CIVL1810 Chap.6 Concrete Materials, Slump and Characteristic Strength
Concrete Materials, Slump and Characteristic Strength
Concrete is stone and sand held in a cement paste that may also contain fly ash or slag. Coarse and fine sizes are blended so the small particles occupy the gaps between the large ones, and the cement itself begins as clay and limestone burnt together into clinker, which is ground with a little gypsum to avoid a flash set.
The examinable content is quantitative: aggregate makes up most of the mix volume, the water cement ratio sits in a narrow band, compressive strength greatly exceeds tensile strength, setting and maturity follow a known timeline, workability is measured by slump class, sampling scales with the volume poured, and characteristic strength is defined statistically rather than as a single test result.
What this chapter covers
- 01
Concrete as aggregate bound with cement, with fly ash and slag as supplementary cementitious materials
- 02
Particle packing: combining fine and coarse aggregate to reduce voids
- 03
How cement is made, and why gypsum is added to the clinker
- 04
Aggregate at 65 to 80 percent of mix volume and water cement ratios of around 0.4 to 0.6
- 05
The timeline: initial set at 30 minutes to an hour, solid at about 10 hours, dense mass at 7 days, full maturity at around 28 days
- 06
Compressive strength of 30 to 50 megapascals, up to 100 for high performance mixes
- 07
Tensile strength of only 3 to 5 megapascals, and why that precludes plain concrete in load bearing elements
- 08
The compressive stress strain curve: highly non linear and increasingly brittle as strength rises
- 09
Cylinder and cube strengths and how a grade designation carries both
- 10
The slump test as an indicator of the amount of water added, and the five slump classes
- 11
Sampling: three, six or twelve samples by volume band, from different mixers, to a certified laboratory within 20 to 32 hours
- 12
Characteristic strength as the mean less 1.64 standard deviations, leaving 95 percent above
- 13
Reinforcing steel: hot rolled deformed bar at 500 megapascals nominal yield, common bar designations, stirrups and the 135 degree hook
- 14
Cover as protection against accelerated weathering and poor fire performance
From batching records to a certified grade
- +1State the definition being used. Characteristic strength is the mean strength less 1.64 standard deviations, which leaves 95 percent of results above the line.
- +1Substitute. 1.64 multiplied by 5.5 is 9.02, so the characteristic strength is 41.0 less 9.02, which is 31.98 megapascals.
- +1Compare with the specification. At 31.98 megapascals the performance falls marginally short of the 32 megapascals required, so as reported it does not satisfy the drawing.
- +1Identify the cheaper improvement. Two levers exist: raise the mean, which means more cement in every cubic metre, or reduce the standard deviation, which means tighter batching and better moisture control. Reducing variability raises the characteristic value without raising the mean, so it is the cheaper route, and it is the reason 1.64 standard deviations appears in the definition at all: variability is penalised.
Key terms
- Aggregate and matrix
- Stone and sand held together by cement paste. The stone and sand are the aggregate and the paste is the matrix; coarse and fine sizes are blended so small particles fill the gaps between large ones, which is called particle packing.
- Clinker
- The fused product of firing clay and limestone at high temperature, around 1200 degrees Celsius. It is ground with a little gypsum, added to avoid a flash set, into the fine powder sold as cement.
- Initial set
- The point around thirty minutes to an hour after mixing at which the concrete is no longer plastic. It marks the end of the window for placing, screeding and trowelling.
- Slump
- A reading of how far the top of a moulded concrete cone settles once the mould has been taken away. It indicates the amount of water added and therefore how workable the mix is, and it is reported as one of five consecutive classes.
- Characteristic strength
- The compressive strength that a defined proportion of the concrete attains, computed as the mean less 1.64 standard deviations, which leaves 95 percent of results above it.
- Deformed bar
- Hot rolled reinforcing bar with surface deformations, the most common reinforcement for non prestressed members, with a nominal yield stress of 500 megapascals.
- Closed stirrup
- A beam tie, usually bent from R10, N10, N12 or N16 bar and hooked back through 135 degrees so it stays shut once the concrete around it has cracked.
Concrete Materials, Slump and Characteristic Strength FAQ
Why does adding water on site cost more than it saves?
Because slump measures the amount of water added, so water bought workability by raising the water cement ratio above the usual band of about 0.4 to 0.6. Strength and durability both fall with the extra water, and the specified characteristic strength may no longer be attained.
The legitimate alternatives are to order a higher slump class in the first place, or to change the placing method, since vibration rather than water is the correct way to get concrete around congested reinforcement.
What makes the sampling rules meaningful?
Two conditions attached to them. Specimens have to come from different mixers, so they stand for the spread of what was actually delivered rather than for one lucky load, and they have to reach an accredited laboratory for controlled curing inside a 20 to 32 hour window. A test result only says something about the pour if both conditions hold.
Why is concrete reinforced at all?
Because the two strengths are wildly different. Compressive strength usually falls between 30 and 50 megapascals, while tensile strength is only of the order of 3 to 5 megapascals, and large tensile stresses develop in load bearing elements from external load and from other causes such as temperature. Plain concrete is therefore precluded, and steel bars carry the tension.
What does cover actually protect against?
Two named consequences. Leave steel exposed and it corrodes far faster and does badly in a fire, so cover answers a durability problem and a fire problem at the same time. It is fixed by bar chairs tied to the reinforcement before the pour, which is why lost cover on a finished element almost always traces back to the steel fixing rather than to the concrete.
Exam move
Treat this chapter as the numbers bank for the whole unit, because its figures make answers elsewhere specific. Write a single card with the aggregate proportion, the water cement band, the two strength ranges, the four stage timeline, the five slump classes, the three sampling bands with their two conditions, and the characteristic strength relation.
Learn to say the tension argument in one sentence, since it appears in every reinforced concrete question in the unit. Practise the characteristic strength calculation both ways: given mean and standard deviation find the characteristic value, and given a required characteristic value and a known variability find the mean the supplier must target.
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