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

Testing concrete for corrosion resistance

Testing Concrete for Corrosion Resistance

Dr J D Bapat

The deleterious agents causing corrosion of reinforcement are water, oxygen, carbon dioxide and salts. They reach reinforcement penetrating concrete surface. The concrete mixtures designed for service exposure environments reduce the opportunities for reinforcement corrosion. The resistance of concrete towards corrosion is measured by its resistance towards the penetration of these agents. The rapid test methods provide a fast and reasonable approximation of the corrosion resistance of concrete. Some of the test methods are given as follows.

Rapid Chloride Permeability Test (RCPT), ASTM C1202: This is quick and the most frequently used test. The RCPT is a measurement of the electrical charge that travels between two sides of a concrete specimen over a six-hour period. This charge is correlated to chloride ions passing through the pore system. Lower values signify a higher resistance to chloride intrusion.

A more rapid test for permeability than the RCPT was recently developed by Florida’s Department of Transportation (FDOT 2004). This procedure uses the Werner probe array method for testing resistivity of concrete on 100x200 mm specimens. ACI 222R (2001) also recommends using this method for assessing the permeability of in-place concrete. The results of the electrical resistivity test have been correlated to the RCPT permeability rating system. The surface resistivity is measured in a matter of seconds thereby allowing for a much larger sample size. The Florida standard requires 8 tests each on 3 specimens, while the RCPT can only provide a single test per specimen because of the inherently destructive preparation requirements.

Determining the Apparent Bulk Diffusion Coefficient of Cementitious Mixtures by Bulk Diffusion, ASTM C1556: The ASTM C1556 is a more rigorous method to calculate concrete permeability. Test results from C1556 typically provide lower variability in test results. The test specimens are moist cured for 28 days and later subjected to unidirectional chloride intrusion. The depth of chloride intrusion is measured over time, beginning at 35 days of soaking period. The successive layers from the specimen are ground. The chloride level of each layer is measured using Standard Test Method for Acid-Soluble Chloride in Mortar and Concrete, ASTM C1152,. This procedure gives a direct correlation to the permeability of the concrete and is considered a useful method for prequalification of concrete mixtures. Unfortunately, C1556 is very time consuming and requires about 3 months to complete.

Standard Test Method for Determining the Penetration of Chloride Ion into Concrete by Ponding, ASTM C1543 (AASHTO T259): The test is commonly used by many highway agencies. A concrete slab (test specimen) is cast and moist cured for 14 days and then air cured to 28 days. The top surface is bermed and ponded with a salt solution for 90 days. Cores are then taken from the exposed surface and sliced into approximately half-inch thick discs. Each disc is crushed and the chloride content of each layer is determined. Unfortunately, this test requires almost six months to complete and there is no clear way provided for interpretation of the results in the method. Transport mechanisms in this test also include undefined components of absorption, diffusion and wick action.

Predicting Chloride Penetration of Hydraulic Cement Concrete by the Rapid Migration Procedure, AASHTO TP 64: In this method a 50-mm long, 100-mm diameter concrete sample is saturated using the vacuum saturation procedure of the RCPT. This test ranks multiple concretes in the same order as ASTM C1202, but has the advantage of not being influenced by strongly ionic admixtures, such as calcium nitrite. The specimen does not experience a temperature rise during the test. The test also has been shown to have a somewhat lower variability than the RCPT.

Question:

Discuss possible ways to protect rebars from corrosion

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 ?