What Is Fresh Concrete?
- Fresh Concrete – Fresh concrete is often molded to a sturdy structural module.
- Fresh concrete is in the plastic state.
- It is often manufactured on spot & provides a vast variety of features.
- It is cohesive & possesses considerable quantities of cement & fines.
Sometimes it’s also referred to as green concrete.
- It could be white concrete with subtle warm gray textures.
- But, Fresh Concrete doesn’t need sealing. It requires more cleaning due to its matte finish, which points out marks & stains effortlessly.
Properties of Fresh Concrete
- Concrete is fresh starting from blending process until it solidifies. At this point, concrete is handled, conveyed, placed & compacted.
- Features of concrete in fresh state are essential because of effect of the standard of hardened concrete.
- The features of fresh concrete are given subsequently:-
- Plastic shrinkage
- Water Cement Ratio
- Slump loss
Also Read: Concrete Vs Cement | Definition of Cement and Concrete | Composition of Cement and Concrete | Types of Cement and Concrete | Properties of Cement and Concrete | Uses of Cement and Concrete | Advantages of Cement and Concrete | Disadvantages of Cement and Concrete
- The term workability denotes the ease or difficulty with which concrete is handled, conveyed, placed, compacted & completed without segregation of the each substances.
- The quantity of water present in concrete needs to be within the proper proportion.
- Ordinarily, a better water-cement proportion is required for permanent workability.
- The concrete which is simple for handling & placing could be a workable concrete.
- There are a number of tests to assess the workability of a concrete blend.
- The tests like flow test, compacting test & slump cone test is completed to calculate the workability of the concrete mixture.
- It’s difficult to evaluate precisely each & every aspects of the workability in just a single definition but the slump test & examination of stone content, cohesiveness & plasticity provides useful indication.
- Workability isn’t the identical thing as consistency especially if they’re made with dissimilar stone sizes – the smaller the stone the more workability the concrete.
Factors Affecting Workability
- Influence of Blend Proportions
- Influence of Aggregate Features
- Influence of Admixtures
- Effect Time
- Test of Workability for Fresh Concrete
- Slump Test
- Competing Factor Test
- Vee-Bee Consistency Test
- Flow Test
- Spread / Flow Table
- Kelly Ball
- K- Slump
- Visual Assessment
Also Read:What Is Grade of Concrete | Concrete Mix Ratio | Type of Concrete Mix | Different Types of Concrete Grade with Concrete Mix Ratio and Compressive Strength | Uses of Different Grades of Concrete
Segregation is generally the separation of the constituents substances of concrete. Segregation is of 3 types:-
- The coarse aggregates are being separated out from concrete.
- The paste of cement is being separated out from coarse aggregates.
- Water is being separated out from the remainder of the substances.
This concrete is weak because of lack of homogeneity & having a deduction of desirable properties.
- Dropping of concrete from lofts furthermore as discharging of concrete from badly designed mixer testifies a bend towards segregation.
- On account of excessive vibration, concrete coarse aggregates attempt to precipitate & it is known as segregation.
- The variety of materials that compose the concrete blend have dissimilar weights, so while the concrete is in a very liquid state, there’s an inclination for ponderous substances to precipitate & therefore the lighter slurry to rise to the loft.
- Isolation means the separation of freshly designed solid components from one another in an exceedingly heterogeneous blend.
- On account of the separation, the hive is created into concrete & which affects the brawn of concrete & its porosity.
- Isolation in concrete can occur on-site, & this affects the sturdiness of structures.
- In fine concrete, all the modules are evenly disseminated & form a homogeneous blend.
- If the concrete has a tendency to separate coarse aggregates from the remainder of the substance, it testifies to the separation within the concrete.
Harmful Effects because of the Segregation of Concrete,
- High porosity of concrete.
- Low compressive brawn.
- Poor finishing of surfaces.
- Bleeding may be a specific type of segregation, within which little water from the concrete oozes out to the surface of the concrete.
- Water rising to the surface possesses sand & cement which on solidification create a scum layer called laitance.
