Measurement of Dry and Autogenous Shrinkages and Thermal Strain of Early Age Concrete Pavement

(1)Background: Early-age concrete shrinkage induces stress that impact the cost and service life of concrete pavements. (2)Methods: In this study, strain measurements of field slabs were conducted and a methodology was presented that independently derived autogenous, drying, and thermal shrinkages in the initial stages of concrete placement. Total strain was measured according to five different environmental conditions and shrinkage strain was calculated for each condition. (3)Results: By measuring the strain of the slab and the specimen, the drying shrinkage strain was measured to be approximately 54% better than that by the conventional non-stressed cylinder method because it was possible to measure the drying shrinkage strain at the surface rather than in the middle part of the slab along its depth direction. When the water-to-cement ratio increased (35→40%), there was a considerable reduction (317με→82με) of autogenous shrinkage strain for the concrete at 28 days of age. Furthermore, calculation of stress-dependent strain allowed the presentation of more intuitive and accurate results. (4)Conclusion: As the measurement of independent shrinkage occurrence is possible, the consequent calculated result of the stress-dependent strain acting on real slabs will facilitate improvement in the construction quality, reduction in the development of defects in the concrete structure, and increase in the service life.

Laws Governing Free and Actual Drying Shrinkage of 50 mm Thick Mongolian Scotch Pine Timber

The relationships between free shrinkage and actual shrinkage of different layers in Mongolian Scotch pine (Pinus sylvestris var. mongolica Litv.) were explored to provide basic data for the further study of drying shrinkage properties. The free shrinkage coefficients at different temperatures and the actual shrinkage strain of each layer were examined under conventional drying. The results showed high precision of free drying shrinkage of corresponding layers of thin small test strips in each layer of sawn timber. The free shrinkage increased linearly as moisture content declined. At the same temperature, the free shrinkage coefficient reached the largest values for the first layer (above 0.267), while the smallest values were recorded for the ninth layer (below 0.249). Except for the ninth layer, the free shrinkage coefficients in width directions of other representative layers decreased as temperature increased. At constant temperature, the difference in free shrinkage coefficient of test materials in the length direction of sawn timber was small for the first layer, but slightly larger and changed irregularly in the fifth and ninth layer direction. At the end of conventional drying, the plastic deformation of each layer in the early stage of drying showed a reducing trend or even reversal due to the effects of reverse stress and later damp heat. In sum, these findings look promising for future optimization of wood drying process.

Time-Dependent Shrinkage Model for Recycled Fine Aggregate Thermal Insulation Concrete

In this study, the shrinkage performance of recycled aggregate thermal insulation concrete (RATIC) with added glazed hollow beads (GHB) was investigated and a time-dependent shrinkage model was proposed. Two types of recycled fine aggregate (RFA) were used to replace natural fine aggregate in RATIC: RFA from waste concrete (RFA1) and waste clay brick (RFA2). Besides, the mechanical properties and thermal insulation performance of RATIC were also studied. Results showed that the pozzolanic reaction caused by RFA2 effectively improved the mechanical properties of RATIC; 75% was the optimal replacement ratio of RATIC prepared by RFA2. Added RFA decreased the thermal conductivity of thermal insulation concrete (TIC). The total shrinkage strain of RATIC increased with the increase of the replacement ratio of RFA. The 150d total shrinkage of RATIC prepared by RFA1 was 1.46 times that of TIC and the 150d total shrinkage of RATIC prepared by RFA2 was 1.23 times. The addition of GHBs led to the increase of early total shrinkage strain of concrete. Under the combined action of the higher elastic modulus of RFA2 and the pozzolanic components contained in RFA2, the total shrinkage strain of RATIC prepared by RFA2 with the same replacement ratio was smaller than that of RATIC prepared by RFA1. For example, the final total shrinkage strain of RATIC prepared by RFA2 at 100% replacement ratio was about 18.6% less than that of RATIC prepared by RFA1. A time-dependent shrinkage model considering the influence of the elastic modulus of RFA and the addition of GHB on the total shrinkage of RATIC was proposed and validated by the experimental results.

