42907 Chap.1 Concrete basics and transport properties
Concrete basics and transport properties
Module 0 and Week 1 of University of Technology Sydney 42907 Design for Durability establish the material this subject actually designs: the cement paste and the pore network its surplus mixing water leaves behind.
It fixes the three transport mechanisms every later model runs on, capillary absorption, diffusion and the connected porosity that carries both, and the one lever that moves all of them, the water to cement ratio. Measured permeable voids rise from 8.42% at a ratio of 0.42 to 16.22% at 0.63, and first hour absorption falls as strength rises. Fick’s second law appears here and is solved in Chapter 5.
What this chapter covers
- 01
Concrete as a composite: a binder of cement and water with fine and coarse aggregate embedded in it
- 02
Compressive strength from 20 to 120 MPa, tensile strength about a tenth of it, density about 2.4 tonnes per cubic metre
- 03
Mix proportions by volume: aggregate 60 to 70%, water 15 to 20%, cement 10 to 15%, admixtures under 2%
- 04
Capillary porosity in the hardened paste governs the transport properties of concrete
- 05
Porosity and degree of saturation as two dimensionless ratios, and why saturation decides which mechanism runs
- 06
Volume of permeable voids from three weighings: oven dry, vacuum saturated and immersed apparent mass
- 07
Measured permeable voids against water to cement ratio: 8.42, 12.43, 15.49 and 16.22 per cent
- 08
Capillary suction into unsaturated pores, and why a narrower pore sucks harder
- 09
First hour absorption as an index of the largest pores, falling with both strength and age
- 10
Fick's second law of diffusion, and the two species that use it: chloride in pore water and carbon dioxide in pore air
Reading the volume of permeable voids off three weighings
- +1The water taken up on saturation is the connected void volume: 2,420 minus 2,296 is 124 g, which at a water density of 1 g per cubic centimetre is 124 cubic centimetres of connected pore.
- +1Saturated mass minus immersed mass gives the displaced water and therefore the total volume: 2,420 minus 1,390 is 1,030 g, so 1,030 cubic centimetres. A 100 mm cube is nominally 1,000 cubic centimetres, so the three weighings are self consistent to about three per cent.
- +1Divide: 124 divided by 1,030, times 100, is 12.0 per cent. That sits between the 8.42 per cent measured at a water to cement ratio of 0.42 and the 12.43 per cent measured at 0.50, so the mix is near the upper end of that pair rather than a low ratio high performance concrete.
Key terms
- Cement paste
- Cement plus water, the phase that binds the aggregate together and the only phase in which the deterioration processes of this subject take place. Its pore structure, not the aggregate, decides how fast aggressive species travel inward.
- Capillary porosity
- The void space left in hardened paste by mixing water that was not consumed by hydration. It governs the transport properties of concrete, which is why the water to cement ratio is the dominant durability variable.
- Volume of permeable voids
- The interconnected fraction of the pore space, measured on a standard cube from its oven dry, vacuum saturated and immersed masses. Unlike total porosity it counts only the pores that form a continuous path.
- Degree of saturation
- Water volume divided by total void volume, running from 0 to 1. It decides which transport mechanism can operate: chloride diffuses in pore water, carbon dioxide diffuses in pore air.
- Capillary suction
- The absorption of water into unsaturated pores by the competition between adhesion to the solid and cohesion between water molecules. It increases as pore diameter decreases, and it carries dissolved chloride inward far faster than diffusion does.
- Diffusion coefficient
- The single coefficient in Fick's second law that sets how fast a concentration profile changes shape. In concrete it lumps together porosity, ion interactions, binding capacity and ageing, and it has to be measured rather than predicted.
Concrete basics and transport properties FAQ
Why does the water to cement ratio matter so much for durability?
Because the mixing water that is not consumed by hydration leaves capillary pores behind, and the more of it there is the more those pores interconnect into a continuous path through the cover.
The measured permeable voids of this subject's own data nearly double, from 8.42 to 16.22 per cent, as the ratio rises from 0.42 to 0.63. Every ingress model later in the guide takes its coefficient from that pore network, so the ratio decided at batching sets the durability of the member for its whole life.
Is porosity the same thing as permeability?
No, and confusing them is a common error. A concrete full of entrained air bubbles can be highly porous and still transport very little, because spherical bubbles do not connect to each other. What carries chloride and carbon dioxide is the connected capillary network, which is why the standard measurement is the volume of permeable voids rather than total void content.
Why are there two different transport mechanisms rather than one?
Because the two aggressive species travel in different media. Chloride ions move dissolved in pore water, so they need saturated pores and they arrive by diffusion, or much faster by capillary suction when a dried surface is rewetted with sea water. Carbon dioxide moves as a gas in air filled pores, so it needs a partly dry network.
The same member can therefore be safe from one process precisely because it is exposed to the other.
What does the first hour absorption figure actually tell me?
It is an index of the largest pores in the concrete, because those fill first when a dried specimen is stood in water. The subject's own data shows it falling both with strength, from 2.581 down to 0.74 kilograms per square metre across 25, 40 and 60 MPa concretes, and with the age at which the specimen was tested. That second direction is the case for curing expressed as a measurement.
Assessment move
Treat this chapter as the vocabulary the rest of the guide is written in, and learn it as a chain rather than as a list: more mixing water gives more capillary pore space, more connected pore space gives a higher diffusion coefficient, and a higher coefficient gives a thicker required cover.
Be able to state the volume of permeable voids formula from the three masses without looking, and sanity check any porosity calculation by confirming that saturated mass minus immersed mass reproduces the specimen volume. Keep the distinction between porosity and connectivity sharp, because the prescriptive tables trade strength against cover on exactly that basis.
Finally, make sure you can say which transport mechanism applies to a given surface before you reach for any formula, since that decision is worth a mark on every member of the design report.
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