Mario Berra

Acoustic tomography for evaluation of unreinforced masonry

A joint U.S./Italian effort is investigating the effectiveness of grout injection for repair of damaged or deteriorated masonry structures. A nondestructive acoustic tomographic imaging technique has been used throughout the program to determine the effectiveness of the repair processes. Pulse velocity information is used for input into a computer program which develops a three-dimensional image of the velocity distribution through the section. Recent work to characterize input waveforms, investigate the effect of mortar joints on measured pulse travel times, and smoothing of analytical data has improved the accuracy of velocity reconstructions.

RAPID EVALUATION OF THE THRESHOLD ALKALI LEVEL FOR ALKALI-REACTIVE SILICEOUS AGGREGATES IN CONCRETE

Abstract A modified version of the ultra-accelerated concrete prism expansion test in alkaline solutions at 150°C (testing concrete mixes at varying alkali content of concrete) was investigated in order to ascertain the reliability of this test method for evaluating the threshold alkali levels (TAL) of alkali-reactive siliceous aggregates in concrete. Five natural sands of known field performance were tested and their TAL values determined by the ultra-accelerated test were compared with those obtained from concrete prism expansion tests at 38°C and 100% RH. A correlation between the threshold alkali level of the sands and their microstructural disorder coefficient, Cd, as measured by infrared spectroscopy, was also attempted. The modified ultra-accelerated concrete prism test proved to be a rapid and reliable method for evaluating the threshold alkali level of reactive siliceous aggregates in concrete, provided that an expansion limit of 0.11% at 3 weeks was taken as the reactivity judgement criterion for this test. The threshold alkali level proved to be an appropriate reactivity parameter for the selection of alkali-reactive siliceous aggregates. Also, the existence of a straight-line relationship between TAL and Cd suggested the possibility of developing a new test methodology for estimating TAL, based on Cd measurements by infrared spectroscopy.

Steel Corrosion Monitoring in Normal and Total-lightweight Concretes Exposed to Chloride and Sulphate Solutions. Part I: Potential Measurements

The paper reports on long time testing of reinforcement corrosion in normal and total-lightweight concretes exposed to cycles consisting of 4 phases: chloride salt fog, drying, sulphate solution immersion, drying. Potential monitoring evidenced a passive condition for all reinforcements embedded in normalweight concretes. The initiation of the corrosive attack in total-lightweight concretes could not be evidenced, although low potential values were found on corroding reinforcements.

APPLICATION OF THE NAOH BATH TEST METHOD FOR ASSESSING THE EFFECTIVENESS OF MINERAL ADMIXTURES AGAINST REACTION OF ALKALI WITH ARTIFICIAL SILICEOUS AGGREGATE

Abstract The use of the mortar bar expansion test in a 1 M NaOH solution at 80°C as a means of assessing the effectiveness of fly ash and silica fume against alkali-silica reaction was investigated using fused quartz as reactive aggregate. Long-term ASTM C227 test results were utilized for comparative purposes. The NaOH bath test provides reasonable results about the minimum contents of fly ash and silica fume needed to prevent deleterious expansion in mortars containing fused quartz. Particularly for silica fume, agreement between the minimum admixture contents predicted by the two test methods is found only when the expansion limits of 0·25% at 12 days and 0·10% at 1 year or, alternatively, 0·15% at 14 days and 0·05% at 1 year are used for the NaOH bath and ASTM C227 test methods, respectively. A consistent relationship exists between the expansion of mortar bars and the Na + ion concentration in the pore solution after 14 days immersion of the specimens in the NaOH solution. With both test methods there is no direct correlation between the performance of fly ashes and their total and available alkali contents.

Influence of Aggregate Size on Strain-Rate Tensile Behavior of Concrete

A study of concrete behavior over a large range of strain rates has been the focus of research conducted at the Joint Research Center-Ispra (JRC). The program was developed to produce new data concerning a few basic but debated subjects of industrial importance. In this paper, the influence of increasing aggregate size on the strain-rate tensile behavior of plain concrete is described. This study is a first step toward calibrating the constitutive law of concrete in the high-strain-rate field.

Steel corrosion monitoring in normal and total-lightweight concretes exposed to chloride and sulphate solutions part II: Polarisation resistance measurements

The paper reports on long time testing of reinforcement corrosion in high strength normal and total-lightweight concretes exposed to cycles consisting of 4 phases: chloride salt fog, drying, sulphate solution immersion, drying. Polarisation resistance monitoring performed for evaluating corrosion rates confirmed passive condition for all reinforcements embedded in normalweight concretes. The initiation of the corrosive attack was detected in all the reinforcements in total-lightweight concretes. Corrosion rates up to 400 μm/year were detected.

Reuse of Woody Biomass Fly Ash in Cement-Based Materials: Leaching Tests

The feasibility of using woody biomass fly ash (WBFA) as a mineral admixture in cement-based materials was investigated. This fly ash was characterized for chemical composition and used to prepare a cement blend with 70 wt% Portland cement and 30 wt% WBFA. Cubic specimens were cast from a blended cement paste (water-to-binder ratio 0.50) and, after 28 days of curing at 20°C and 100% relative humidity, these specimens were tested for heavy metal leachability through the use of a sequential leaching protocol, at a constant pH of leachant (deionized water; pH 6.0). It was found that, except for the chloride content, the WBFA is able to meet the European chemical requirements established for reuse of coal fly ash in cement-based materials. Although the WBFA is characterized by a significant content of heavy metals of particular environmental concern (Cd, Cr, Cu, Ni, Pb, Zn), the results of the monolith leaching test have shown a good immobilization capacity of such metals by the cementitious matrix and, consequently, a good environmental quality of the blended cement investigated.

