking, hot weather can increase the rate of slump loss and increase the rate of water evaporation. The reducing of concrete temperature can be done by covering the aggregates, storing the cement in a shaded area, and using ice to cool the water. Recently liquid nitrogen used to reduce the temperature of concrete. construction management: concrete construction bridge construction:How to become a bridge engineer The LN has a temperature of -196 C˚ (-320F˚). The LN used to cool the concrete directly on the concrete mixer. LN is stored in a special tank that can endure the low tempe

A slip of a strand can occur during the stressing process or while anchoring the tendon. The slip of a strand can occur due to rusted wedges and rusted wedge plate holes. The rust or dirt will prevent the wedges from firmly gripping the strands. Worn teeth of jack wedge can be another reason for strand slipping. Strands slippage can be prevented by using of well-maintained wedges, wedge plates, and jacks.  construction management: concrete construction bridge construction:How to become a bridge engineer Slippage of strands will be discovered from the marks made on strands tail for this purpose. If the slip is significant (more than 1 in (25mm)), the effects of strand slip should be considered. Slipped strand will have lower stress than other strands. This will reduce the overall force of the tendon. In order not to overstress other strands, the prestressing force should be reduced proportionally to account for the strand slip. For example, if one strand out of ten slip

High-performance concrete possesses better properties and strength than typical concrete. High-Performance concrete mixtures are highly durable, and they have high ultimate strength. HPC has been used in tunnels, bridges, and tall building structures. Also, in shotcrete repairs, parking garage, and agricultural applications. ACI defined high-performance concrete as a concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing, and curing practice.  Figure 1 High-performance concrete mixtures developed for a specific application. The characteristics of high-performance concrete are developed to suit the specific application. Some of the properties that may be required include: • High strength • High early strength • High modulus of elasticity • High abrasion resistance • High durability and long life in severe environments • Low permeability and diff

In 1968 the 22 levels residential building Ronan point in London collapsed partially as a result of a limited gas explosion on the 18th floor. The small explosion prompts the collapse of the entire corner of the building, as shown in figure no: 1, which killed 4 people and injured 14. The cause of the explosion was a resident striking match to boil water, which ignites a gas that was leaking from a newly installed joint for a food cooker. Figure 1 The building was constructed using a technique known as a large panel system. In this technique, precast panels are prepared off-site. Then these panels are lifted and placed on their specified positions. Finally, precast panels joined to each other by bolts. The gas explosions reveal the weakness of this technique. The bolting of the precast panel proves to be weak as it could not stand a small blast. The explosion on the 18th floor blown out a structural wall panel. This wall supporting the upper 4 stories of the building. The f

Carbonation of concrete is a significant defect that cause reinforcement to corrode. Figure no: 1 showing the stages of the carbonation process. Carbon dioxide (Co2) is natural gas, and Sulphur dioxide combined with the moisture from the surrounding environment will react with calcium hydroxide in concrete producing calcium carbonate and calcium sulfate. This chemical reaction will reduce concrete alkalinity, which results in a rapid increase in the rate of reinforcement corrosion. Figure 1 Cracking and spalling of concrete are the symptoms of concrete carbonation. The cracks and spalling occur due to reinforcement corrosion. Steel rust occupies a bigger volume, which exerts stresses on concrete, and this will result and concrete cracks and spalling. construction management: concrete construction bridge construction:How to become a bridge engineer The depth of concrete carbonation can be determined using phenolphthalein. Phenolphthalein is a powder,

Flaky and elongated aggregates can adversely affect concrete. Flaky and elongated aggregates are difficult to compact. Therefore using flaky and elongated aggregates will reduce concrete workability, which required more amount of water to produce a workable mix. Increasing water content will reduce concrete strength and durability. Moreover, these aggregates are not robust, which leads to lowering concrete strength. Figure 1 construction management: concrete construction bridge construction:How to become a bridge engineer The Flakiness Index of aggregates is the percentage by weight of particles whose least dimension (thickness) is less than three- fifths (0.6times) of their mean dimension. This test does not apply to sizes smaller than 6.3mm. The Elongation index of an aggregate is the percentage by weight of particles whose greatest dimension (length) is greater than nine-fifths (1.8times) their mean dimension. Flakiness and elongation tests ar

