Summary
Highlights
This module introduces Portland cement concrete, which uses cement paste as a binder. The discussion begins by differentiating between fresh and hardened concrete, focusing first on the properties of fresh concrete. Key aspects include consistency, defined as the ability to flow, and its impact on the concrete's strength, which is affected by the degree of compaction. The video emphasizes that proper consistency is vital for easy transport, placement, and finishing without segregation.
Workability describes the ease with which concrete mixes can be compacted using the lowest possible water-to-cement (WC) ratio. Factors affecting workability include water content (higher water content increases workability but decreases strength), maximum aggregate size (larger aggregates require less water to wet their surface), aggregate grading (poor grading reduces consistency and requires more water), and the shape and texture of aggregates (rounded aggregates are easier to mix and flow than angular ones).
The moisture content of aggregates significantly impacts the water required for the concrete mix. Different states include oven-dry, air-dry, saturated surface dry (SSD), and damp/wet. These states affect how much water the aggregates absorb or contribute to the mix. The video then outlines methods for measuring workability: slump test (most common), compacting factor test, flow table test (often for self-compacting concrete in labs), and Kelly ball test.
The slump test detects variations in the uniformity of a mix. Concrete is placed in a cone mold, compacted with a tamping rod, and then the mold is raised. The difference in height between the original cone and the settled concrete indicates the slump. Types of slump include stiff (zero slump), rich (ideal, satisfactory slump), shear (partial collapse), and collapse (total fall), each indicating different water content and mix quality.
The compacting factor test measures the density ratio of a fully compacted mix to its actual recorded density, often used in laboratories due to apparatus size. The flow table test measures the spread diameter after vibration, suitable for self-compacting concrete and lab experiments. The Kelly ball test involves penetrating a concrete mix with a hemisphere attached to a vibrator, measuring the depth of penetration to assess consistency.
Segregation is the separation of concrete constituents, a critical factor affecting strength. Two forms exist: coarse particle segregation, where denser aggregates settle (due to excessive vibration or dropping concrete from high elevations), and segregation in wet mixes, where excess water carries away cement paste and fine aggregates. Prevention involves proper vibration and controlled placement methods like using downspouts or pipes.
Bleeding (water gain) occurs when excess water rises to the surface of freshly placed concrete, often due to over-vibration. This can lead to laitance, a weak, pliable layer on the surface that appears clean but easily deteriorates under friction, particularly on pavements. Compaction aims to eliminate entrapped air, preventing voids (honeycombs), and can be achieved through internal vibration, external vibration (on forms), or vibrating tables (mainly for lab and precast applications).
Curing involves controlling the environment to allow concrete to attain its design strength over a specified period, typically 28 days for common concrete. This requires maintaining suitable temperatures (not too cold or hot) and preventing moisture loss. Methods include ponding (maintaining a water layer), spraying water, or using impermeable membranes. The quality of mixing water is crucial; ideally, it should be fit for drinking, as impurities can affect strength and durability.
Pumped concrete is designed to be easily pumped, requiring high workability (6-8 inch slump) to flow through pipes, often for large-scale pours or reaching inaccessible areas. Shotcrete is concrete sprayed onto vertical surfaces, requiring specific consistency to adhere. Underwater concreting uses specialized techniques like the tremie method, where concrete is delivered through a watertight pipe immersed in water, preventing mixing with external water.
Hardened concrete results from the binding and hardening of a concrete mixture with appropriate proportions. Its primary constituents are binder (cement, like Portland cement), coarse aggregate (gravel), fine aggregate (sand), and water. Optional additives are inherent components of the cement, while admixtures are separate ingredients added during mixing to modify properties, such as accelerating or retarding curing time or improving workability.
Compressive strength is the hardened concrete's ability to resist compressive loads, measured by crushing cylindrical specimens in a universal testing machine (UTM). Concrete gains strength over time, with approximately 16% on day 1, 40% on day 3, 65% on day 7, and 90% by day 14. The 28-day strength is considered the design strength, as further strength gain becomes negligible, providing a factor of safety.
Flexural strength is the concrete's ability to resist bending, typically measured using unreinforced rectangular concrete beams subjected to two-point or center-point loading. This is important for elements like pavements and beams. Concrete has very low tensile strength compared to its compressive strength, which is why steel reinforcement is used (reinforced concrete). Tensile strength is indirectly measured using a splitting tensile test on cylindrical specimens, where the sample is crushed on its side, causing it to split.
Shear strength refers to the concrete's resistance to sliding or shearing along another part. Torsion involves resistance to twisting, which is complex to evaluate. Concrete failure often results from combined stresses (compression, tension, shear, torsion) rather than a single type. Durability is the ability of hardened concrete to resist deterioration from external (environment, chemical action, wear) and internal (alkali-aggregate reaction, volume changes, absorption, permeability) forces.
External causes include leaching out of cement (often due to excess water), corrosive action of sulfates or acidic water, extreme temperatures, abrasion, and electrostatic action. Internal causes include alkali-aggregate reactions (reactivity between cement and certain aggregate minerals), volume changes due to differing thermal properties of aggregates and cement paste (leading to temperature cracks), and permeability (allowing water ingress, especially with air-entrained concrete).
Shrinkage is caused by settling of solids, loss of free water, chemical combination of cement with water (autogenous shrinkage), and drying. Rapid or uncontrolled drying leads to cracks. Common defects include cracks (deep cracks are unsafe), crazing (closely spaced shallow cracks due to rapid surface hardening or excess water), blistering (hollow bumps from entrapped air), delamination (top layer separating due to uneven curing rates), dusting (weak surface layer from bleeding), curling (slab edges lifting due to temperature differences), efflorescence (white salt deposits from soluble salts in water), and scaling/spalling (surface deterioration due to water penetration, especially if leading to rebar corrosion).