Fresh Cement Concrete
Fresh cement concrete is that stage of concrete in which concrete can be moulded and it is in plastic state condition. This is also may be called “Green Concrete”. Another term which is used to describe the state of fresh cement concrete is known as consistence, which is the ease with which concrete will flow naturally.
Properties of Fresh Cement Concrete
a. Setting time of Cement Concrete
The hardening of cement concrete before its heat of hydration is known as setting of cement concrete. OR may be defined,
The hardening of concrete before it gaining of its strength. OR may be defined,
The transition process of changing of form of concrete from plastic state medium to hardened state medium. Setting of cementcious based concrete is based or related to the setting of cement paste or gel formation. Thus as a result cement properties greatly affect with the setting time.
Factors which affecting setting time:
Following are the major factors that affect the setting of cement concrete.
1. Water Cement ratio of concrete
2. Suitable Temperature of atmosphere
3. Cement content used in concrete
4. Type of Cement used in concrete
5. Fineness of Cement material
6. Relative Humidity of concrete
7. Admixtures used in concrete
8. Type and amount of Aggregate used in concrete
b. Workability of Cement Concrete
Workability of concrete is almost referred to as the process with which a concrete can be transported easily, placed and consolidated without excessive bleeding or segregation and wastage of concrete.
OR some times may be described,
The internal work which was required to overcome or solve the frictional forces between concrete intermediate ingredients for full form of compaction. It is obvious observed that no single test can evaluate all these different factors. In fact, most of these result cannot be easily assessed even though some kind of standard tests have been established or completed to evaluate them under such specific conditions.
In the case of cement concrete, consistence is sometimes taken to mean the degree of wetness of concrete; within specific limits, wet concretes are more workable than dry type of concrete, but concrete of same consistence may vary in workability condition.
Because the strength of concrete is most adversely and significantly affected by the presence of voids or air in the compacted mass of concrete, it is too much vital to achieve a maximum possible density of concrete. This requires sufficient workability for virtually full compaction of concrete to be possible using a reasonable amount of work under the given such specific conditions. Presence of voids or air bubbles in concrete reduces the density and greatly reduces the strength of concrete: 6% of voids can lower the strength by as much as 32% as observed in experimental data.
Slump Test of concrete can be used to find out the workability of concrete.
Factors affecting cement concrete workability:
- Water-Cement ratio of concrete
- Amount and type of Aggregate used in concrete
- Amount and type of Cement used in concrete
- Weather conditions of atmosphere
- a)Temperature b)Wind
- Chemical Admixtures addition
- Sand to Aggregate ratio used in concrete
i. Water content or Water Cement Ratio in cement concrete
Maximum amount of water content , the water cement ratio in concrete as a result more will be workability of concrete. Simply adding extra amount of water the inter particle lubrication of concrete is increased suddenly or quickly. Higher water content affects in a higher fluidity condition and greater workability condition of concrete but reduces the strength of concrete also rapidly. Because with the increasing of w/c ratio the strength of concrete decreases systematically as more amount of water will result in higher percentage of concrete porosity which is visible after hardening of concrete. So, the lower amount of water content the w/c, the lower is the void or air volume/solid volume, and also stronger the hardened cement paste or cement gel as quickly as possible.
Increased water content also results in bleeding, hence, increased water content can also mean that cement slurry will escape through the joints of the form work(Shuttering)
ii. Amount and type of Aggregate
In case larger Aggregate sizes have comparatively smaller surface areas (for the cement paste to coat) and on the other hand less water means less cement, it is often observed that one should use the largest aggregate size and the stiffest concrete mix. Most of the building elements are constructed with a maximum aggregate size of dimension 3/4″ to 1″, also larger sizes being required to be prohibited by the closeness of the reinforcing bars.
Because cement concrete is continuously shrinking for long years after it is initially placed, it is generally accepted that under thermal loading it will never be expand to it’s originally-placed volume. Maximum amount of aggregate results less will be workability of concrete.
- By using smooth and round shape of aggregate and increases the workability. It also reduces if angular and rough aggregate is used concrete.
- Greater size of aggregate- less amount of water is required to lubricate the concrete, the extra available water is for workabilityof concrete.
- Angular shape of aggregates increases flakiness or elongation as a result reduces workability of concrete. Rounded and smooth shape aggregates require comparatively less water and less lubrication and maximum workability in a given w/c ratio.
