Concrete Technology - Fresh Concrete PDF

# CONCRETE TECHNOLOGY - CE33125 ## Module 2: Fresh Concrete ### Detailed Syllabus - **Fresh Concrete:** - Workability - Factors affecting workability - Measurement of workability by different tests - Setting times of concrete - Effect of time and temperature on workability...

# CONCRETE TECHNOLOGY - CE33125 ## Module 2: Fresh Concrete ### Detailed Syllabus - **Fresh Concrete:** - Workability - Factors affecting workability - Measurement of workability by different tests - Setting times of concrete - Effect of time and temperature on workability - Segregation - Bleeding - Mixing & vibration of concrete - Steps in manufacturing of concrete **Dr. Govind Mohan** **Assistant Professor** **Department of Civil Engineering** ## OUTLINE - Introduction - Workability - Measurement of workability - Factors affecting workability - Requirements of workability - Steps in manufacture of concrete ## INTRODUCTION Fresh concrete or Plastic concrete is a freshly mixed material which can be moulded into any shape. The relative quantities of cement, aggregates, admixtures, mineral additives and water mixed together, control the properties of concrete in the wet state as well as in the hardened state. It is the plastic phase of concrete, in which the time of the water added to concrete mix to the end of time where concrete surface is finished in its final location in the structure. **Composition of Concrete** - Water - Aggregates - Supplementary cementitious materials - Cement - Chemical Admixture ## WORKABILITY - **As per IS: 6461 (Part-VII) - 1973:** - "The property of freshly mixed concrete or mortar which determines the ease and homogeneity with which it can be mixed, placed, compacted and finished." - **Rheology:** - "The science of the deformation and flow of materials, and is concerned with relationships between stress, strain, rate of stain and time." - The term rheology deals with the materials whose flow properties are more complicated than those of simple fluids. The rheological principles and techniques as applied to concrete include the deformation of hardened concrete, handling and placing of freshly mixed concrete, and the behaviour of its constituent parts, namely, cement slurries and pastes. The rheology of fresh concrete like workability includes the parameters of stability, mobility and compactability, which are necessary to determine the suitability of any concrete mix. - For the purpose of discussion of rheological properties of fresh concrete, these parameters are redefined in terms of forces involved in the transmission of mechanical stresses on the concrete. The fresh concrete is subjected to normal and shearing forces during its handling and placing. ### Stability - Stability is defined as a condition in which the aggregate particles are held in homogeneous dispersion by matrix, and random sampling shows the same particle size distribution during transportation, placing and compaction. - The stability of concrete is measured by its segregation and bleeding characteristics. ### Mobility - Mobility of fresh concrete is its ability to flow under momentum transfer, i.e., under mechanical stresses. - The flow is restricted by cohesive, viscous and frictional forces. - The cohesive force develops due to adhesion between the matrix and aggregate particles. - The viscosity of the matrix contributes to the ease with which the aggregate particles can move and rearrange themselves within the matrix. - The internal friction occurs when a mixture is displaced and the aggregate particles translate and rotate. ### Compactability - The compactability measures the ease with which fresh concrete is compacted. - Compacting consists of expelling entrapped air and repositioning the aggregate particles in a dense mass without causing segregation. - Compactability is measured by the compacting factor test. - **In the case of concrete, consistency is sometimes taken to mean the degree of wetness; within limits, wet concretes are more workable than dry concrete, but concrete of the same consistency may vary in workability.** - The strength of concrete is adversely and significantly affected by the presence of voids in the compacted mass, it is vital to achieve the maximum possible density. This requires sufficient workability for virtually full compaction to be possible using a reasonable amount of work under the given conditions. - **Presence of voids in concrete reduces the density and greatly reduces the strength: 5% of voids can lower the strength by as much as 30%.