Thermoelastic stress analysis: temperature-strain relationships in concrete and mortar

This paper reports the results of tests carried out on concrete and mortar in order to determine the variations in temperature brought about by applied loads. Attention is called to the methodological aspects, and the influence of material heterogeneity is investigated by comparing the values obtained in concrete and mortar with those recorded for a comparatively homogeneous material (PMMA), the aim being to provide an initial assessment of the applicability of thermoelastic analysis to the materials being considered. The temperature-strain (T, ε)—or alternatively, temperature-stress (T, σ)—diagrams obtained in adiabatic conditions under mono-axial tensile and compressive forces are examined and Grüneisen’s parameters are calculated. A clear parallelism between (σ, ε) curves and Young’s elastic modulus, E, on the one hand, and (T, ε) curves and Grüneisen’s parameter on the other is shown to exist. Finally the heating-cooling phenomena occurring in concrete and mortar in compression, as observed both in static tests and loading cycles of short duration, are investigated.ResumeDans le domaine structural, on aborde souvent le problème de la détermination des contraintes ou des déformations de structures hyperstatiques ou isostatiques sous l’effet des variations de la température ambiante. Le problème affronté ici est inverse, c’est-à-dire déterminer les variations thermiques provoquées par les charges. Le but est d’évaluer la possibilité d’adapter aux bétons les applications de l’analyse thermoélastique.Dans cette étude, nous présentons les résultats d’essais menés sur les bétons et les mortiers de ciment, afin de déterminer les courbes température-deformations (T, ε) ou bien les courbes température-contraintes appliquées (T, σ), obtenues par des sollicitations mono-axiales de traction et compression en conditions adiabatiques. On étude l’influence exercée par l’hétérogénéité du matériau en comparant les valuers obtenues avec celles d’un matériau plus homogène, tel que le polyméthylméthacrylate (PMMA).On relève l’étroit parallélisme existant entre les courbes (σ, ε) et le module élastiqueE de Young d’une part, et les courbes (T, ε) et le paramètre de Grüneisen d’autre part. Ce parallélisme se manifeste à la fois par la fiabilité, la répétitivité et les dispersions des valeurs obtenues. Pour terminer, on met en évidence le phénomène de refroidissement-réchauffement dans les bétons pendant l’essai de compression statique. Ce phénomène est encore plus manifeste lors des essais par charges cycliques de compression de courte durée.

Alkali Release from Aggregates in Long-Service Concrete Structures: Laboratory Test Evaluation and ASR Prediction

This paper proposes a simple model for predicting the development of deleterious expansion from alkali-silica reaction (ASR) in long-service concrete structures. This model is based on some composition and reactivity parameters related to ASR, including the long-term alkali contribution by aggregates to concrete structures. This alkali contribution was estimated by means of a laboratory extraction test, appositely developed in this study in order to maximize the alkali extraction within relatively short testing times and with low leaching solution/aggregate ratios. The proposed test is a modification of the Italian Standard test method UNI 11417-2 (Ente Nazionale Italiano di Normazione) and it consists of subjecting an aggregate sample to leaching with saturated calcium hydroxide solution in a laboratory autoclave at 105 °C. Nine natural ASR-susceptible aggregates (seven sands and two coarse aggregates) were tested and the following optimized test conditions were found: leaching solution/aggregate weight ratio = 0.6; solid calcium hydroxide/aggregate weight ratio = 0.05; test duration = 120 h. The results of the optimized alkali extraction tests were used in the proposed model for predicting the potential development of long-term ASR expansion in concrete dams. ASR predictions congruent with both the field experience and the ASR prevention criteria recommended by European Committee for Standardization Technical Report CEN/TR 16349:2012 were found, thus indicating the suitability of the proposed model.

Residual Capability of Alkali Binding by Hydrated Pozzolanic Cements in Long-Service Concrete Structures

An experimental procedure was developed and applied to cement pastes made with two different pozzolanic cements (CEM IV/B (P) and CEM IV/B (V)) in order to ascertain the existence of a residual capability of alkali binding by long-term hydrated pozzolanic cements and, at the same time, to evaluate the alkali retention capability and the concentration of OH− ions in the pore solution of such cementitious matrices. The developed procedure consisted of accelerated curing of cement paste specimens (150 days at 60°C and 100% RH), subsequent leaching tests at 60°C for 30 days by using deionized water or basic solutions (NaOH or KOH at different concentrations) as leaching media, and correlation of the leaching test results with a simple mass balance equation for sodium and potassium ions. The developed procedure was found to be appropriate for evaluating both the pore liquid alkalinity and the alkali retention capability by long-term hydrated pozzolanic cement pastes. A residual capability of alkali binding was also identified for both tested pozzolanic cements, thus indicating their potential ability to prevent (CEM IV/B (V)) or minimize (CEM IV/B (P)) the risk of deleterious expansion associated with alkali-aggregate reaction in long-service concrete structures, like concrete dams.