Concrete - Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

Chapter 7: Concrete

Workability

Workability is the property of freshly mixed concrete that determines the ease and homogeneity with which it can be mixed, transported, placed, compacted and finished. It may also be defined as the amount of useful internal work required to be done by the concrete to achieve full compaction.

Workability of Concrete - Factors Affecting Workability

The workability of concrete depends on several factors. The principal ones are:

• Water content: Workability mainly depends on the total water content and water-cement ratio, which control paste volume and lubrication of aggregates. Generally, a water-cement ratio in the range of about 0.45 to 0.6 yields workable concrete without admixtures for nominal mixes. A higher water-cement ratio increases the paste volume and thus improves workability. Manual mixing often uses a higher water-cement ratio to ease the process; machine mixing permits lower water content. For designed mixes the water-cement ratio is chosen to meet required strength and durability and is therefore usually lower.
• Mix proportion: The ratio of fine and coarse aggregates to cement (the aggregate-cement ratio) affects the amount of paste available to lubricate aggregate particles. Richer mixes (more cement) provide more paste and improve workability; lean mixes (less cement relative to aggregate) reduce available paste and restrict mobility.
• Shape of aggregate: Rounded aggregates reduce inter-particle friction and are easier to mix and place than angular, elongated or flaky particles. Angular and flaky aggregates lower workability.
• Size of aggregate: For the same volume, larger aggregates have less surface area than smaller aggregates. Increased surface area requires more paste (and water) to coat aggregates, which reduces workability for a given water content. Hence, concretes with finer aggregates tend to be less workable at equal water content than concretes with coarser aggregates.
• Grading of aggregate: Well-graded aggregates containing an appropriate distribution of particle sizes reduce voids in the aggregate skeleton and lower paste demand. Good grading improves workability by reducing the paste required to fill voids and to lubricate particles.

Measurement of Workability

The workability of concrete is measured by standard tests. Common tests are:

• Slump test
• Compaction factor test
• Flow table test
• Vee-Bee consistometer test

Slump Test

The slump test is the most common field and laboratory test for workability of fresh concrete. It gives a measure of the consistency and relative workability of concrete.

• The test mould is a frustum of a cone with top diameter 10 cm, bottom diameter 20 cm and height 30 cm.
• The mould is filled in three equal layers; each layer is compacted using 25 strokes of a standard tamping rod.
• After filling and compacting, the mould is lifted vertically upwards. The concrete subsides; the vertical difference between the height of the mould and the highest point of the subsided concrete is the slump.
• The slump value indicates workability - a larger slump generally means higher workability.
• This test is not suitable for concretes having very high workability (fluid mixes) or very low workability (stiff mixes). In addition, slump may show different forms of failure: true slump, shear slump and collapse slump; interpretation should be made accordingly.

Compaction Factor Test

The compaction factor test is used to assess the workability of relatively low-workability concretes for which the slump test is unsuitable.

• The principle is to determine the degree of compaction achieved by allowing fresh concrete to fall through a standard height and measuring the mass of partly compacted concrete compared with fully compacted concrete.
• The ratio of the weight of partially compacted concrete to that of fully compacted concrete is called the compaction factor.
• Compaction factor values typically range from about 0.7 (very low workability) to 0.95 (high workability).
• Results from the compaction factor test are generally more uniform for stiff mixes than slump results.

Flow Table Test

The flow table test is used for concretes of very high workability (very fluid or self-compacting types where slump test is not appropriate).

• The apparatus comprises a circular table (about 76 cm diameter) and a mould in the form of a frustum (top diameter ≈ 17 cm, bottom ≈ 25 cm, height ≈ 12 cm).
• Concrete is filled in two layers and the mould is removed. The flow table is raised and dropped by a standard height (for example 12.5 mm) a prescribed number of times (commonly 15 times in 15 seconds).
• The spread of concrete on the table is measured in at least six directions and the average spread is used to represent workability.

Vee-Bee Consistometer Test

The Vee-Bee consistometer test is suitable for measuring the workability of concretes with slump less than about 50 mm and for mixes that consolidate slowly under vibration.

• The concrete is placed in a mould which is located inside a cylindrical container mounted on a vibrating table.
• After removing the mould, vibration is applied and the time required for the concrete to change shape from slump form to a cylindrical form (i.e. to flow and consolidate) is recorded as Vee-Bee time (seconds).
• The Vee-Bee time is used to indicate workability; larger times indicate lower workability.

