Chapter 3-Properties of Fresh Concrete PDF

Properties and Strength of materials - ARCH 214 Eng. Lara Rahim Al yamama university ARCH214 – Chap 3–Fundamentals of Concrete Chapter 3 Fundamentals of concrete – Properties of Fresh Concrete Outline 1. Introduction 2. Fresh concrete properties a. Consistency b. Workability c. Segregation d. Bleedi...

Properties and Strength of materials - ARCH 214 Eng. Lara Rahim Al yamama university ARCH214 – Chap 3–Fundamentals of Concrete Chapter 3 Fundamentals of concrete – Properties of Fresh Concrete Outline 1. Introduction 2. Fresh concrete properties a. Consistency b. Workability c. Segregation d. Bleeding and settlement e. Consolidation f. Hydration setting time and hardening Introduction Concrete = aggregates + paste Paste = Portland cement + water The paste binds the aggregates into a rocklike mass as the paste hardens because of the chemical reaction between the cement and water. The paste is composed of cementitious materials, water and entrapped air or purposely entrained air. It constitutes about 25% to 40% of the total volume of concrete. Freshly mixed Concrete Freshly mixed concrete should be plastic or semi-fluid In plastic concrete mixtures: All grains of sand and pieces of gravel are held in suspension The ingredients do not segregate during transport when the concrete hardens, it becomes a homogeneous mixture of all the components During placing, concrete of plastic consistency does not crumble but flows sluggishly without segregation. Plastic mixtures are suitable for most concrete work. For thin concrete members, plasticizing admixtures may be used to make concrete more flowable. Main Prop. OF Fresh Concrete a. b. c. d. e. f. Consistency Workability Segregation Bleeding and settlement Consolidation Hydration setting time and hardening 1-Concrete Consistency Consistency or fluidity of concrete is an important component of workability and refers in a way to the wetness of the concrete. However, it must not be assumed that the wetter the mix the more workable it is. If a mix is too wet, segregation may occur. 1-Concrete Consistency On the other hand, if a mix is too dry it may be difficult to place and compact, and segregation may occur because of lack of cohesiveness and plasticity of the paste. 3 Ways to determine Consistency of Fresh Concrete Slump Test Consistency Tests Flow Test Ball penetration test Slump Test — Definition A slump test is a method used to determine the workability and consistency of concrete. The stiffness of the concrete mix should be matched to the requirements for the finished product quality — Slump is a measurement of concrete’s workability, or fluidity. — It’s an indirect measurement of concrete consistency or stiffness. — Principle The slump test result is a measure of the behavior of a compacted inverted cone of concrete under the action of gravity. It measures the consistency or the wetness of concrete. Slump Test — Apparatus — Slump cone : frustum of a cone, 300 mm (12 in) of height. The base is 200 mm (8in) in diameter and it has a smaller opening at the top of 100 mm — Scale for measurement, — Tamping rod(steel) 15mm diameter, 60cm length. Slump Test — Procedure — The base is placed on a smooth surface and the container is filled with concrete in three layers, whose workability is to be tested. — Each layer is tamped 25 times with a standard 16 mm (5/8 in) diameter steel rod, rounded at the end. — When the mold is completely filled with concrete, the top surface is struck off (leveled with mold top opening) by means of screening and rolling motion of the temping rod. — The mold must be firmly held against its base during the entire operation so that it could not move due to the pouring of concrete and this can be done by means of handles or foot – rests brazed to the mold. Slump Test — Procedure — Immediately after filling is completed and the concrete is leveled, the cone is slowly and carefully lifted vertically, an unsupported concrete will now slump. — The decrease in the height of the center of the slumped concrete is called slump. — The slump is measured by placing the cone just besides the slump concrete and the temping rod is placed over the cone so that it should also come over the area of slumped concrete. — The decrease in height of concrete to that of mold is noted with scale. (usually measured to the nearest 5 mm (1/4 in). Slump Test — Precautions — In order to reduce the influence on slump of the variation in the surface friction, the inside of the mold and its base should be moistened at the beginning of every test, and prior to lifting of the mold the area immediately around the base of the cone should be cleaned from concrete which may have dropped accidentally. Slump Test — Types Of Slump The slumped concrete takes various shapes, and according to the profile of slumped concrete, the slump is termed as; — Collapsed Slump — Shear Slump — True Slump — Zero Slump Slump Test — Types Of Slump — Collapsed Slump In a collapse slump the concrete collapses completely. — A collapse slump will generally mean that the mix is too wet or that it is a high workability mix, for which slump test is not appropriate. — Shear Slump If one-half of the cone slides down an inclined plane, the slump is said to be a shear slump. — If a shear or collapse slump is achieved, a fresh sample should be taken and the test is repeated. — If the shear slump persists, as may the case with harsh mixes, this is an indication of lack of cohesion of the mix. — Shear slump indicates that the concrete lacks cohesion. It may undergo segregation and bleeding and thus is undesirable for the durability of concrete. Slump Test — Types Of Slump — True Slump True slump refers to general drop of the concrete mass evenly all around without disintegration. This type of slumps is most desirable — Zero Slump If concrete maintains the actual shape of the mould, it is called zero slump which represents stiff, consistent and almost no workability. Slump Test — Uses — The slump test is used to ensure uniformity for different batches of similar concrete under field conditions and to ascertain the effects of plasticizers on their introduction. — This test is very useful on site as a check on the day-today or hour- to-hour variation in the materials being fed into the mixer. — An increase in slump may mean, for instance, that the moisture content of aggregate has unexpectedly increases. Other cause would be a change in the grading of the aggregate, such as a deficiency of sand. — Too high or too low a slump gives immediate warning and enables the mixer operator to remedy the situation. Slump Test Degree of workability Slump (mm) Very low 0 - 25 Low 25 - 50 Medium 50 - 100 High 100 - 175 Slump Test Slump (mm) 0 - 20 20 - 40 40 - 120 120 - 200 200 - 220 Consistency Dry Stiff Plastic Wet Sloppy >Table : Relation between Consistency and Slump values The concrete should have a peanut butter-like consistency and should hold its shape when it is squeezed in a gloved hand. It should not be soupy. If so, add a small amount of dry concrete to stiffen the mix. Nor should the mix be powdery. Flow Test — Equipment — Flow table with a grip and a hinge, 70 cm x 70 cm. — Abrams cone, open at the top and at the bottom - 30 cm high, 17 cm top diameter, 25 cm base diameter — Water bucket and broom for wetting the flow table. — Tamping rod, 60 cm height — Scale for measurement Flow Test — Conducting — The flow table is wetted. — The cone is placed on the flow table and filled with fresh concrete in two layers, each layer 25 times tamp with tamping rod. — The cone is lifted, allowing the concrete to flow. — The flow table is then lifted up several centimeters and then dropped, causing the concrete flow a little bit further. — After this the diameter of the concrete is measured in a 6 different direction and take the average. Flow Test Flow Test Percent of Flow 0 – 20 % 20 – 60 % 60 – 100 % 100 – 120 % 120 – 150 % Consistency Dry Stiff Plastic Wet Sloppy A concrete mix sample has a diameter flow of 35 cm, describe it’s consistency. Concrete Workability — Definition The ease of placing, consolidating, and finishing freshly mixed concrete and the degree to which it resists segregation is called workability. Concrete should be workable but the ingredients should not separate during transport and handling — Workability is one of the physical parameters of concrete which affects the strength and durability as well as the cost of labor and appearance of the finished product — Concrete is said to be workable when it is easily placed and compacted homogeneously i.e without bleeding or Segregation. Unworkable concrete needs more work or effort to be compacted in place, also honeycombs &/or pockets may also be visible in finished concrete. Concrete Workability — Factors