- Porous type of aggregates required more water content compared to non absorbent type of aggregates for achieving same degree of workability of concrete.
iii. Aggregate Cement ratio of concrete
Maximum aggregate cement ratio happens less workability of concrete. So, less cement content means less water addition , so the paste/mix is stiff condition.
iv. Weather/atmospheric Conditions
1. Temperature condition
In case of temperature is high, evaporation increases, then value of workability decreases.
2. Wind condition:
If wind is blowing with greater velocity/speed, the rate of evaporation of concrete also increase reduces the amount of water content and ultimately reducing workability of concrete.
v. Admixtures or additives
Chemical type admixtures or additives may be used to increase workability of concrete.
Addition of air entraining agent or chemical produces air bubbles which acts as a soft of ball bearing between ingredient particles of concrete and also increases mobility,fluidity, workability of concrete and decreases bleeding, segregation. The use of super fine pozzolanic materials also gives better lubricating effect and more workability in concreteing effect.
vi. River Sand to Aggregate ratio
Whenever the amount of sand is used more the workability of concrete will reduce because river sand has more surface area and more outer contact area which causing more resistance with ingredient particles. The ingredients of concrete can be well proportioned by weight or volume method. the aim is to provide the desired strength and workability of concrete with minimum expense. Low water-cement ratio is always used to achieve a higher strength value of concrete. It can be concluded that now by keeping the cement content high amount may be use enough for god workability of concrete and still have a very low w/c ratio, the actual reason is that cement is the most costly component of the basic ingredients of concrete.
C(i). Bleeding of Concrete
Bleeding in cement concrete is sometimes defined as water excessiveness in concrete. It is a different form of segregation of concrete, in which some quantity of the water from the concrete mixes comes out to the surface of the concrete, as lowest specific gravity among all the ingredients or components of concrete. Bleeding of concrete is excessively observed in a highly wet conditioned mix of concrete, badly proportioned concrete and insufficiently mixed type of concrete. In case of thin concrete structure like roof slab or road slabs and when concrete is placed in direct sunny weather as results show excessive bleeding.
Due to concrete bleeding, water comes up on surface area and accumulates at the surface. Manytimes, along with this surface water, certain quantity of cement content also comes up to the surface of concrete. When the open surface of concrete is worked up with the trowel, the aggregate goes inside down and the cement mixed water come up to the top surface. This transformation of cement paste or slurry at the concrete surface is known as “Laitance”. In that cases, the top surface of the slabs and concrete pavements will not have good wearing quality as desired. This laitance phenomena formed on roads produces cementcious dust in summer and mudy or clayey form in rainy season.
Whenever water flowing from bottom to top direction, the microscopic flowing path makes continuous channels. If the water cement ratio used in concrete is more than 0.7, the bleeding channels of water will remain continuous way and un segmented position. These continuous concrete bleeding channels are basically responsible for causing of permeability of the concrete structures.
Whenever the mixing water is in the process of coming up continously, it may be internally resisted by aggregates as borderline. The bleeded water is try to accumulate below the aggregate of concrete. This accumulation of excess water creates water voids ,bubbles and reduces the bond between the aggregates and the cement paste.
The above mentioned case is mostly observed in the case of flaky shape and size of aggregate. Simultaneously, the water which accumulates just below the reinforcing bars also reduces the bond strength between the reinforcement and the concrete mixes. The poor bond between the aggregate and the cementcious paste or the reinforcement and the cementcious paste due to bleeding can be remedied by re vibration of concrete. The constant formation of laitance and the consequent bad effect for that can be reduced by delayed finishing operations of concrete surface.
Rate of bleeding of concrete increases with respect to time up almost one hour so that, the bleeding rate decreases but continues under constant slowly condition more or less till the final setting time of cement concrete.
Prevention of Bleeding in cement concrete
- Bleeding of concrete can be reduced by proper ingredient proportioning and uniform and complete mixing ratio.
- The use of finely divided or grained pozzolanic materials reduces bleeding of concrete by creating a longer path for the water to traverse form the mix.
- Air-entraining chemical or agent is very much effective in case reducing the bleeding of concrete.
- Bleeding of concrete can also be reduced by the use of finer cement or cementcious element with low alkali content properties. Always rich mixes are less susceptible to bleeding of concrete than lean type mixes.
The bleeding of concrete is not totally harmful phenomena of concrete, if the rate of evaporation of water content from the concrete surface is equal to the rate of water exertion as bleeding of concrete. Discharge of excess water, after it had played its major role in case of providing workability of concrete, from the body of concrete by way of bleeding which will be good for the concrete.
