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ABDULLAH ABD AL-KAREEM CEEN 3244 Assignment No 3 Question 1: What is the effect of water-cement ratio on the permeability of hardened concrete? How does permeability affect the durability of the concrete structure?

Answer: Water to cement ratio has direct effects on the permeability of hardened concrete. The general trend of increasing the water to cement ratio on the coefficient of permeability is increasing in nature. As the water to cement ratio increases from 0.3 to 0.7, the coefficient of permeability increases by 1000 times. The durability of the hardened concrete is largely affected by the permeability coefficient of the concrete. The greater the permeability constant of the concrete, the greater the amount waters and chemicals constituents that will sweep in and out of the hardened concrete. Such permeability will have reduced resistance to frost, reactivity of alkali with silica gel and other chemical attacks. Another important factor is the rusting of the steel rebars in case of Reinforced cement concrete due to increased permeability. In general, the greater the permeability, the smaller the durability of hardened concrete.

Question 2: What does the term “curing concrete” mean? What will happen if concrete is not cured adequately? Discuss five different methods of concrete curing.

Answer: Curing Concrete means to provide adequate amount of moisture and optimum temperature conditions for the long-term cement hydration reaction to take place efficiently in the concrete being hardened for a definite period such that designed properties of concrete are achieved successfully. If the curing of concrete is not adequate, the concrete will dry in air and the gain in strength will stop as long as the curing is stopped. The concrete will not gain it’s designed strength if the curing is not performed as per requirements and the resultant hardened concrete will have lower compressive strength as compared to designed values. Following are the five different methods of curing concrete;

1. Ponding or Immersion In this technique the exposed surfaces of concrete are covered with water by forming small size earthen dikes around the concrete surface such that the poured water is retained and a layer of water retains on the surface of the structure. This type of curing is suitable for flat concrete structures such as slabs, floors and pavements. 2. Spraying or Fogging In this curing method a system are nozzles are used to continuously spray water onto the concrete surface meant for curing and requires large amounts of water. 3. Wet Coverings In this method, fabrics of moisture retaining properties such as jute, burlap, cotton mats or rugs are saturated with water and are placed on the surfaces meant for curing. During this, the fabrics are continuously kept wet by periodic watering. To avoid water losses by evaporation from the fabrics, they are covered with polyethylene sheets. A disadvantage of such curing method is that stains or discoloring of concrete could occur with some types of wet coverings. 4. Steam Curing This type of curing is used when additional amount of heat is required for early gain in strength of concrete in cold weather conditions. Steam at high or low pressure is sprayed on enclosed cast in place structures and large precast members. 5. Insulating Blankets or Covers In weathers where the temperature falls below the freezing point of water, the water is required to be kept from freezing and for that purpose the concrete is insulated by using dry and porous material layer acting as an insulating blanket. Materials such as hay or straw fiberglass, cellulose fibers, sponge rubber, mineral wool, vinyl foam, or open-cell polyurethane foam can be used.

Question 3: Why is extra water harmful to fresh concrete but good for concrete after it reaches its final set?

Answer: Extra water accounts for the amount of water that is in excess in comparison to the amount of water required to make hydration gel. Since in fresh concrete apart from increasing the workability, it can lead to problems like aggregate segregation and bleeding. However, it can be useful when concrete reaches its final set as the extra water will be available for curing of the concrete and facilitating the hydration process.

Question 4: Discuss the change in volume of concrete at early ages.

Answer: A cement paste in elastic state undergoes 1% decrease in volume known as plastic shrinkage and is due to the loss of water from the cement paste either by evaporation or by the dry concrete below the fresh concrete. This can lead to cracks and can be prevented by controlling water losses. The second form of volume changes in concrete at early ages is the one occurring after setting due to improper curing that causes dry shrinkage. This dry shrinkage occurs due to lack of curing, using high water to cement ratio and high cement concrete. Swelling can also take place in the continuously cured concrete due to increased amount of hydration and hydration gel formation but does not causes significant problems

Question 5: What is shotcrete? Describe the difference between the wet and dry processes.