** ### Ease - Ease is related to rheology of fresh concrete which includes performance parameters of stability, mobility and compactability. - **Consistency** is the relative mobility or ability of a freshly mixed concrete to flow and the usual measurements are: slump for concrete, flow for mortar or grout, and penetration resistance for neat cement paste. - **Homogeneity**, which means uniform and stable distribution of cement, aggregate and water, and resistance to segregation is a critical physical property of plastic concrete. ## TYPES OF WORKABILITY ### Unworkable Concrete: - An unworkable concrete also known as harsh concrete, is concrete with a very little amount of water. - The hand mixing of such concrete is difficult. - Such type of concrete has high segregation of aggregates, and it is very difficult to maintain the homogeneity of the concrete mix. - The water-cement ratio of such concrete is below 0.4. ### Medium Workable Concrete: - Medium workable concrete is used in most of the construction works. - This concrete is relatively easy to mix, transport, place, and compact without much segregation and loss of homogeneity. - Water cement ratio for medium workable concrete is 0.4 to 0.55. ### Highly Workable Concrete: - This type of concrete is very easy to mix, transport, place, and compact. - It is used where effective compaction of concrete is not possible. - High chances of segregation and loss of homogeneity in highly workable concrete. - Water-cement ratio of such concrete is more than 0.55. ## MEASUREMENT OF WORKABILITY The following tests are commonly employed to measure workability: - Slump Test - Compacting Factor Test - Vee Bee Consistometer Test - Flow Test - Kelly Ball Test (ASTM method) - **NOTE:** - The standardized tests for measurement of workability, namely flow, spread, and slump tests, measure consistency, not the homogeneity. To measure the homogeneity properties it is necessary to use a rheometer, an instrument that measures yield stress and plastic viscosity. - **NOTE:** - A reduction in viscosity increases flow and improves consistency. Low viscosity mortar with super-plasticizer has relatively high stress and high plastic viscosity and therefore is more resistant to segregation and bleeding. Low viscosity is essential for ease of placement with cohesion, and for resistance to segregation and bleeding that is necessary for homogeneity. **As per IS: 456 - 2000; Suggested ranges of workability of concrete measured in accordance with IS 1199 are given below:** | Placing Condition | Degree of Workability | Slump (mm) | |---|---|---| | Blinding concrete; Shallow sections; Pavements using pavers | Very low | See 7.1.1 | | Mass concrete; Lightly reinforced sections in slabs, beams, walls, columns; Floors; Hand placed pavements; Canal lining; Strip footings | Low | 25-75 | | Heavily reinforced sections in slabs, beams, walls, columns; Slipform work; Pumped concrete | Medium | 50-100 | | Trench fill; In-situ piling | Medium | 75-100 | | Tremie concrete | High | 100-150 | | | Very high | See 7.1.2 | - **NOTE:** - For most of the placing conditions, internal vibrators (needle vibrators) are suitable. The diameter of the needle shall be determined based on the density and spacing of reinforcement bars and thickness of sections. For tremie concrete, vibrators are not required to be used (see also 13.3). ## SLUMP TEST - The slump test is perhaps the most widely used, primarily because of the simplicity of the apparatus required and the test procedure. - The slump test indicates the behaviour of a compacted concrete cone under the action of gravitational forces. - The test is carried out with a mould called the slump cone. - The slump cone is placed on a horizontal and non-absorbent surface and filled in three equal layers of fresh concrete, each layer being tamped 25 times with a standard tamping rod. The top layer is struck off level and the mould is lifted vertically without disturbing the concrete cone. The subsidence of concrete in 'mm' is termed the slump. - The test is suitable only for concretes of medium to high workability (i.e., having slump values of 25 mm to 125 mm). - The slump test is limited to concretes with maximum size of aggregate less than 38 mm. **As per IS: 7320-1974 & IS: 1199 1959; Specification for concrete slump test apparatus are shown below:** - *Diagram of concrete slump test apparatus* - **True Slump:** True slump is the only slump that can be measured in the test. - **Zero Slump:** Zero slump is the indication of very low water-cement ratio, which results in dry mixes. This type of concrete is generally used for road construction. - **Collapsed Slump:** This is an indication that the water-cement ratio is too high, i.e. concrete mix is too wet or it is a high workability mix, for which a slump test is not appropriate. - **Shear Slump:** The shear slump indicates that the result is incomplete, and concrete to be retested. ## COMPACTING FACTOR TEST - The compacting factor test gives the behaviour of fresh concrete under the action of external forces. - It measures the compactability of concrete which is an important aspect of workability, by measuring the amount of compaction achieved for a given amount of work. - The compacting factor test has been held to be more accurate than slump test, especially for concrete mixes of medium and low workability, (i.e., compacting factor of 0.9 to 0.8), because the test is more sensitive and gives more consistent results. - The test has been more popular in laboratory conditions. - For concrete of 'very low' workability of the order of 0.70 or below, the test is not suitable, because this concrete cannot be fully compacted for comparison in the manner described in the test. - **As per IS: 1199 1959; Specification for compaction Factor test apparatus are shown below:** - *Diagram of compaction factor test apparatus* - **As per IS: 1199 1959; The compacting factor is defined as "the ratio of the weight of partially compacted concrete to the weight of fully compacted concrete."