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