Strength of Concrete

Strength is a principal property of hardened concrete. Tests are commonly conducted to determine compressive and tensile strengths.

Compressive Strength

The compressive strength of concrete is the most commonly specified strength parameter and is typically determined on standard moulded specimens (cubes or cylinders) after specified curing.

• Remove the specimen from water after the specified curing period and wipe off excess surface water.
• Measure specimen dimensions to the nearest 0.2 mm.
• Clean the bearing surfaces of the testing machine.
• Place the specimen in the machine so that the load is applied to opposite sides of the specimen; align the specimen centrally on the base plate.
• Rotate the movable portion gently by hand so that it just touches the top surface of the specimen.
• Apply load gradually, without shock, at a uniform rate - commonly 140 kg/cm² per minute - until the specimen fails.
• Record the maximum load and note any unusual features of failure.

Tensile Strength

Concrete has relatively low direct tensile strength compared with its compressive strength. Because applying uniaxial tensile load directly on concrete specimens is difficult, indirect methods are normally used to estimate tensile strength:

• Split cylinder (Brazilian) test
• Flexural (modulus of rupture) test

Split-Cylinder Test

In the split-cylinder test a standard cylindrical specimen (commonly 300 mm long × 150 mm diameter) is loaded diametrically in compression, which induces tensile stresses along the vertical plane and causes the cylinder to split.

• The cylinder is placed horizontally between the loading platens of the compression testing machine.
• To distribute load and reduce high local compressive stresses at the loading strips, plywood or bearing strips are often placed between the specimen and the loading platens.
• The compressive load applied diametrically produces tensile stresses across the vertical diameter; failure occurs by splitting along this plane.
• Assuming elastic behaviour, the average tensile stress at failure, ft, for a cylinder of diameter D and length L, under maximum load P is given by a standard expression. (Reference image and formula follow.)

Non-Destructive Tests (NDT)

Non-destructive testing methods assess the quality, uniformity and estimated strength of existing concrete structures without damaging the component. NDT is useful for evaluating in-situ concrete strength, locating cracks, detecting voids, assessing deterioration and checking uniformity. Skilled interpretation is required to convert measured values to strength or condition.

• Schmidt rebound hammer test (surface hardness)
• Ultrasonic pulse velocity (UPV) test (through-transmission)

Schmidt Rebound Hammer Test

The rebound hammer measures the surface hardness of concrete and gives an index that can be correlated to compressive strength.

• Begin with calibration of the rebound hammer against a test anvil of known hardness (anvil Brinell hardness value often cited around 5000 N/mm²).
• Hold the rebound hammer at right angles to the concrete surface; readings can be taken on vertical or horizontal surfaces (upward or downward orientations are possible), but intermediate angles produce different rebound numbers and must be avoided.
• Take a sufficient number of readings over a small area and use the average rebound number; correlate the average rebound number with compressive strength using a calibration curve or correlation chart appropriate to the concrete and hammer.
• Note that surface condition, moisture, carbonation and coarse aggregate can affect results; use NDT data as an index and supplement with other tests where necessary.

Ultrasonic Pulse Velocity (UPV) Test

The ultrasonic pulse velocity test measures the time taken by an ultrasonic pulse to travel through a known path in concrete and uses the pulse velocity as an indicator of concrete quality.

• An electroacoustic transducer generates a mechanical pulse at one point; another transducer detects it at a known distance. The transit time is recorded and the pulse velocity calculated as distance divided by time.
• Three transmission arrangements are commonly used: direct (transmitter and receiver on opposite faces), semi-direct (on adjacent faces) and surface transmission (both on same face).
• Higher pulse velocities indicate denser, more homogeneous and generally better quality concrete; lower velocities suggest cracks, voids, poor compaction or deterioration.
• UPV is often combined with rebound hammer results and core testing to provide a more reliable assessment of in-place strength and condition.

Summary

Workability is the key fresh property of concrete that influences ease of handling and compaction and is controlled by water content, mix proportions, aggregate shape, size and grading. Standard tests - slump, compaction factor, flow table and Vee-Bee - are used to measure workability for different ranges of consistency. Strength tests (compressive and tensile) and non-destructive tests (rebound hammer, UPV) provide measures of hardened concrete performance and in-situ assessment. Appropriate selection and interpretation of tests ensure concrete meets the required durability and structural performance.