Slag cement for durable structures

SLAG CEMENT FOR DURABLE STRUCTURES

Dr J D Bapat

The corrosion of reinforcement is the major factor responsible for distress and deterioration of concrete structures under marine conditions. The ingress of chlorides is the principal cause of corrosion; carbonation and sulphate attack are the main factors assisting it. The ground granulated blast furnace slag (GGBS) (like fly ash and silica fume)is a mineral admixture and can be added to the ordinary Portland cement (OPC) , in the cement plant, and marketed as Portland slag cement (PSC) or as an admixture to cement concrete, at the site. The paper reviews the impact of all the three deteriorating factors on the concrete , considering corrosion of reinforcement as principal factor , and the contribution of mineral admixture like GGBS towards improving its durability, under marine conditions. The carbonation depends upon the relative humidity of the environment, permeability of the concrete and the curing conditions. The data on the carbonation of structures available through a number of long term researches carried out world over, on different types of cements, have established that there is no connection between the degree of carbonation and the type of cement. The sodium and the magnesium sulphate, both deleterious to the concrete, are obtained under marine conditions along with high content of chlorides. The deterioration caused due to magnesium sulphate is mainly seen in terms of the reduction in the compressive strength and that due to sodium sulphate in terms of expansion, in the concrete structures. The long term experiments carried out under actual sea water conditions reveal that the concrete with PSC/GGBS has better resistance towards sulphate attack. The refinement of pore structure and the chloride binding capacity of concrete with PSC/GGBS are the principal causes behind the reduced permeability, higher electrical resistance and the resultant higher corrosion resistance and the durability of such concrete under marine conditions.

FAQ:

What are admixtures ?

Admixtures are ingredients other than water, aggregates and hydraulic cement, that are mixed to make concrete. The proper use of admixtures offers certain beneficial effects to concrete, including improved quality, acceleration or retardation of setting time, enhanced frost and sulfate resistance, control of strength development, improved workability and enhanced finishability. It is estimated that 80% of concrete produced in North America these days contains one or more types of admixtures. According to a survey by the National Ready Mix Concrete Association, USA, 39% of all ready-mixed concrete producers use fly ash and at least 70% of produced concrete contains a water-reducing admixture. Admixtures vary widely in chemical composition and many perform more than one function. Two basic types of admixtures are available: chemical and mineral. The pulverised fuel ash or fly ash, blast furnace slag or ggbs, silica fume come under the category of mineral admixtures. All admixtures to be used in concrete construction should meet specifications; tests should be performed to evaluate how the admixture will affect the properties of the concrete to be made with the specified job materials, under the anticipated ambient conditions and by the anticipated construction procedures.

Which is the one property of hardened concrete that affects the durability ?

Permeability is the one property that affects the durability of concrete

Is it possible to measure the permeability of concrete, in situ ?

Yes, it is possible to make the in situ measurement, using Concrete Permeability Tester. The equipment measures air permeability and relative humidity of cover concrete on site. A blind hole drilled or formed in the concrete is sealed with a special plug, and the resulting cavity pressurized with air. The permeability of the concrete is calculated from the time taken for the pressure to drop. If the capillary pores are partially blocked with moisture, the permeability is reduced. Therefore the relative humidity of the cavity is also measured.

References:

(1) Bapat J. D., “ Portland slag cement / ggbs for durable structures under marine conditions ” Cement J., Vol. XXXV, No. 1, January 2002, pp 7-18

(2) Bapat J.D., “ GGBS/PSC for concrete structures with corrosion resistance ”, The Masterbuilder, Vol. 4, No. 1, Feb.-Mar. 2002, pp 26-33

(3) Bapat J. D., “ Performance of cement concrete with mineral admixtures ”, J. Adv.Cem.Res. , Vol. 13, No. 4, October 2001, pp 139-155

Question:

How does slag make concrete durable ?