- Bleeding takes place in an exceedingly wet blend, badly proportioned & incorrectly blended concrete.
- Bleeding in concrete is typically named water gain.
- This is often a special kind of separation, within which some water from the concrete flows to the surface of the concrete, with the minimum relative density among other concrete modules.
Prevention of Bleeding in Concrete
- Bleeding will be diminished with appropriate proportions & uniform & complete blending.
- Bleeding is diminished by the utilization of homogeneously divided pozzolanic materials, resulting to a lengthy process to cross the water.
- The air-penetrating agent is incredibly effective for diminishing blood.
- Bleeding is often diminished by the utilization of fine cement or cement with low alkali.
4. Plastic Shrinkage
- Concrete goes through a volumetric contraction while it’s during a plastic state i.e. before solidification. This can be called plastic Shrinkage.
- Plastic shrinkage is due to the shape while the concrete remains plastic, i.e. has not yet set.
- Drying of plastic concrete may result in shrink & crack. They rarely occur near the sides of a slab as in those locations the concrete is generally movable.
- It starts after half an hour of concrete pouring.
- If water is withdrawn from the concrete before it sets, the quantity of the concrete is diminished by the quantity of water withdrawn. This volume loss is referred to as plastic shrinkage.
- Water could also be withdrawn from the plastic concrete by vaporization or by being soaked by dry soil or old concrete or by the dry wooden formwork.
5. Setting Time
- When concrete transforms its state from plastic to hardened state, this is named setting & additionally the duration which concrete takes to vary its state is named setting time.
- Setting time depends on the sort of cement & it may be surge or diminish by addition of admixture in concrete.
- Concrete is ought not to be kept any higher than 40°C without proper safeguard.
- High-temperature leads to rapid hydration of cement, surged vaporization of water, good blending water demand, & huge volume changes leading to cracks.
- The 1st 24 to 72 hours after placing fresh concrete are of maximum importance as it results in brawn & sturdiness since it is solely dependent on degree of compaction.
- In this period, hydration occurs, which could be a chemical reaction during which concrete achieves its sturdiness.
- If the temperature falls below the minimum temperature, then concrete takes an oversized time to line, & therefore the hydration process slows down.
7. Water Cement Ratio
- The ratio of the quantity of water to the quantity of cement by weight is termed the water-cement ratio.
- The strength & quality of concrete relies upon this ratio.
- The quantity of water is expressed in a liter/bag of cement. If water required for 1 bag of cement is 30 liters, the water-cement proportion is 30/50 = 0.6 (approximately).
- If the proportion of water utilized is a bijou amount, then there’ll not be enough quantity of water to hydrate cement which may lead to weak & porous concrete.
- Again an excessive amount of water leads to the segregation of aggregates & offers porous concrete of low strength & concentration.
- A certain minimum quantity of water is important to hydrate the cement completely.
- The water-cement ratio shouldn’t be permitted to exceed the required limits for a range of varieties of concrete & will be kept low.
8. Hydration In Fresh Concrete
- When cement comes in touch with water, a chemical action occurs. This reaction is thought of as hydration.
- If the blending water dries quickly before the cement has been fully hydrated, the curing process will be checked, & therefore the concrete won’t coagulate to its desired brawn.
- Hydration occurs quicker at loftier temperatures & produces heat. This heat is cooperative during cold-weather construction but detrimental during hot-weather construction.
- The consistency of a concrete mix could be an estimate of the stiffness or sloppiness or fluidity of the combo.
- For effective handling, placing & compacting the concrete, consistency must be homogeneous for every batch. It’s necessary to live the consistency of concrete at regular intervals.
- Slump test is often accustomed to estimate the consistency of concrete.
10. Slump loss
- From the time of blending, fresh concrete gradually loses consistency.
- This provides rise to the issues providing the concrete becomes too stiff to handle, place & compact properly.
Slump loss in concrete is caused by the subsequent reasons:-
- Hydration of cement
- Loss of water by vaporization
- Absorption of water by dry aggregates
- Absorption of water by surfaces in touch with the concrete.