News

Last year we acquired the premises next door to us which has been a great improvement for the smooth running of the laboratory.

Although Jim Horton has retired, we have retained his services in a consultative capacity. His expertise particularly in the field of construction defects is being passed onto Nathi Memela who joined the company early in 2011, and with the experience gained, now runs our field work section.

Technical notes

>> WATER ADDITION TO CONCRETE

Water is a very necessary component of concrete as it allows the chemical process of hydration of the cement to take place. This causes the cement to react and harden thus providing the concrete strength.

Typically, extra water is added to concrete to improve the workability and therefore make it easier to move and compact.

When designing a mix it is important to ensure the water : cement ratio (W/C) is correct. The standard W/C ratio curve of strength against W/C ratio for any cement shows how strength increases with reducing W/C ratio, i.e. high strength mixes have a low W/C ratio and lower strengths higher W/C ratios.

From this it is obvious if extra water is added to the mix the strength will be reduced and this will have consequences if the design W/C ratio is exceeded.

An addition of 20 litres / m3 of concrete could reduce the compressive strength by 5 to 10 MPa and a 10 litre addition by up to 5MPa.

This is the main concern of uncontrolled addition of water to concrete. There are however other dangers that the concreter should be aware of: 

  • Excessive cracking due to plastic settlement
  • Weak wearing surfaces due to the presence of excessive surface water
  • Increased drying shrinkage and creep which can lead to long term cracking and deflection problems

Control of workability and therefore water is normally carried out by means of the slump cone. (This is described in SANS Standard method 862-1:1994)

Slump is not an exact test and also one cannot design concrete to achieve the same slump for each batch manufactured. The allowed variation is 25mm or a third of the target value, whichever is the greater.

>> DURABILITY INDEX TESTING   

Testing of the potential durability of concrete using the 3 tests developed at the Universities of Cape Town and Witwatersrand has become a common requirement for particularly Civil Engineering structures such as bridges and reservoirs.

The three tests are the oxygen permeability index (OPI), the water sorptivity (WS) and chloride conductivity (CC). Details of the tests and their development may be found in a series of Monographs issued by the universities as well as draft standard SANS 5.16 - 1 to 4: 2006.

Simply, the OPI test measures the permeability of the concrete which correlates well with the rate of carbonation of concrete. High values of OPI lead to slow carbonation front advancement whereas low values correlate to faster and deeper carbonation occurring. The W/C ratio of the mix and early age curing significantly affect the OPI value.

The water sorptivity measures the rate of uptake of water. It also correlates fairly well with carbonation values and is particularly sensitive to efficiency of early age curing.

Finally the chloride conductivity measures the ability of the concrete to reduce chloride ingress. This is particularly important for reinforced concrete exposed to marine conditions or where chlorides are produced in industrial environments. The chloride conductivity value is most significantly affected by the cement type used, although W/C ratio and curing will also impinge on this value.

If durability index testing is included in your specification pay due attention to the requirements or run the risk of reduced payments, the cost of remedial measures or even demolition.

Contest can assist in ensuring that the right approach is used in achieving your Durability Index targets.

Remember, two important factors;

1. Design of the mix
2. Early age curing

 

 

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