** <start_of_image>-*Formula for compacting factor* <start_of_image>-*Table showing relation between slump value and compacting factor value* ## VEE BEE CONSISTENCY TEST - The Vee-Bee test is suitable for stiff concrete mixes having low and very low workability. - Compared to the slump test and compacting factor test, the Vee-Bee test has an advantage that the concrete in the test receives a similar treatment as it would in actual practice. - The concrete cone when subjected to vibration by starting the vibrator starts to occupy the cylindrical container by the way of getting remoulded. The remoulding is considered complete, when the concrete surface becomes horizontal. The time required for complete remoulding in seconds is considered as a measure of workability and is expressed as the number of Vee-Bee seconds. - The end point of the test is to be ascertained visually, it introduces a source of error which is more pronounced for concrete mixes of high workability and consequently records low Vee-Bee time. - For concrete of slump in excess of 125 mm, the remoulding is so quick that time cannot be measured. Therefore, this test is not suitable for concrete of higher workability, (i.e., slump of 75 mm or above). - **Specification for Vee Bee Consistency test apparatus are shown below:** - *Diagram of Vee Bee Consistency test apparatus* - **Relation between Slump value and Vee Bee time:** -*Diagram showing relation between slump value and Vee Bee time* -*Table showing relation between slump value and Vee Bee time* ## FLOW TEST - The Flow test gives the satisfactory performance for concretes of the consistencies for which slump test can be used. - The test consists of moulding a fresh concrete cone on the top of the platform of flow table. Then, table shall be raised and dropped 12.5 mm, 15 times in about 15 seconds. - The spread of the concrete, measured as the increase in diameter of cone, is taken as a measure of the flow or consistency of the concrete. - The test suffers from the drawback that the concrete may scatter on the flow table with a tendency towards segregation. - **As per IS: 1199 1959; Specification for Flow Table test apparatus are shown below:** - *Diagram of Flow Table test apparatus* - **As per IS: 1199 1959; The flow of the concrete shall be recorded as the percentage increase in diameter of the spread concrete over the base diameter of the moulded concrete.** -*Formula for flow test* -*Table showing relationship between % of flow and concrete consistency* ## KELLY BALL TEST - **Field test:** measurement of the indentation made by 15 cm diameter metal hemisphere weighing 13.6 kg. when freely placed on fresh concrete. - This test is not covered by Indian Standards Specification. - The advantage of this test is that it can be performed on the concrete placed on site. - This test can be performed faster with a greater precision than a slump test. **Advantages:** - Ball penetration test can be performed on the concrete in a hopper, buggy, wheelbarrow, or other suitable container. - This method is simple and the rapidity with which the consistency of the concrete can be determined. - It is also not dependent on a procedure of filling and rodding a container like the slump test. ## FACTORS AFFECTING WORKABILITY The workability of fresh concrete depends primarily on the properties of individual constituent materials such are: - Water Content - Mix Proportions - Size of Aggregate - Shape of Aggregate - Surface Texture of Aggregate - Grading of Aggregate - Use of Admixture - Water Absorption by Aggregate - Environment Conditions - Influence of cement and other mineral admixtures ### Water Content: - Adding more water increases the fluidity of concrete but increasing the water content should be considered as the last resource. - More water can be added by adding more quantity of cement to keep the water/cement ration constant so that the strength remains the same. ### Mix Proportions: - Aggregate/cement ratio influences the workability, A higher ratio of A/C leads to leaner concrete. - Lean concrete has less quantity of paste affecting the mobility of aggregate. - Rich concrete has a lower A/C ratio, more paste is available