affecting workability — — — — — — — — — Water content in the concrete mix Amount of cement & its Properties Aggregate Grading (Size Distribution) Nature of Aggregate Particles (Shape, Surface Texture, Porosity etc.) Temperature of the concrete mix Humidity of the environment Mode of compaction Method of placement of concrete Method and duration of transportation of concrete Concrete Workability — How To improve the workability of concrete — increase water/cement ratio — increase size of aggregate — use well-rounded and smooth aggregate instead of irregular shape — increase the mixing time — increase the mixing temperature — use non-porous and saturated aggregate — with addition of air-entraining mixtures Compacting Factor Test — Apparatus — Trowels — Hand Scoop (15.2 cm long) — Rod of steel or other suitable material (1.6 cm diameter, 61 cm long rounded at one end ). — Balance. Compacting Factor Test Workability Slump (mm) C.F Uses Very Low 0 - 25 0.78 Roads - Pavements Low 25 - 50 0.85 Foundations Concrete Medium 25 - 100 0.92 Reinforced Concrete High 100 - 175 0.95 Reinforced Concrete (High Reinforcement) Concrete Segregation — Definition — Segregation is when the coarse and fine aggregate, and cement paste, become separated. Segregation may happen when the concrete is mixed, transported, placed or compacted — Segregation makes the concrete — WEAKER, — LESS DURABLE, and will leave A POOR SURFACE FINISH Concrete Segregation — Basic types of segregation —Coarse segregation : Occurs when gradation is shifted to include too much coarse aggregate and not enough fine aggregate. —Fine segregation : Occurs when gradation is shifted to include too much fine aggregate and not enough course aggregate. Concrete Segregation — To Avoid Segregation — Check the concrete is not 'too wet' or 'too dry'. — Make sure the concrete is properly mixed. It is important that the concrete is mixed at the correct speed in a transit mixer for at least two minutes immediately prior to discharge. — The concrete should be placed as soon as possible. — When transporting the mix, load carefully. — Always pour new concrete into the face of concrete already in place. — When compacting with a poker vibrator be sure to use it carefully Concrete Segregation — To Avoid Segregation — If placing concrete straight from a truck, pour vertically and never let the concrete fall more than one-and-a-half meters. Honeycomb Seggregation Concrete Bleeding Concrete Bleeding — Introduction — This refers to the appearance of water along with cement particles on the surface of the freshly laid concrete. — This happens when there is excessive quantity of water in the mix or due to excessive compaction. Bleeding causes the formation of pores and renders the concrete weak. — Bleeding can be avoided by suitably controlling the quantity of water in the concrete and using finer grading of aggregates. Concrete Bleeding — Bleeding Process — Almost all freshly placed concrete bleeds. As aggregate and cement particles settle, they force excess mixing water upward. The process continues until settlement stops, either because of solids bridging or because the concrete has set. — The total amount of bleeding or settlement depends on mix properties, primarily water content and amount of fines (cement, fly ash, fine sand). — Increasing water content increases bleeding, — Increasing the amount of fines reduces bleeding. — Amount of bleeding is also proportional to the depth of concrete placed. More bleed water rises in deep sections than in thin ones. Consolidation Consolidation of Concrete is the process of reducing or eliminating the volume of voids. it is also the process of removing this excessive air and making the concrete into a uniformly solid mass. Good consolidation is needed to achieve a dense and durable concrete. Poor consolidation can result in early corrosion of reinforcing steel and low compressive strength Consolidation Vibration large max size of aggregates + well-graded aggregates Sets into motion the particles in freshly mixed concrete Reduces friction between the particles less surface area to coat with paste + less volume there is to fill with paste Gives the mixture the mobile qualities of a thick fluid Less water and cement are needed Consolidation Effect of voids in concrete due to a lack of consolidation on modulus of elasticity, compressive strength, and flexural strength The End