POST-DEMOULD CURING OF HEAT-TREATED CONCRETE: NECESSITY OR COMPLICATION? Neil Lee, BRANZ Ltd

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1 POST-DEMOULD CURING OF HEAT-TREATED CONCRETE: NECESSITY OR COMPLICATION? Neil Lee, BRANZ Ltd Preamble Modern concrete technology offers both a versatile and highly durable building material, easily capable of delivering amenities with useful service lives of a century or more. This durability potential can only be fully achieved if the cover concrete protecting the reinforcement is sufficiently impermeable, an attribute dependent on good curing practice. Indeed, paying adequate care and attention to curing procedures is more important to the success of a concrete structure now than ever before, for a number of reasons: Current cements are typically optimised for high early strength. This has the great advantage of permitting rapid stripping and reuse of formwork, but also eliminates the passive curing benefit formerly conferred by protecting the concrete s surface for a longer period prior to exposure. Super-plasticisers allow the production of high performance concrete with water-tocement ratios at, or below, the minimum required to completely hydrate all the cement present, leading to the potential for self-desiccation of the cover concrete. Concrete mixes supplied for demanding applications routinely contain supplementary cementitious materials such as silica fume and blast-furnace slag. These react more slowly than Portland cement, requiring an extended curing duration to achieve the desired performance. For these reasons, NZS 3101: Part 1: 2006 The Design of Concrete Structures specifies minimum curing requirements for environmental exposure classifications of varying severity (Clause 3.6). Concrete intended for the B2 and C marine environments must be cured for a minimum of seven days under ambient temperature conditions. The more aggressive C classification has the additional requirement that curing may only be achieved through the direct application of water by ponding or continuous mist sprays i.e. the use of liquid-applied membranes, polythene sheeting and other techniques that simply retain existing moisture within green concrete are not permitted. A similar specification is in place for concrete in contact with aggressive soils or ground-water (exposure classifications XA2 and XA3). The reference to ambient temperature suggests that Clause 3.6 is intended to apply primarily to in situ construction. However, the associated commentary in NZS 3101: Part 2 makes the following observation about pre-cast concrete production: Accelerated curing generally has a detrimental effect on durability, this is more significant for SCM concretes. Thus seven days water curing is still recommended after the completion of the accelerated curing cycle. (Clause C3.6) Anecdotal reports from the industry suggest this recommendation is not widely observed. Consequently BRANZ Ltd, funded through the Building Research Levy, has undertaken a limited investigation of the durability detriment resulting from the failure to follow this recommended practice.

2 Experimental programme Four concrete mixes were produced in the laboratory to evaluate the consequences of failing to apply further water curing to concrete that has initially been subjected to an accelerated heat curing regime. The regime chosen was intended to simulate low pressure steam curing practice, as used by commercial pre-cast concrete producers. The properties of durability and compressive strength of the resulting hardened concrete were compared with, and without, subsequent additional water curing at ambient temperatures. The mixes studied comprised a pure Type GP cement concrete and three SCM-containing concretes employing either Microsilica 600 pozzolan, Duracem blast-furnace slag cement, or an imported Class F (low-lime) fly-ash. Details of the concrete mix designs are given in Table 1, and the resulting fresh concrete properties of each mix are shown in Table 2. In all cases, the total cementitious content binder content was 400 kg/m 3, and the SCM was added at a replacement level compliant with the requirements for C zone concrete in Table 3.6 of NZS Table 1. Concrete mix designs Constituents Mix type (Quantity per m 3 ) GP Microsilica Duracem Fly-ash 19 mm Belmont chip (kg) mm Belmont chip (kg) Kakariki sand (kg) Golden Bay type GP (kg) Microsilica 600 (kg) 32 Holcim type GP cement (kg) Class F fly-ash (kg) 120 Holcim Duracem (kg) 350 SCM replacement none 8% MS 65% slag 30% fly-ash Sika BV50 (l) Sikament NN (l) Total water (l) Table 2. Fresh concrete properties Property GP Microsilica Duracem Fly-ash Target slump (mm) Achieved slump (mm) Air content (%) Measured yield Fresh density (kg/m 3 ) W/C ratio Accelerated curing was achieved by placing the freshly-cast cylinders, still in their moulds, into a thermostatically-controlled water bath. These cylinders were subjected to a heating

3 regime designed to achieve 30 MPa compressive strength within 18 hours of casting, a typical requirement for pre-stressed concrete in a production pre-cast plant. Recorded temperature profiles measured within the concrete test cylinders are shown in Figure 1. Heat curing commenced after a delay of approximately 3.5 to 4 hours, to allow the concrete to achieve initial set. Accelerated Curing Temperature Profile Rise = +12.7º / hr Initial Cooling = -5.6º / hr Curing Temperature (ºC) Duracem series Microsilica series GP series Elapsed Time After Casting (hours) Figure 1. Temperature profiles during accelerated curing. The temperature profile for the fly-ash concrete was not recorded due to equipment malfunction, but is believed to be similar. While lower heating temperatures of around 65ºC are more typically employed, the higher temperature of 80ºC was used in this research to take account of the enthalpy of cement hydration reactions, which serve to boost the temperature experienced by large mass concrete elements. The Concrete Institute of Australia s guide to curing of concrete recommends a maximum rate of temperature rise of 20ºC per hour, a maximum peak temperature of 80ºC, and a maximum initial rate of temperature fall of 6ºC per hour. The heating regime met these requirements. A number of pre-cast producers have subsequently commented that they believe this temperature is unlikely to be reached in commercial practice. At the conclusion of the heat treatment, approximately 24 hours after casting, cylinders were stripped when cool and either received no further treatment [Accelerated curing] or an additional six days water curing at 21ºC [Accelerated+Additional curing] to comply with the seven day curing requirements of NZS At the completion of the appropriate curing procedure, all of the cylinders were stored in a controlled climate of 23ºC and 50% RH until the required test date. Test procedures Compressive strength testing in accordance with NZS 3112: Part 2 was carried out 56 days after casting on test cylinders that had been subjected to each of the curing regimes. All cylinders were re-saturated with water 48 hours prior to testing.