making it cohesive and fatty, giving better workability. - For better workability total surface area of aggregate should be minimized by the proper choice of size, shape, texture, and proportions of fine and coarse aggregate. - If voids are fewer, more cement particles are available to lubricate the aggregate particles. ### Size of Aggregate: - The bigger the size, the less the surface area, less amount of water is required to wet, and less matrix or paste is required to lubricate. - Lesser surface area – less water to wet - less cement paste to lubricate. - Big-size aggregates will give higher workability for a given quantity of paste and water. - Grading of fine aggregate is more critical as it contributes more surface area. - Excess of fine aggregate and lack of fine aggregate may result in segregation or honeycombing. ### Shape of Aggregate: - Angular, elongated, and flaky aggregate makes very harsh concrete, compared to rounded aggregate. - Round aggregate has less surface area for a given volume, and frictional resistance is also lesser. - River sand and gravel provide greater workability compared to crushed sand and aggregate. - Crushed sand should be designed to yield well-shaped and well-graded aggregates. ### Surface Texture: - Total surface area of rough aggregate is more than the smooth aggregate. - Rough textured - poor workability:: smooth or glassy textured - better workability. ### Grading of Aggregates: - Maximum influence on workability. - Well graded - least amount of void in a given volume. - When the total void is less, the excess paste is available to lubricate. - Excess paste: mixture becomes cohesive and fatty, preventing segregation. ### Use of Admixtures: - Use of plasticizers and superplasticizers to increase the workability of concrete. - Increase of slump to many folds at a minimum dose of admixture. - Use of an air-entraining agent to reduce the internal friction between particles. ### Water Absorption by Aggregate: - Aggregate having high porosity has high water absorption. - Reduces availability of water for the paste and affects the workability of concrete. - Water absorption of coarse and fine aggregate should be kept at less than 2%. ### Cement and Mineral Admixtures: - Fineness of cement, C3S, C3A and SO3 content affects the workability of concrete. - Rapid hardening cement has lower workability compared to cement having fly ash and ggbs. - Influence of mineral admixture on workability is lesser compared to aggregate and its grading. ### Environmental Condition: - Ambient temperature, wind velocity, and relative humidity have a significant effect on workability. - Hot day: increased water requirement and negative effect on workability. - High wind (>15 km/h): increases water demand and faster evaporation of water, much faster slump loss. - Relative humidity (<50%): negative effect on workability and slump. - Suitable precautions are necessary to avoid slump loss and plastic shrinkage cracks. ## REQUIREMENT OF WORKABILITY - The workability of fresh concrete should be such that it can be placed in the formwork and compacted with minimum effort, without causing segregation and bleeding. - The choice of workability depends upon the type of compacting equipment available, the size of the section and concentration of reinforcement. Compaction by hand using rodding and tamping is not possible when compacting factor is less than 0.85. Ordinary techniques of vibration are not applicable if the compacting factor falls below 0.70. In such cases, techniques like vibro-pressing have to be adopted. - For heavily reinforced sections or when the sections are narrow or contain inaccessible parts or when the spacing of reinforcement makes the placing and compaction difficult, the workability should be high to achieve full compaction with a reasonable amount of effort. - The value of workability will generally increase with the increase in the size of aggregate and will be somewhat lower for aggregate of smaller size than indicated. - An insufficient workability may result in incomplete compaction, thereby severely affecting the strength, durability and surface finish of concrete and may indeed prove to be uneconomical in the long run. ## SEGREGATION AND BLEEDING - The stability of a concrete mix requires that is should not segregate and bleed during transportation and placing. - Segregation can be defined as separating out of the ingredients of concrete mix so that the mix is no longer in a homogeneous condition. - **Two types of segregation can occur:** - The separating out of coarser particles in a dry mix, termed segregation. - Separation of cement paste from the mix in the case of lean and wet mixes, termed bleeding. - Segregation depends upon handling and placing operations. The tendency to segregate increases with the maximum size of the aggregate, amount of coarse aggregate, and with the increased slump. - **The tendency to segregate can be