Jacques Harb

Relationships Between Key ASTM Test Methods Determined on Concrete and Concrete-Equivalent-Mortar Mixtures

Concrete batching requires considerable amounts of materials, energy, and time for testing. This highlights the importance of using alternative easier approaches based on mortars to simplify and speed up the experimental testing programs. This paper seeks to establish relationships between responses of ASTM Test Methods conducted on concrete and concrete-equivalent-mortar (CEM). Different series of mixtures having various cement contents of 300, 350, 400, and 450 kg/m3 and water-to-cement ratios of 0.4, 0.45, 0.5, 0.55, and 0.6 were tested. Test results showed that the CEM approach can adequately predict the slump, slump variations, water reduction, air content, setting time, and compressive strength of concrete with coefficients of correlation (R2) greater than 0.86. Conversely, moderate relationships were obtained when correlating the flexural strength and length change responses of concrete to those determined on CEM. This was mainly attributed to variations in the specimen dimensions and effect of the interfacial transition zone resulting from the presence of coarse aggregates.

Use of Triaxial Compression Test on Mortars to Evaluate Formwork Pressure of Self-Consolidating Concrete

To evaluate triaxial testing suitability to determine formwork pressure developed under both drained and undrained conditions by self-consolidating concrete (SCC), a comprehensive research project was undertaken. For ten concrete-equivalent mortar (CEM) mixtures made with various water-cementitious material ratios (w/cm) and binder types, there was determination of the cohesion (C) and angle of internal friction (Φ). Formwork pressure was determined using SCC and correlations were established with respect to thixotropy. Test results show that the drained and undrained conditions the tests are conducted under greatly affect CEM mixture cohesiveness. Higher C and lower Φ values were exhibited by mixtures made with ternary cement and/or lower w/cm in comparison to those made with binary or Type 1 cement, and those prepared with increased water content. Either thixotropy or C values can be used to predict SCC lateral pressure developed right after casting. The CEM triaxial approach was not appropriate, however, for pressure drop evaluation over time, given the absence of coarse aggregate, which underestimates internal friction development between solid particles.

Assessment of Thixotropy of Fresh Mortars by Triaxial and Unconfined Compression Testing

Triaxial and unconfined compression tests are widely used in geotechnical applications to analyze soil’s shear strength properties, including its cohesion (C) and angle of internal friction (ϕ). A research program was undertaken to evaluate the suitability (advantages and limitations) of such tests for assessing the thixotropy of freshly mixed mortars. Validation of the results was achieved via the establishment of correlations between thixotropy and various static stability indexes, including water bleeding, surface settlement, and coarse aggregate segregation. The test results show that variations in structural build-ups (or thixotropy) can be detected well using triaxial and unconfined compression tests. Testing is realized under quasi-static conditions, with which the discrepancies related to rotational speed and resting periods prior to shearing in mortar rheometers can be eliminated. Tests conducted under drained conditions yielded higher C values than those realized under undrained conditions, indicating that water drainage increases the rate of restructuring (or thixotropy) of cementitious-based materials. A cohesion threshold of around 4kPa was detected on tested mortars, below which the use of such tests becomes inappropriate.

Using ambient vibration measurements for risk assessment at an urban scale: from numerical proof of concept to Beirut case study (Lebanon)

Post-seismic investigations repeatedly indicate that structures having frequencies close to foundation soil frequencies exhibit significantly heavier damages (Caracas 1967; Mexico 1985; Pujili, Ecuador 1996; L’Aquila 2009). However, observations of modal frequencies of soils and buildings in a region or within a current seismic risk analysis are not fully considered together, even when past earthquakes have demonstrated that coinciding soil and building frequencies leads to greater damage. The present paper thus focuses on a comprehensive numerical analysis to investigate the effect of coincidence between site and building frequencies. A total of 887 realistic soil profiles are coupled with a set of 141 single-degree-of-freedom elastoplastic oscillators, and their combined (nonlinear) response is computed for both linear and nonlinear soil behaviors, for a large number (60) of synthetic input signals with various PGA levels and frequency contents. The associated damage is quantified on the basis of the maximum displacement as compared to both yield and ultimate post-elastic displacements, according to the RISK-UE project recommendations (Lagomarsino and Giovinazzi in Bull Earthq Eng 4(4):415–443, 2006), and compared with the damage obtained in the case of a similar building located on rock. The correlation between this soil/rock damage increment and a number of simplified mechanical and loading parameters is then analyzed using a neural network approach. The results emphasize the key role played by the building/soil frequency ratio even when both soil and building behave nonlinearly; other important parameters are the PGA level, the soil/rock velocity contrast and the building ductility. A numerical investigation based on simulation of ambient noise for the whole set of 887 profiles also indicates that the amplitude of H/V ratio may be considered as a satisfactory proxy for site amplification when applied to measurements at urban scale. A very easy implementation of this method, using ambient vibration measurements both at ground level and within buildings, is illustrated with an example application for the city of Beirut (Lebanon).Graphical abstract.

