Nicholas J. Carino

Effects of Test Conditions and Mixture Proportions on Behavior of High-Strength Concrete Exposed to High Temperatures

In this research, mechanical properties of high-strength concrete exposed to elevated temperatures were measured by heating 100 x 200 mm cylinders at 5 deg C/min to temperatures of up to 600 deg C. Heating was conducted with/without sustained stress, and properties were measured at elevated temperatures and after cooled to room temperature (RT). 4 mixes with water-cementitious materials (w/cm) ratios ranging from 0.22-0.57 and RT strengths ranging from 51-98 MPa were used. 2 of the mixtures contained silica fume. Measured compressive strengths and elastic moduli were normalized with respect to RT values, and analysis of variance was used to determine whether the test condition, the value of w/cm, or the presence of silica fume affected results. The influence of these variables on the tendency for explosive spalling was also examined. Results show that losses in relative strength due to high-temperature exposure were affected by the test condition and w/cm, but there were significant interactions among the main factors that resulted in complex behaviors. The presence of silica fume does not appear to have a significant effect. Measurements of temperature histories in the cylinders revealed complex behaviors believed to be linked to heat-induced transformations and transport of free/chemically combined water.

The Maturity Method: Theory and Application

The maturity method may be used to predict the in-place strength of hardening concrete based on its thermal history. A theoretical basis for the maturity method is presented. The general form of the time-temperature function is found to be the time integral of the rate constant. For the case of linear dependence between temperature and the rate constant, the time-temperature function becomes the traditional maturity function. The Arrhenius equation is shown to be an accurate representation of the temperature dependence of the rate constant, and the concept of equivalent age is explained for practical application of the Arrhenius equation. It is explained how the accuracy of strength prediction by the traditional maturity method can be improved by using the proper datum temperature. Results illustrate that the appropriate value of apparent activation energy or datum temperature for concrete may be obtained from strength-gain data of isothermally cured mortar specimens.

Detecting Delaminations in Concrete Slabs With and Without Overlays Using the Impact-Echo Method

The paper demonstrates the feasibility of detecting delaminations in reinforced concrete slabs using the impact-echo method, a nondestructive testing technique based on transient stress wave propagation. The results of two laboratory studies are discussed. One study involved detecting artificial delaminations embedded at unknown locations in a reinforced concrete slab. All the artificial delaminations in the slab were located. The second study was aimed at showing the feasibility of detecting delaminations in reinforced concrete slabs with asphalt concrete overlays. Two reinforced concrete slab specimens with corrosion-induced delaminations were tested. Prior to overlaying the slabs with asphalt concrete, the depths of delaminations as determined by impact-echo testing were verified by drilling at selected points. After the asphalt concrete overlays were applied, the slabs were retested. It was found that the impact-echo method could successfully locate the delaminations in the slabs through the asphalt concrete overlays.

RATE CONSTANT FUNCTIONS FOR STRENGTH DEVELOPMENT OF CONCRETE

The rate constant for strength of a particular concrete mixture is the initial slope of the relative strength-versus-age curve at constant temperature curing. The form of the rate constant versus temperature function is needed to describe the combined effects of time and temperature on strength development. This study investigates the relationship between the rate constant and curing temperature. Study conclusions are presented that are based on strength gain data for concrete and mortar specimens made with Type I cement and cured at 10, 23, and 40 C.

Nondestructive Testing of Concrete: History and Challenges

A brief history of nondestructive testing of hardened concrete over the past 50 years is presented. The contributions of V.M. Malhotra towards the development and promotion of nondestructive testing are emphasized. The underlying principles and inherent limitations of the methods are reviewed, and historical highlights of their development are presented. Test methods are grouped into those which assess in-place strength and those which evaluate non-strength characteristics, such as flaws and deterioration. The paper concludes with a discussion of the challenges for the 21st century in the area of nondestructive testing.(AU)

FINITE-ELEMENT ANALYSIS OF THE PULLOUT TEST USING A NONLINEAR DISCRETE CRACKING APPROACH

An axisymmetric finite-element model, in which fracture was simulated by means of a nonlinear cracking approach, was used to study the pullout test. The pullout test involves measuring the force required to extract a conical frustum of concrete by pulling on an embedded steel disk in opposition to a concentric steel reaction ring at the concrete surface. The precise mechanism of failure and therefore the strength property of concrete, which is actually being measured by the pullout test, has been the subject of several recent studies. The stable, primary crack system extending from the outer edge of the insert to a point beneath the reaction ring where it is arrested. A stress redistribution resulting from this cracking leads to the development of a secondary crack system which initiates below the concrete surface at the inner edge of the reaction ring and propagates towards the outer edge of the insert. This secondary crack system becomes the eventual failure surface defining the conical frustum. The failure surface appears to be completed by shear fracture of the remaining uncracked ligament. The ultimate load-carrying mechanism is aggregate interlock across the completed failure surface.

CAPO-TEST to Estimate Concrete Strength in Bridges

This paper addresses whether carbonation in existing concrete structures affects the compressive strength estimated using the CAPO-TEST, a post-installed, pullout test conforming to ASTM C900 and EN 12504-3. Fifteen bridges, ranging from 25 to 52 years of age at the time of testing, were investigated. For each bridge, average values of core strengths and CAPO pullout strengths were obtained. Carbonation depth, which varied from 2 to 35 mm (0.08 to 1.4 in.), was measured using chemical staining methods. It was anticipated that, as the depth of carbonation increased, the pullout strength would increase for the same underlying concrete strength. Thus, the in-place compressive strength estimated on the basis of the manufacturer’s general correlation would be expected to systematically exceed the strength measured by the cores. It was found that, on average, the compressive strength estimated from the CAPO-TEST and the general correlation was only 2.8% greater than the measured core strength. More importantly, there was no correlation between depth of carbonation and the relative error of the estimated strength based on the CAPO-TEST.