Initial stage of concrete bleeding occur , whenever the concrete mass is in fully plastic condition , it may not cause much more harmful, because the concrete being in a fully under plastic state condition now at that stage, now it will get subsided and compacted properly. It is confined as the delayed concrete bleeding, whenever the mass concrete has lost its plasticity condition , which may causes severe harmful to the concrete. Controlled re vibration process of concrete may be adopted to overcome the bad effect of concrete bleeding.
C(ii). Segregation in cement concrete
Segregation of concrete can be well defined as the separation of the constituent or ingredient materials of concrete. A well conditioned concrete is one in which all the concrete ingredients are properly mixed and distributed to make a homogeneous concrete mixture. There are acceptable differences in the form of sizes , shape and specific gravities of the constituent ingredients of cement concrete. Therefore, it is very much natural that the ingredients materials show a tendency to fall apart from the mixture of concrete.
Segregation may be classified in three types
- Coarse aggregate separation : When coarse aggregate separating out or settling down from the rest of the concrete mixture.
- Paste separation : Cement paste whenever separating away from coarse aggregate or fine aggregate mix.
- Water separation: Whenever mixed water separating out from the rest of the concrete material being a material due to lowest specific gravity.
A well conditioned concrete, taking into consideration for various parameters such as grading, size, shape,quality and surface texture of aggregate with optimum quantity of water which makes a cohesive mix of concrete. Such type of concrete will not provide any tendency for segregation at any stage. The sticky cohesive and bulging characteristics of concrete mix do not allow the aggregate portion to fall apart from the mixture, at the same time; the concrete mixture matrix itself is sufficiently contained by the aggregate. As well as , extra water also does not find it easy way to move out freely from the rest of the ingredients portions.
The conditions which favorable for segregation of concrete:
- Badly proportioned concrete mix where sufficient concrete matrix is not available to bind and contain the aggregates with compactly.
- Insufficiently mixed concrete with excess water content
- Dropping of prepared concrete from certain heights as in the case of placing of concrete in column concreting times.
- When cement concrete mixture is discharged from a badly designed mixer or mixing machine, or from a mixer with which blades worn out condition.
- Conveyance of cement concrete by conveyor series belts, wheel barrow, long distance hauling by dumper, long lifting by skiping and hoist are the other different situations promoting segregation of concrete from the concrete mixture.
Vibration of cement concrete is one of the most important methods of compaction of concrete. It must be always remembered that only relatively dry mix need to be vibrated properly. If too wetty mix is excessively vibrated in a vibrator; it is mostly that the cement concrete gets segregated totally. It should also need to be remembered that vibration of concrete is continued just for specific required time for optimum conditioned results. If the vibration process is continued for a long span of time, in case of too much wety mix, it is likely to expected result in case of segregation of cement concrete due to settlement of coarse aggregate in bottom portion mixture.
D. Heat of Hydration in cement concrete
Concrete achieve its strength by the heat of hydration of cement particles or molecules. The hydration process of cement is not a sudden momentary action but its a process continuing for long span of time. Obviously, the rate of cement hydration is rapid to start, but the process continues over very long time period at a decreasing rate in the field and in actual practical work, even a higher value of water/cement ratio is used, the cement concrete surface which is open to atmospheric condition, the water which is used in the concrete that evaporates and the excess water which is available in the concrete will not be sufficient for effective hydration of cement to take place particularly in the top layer of concrete.
If the cement hydration process is continue, extra amount of water must be added to refill the loss of added water in case of absorption and evaporation of water. So that, the curing of concrete can be considered as creation of a favorable environment condition during the early period for uninterrupted cement hydration process. The most desirable conditions are, perfectly suitable temperature and ample quantity moisture availability.
During heat of hydration process of concrete, it releases high heat of hydration by exothermic reaction. This released heat is harmful from the point of view of volume stability of concrete. Heat of hydration of cement concrete may also cause shrinkage in concrete structure, on that times it produces cracks on structures. If the generated heat is removed by some way, the most adverse effect due to generation of exothermic heat it can be reduced. This process can be done by a thorough water curing full portion of the structure.
E. Air Entrainment of concrete
Air entrainment or addition of air entraining agent/chemicals for water reduction purpose the density of concrete and consequently reduces also the strength of concrete. Air entrainment agent is used to produce a number of significant effects in both the plastic and the hardened concrete structures. It includes the following:
- Resistance against freeze–thaw action in the hardened cement concrete.
- Increased cohesion properties of concrete, reducing the tendency to bleeding and segregation in the plastic condition of cement concrete.
- Compaction of low workability concrete mixes with including of semi-dry concrete state.
- Stability of extruded cement concrete.
- Cohesion and compaction for handling properties in bedding mortars of concrete.