minimized by:** - reducing the height of drop of concrete, - not using vibration as a means of spreading a heap of concrete into a level mass over a large area, and - reducing the continued vibration over a longer time, as the coarse aggregate tends to settle to the bottom, and the scum rises to the surface (this formation of scum is termed laitance). - **NOTE:** - The segregation of coarse particles in a lean dry mix may be corrected by the addition of a small quantity of water which improves cohesion of the mix. - **NOTE:** - Bleeding is due to rise of water in the mix to the surface because of the inability of the solid particles in the mix to hold all the mixing water during the settling of the particles under the effect of compaction. - **NOTE:** - Bleeding causes the formation of a porous, weak and non-durable concrete layer at the top of placed concrete. - **NOTE:** - In case of lean mixes bleeding may create capillary channels increasing the permeability of concrete. - **NOTE:** - When concrete is placed in different layers and each layer is compacted after allowing certain time to lapse before the next layer is laid, bleeding may also result in a plane of weakness between two layers. - **NOTE:** - Any laitance formed should be removed by brushing and washing before a new layer is added. - **NOTE:** - Over-compacting the surface should be avoided. - **CAUSES OF BLEEDING:** - Poorly graded aggregate with a lack of fine material with particle size < 300µm - High workability mixes - **REMEDIES:** - Increase sand content - Air entrain concrete as substitute for fine materials - Provide high workability with superplasticizers rather than high water contents - Use very fine materials such as silica fume ## SETTING TIME OF CONCRETE - The transition process of changing concrete from a plastic state to a hardened state. The setting of concrete is based on or related to the setting of cement paste. Thus, cement properties greatly affect the setting time. - **Factors affecting setting time:** - Water-cement ratio - Suitable Temperature - Cement content - Types of Cement - Fineness of Cement - Relative Humidity - Admixtures - Type and amount of Aggregate - *Diagram showing penetration resistance vs. time for setting of concrete* ## STEPS IN MANUFACTURING OF CONCRETE The concrete manufacturing process involves seven stages: - Batching - Mixing - Transporting - Placing - Compacting - Finishing and - Curing ### 1) Batching: - Proper and accurate amounts of each ingredient should be utilised to produce high-quality concrete. - The factors affecting the choice of proper batching system are: - size of job, - required production rate, and - required standards of batching performance. - The batching equipment falls into three general categories: - Manual batching - Semi-automatic batching - Automatic batching - *Diagram of batching plant* - **Volume batching and Weight batching are the two most common methods of batching materials. The size of the task, the needed production rate, and the required criteria for batching performance are the elements influencing the choice of batching method. It is advised to weigh batches of the most important works.** ### 2) Mixing: - The object of mixing is to coat the surface of all aggregate particles with cement paste, and to blend all the ingredients of concrete into a uniform mass. - The mixing action of concrete thus involves two operations: - a general blending of different particle sizes of the ingredients to be uniformly distributed throughout the concrete mass, and - a vigorous rubbing action of cement paste on to the surface of the inert aggregate particles. - Concrete mixing is normally done by: - Hand Mixing: the mixing of concrete is done by hand. - Machine Mixing: the mixing of concrete is done by mechanical means called mixer. - **NOTE:** - Machine mixing is more efficient and economical compared to hand mixing. - **The classification of the mixers based on discharging technique:** - Tilting mixers - Non-tilting mixers - Pan-type or stirring mixer - *Diagram showing tilting, non-tilting and pan type mixers* - **(a) In the tilting-type mixer, the chamber (drum) which is generally bowl-shaped or double-conical-frustum type is tilted for discharging. Before the beginning of mixing the first batch of concrete, a certain amount of mortar is mixed in the mixer. This process is called buttering the mixer.** - **(b) The non-tilting-type mixer consists of a cylindrical drum with two circular openings at the ends and blades fixed inside the drum. The drum rotates about a horizontal axis and cannot be tilted. Owing to a rather slow rate of discharge, the concrete is sometimes susceptible to segregation.** - **(c) The pan mixer consists of a circular pan rotating about a vertical axis. One or two stars of paddles also rotate in the pan about a vertical axis not coincident with the axis of the pan.