International Concrete Abstracts Portal

This study employs machine learning (ML) to predict ultrasonic pulse velocity (UPV) based on the mixture composition and curing conditions of concrete. A data set was compiled using 1495 experimental tests. Extreme gradient boosting (XGBoost) and support vector regression (SVR) were applied to predict UPV in both direct and surface transmissions. The Monte Carlo approach was used to assess model performance under input fluctuations. Feature- importance analyses, including the SHapley Additive exPlanation (SHAP), were conducted to evaluate the influence of input variables on wave propagation velocity in concrete. Based on the results, XGBoost outperformed SVR in predicting both direct and surface UPV. The accuracy of the XGBoost model was reflected in average R2 values of 0.8724 and 0.9088 for direct and surface UPV, respectively. For the SVR algorithm, R2 values were 0.8362 and 0.8465 for direct and surface UPV, respectively. In contrast, linear regression exhibited poor performance, with average R2 values of 0.6856 and 0.6801 for direct and surface UPV. Among the input features, curing pressure had the greatest impact on UPV, followed by cement content. Water content and concrete age also demonstrated high importance. In contrast, sulfite in fine aggregates and the type of coarse aggregates were the least influential variables. Overall, the findings indicate that ML approaches can reliably predict UPV in healthy concrete, offering a useful step toward more precise health monitoring through the detection of UPV deviations caused by potential damage.

The reduction in shear capacity when using recycled coarse aggregate (RCA) made from crushed concrete is evaluated in terms of tensile cracking and fracture-surface characteristics. An experimental investigation into the fracture and flexure-shear behaviors of recycled aggregate concrete (RAC) is presented. Replacing natural aggregate in concrete proportioned for 30 MPa (4350 psi) compressive strength with RCA results in lower compressive and tensile strengths. The tensile fracture-surface characteristics vary between RAC and natural aggregate concrete (NAC). While the surface area created in the tensile fracture of RAC is larger than that of NAC, the fracture surface profile in RAC has a smaller roughness than NAC. In the flexure-shear response of reinforced concrete beams, the dilatancy determined from the slip and crack opening displacements measured across the shear crack is smaller in RAC than in NAC. The failure in the reinforced beam is due to the frictional stress transfer loss across the primary shear crack. There is a larger decrease in the shear capacity with the use of RAC than indicated by the reduction in compressive strength. The reduced shear capacity of reinforced RAC is due to the combined influences of reduced tensile strength and crack surface roughness. The design provisions require calibration for crack surface roughness when using RAC in structural applications.

The use of parametric/natural fires in the design of reinforced concrete structures in fire conditions requires an accurate definition of the temperature-induced evolution of the thermal and mechanical properties. Within this context, the characterization of four normal-strength concretes (f_c²⁰ = 4220-7000 psi [29-47 MPa]), with siliceous and carbonate aggregates are studied here as concerns the thermal diffusivity D (between 68 and 1644°F [20 and 900°C]) and under uniaxial compression after different thermal cycles, with reference maximum temperatures of 392, 752, and 1112°F [200, 400, and 600°C]. The results show that thermal diffusivity exhibits mostly irreversible behavior after exposure to temperatures over 1382°F [750°C]. As concerns the compressive strength, the hot and residual values (when TT_test = 68°F [20°C]) are, overall, in line with the most common standard provisions. Quite interestingly, the tests carried out at intermediate temperatures (with T_test does not = T_max and T_test > 68°F [20°C]) highlighted a strength decay, which is not simply an interpolation between hot and residual values.

ASTM C31 describes the procedure for making concrete specimens in the field. Its origin can be traced to 1920, proposing rodding or stroking each 100 mm thick layer 25-30 times. Concrete technology has evolved tremendously over the last century, but specimens are still prepared following this 100-year-old methodology. This paper investigates the density and compressive strength of concrete cylinders for different consolidation procedures. Mix design variations include paste volume, w/c, aggregate grain size distribution, fly ash, and plasticizer. An increase in compressive strength of approximately 5 MPa can be obtained if 100 × 200 mm cylinders are rodded in 4 layers, 25 rods each, if the slump is not over 100 mm. For all other mixtures, the current rodding procedure of 2 layers, 25 rods each, is recommended. For mixtures with higher slump, 2 layers with less rodding per layer deliver similar strength values, but the variability is high.

This paper proposes new procedures for determining allowable loads for power-actuated fasteners that are consistent with ASCE/SEI 7-22. Thirty new load test datasets for single fasteners in shear and tension, and fastener groups in shear, are analysed statistically. The current ICC-ES AC70-2021 procedure yields allowable loads that are quite variable, even negative, and very sensitive to “reject-as-outlier” decisions. In addition, ICC-ES AC70-2021 procedures to determine allowable loads can currently not be clearly linked to the reliability requirements per ASCE/SEI 7-22. Monte Carlo simulation demonstrates that the proposed Simplified Method, derived from the described Detailed Method, is robust for sample sizes as small as ten specimens. It yields Allowable Fastener Loads that are 10 to 25% greater than those obtained using the current ICC-ES AC70-2021 procedure, yet are typically 60 to 90% of the actual Allowable Fastener Loads, derived from the described Detailed Method to assess allowable loads in line with ASCE 7 reliability requirements. The new provisions are extended to cases where the coarse aggregate hardness in the test specimens differs from that in the structure, which is not addressed in ICC-ES AC70.