Concrete is a composite material made homogenous from raw materials- cement, sand, stone, water and admixture and steel (in case of RCC). It is universally used as a construction material because it can be moulded into any shape one desires and as compared to other structural building materials – steel, timber, plastics- provides at a reasonable cost, strength and durability for a large mass. The material needs to testimonials other than the magnificent construction, dams, powerhouses, bridges, buildings, highways, airfields, flyovers, water reservoirs and sports stadium which are the living monuments and yet the engineer has often known less about the concrete of which these structures are made. The principal properties of concrete, their interrelationships and the elements which control these properties are as follows:
- Unlike other structural materials, concrete of a specific property is not available in the market as a ready-made product. Still, it has to be tailor-made at the site to suit a particular purpose.
- No doubt the manufacturer guarantees the quality of cement, which is a hydraulic binder responsible for the strength and other properties of concrete.
- The properties of the concrete depend upon several factors such as type and quantity of cement, water-cement ratio, max. Size of aggregate, its quality, grading, shape and surface texture, the type of admixture and quality control and supervision at all the stages in the manufacture, placing and curing.
- It is worth noting that the ingredients of good concrete and bad concrete are the same and it is the know-how often without additional cost or labour which differentiates between the two.
What is good concrete practice?
To understand good concrete practice is necessary to know first what are the requirements of good concrete and secondly how can they be achieved in actual practice.
- Good concrete should be satisfactory both in the plastic state and in the hardened state.
- It must have adequate workability for handling and placing inside the forms with the proper flow around reinforcement without segregation and should be capable of being fully compacted without excessive effort.
- It is also necessary that it should develop the required strength and durability as per the design requirement and all these properties should be achieved at a reasonable cost to ensure maximum possible economy in the cost of construction.
The principal properties of concrete:
Properties of Concrete in Plastic state:
i) Good workability
ii) Freedom from segregation, cohesiveness
iii) Freedom from bleeding
Properties of Concrete in Hardened State:
i) Strength in crushing tension, flexure and bond
- resistance to weathering
- resistance to adverse chemical reactions
- resistance to wear,
- water tightness
- resistance to corrosion of reinforcement
(iv) Properties involving dimensional changes:
- minimum drying shrinkage
- minimum length change due to temperature
- freedom from cracks
The above properties of concrete and the various factors affecting them are discussed below :
The requirement for Good Concrete:
One of the most important properties of concrete is workability, and it has been defined as the case with which the given ingredients can be mixed into concrete and subsequently handled, transported, and placed with minimum loss of homogeneity and capable of being appropriately finished.
Workability mainly depends upon the water content of the mix and also depends on the following factors:
- Consistency required for placement
- Type of aggregate-shape, size and texture
- Maximum size of aggregate, it’s grading.
- The ratio of fine to coarse aggregate
- Characteristic of cement
- Type and a dose of admixture and air-entrained
- Ambient temp. and temperature of the concrete
The choice of workability also depends upon the location and conditions and method of placement.
The second Requirement of good concrete in the plastic state is cohesiveness or Freedom from segregation. In a well-proportioned concrete with well-graded aggregates of good quality, containing the right quantity of cement and water and transported and handled segregation carefully is not likely to occur. Segregation results in the lack of homogeneity in the final product.
Bleeding of Concrete:
The third Requirement of good concrete is Freedom from bleeding or at least reduced tendency of bleeding. Bleeding is a form of segregation where water in the mix tends to rise to the surface as the solid constituents cannot hold all the mix water when they settle downwards. The bleed water when mixed with top layers concrete during finishing operations or when it brings with it fine cement particles result in the formation of laitance which being porous and weak results in weak joints or surface with poor resistance to abrasion or dusty surface. The tendency to bleeding depends upon the properties of cement. Finer grinding, high C3A content or alkali content reduces bleeding while too high water-cement ratio causes sore bleeding. Addition of finery materials such as fine silica, fine sand, clay or pulverized coal, help in reducing bleeding, air entrainment is also useful.
Concrete Compressive Strength:
The usual primary Requirement of good concrete in its hardened state is a satisfactory compressive strength since many other desired properties such as tensile strength, flexural strength, density, impermeability, durability etc. are concomitant with high strength. Crushing strength of a fully compacted concrete depends upon
- W/C ratio
- The quality and characteristics of cement
- Degree of compaction
- Age of concrete
- Temperature and period of curing
The Durability of Concrete:
Good concrete should be able to withstand in a satisfactory manner the is effects of service conditions to which it will be subjected such as
- Chemical reactions
Weathering is caused by the disruptive action of freezing or thawing and by expansion and contraction under restraint resulting from temperature variation or by alternate wetting and drying.
Chemical reaction :
Concrete Gets Deteriorated due to,
- Chemical reactions between alkalis in cement and mineral constituents of aggregates containing reactive silica such as opal, chalcedony
- Expansive forces created by the reaction of soluble sulphates of sodium, magnesium or calcium with C3A hydrates the cement in the presence of lime to form monosulphate variety of calcium sulpho aluminate. The reaction is accompanied by considerable expansion and disruption.
- Corrosion of concrete due to the formation of soluble products which are removed by leaching as in case of an attack by inorganic and organic acids. Acidic conditions exist in foundation surrounded by peat human or decomposed vegetation. Humic acid, carbonic acid and sulfuric acid formed by pyrites present in shale or clays can attack concrete if not protected
- continuous removal of lime formed during hydration by water often made aggressive by dissolved CO) pressing through cracks or interconnected voids as in case of honeycombed concrete or along improperly treated construction joints.
The principal cause of wearing or erosion of concrete surface is due to the action of flowing water containing abrasive materials or due to the attrition and impact of traffic wear is also caused by cavitation in hydraulic structure or by wind blasting.
Permeability of Concrete:
Problems of durability arise when the concrete is permeable to sulphates, chlorides and acid bearing solutions. Impermeability imports automatic resistance to all destructive influences. Good, strong, homogenous concrete prepared from good quality aggregates at low w/c ratios properly compacted and cured is practically impervious.
Corrosion of Reinforcement:
As most of the structural concrete is reinforced the essential Requirement of good concrete is its ability to provide protection to the embedded steel even in aggressive marine environments. One of the prerequisites to provide protection and to stall the carbonation from reaching the reinforcement in a polluted atmosphere is to ensure good coverage in addition to fixing the limits of chloride level.
Good concrete is capable of resisting dimensional changes which lead to cracking. The likelihood and the amount of cracking depends on the following :
- Contraction by drying or cooling
- Restraint – may be internal, external, full or partial,
- Elasticity or stiffness,
- Tensile strength
For reduced tendency of cracking, essential properties of the concrete are :
- Low thermal coefficient of expansion
- Low drying shrinkage
- Absence of internal or external restraint,
- Low elasticity
- High creep
- High extensibility
- High tensile strength
The principal properties, their interrelationships and the elements which control these properties are discussed above. A good concrete should meet the above requirements.