** - **Mixing Time:** It is the time required to produce uniform concrete. - *Table showing recommended mixing time* ### 3) Transportation: - Concrete from the mixer should be transported to the point where it has to be placed as rapidly as possible by a method which prevents segregation or loss of ingredients. - The requirements to be fulfilled during transportation are: - No segregation or separation of materials in the concrete. - Concrete delivered at the point of placing should be uniform and of proper consistency. - The principal methods of transporting concrete from the mixer are the following: - Barrows - Wheel barrows and handcarts - Power barrows or dumpers - Tippers and lorries - Truck mixers and agitator lorries - Dump buckets - The monorail system or trolley or rails - *Diagram showing wheelbarrow, motorized wheelbarrow, powered barrow, tipper truck, and truck mixer* ### 4) Placing: - The methods used in placing concrete in its final position have an important effect on its homogeneity, density and behaviour in service. The same care which has been used to secure homogeneity in mixing and the avoidance of segregation in transporting must be exercised to preserve homogeneity in placing. ### 5) Compacting: - During the manufacture of concrete a considerable quantity of air is entrapped and during its transportation there is a possibility of partial segregation taking place. If the entrapped air is not removed and the segregation of coarse aggregate not corrected, concrete may be porous, non-homogeneous and of reduced strength. The process of removal of entrapped air and of uniform placement of concrete to form a homogeneous dense mass is termed compaction. - **Compaction is necessary for the following reasons:** - The internal friction between the particles forming the concrete, between concrete and reinforcement, and between concrete and formwork, makes it difficult to spread the concrete in the forms. The friction also prevents the concrete from coming in close contact with the reinforcement, thereby leading to poor bond between the reinforcement and surrounding concrete. The compaction helps to overcome the above frictional forces. - Friction can also be reduced by adding more water than can combine with cement. The water in excess to that required to hydrate the cement fully forms water voids which have as harmful an affect in reducing strength as air voids. Nevertheless, it is preferable to use slightly more water than run the risk of securing inadequate compaction. The compaction reduces the voids to minimum. - **Compaction Methods:** - hand rodding, - mechanical vibrations, - Immersion or needle vibrators - External or Shutter Vibrators - Surface Vibrators - Vibrating Table - centrifugation or spinning, and - high pressure and shock (Ex. Vibropressing) - Other methods such as Jolting & Rolling ### 6) Finishing: - The requirements of finishing concrete depend on the type of structural element and its intended service use, e.g., minimal finishing is required for a beam, whereas careful finishing is required for flat surfaces like roads and airport runway pavements; domestic and office floors. ### 7) Curing: - The physical properties of concrete depend largely on the extent of hydration of cement and the resultant microstructure of the hydrated cement. Upon coming in contact with water, the hydration of cement proceeds both inwards and outwards in the sense that the hydration products get deposited on the outer periphery of cement grains, and the nucleus of un-hydrated cement inside gets gradually diminished in volume. - This process of creation of an environment during a relatively short period immediately after the placing and compaction of the concrete, favourable to the setting and the hardening of concrete, is termed curing. - **The favourable conditions are:** - Adequate moisture within concrete to ensure sufficient water for continuing hydration process. - Warm temperature to help the chemical reaction. - **Maturity of Concrete:** - The strength of concrete depends on both the period of curing (i.e., age) and temperature during curing, the strength can be visualized as a function of period and temperature of curing. - The product (period × temperature) is called the maturity of concrete. - **Curing Periods:** - To develop design strength, the concrete has to be cured for up to 28 days. As the rate of hydration, and hence the rate of development of strength, reduces with time, it is not worthwhile to cure for the full period of 28 days. - **Methods of Curing Concrete:** - Ponding of water over the concrete surface - Covering the concrete with wet straw or damp earth - Covering the concrete with wet burlap - Sprinkling of water - Covering the surface with waterproof paper - Membrane curing of the concrete - Chemical curing - Steam Curing - Curing of Concrete by Infrared Radiation - Electrical Curing of Concrete - *Diagram showing different methods of curing concrete* ## THANK YOU