Estimating early-age in situ strength development of concrete slabs

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Abstract

Post-tensioned concrete members are becoming very popular around the world due to structural efficiencies. However, some important issues related to concrete properties are not resolved yet. A more accurate prediction of the in situ early-age strength is needed to allow for the application of the post-tensioning (PT) load without causing any damage to the PT anchorage zone concrete. Strength–maturity functions are conventionally used to estimate the in situ concrete strength on the basis of a datum temperature. Temperature Match Curing (TMC) is an alternative method which provides a more representative estimate of the in situ strength. In this paper, the prediction of in situ strength of slabs using maturity functions is compared with the results from the TMC and those cured following standard recommendations. The comparisons generally reveal that the specimens cured and tested according to the standard methods can be used for estimating the strength development of concrete with some reservations.

Highlights

▸ Prediction of in situ strength of suspended post-tensioned slabs is made using different methods. ▸ Temperature match, standard method of testing and prediction using maturity methods are compared. ▸ Study shows that standard methods can be used for estimating the strength development of concrete. ▸ The research findings are valid for average in situ temperature variation between 15 and 35 °C.

Introduction

Concretes used in post-tensioned (PT) concrete slabs have early-age strength requirements for initial and final stressing processes. This is typically at 1 day age for initial stressing and three to 4 days age for final stressing. The age at which the final stressing is applied depends upon a confident prediction of in situ strength. The normal practice is to use a minimum compressive strength of 22 MPa [1].

The Australian Standard on specification and supply of concrete AS1379 [2] requires concrete samples to be tested in accordance with AS1012 Parts 1, 8 and 9. According to AS1012 [3], the concrete samples are cured under standard-moist conditions where the samples are subjected to constant temperatures of 23 ± 2 °C and 27 ± 2 °C for a standard temperate zone and standard tropical zone, respectively. In ASTM C192/C192M [4], the curing temperature is set at 23 ± 2 °C. On the other hand, the Australian Concrete Structures Standard AS3600 [5] and ACI 318-02 [6] state that accompanying cylinder specimens of the same concrete batch should be stored and cured under conditions similar to those of the in situ concrete. What constitute “conditions similar” to the in situ concrete are open to interpretation.

There is a major technical problem in the construction industry in that there are no standard methods for assessing in situ strength of concrete used in post-tensioned concrete slab construction. Temperature Match Curing (TMC) is well established for assessing strength in situ and is widely used in the industry. TMC is a process by which test specimens of concrete, from a representative sample of the concrete that is placed in an in situ or precast element, are cured by keeping them in a chamber at the same temperature as a pre-selected point in the element [7]. The TMC system can function under isothermal, near adiabatic, pre-recorded, and even real-time profile conditions. It has been successfully used by a number of investigators [8], [9], [10] to predict the in situ strength. In TMC and core tests performed by Mak and Ritchie [11] on 200 mm thick slabs, a good match was obtained between the cores from in situ concrete slab and the TMC specimens, with less than 1% error overall. This finding indicates that both TMC and core cylinders subjected to a similar temperature history have the same maturity, when considering an allowance for converting from core to cylinder strengths. Other authors have reached similar conclusions that the TMC system provides the closest correlation with in situ strength of early-age concrete [12]. However, the technology remains costly and inaccessible to small to medium manufacturers.