4 The durability of each concrete was evaluated by determining its chloride-ion diffusion coefficient i.e. the ability of the saturated concrete to resist the ingress of chlorides (salt) penetrating the concrete in response to a concentration gradient. This is considered to be the most significant control on concrete durability performance in maritime environments. Because of time constraints, it was not possible to carry out natural diffusion experiments to determine the coefficient. Instead the NT-Build 492 Rapid Migration Test method was employed, which is considered to correlate well with true chloride-ion diffusion coefficients across a wide variety of concrete. A schematic of the test arrangement is shown in Figure 2. The applied potential causes chloride ions to migrate from a concentrated chloride solution into the water-saturated test sample. At the conclusion of the test, the sample is split open and sprayed with silver nitrate solution, which reacts to give white insoluble silver chloride on contact with chloride ions. This provides a simple physical measurement of the degree to which the sample has been penetrated and has the advantage of being unaffected by the chemistry of pore solution within different varieties of concrete. As with compressive strength, cylinders cured by both regimes [Accelerated and Accelerated+Additional] were tested 56 days after casting. variable DC voltage silicone rubber sleeve 0.3 N NaOH 10% NaCl solution Cl - Figure 2. Schematic arrangement of an NT Build 492 Rapid Migration Test. Results Figure 3 shows the chloride-ion diffusion coefficients and compressive strength measured on each concrete type after receiving heat curing followed by subsequent water curing. The result is expressed as a percentage of the equivalent value for concrete that received the initial accelerated heat curing only. Very little increase in compressive strength is obtained, and the improvements in chloride-ion diffusion coefficients, while numerically larger, are not of sufficient magnitude to significantly improve the predicted durability of the concrete under typical in-service conditions.

5 Benefit Conferred by Additional Water Curing Improvement over accelerated curing only (%) Compressive Strength Chloride-ion Diffusion Coefficient GP Microsilica Duracem Fly-ash Cement Binder / SCM Type Figure 3. Improvement gained by additional water curing of the heat-cured concrete. Note that the bar magnitude reveals only the benefit conferred by the application of further curing, not the relative performance of each binder type. Accelerated curing was generally observed to have a detrimental effect on the absolute chloride-ion diffusivity (and strength) measured, compared with concrete water curing at ambient temperatures, as suggested by Clause C3.6 of NZS 3101:Part 2. This is probably explainable by the rapidity with which the cement hydration reaction proceeds under higher temperatures, resulting in the development of a cement matrix characterised by coarser and more porous calcium-silicate-hydrate gel. However, a limited number of test results indicated that the right combinations of SCM type and cement may better tolerate, or even benefit from, curing at an elevated temperature, which may merit further investigation. In particular, it is suggested that the durability response as a function of curing temperature is examined. It should be noted that the pre-cast industry currently makes allowance for any detrimental effect of accelerated curing in the design of pre-cast structural elements. Few, if any, premature durability failures of in-service pre-cast items have been reported in New Zealand. Conclusions Only small benefits to concrete performance are likely to be achieved by subsequent water curing of heat-cured concrete elements. This would appear to justify current pre-cast industry practice, although the procedure recommended by NZS 3101 should still be considered where durability is critical. It must be stressed that any suggestion that water curing can be dispensed with applies strictly to concrete cured at elevated temperature. For concrete cured under ambient conditions, the minimum curing requirements specified in Clause 3.6 of NZS 3101 should be rigorously observed.

6 The chloride-ion diffusivities measured on heat-cured concretes were sufficiently different to those expected for comparable mix designs cured at 21ºC to suggest that durability solutions consisting of prescriptive tables of strength and cover (e.g. Tables 3.6 and 3.7 of NZS 3101) may ultimately need to be revised for elevated temperature curing regimes. However a larger body of test results would be required to support this contention. REFERENCES Lee NP and Chisholm DH Durability of Reinforced Concrete Structures Under Marine Exposure in New Zealand. BRANZ Study Report 145. BRANZ Ltd, Judgeford, New Zealand. Concrete Institute of Australia CIA Z Curing of Concrete. Crow s Nest, NSW, Australia. Meeks KW and Carino NJ NISTIR 6295 Curing of High Performance Concrete: Report of the State-of-the-Art. National Institute of Standards and Technology, Gaithersburg, MD, USA. Nordtest NT Build 492 Chloride Migration Coefficient from Non-Steady-State Migration Experiments. Espoo, Finland. Standards New Zealand NZS 3101: 2006 Concrete Structures Standard. Part 1 The Design of Concrete Structures & Part 2 Commentary. SNZ, Wellington, New Zealand. Standards New Zealand NZS 3112: Part 2 Tests Related to the Determination of Strength of Concrete. SNZ, Wellington, New Zealand.