Coating of concrete performed by applying liquid or semi-solid materials to a concrete surface. The concrete coating can be paints, cement-based topping, overlay, and epoxy aggregate system. The purpose of the concrete coating is to stop the ingress of water inside concrete. The prevention of water ingress will prevent the ingress of aggressive chemicals such as chloride and sulfate. Chloride diffuses into concrete in solution. The ingress of chloride will adversely affect concrete durability. Chloride will cause steel corrosion. Corroded steel will expand and exert stresses on concrete which can cause concrete cracking and spalling. Also, the effective cross-section and tensile strength of steel will be reduced. The ingress of sulfate will affect concrete durability also. Sulfate will react with concrete causing destructive expansion.  Figure 1 construction management: concrete construction bridge construction:How to become a bridge engineer The concret

Rapid hardening cement develops strength at a higher rate compared to ordinary portland cement. The strength produced at 2 days by RHC equal 7 days strength of OPC if some w/c ratio used. The strength of RHP will equalize with OPC after 2 to 3 months. The reason behind developing early strength is the increase of C3A and C3S and due to increasing the fineness (min 3250 cm2/gm)of the cement. Increase the fineness of cement, and the amount of C3A and C3S will result in a higher rate of hydration and rapid gaining of strength.  Figure 1 construction management: concrete construction bridge construction:How to become a bridge engineer The RHC is chemically similar to OPC. Therefore the designing of concrete mix will be similar as if OPC used. The initial setting of rapid hardening concrete is 60 minutes or more. The RHC will gain 30% of it is compressive strength in the first 24 hours. RHC has the following advantages:  RHC curing period is less than

Grouting usually used for grouting of crane rails, anchor bolts, stanchion (steel column) base pates, bridge bearings, etc. also it can be used for filling precast joints, tie holes, fast repairing of potholes on the roads, runways, and other concrete structures. There are two types of grout: epoxy grout and cement grout. Figure 1 Epoxy grout consists of resin, hardener, and sand or aggregates. We have various type of resin, such as epoxy, polyester, and polyurethane. The second component of epoxy grout initiates the hardening process of epoxy grout. Some types of epoxy grout can gain compressive strength of 80 Mpa during the first week. On the other hand, cement grout is a mix of cement and water. A plasticizing agent can be used to reduce the amount of water. Cement grout and epoxy grout have similar applications. The cement grout will produce a seven-day compressive strength of 32 MPa, which is less than epoxy grout strength at the same age. The pot life term used in g

Lightweight concrete made simply by using lightweight aggregates. Lightweight aggregates are produced by expanding shale, clay, and slate. These aggregates are manufactured in high-temperature rotary klin. During the production, gas bubbles are formed inside the aggregates and remain after cooling. These bubbles will reduce the weight of aggregates without reducing strength. Figure 1 construction management: concrete construction bridge construction:How to become a bridge engineer The using of lightweight concrete is limited due to cost and durability concerns. Despite the limited use of lightweight concrete, many structures around the world are built using lightweight concrete.  Using lightweight concrete to construct a building will significantly reduce the dead load. Cutting the dead loads means reducing the size of foundations. For precast elements. Using lightweight concrete will reduce the weight of prefabricated elements and facilitate the h

Cracks that are growing in size and depth over time are known as active crack. There are several methods used to repair cracks, but the reason behind the cracks should be investigated and removed. Overloading and thermal expansion or contraction could be a reason for developing a crack. Repairing of active cracks is insufficient without fixing the cause of the crack. In some cases, a new crack will develop near the old repaired crack. Drilling and plugging through the crack: in this method, a hole is drilled near the crack, as shown in figure 1. this method suitable for straight-line cracks. The hole width and length should be equal or more than the crack width and length. Then the hole filled with grout. The grout forms a key that preventing the crack from growing and propagating. The diameter of the drilled hole range from 50 mm to 75 mm depends on the size of the crack. The hole can be filled with reinforcement and epoxy grout. The size of rebar should be predetermined