Answer: Shotcrete is small sized aggregate concrete or mortar sprayed with a very high velocity on the required horizontal or vertical surfaces that consolidates due to impact force upon spraying. Dry and wet processes are the two methods of applying shotcrete. The former involves a premixed blend of the cement and moist aggregate and is applied through a compressed nozzle hose where the water is added to the dry mix at the time of its ejection from the nozzle at the nozzle. The later involves the addition of water to the premixed blend and is applied via the compressed nozzle hose without any addition of water as the mix ejects the nozzle.

Question 6: What is consolidation of concrete? Describe different methods for consolidating concrete for small and large jobs.

Answer: It is the removal of the air voids and bubble from the permanently placed concrete in order to avoid excess pores and permeability and honeycombing by the application of vibration or compactor.

For small jobs of consolidating the concrete, the manual ramming and tampering of concrete is adequate. For large scale applications, vibrators of different forms are used to consolidate the concrete.

Question 7: Describe five tests of hardened concrete.

Answer: The five common tests performed on the hardened concrete are as follow; 1. Compressive Strength Test Compressive Strength test is used to measure the compressive strength fc’ of hardened concrete on cylindrical specimens of standard sizes. The ASTM C31 standard is followed for the preparation of the standards and ASTM C39 is followed for testing procedures and reporting’s. This test involves the application of axial compressive load usually applied by the Universal Testing Machine “UTM” on a specimen of standard cylindrical size until the specimen fails. This test is performed on hardened concrete of age 3 days, 7 days and 28 days respectively. 2. Split-Tension Test This test is used to measure the tensile strength of the hardened concrete on a standard cylindrical specimen of size 0.15m by 0.30 m. It involves the application of axial compressive load along the length of the vertical diameter usually by Universal Testing Machine “UTM” until the specimen fails. The ASTM C496 standard provides the procedure and specimen preparation guidelines to perform this test. 3. Flexure Strength Test This test is performed on specimens of standard sizes using the guidelines of ASTM C31 and the procedure is adopted using ASTM C78. This test involves application continuous application of axial compressive strength on a third point loading apparatus on a square cross section specimen with a span of three times the depth of the specimen until the specimen ruptures. Depending on the propagation of the initial fracture, varying formulas are used to estimate the flexure strength. 4. Rebound Hammer Test It is a Non-destructive testing technique which involves the use of the Schmidt Hammer to determine the hardness of the hardened concrete surface. The test procedure involves the guidelines provided in ASTM C805 and involves the spring and mass enclosure of the hammer to be rebounded against a clean concrete surface at a perpendicular angle. The amount of rebound is an indication of the concrete strength and the magnitude of the rebound is related to the concrete strength in graphical forms usually provided the manufacturers of the hammer. 5. Penetration Resistance Test

This test is standardized by ASTM C803 and involves a gun like device that shoots probes into the concrete surface for determining its strength. The test is performed on a three-hole template where each hole has specific location and measuring the probe penetration in each of the hole using the scales and special plate and averaging it give the value of the probe penetration which is inversely related to the strength of the concrete.

Question 8: What is the mortar made of? What are the functions of the mortar?

Answer: Mortar is a made up of a cementitious material, fine aggregate and water and varying according to the type of the cementitious material used. Thus, mortar is classified into following three types; 1. Masonry Cement Mortar 2. Cement Mortar. 3. Cement Lime Mortar. Following are the functions of the mortar; 1. To provide aesthetic quality to the structure by leveling the uneven surfaces. 2. To proving the leveling surface to the concrete masonry units. 3. To proving seating to the concrete masonry units. 4. To provide bonding between the underlying and adjacent masonry units either reinforced or non-reinforced.

Question 9: What is the manufacturing process of CMUs?

Answer: Concrete Masonry Units “CMU” are manufactured using no slump concrete mixture of Portland cement, aggregate and admixtures. The mixture is then poured into molds of standard sizes under pressure and are then steam cured.

Question 10: Define solid and hollow masonry units according to ASTM C90.

Answer: According to ASTM C90, the solid masonry units are the concrete bricks whose net cross-sectional area of in every plane parallel to the bearing surface shall be not less than 75 % f the gross crosssectional area measured in the same plane.

Whereas the hollow masonry units are the concrete blocks, hollow blocks or cinder blocks whose net cross-sectional area of in every plane parallel to the bearing surface in more than 75 % of the gross cross-sectional area measured in the same plane and conforms to the minimum face shells and web requirements of the Table 1 of the ASTM C90 standard....