Influence of thixotropy on performance of grouts placed using vacuum injection techniques

The injection of cementitious grouts by vacuum techniques is increasingly used in repair and new construction of post- tensioning members. A research program was undertaken to assess the effect of thixotropy on water retention and behavior of grouts placed using such techniques. Tests were conducted using specimens sampled right after mixing as well as after being subjected to vacuum to extract part of the free mixing water. Test results have shown that the partial or complete extraction of free mixing water due to vacuuming decreases flowability (flow time and mini-slump cone) and increases thixotropy of grouts, mostly due to increased cohesiveness and internal friction within the solid particles. Good correlations were established between extracted water and variations in thixotropy, air entrainment, wick-induced bleeding, unit weight, setting time, and compressive strength. Mixtures exhibiting more than 35% normalized extracted water are not suitable for grouting by vacuum techniques due to increased cohesiveness that hinders proper penetrability. A set of regression models was proposed to simplify product development and predict grout properties either in the fresh or hardened states.

STATIC AND DYNAMIC ANALYSES OF DAHR EL BAIDAR SLOPE

The slope of Dahr El Baidar located on the central mountain of Lebanon hosts a section of the Arab Highway that is under construction to connect Beirut to neighboring Arab countries. This slope has experienced several failures; yet most involved geotechnical companies have investigated the slope with classical geotechnical procedures. In this paper, the analysis of the slope is performed using an integrated geo-assessment approach to identify the principal causes of failure and understand its dynamic behavior. The study includes geological, geophysical and geotechnical soil characterization of the designated area. The geological analysis reveals the presence of faults in the vicinity, in addition to a layer of weak clay at the surface. The geotechnical investigation is based on the interpretation of several boreholes. Geophysical tests are performed using ambient noise vibration technique in order to reveal the resonant frequency and thickness of the soil deposit material. A correlation between geotechnical and geophysical tests was used to establish soil properties in the studies. The above data is used towards a better understanding of the cause and occurrence of the failing zone. A dynamic analysis is conducted to determine the slope amplification and compare the simulated frequencies with the measured ones.

Deterministic and Probabilistic Seismic Analyses of a Slope-Footing System

This paper details deterministic and reliability-based analyses at the ultimate limit state of a slope-footing system. The seismic effect is simulated by a pseudostatic approach. The deterministic analysis aims at determining the safety factor Fs of the soil-footing system while the probabilistic one is devoted to the computation of the Hasofer-Lind reliability. The deterministic models used for both deterministic and probabilistic analyses are based on the upper-bound approach in limit analysis using three failure mechanisms. The deterministic results obtained from the limit analysis are validated by comparison with those obtained from numerical simulations based on the finite element software PLAXIS. Also, a parametric study is performed to analyze the effect of the different geometrical and mechanical parameters on both the safety factor and the reliability index.

Application of reliability analysis on seismic slope stability

Seismic slope stability is performed using a reliability-based analysis. In this approach, a log-spiral failure mechanism of the limit analysis is adopted. The study is carried using two deterministic models of earthquake loadings: The first model uses the seismic coefficient concept to identify the quasi-static representation of earthquake effects. In the reliability analysis, the performance function is based on the safety factor of the slope obtained by the strength reduction method. The second model uses Newmark method to calculate the permanent displacement at the toe of the slope as a result of the application of a time-history acceleration earthquake record at the base of the slope. The performance function is then defined with respect to the permanent displacement of the toe. For the assessment of the slope reliability, the Hasofer-Lind reliability index is considered. Therefore, the random variables are the cohesion and the angle of internal friction of the soil. Results show that the pseudostatic approach produces more conservative results than the Newmark sliding block analysis. The optimal slope height for a target reliability index is identified based on a reliability design procedure. Finally, the analysis is applied with the use of a case study of typical Lebanese slope to illustrate the above approaches.