The standard method of testing according to AS1012 [3] prescribes a curing regime and boundary conditions that are essentially different from those under which the in situ concrete is cured. Slabs have a high surface to volume ratio and while the top surface is exposed to variable temperature profiles, the other surfaces are sealed by formwork. It is well known that the curing temperature of concrete is an important factor governing early-age strength development, as the progress of cement hydration is temperature sensitive. The maturity method has reportedly been used as an effective means of predicting the early-age of in situ concrete in spite of its inherent limitations [13], [14], [15], [16], [17], [18], [19]. The maturity method is used to account for the combined effect of temperature and time on the development of the hydration reaction and the mechanical properties of concrete. The nonlinear maturity function based on the Arrhenius law, which is also known as equivalent age function [Eq. (1)], is considered the most accurate, as it better represents the effect of temperature on strength development over a wide range of temperatures [20]:te=0te-EaR1T-1TrΔtwhere te is the equivalent age at the reference temperature, days; Tr is the reference temperature, K; Ea is the apparent activation energy, J mol−1; R is the universal gas constant, 8.314 J mol−1  K−1; T is the temperature of the concrete during time interval Δt, K

The reference temperature (Tr ) is the standard cured temperature at which test specimens are cured. In many parts of the world it is 20 °C (293 K). In Australia it is 23 °C in temperate zones and 27 °C in tropical zones.

Once the equivalent age is found, strength maturity functions can be used to find the strength at a particular age. Carino and Lew [19] have reviewed the basic concepts underlying a number of strength–maturity functions for the strength gain of concrete under isothermal curing. Table 1 lists these functions. An evaluation of the functions and how they relate to the in situ strength of concrete as measured by other means such as Temperature Match Curing (TMC), has received very little attention.

Given that an accurate prediction of in situ strength is required prior to the application of a pre-stressing load, the main objective of this paper is to establish whether the existing procedures can reasonably predict the in situ strength of concrete. To that end, a comparison of the compressive strength values obtained from specimens cured under TMC, standard, ambient conditions, and the strength–maturity functions is carried out. In this paper, the TMC cured specimens will be considered to represent the in situ strength development accurately. The assessment of the strength values is limited to 4 days age, corresponding to the age of final stressing. It is noted that the variation of temperature ranging from 10 to 35 °C used in this study represents temperature variation in temperate zones. Therefore, the results outcomes presented in this paper are limited to those regions, such as eastern and southern part of Australia, New Zealand, southern part of Europe, California, southern part of South America and South Africa, and other regions with similar climate.

Two building projects in Melbourne were investigated to obtain typical temperature profile of the slab. The temperature profile was then used as an input temperature for the TMC specimens. Details of the experimental procedure are presented in Section 2. The procedure to determine the parameters associated with the strength–maturity functions are the same as those reported by Carino and Lew [19], the parameters kT , t 0 and S are obtained by fitting the strength–age relationship to each set of strength–age data. Cylinder specimens are cured at different constant temperatures and their compressive strengths are determined at different age intervals. The compressive strength predicted by maturity method and standard procedures (by curing specimens under ambient and standard conditions) were compared with the in situ strength indicated by the TMC results in Sections 3 Prediction of in situ strength using maturity functions, 4 Conclusions respectively.

Section snippets

In situ temperature measurement and data logging

Ten post-tensioned suspended slabs were instrumented in two construction projects, Sentinel Apartment and Austin Hospital in Melbourne during the winter time [24]. These represent typical slabs cast for multi-story buildings in Australia. The winter time is of particular interest as the range of average daily temperature could be lower than that of standard 23 °C. In this case, the standard testing procedure following the Australian Standards AS1012 [3] may not accurately represent the in situ

In situ slab temperature profiles

As stated earlier, slabs are structural members with a very high surface area to depth ratio compared to other members. There are no curing compounds or other surface treatment applied to these slabs. With no means to prevent heat exchange between the hydrating concrete and the ambient, the surface area of the slab remains exposed to the ambient conditions. From Fig. 2, it can be seen that although the in situ concrete slab temperature profiles do not vary considerably over the slab depth, the

Conclusions

The maturity function has been shown to provide a reliable estimate of the strength development for the early-age concrete. A further sensitivity study found that the strength predictions are not significantly affected by the activation energy parameter. This could potentially simplify the procedure of prediction of the strength using strength–maturity functions. More significantly, comparisons of the strength values of TMC and standards cured specimens have not shown significant differences.

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