International Concrete Abstracts Portal

The use of pozzolanic material, such as fly ash and silica fume, is becoming more popular in producing high performance/high strength concrete (HP/HSC) for various structural applications. Many studies have addressed the mechanical properties as well as durability of HP/HSC, however, the effect of pozzolans on the shrinkage and creep behaviors are not clearly addressed. There is a need to understand and identify how changes in the composition and porosity of HP/HSC, and consequently the elastic modulus, would affect its early age as well its long term performance. The main objective of this paper is to examine the effect of using various models for modulus of elasticity on the prediction of creep of high strength concrete (HSC) containing pozzolans. The study included an experimental program and a comparison of available analytical models for predicting the compressive creep and modulus of elasticity of HSC. Results from creep tests performed on different mixes (with compressive strength up to 90 MPa) were compared with those from prediction models available in the literature. Three creep models, ACI 209, CEB 90, and GL 2000, were used. In addition, various values of modulus of elasticity obtained from experimental calculation, ACI 318, ACI 363, CEB 90, Gardner, and from an equation proposed by the authors were evaluated. Results show that the modulus of elasticity has high impact on the accuracy of predicted creep and that available modulus of elasticity models needs to be revised to reflect HSC containing pozzolans.

Reinforced concrete columns under compression loads and under little or no moment may exhibit cracking. Some cracks develop at early ages and others years later under sustained axial loads or no significant loads at all. Flexural cracking may be expected from externally applied loads on columns within the tension-controlled zone in the axial load-moment diagram. However, for columns within the compression-controlled zone of the diagram, cracking is not normally expected to occur under allowable service loads. Concrete shrinkage and creep, temperature variations and loading history cause all these cracks. In this paper, the causes of these cracks are described, analyzed and illustrated with photos of cracked columns. Design and construction recommendations to prevent or reduce these cracks are provided.

Continuous highway overpass structures are often governed by serviceability rather than ultimate conditions. Deflection prediction and control is vital to avoid cracking. A two span overpass in Calgary was chosen as a case study. Deflections and strains in two precast prestressed girders were monitored from fabrication to erection, and a comprehensive material testing program was done on the concrete mix. The results of the case study show that the CEB MC-90 model code underestimated the time-dependent response by a maximum of 16% while ACI 209 overestimated by 19%. By tuning ACI 209 and CEB MC-90 to the concrete material testing data, predictions were increased to within 8% and 7%, respectively. A variability analysis on the two tuned models showed that while they give nearly the same prediction, the CEB MC-90 format induces less uncertainty in predictions. In addition, extrapolation to long-term ages shows a substantial divergence between predictions of the two models.

The study included A3 – General Paving (21 MPa at 28 days), A4 – General Bridge Deck (28 MPa at 28 days), and A5 – General Prestress (35 MPa at 28 days) concrete mixtures approved by the Virginia Department of Transportation (VDOT). The study also included a lightweight, high strength concrete mixture (LTHSC) used in the prestressed beams of the Chickahominy River Bridge, and a high strength (HSC) concrete mixture used in the prestressed beams of the Pinner’s Point Bridge. For the A3, A4, and A5 portland cement concrete mixtures, the CEB 90 model appears to be the best predictor. However, there is little difference in prediction capabilities between the CEB 90, GL2000 and B3 models. For mixtures containing supplemental cementitious materials, slag and fly ash, the GL2000 model appears to be the best predictor. For the LTHSC concrete mixture, the CEB-C90 model appears to be the best early age predictor, while the Bazant B3 model appears to be the best predictor a later ages. And for the HSC concrete mixture, the Gardner/Lockman model appears to be the best predictor.

The long-term service behavior of modern reinforced or prestressed concrete structures, whose final static configuration is frequently the result of a complex sequence of phases of loading and restraint conditions, are influenced largely by creep. Creep substantially modifies the initial stress and strain patterns, increasing the load induced deformations, relaxing the stresses due to imposed strains, either artificially introduced or due to natural causes, and activating the delayed restraints. The resulting influences on serviceability and durability are twofold, creep acting both positively and negatively on the long-term response of the structure. The paper shows that use of the four fundamental theorems of the theory of linear viscoelasticity for aging materials, and the related fundamental functions, offers a reliable and rational approach to estimate these effects. Extremely compact formulations are obtained, which are particularly helpful in the preliminary design, as well as in the control of the output of the final detailed numerical investigations and safety checks, and suitable for codes and technical guidance documents. Particular attention is dedicated to the problem of change of static system.