Guidelines to select cement for concrete

GUIDELINES TO SELECT CEMENT FOR CONCRETE

J D Bapat

Concrete is used as a material to build varieties of structures under different surrounding conditions. Here are some guidelines to select appropriate cement or binding material for concrete used for different structural applications.

Sl No	Application	Comments
1	Conventional structural	Select cement for economy
2	Concrete in non-aggressive environment	Any cement as per the Standards. Blends of OPC with GGBS or FA or alternatively PPC or PSC, as per the Standards, have been extensively and successfully used in non-aggressive environment
3	Large placements where temperature rise, due to heat of reaction, is to be kept as low as possible	Best results are likely to be achieved with cements with mineral admixtures contents in excess of 50% GGBS or 25% FA.
4	Structural precast	Choice of cement for precast elements will depend mainly on strength requirements at early ages. High early strengths, without steam curing, will be achieved most economically with cements of strength Grade 43 and higher and with low mineral admixture content. Cements with higher mineral admixture content are better suited to steam curing. Where there is no requirement for rapid strength gain, the choice of cement should be based on economy
5	Precast bricks, blocks and pavers	Provided the elements have sufficient strength to allow handling at an early age, typically the day after casting, the choice of cement should be based on economy.
6	High-performance or High-strength concrete	Strength Grade 43 or higher may be used. The inclusion of about 8% silica fume is common practice in this application. Other mineral admixtures may also be used for durability or economic benefits. Superplasticiser is an essential ingredient in high-performance concrete. The compatibility of the cement/binder material and the Superplasticiser is important and needs to be checked.
7	Reinforced concrete in marine environment	Research and long-term field experience shows that best results in marine environment are obtained, using concrete with cement replacement > 50% by GGBS or FA around 30%
8	Concrete with alkali-reactive aggregates	Cement replacement should be > 40% GGBS or > 20% FA, for alkali-reactive aggregates
9	Concrete exposed to sulphate attack	OPC with 5-8% C3A possesses good resistance towards sulphate attack. Using high levels of GGBS as cement replacement (above 50%) improves sulphate resistance

Shrinkage compensating concrete

SHRINKAGE COMPENSATING CONCRETE

Dr J D Bapat

Shrinkage compensating concrete can be produced using expansive cement or expansive constituents.

The expansive cement, also known as sulfoaluminate cement or modified Portland cement, comes in the category of hydraulic cements that expand slightly during the early hardening period after setting. They meet the requirements of ASTM C 845 in which the group of expansive cement varieties is designated as Type E-1. Expansive cement is used to make shrinkage-compensating concrete that is used to (a) compensate for volume decrease due to drying shrinkage, (b) induce tensile stress in reinforcement and (c) improve dimensional stability of post-tensioned concrete structures. One of the major advantages of using expansive cement is in the control and reduction of drying-shrinkage cracks. In recent years, shrinkage-compensating concrete has been of particular interest in bridge deck construction, where crack development must be minimized. It is also used to minimize cracks in the concrete slabs and structures.

Three types of expansive cement are defined in ASTM C 845.

* Type K: Contains anhydrous calcium aluminate along with calcium sulphate and free lime in Portland cement

* Type M: Contains calcium aluminate along with calcium sulfate in Portland cement

* Type S: Contains tricalcium aluminate along with excess of calcium sulfate in Portland cement

The Type K is more commonly used

Concrete is generally mixed with more water than what is needed to hydrate the cement; that is done to obtain desired workability. After the hydration, i.e. during the process of concrete hardening, remaining water evaporates, causing the concrete to shrink. The amount of shrinkage due to drying depends on the characteristics of the constituent materials, mixture proportions and placing methods. When drying in pavements or structural members is restrained by sub-grade friction, reinforcement or other parts of the structure, drying shrinkage will induce tensile stresses. The tensile stress due to drying shrinkage usually exceeds the concrete tensile strength, which results in cracking. When expansive cement is used in concrete it induces stress, in opposite direction, large enough to compensate for drying shrinkage stress and minimize cracking. When the magnitude of expansion is small but usually adequate to offset the tensile stress (about 0.2 to 0.7 MPa) from restrained drying shrinkage, the concrete is known as Shrinkage Compensating Concrete.

The expansive cement can also be used to produce self-stressing concrete. That is done by restraining the expansion to induce a compressive stress high enough to result in a significant residual compression in the concrete after drying shrinkage has occurred.

Shrinkage-compensating concrete can be produced by using an expansive agent provided that an adequate wet curing, for at least one week, is carried out after demolding. A synergistic effect on the shrinkage reduction is observed, when the shrinkage reducing admixture and the expansive agent are used together.

Some water-reducing admixtures may be incompatible with expansive cement. Fly ash and other pozzolans may affect expansion and may also influence strength development and other physical properties. The air-entraining admixtures are as effective with shrinkage-compensating concrete as with Portland Cement in improving freeze-thaw durability.

The structural properties, namely tensile, flexural, and compressive strength of shrinkage compensating concrete are comparable to those of Portland cement concrete (PCC). Some superplasticisers, when added, do not require concrete to be wet cured, it is claimed.

This ACI Standard Practice, ACI 223, gives details on the use of shrinkage-compensating concrete in structures (reinforced and post-tensioned slabs, both on grade and elevated) and pavements. The recommendations cover proportioning, mixing, placing, finishing, curing, and testing of concrete. The scope of this standard practice is limited to shrinkage-compensating concrete made with expansive cements. The recommendations of this standard practice are not applicable to self-stressing expansive cement concretes proportioned to produce a prestressed concrete structure for load-bearing purposes. Procedures for proportioning, handling, and curing of self-stressing concretes are often radically different from procedures for shrinkage-compensating concretes used to compensate for normal drying shrinkage.

Bibliography:

[1] Standard practice for the use of shrinkage — Compensating concrete (ACI 223) : Published by the American Concrete Institute, Publications Department, P.O. Box 19150, Detroit, Michigan 48129, USA, Jan 1, 1998

[2] Fu Y., Xie P., Gu P., Beaudoin J.J., “ Characteristics of shrinkage compensating expansive cement containing a pre-hydrated high alumina cement-based expansive additive” J. Cement and Concrete Research, Vol. 24, Iss. 2, 1994, pp 267-276

[3] Maltese C., Pistolesi C., Lolli A., Bravo A., Cerulli T., Salvioni D. “ Combined effect of expansive and shrinkage reducing admixtures to obtain stable and durable mortars”, J. Cement and Concrete Research, Vol. 35, Iss. 12, 2005, pp 2244-2251

[4] Nagataki S., Gomi H., “Expansive admixtures (mainly ettringite)”, J. Cement and Concrete Composites, Vol. 20, Iss. 2-3, 1998, pp. 163-170

[5]Collepardi M., Borsoi A., Collepardi S., Olagot J.J.O., Troli R., “Effects of shrinkage reducing admixture in shrinkage compensating concrete under non-wet curing conditions”, J. Cement and Concrete Composites, Vol. 27, Iss. 6, 2005, pp. 704-708

[6] Shuguang H., Yue L., “Research on the hydration, hardening mechanism, and microstructure of high performance expansive concrete”, J. Cement and Concrete Research, Vol. 29, Iss. 7, 1999, pp. 1013-1017

Question:

Give three possible applications of shrinkage compensating concrete

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 ?