Suitability of Shale Treated with Palm Kernel Shell Ash and Pulverized Palm Kernel Shell As Landfill Liners at Nguzu Edda, Southeastern Nigeria

Wastes are mostly disposed of in landfills, which pose threat to soil and groundwater as a result of percolation of leachate. Therefore, barrier soils are required for the lining of a landfill to prevent seepage of leachate into the surrounding groundwater system. In this regard, the suitability of ten shale samples treated with 0-12 % (increments of 2 %) proportion of palm kernel shell ash (PKSA) and pulverized palm kernel shell (PPKS) by dry weight of the shale samples were evaluated for uses as landfill liner. The samples were subjected to series of geotechnical tests to determine the index properties, and strength characteristics of the natural and treated shale using West African standard (WAS) and Modified AASHTO (MAS) compactive energy levels for comparison purposes. The shale was classified as A-7-5. The atterberg limits test results show that liquid and plastic limits generally decreased, while the plasticity index (PI) increased with increase in PKSA and PPKS contents. The results also demonstrated that maximum dry density (MDD), volumetric shrinkage strain (VSS) and hydraulic conductivity decreases significantly while optimum moisture content (OMC) increases with increase in PKSA and PPKS contents for the both energy levels. The maximum strength of 380.30 and 448.70 KPa were recorded at 4 % of the stabilizers. The findings affirmed that the samples met the requirements for landfills liner, although PPKS was more effective than PKSA for both energy levels. Moreover, the addition of PKSA and PPKS to liners can also be an alternative means of its disposal.

Influence of Rapid Heat Treatment on the Shrinkage and Strength of High-Performance Concrete

Resource-efficient precast concrete elements can be produced using high-performance concrete (HPC). A heat treatment accelerates hardening and thus enables early stripping. To minimise damages to the concrete structure, treatment time and temperature are regulated. This leads to temperature treatment times of more than 24 h, what seems too long for quick serial production (flow production) of HPC. To overcome this shortcoming and to accelerate production speed, the heat treatment is started here immediately after concreting. This in turn influences the shrinkage behaviour and the concrete strength. Therefore, shrinkage is investigated on prisms made from HPC with and without steel fibres, as well as on short beams with reinforcement ratios of 1.8% and 3.1%. Furthermore, the flexural and compressive strengths of the prisms are measured directly after heating and later on after 28 d. The specimens are heat-treated between 1 and 24 h at 80 °C and a relative humidity of 60%. Specimens without heating serve for reference. The results show that the shrinkage strain is pronouncedly reduced with increasing temperature duration and rebar ratio. Moreover, the compressive and flexural strength decrease with decreasing temperature duration, whereby the loss of strength can be compensated by adding steel fibres.

Comparison Between Shrinkage Strain and FEM Thermo-Mechanical Method for Estimation of Welding Induced Residual Stresses

Residual stress estimation in structural integrity procedures plays an important role during the fitness-for-service (FFS) assessment of girth welds. Various FFS codes and standards, such as API 579 and BS 7910, recommend predetermined residual stress profiles based on finite element modeling (FEM) coupled with experimental results. Nonlinearity associated with non-uniform temperature gradients’ distribution during welding can develop residual stress up to the yield strength of the material, in weld shrinkage and plastic zones. Plastic zone size, shape, and locations are critically important in reducing or controlling final distortions, decreasing the residual stress according to length scale, and defining the optimum sequence of the welding process. However, in practice, estimation of finally developed residual stresses is used in structural integrity procedures for determining the FFS of welded joints. Various FEM models employed in its assessment require large computational time in solving the complex thermo-mechanical phenomenon involved in the welding process. Shrinkage strain models have been found to be fast and effective in determining final residual stresses, once the size, location and shape of the plastic zone can be predetermined. This manuscript demonstrates a comparison between the shrinkage strain method and the moving heat source method, based on transient temperature development as a function of time. The results (or findings) reveal a high compromise between FEM thermo mechanical model and shrinkage strain method in determining final residual stresses with later consuming less computational time. The findings provide significantly important feedback to welded joints’ structural integrity assurance practitioners.

Use of Fiber Brag Gating Sensors in civil engineering applications

This research presents an overview of development and application of Fiber Bragg Grating sensors (FBG) technology in civil engineering applications. The primary focus of this research is the use of FBGs to investigate two most important time-dependent properties of concrete namely: creep strain and shrinkage strain. The first phase of this investigation is focused on using FBG sensors to measure the concrete strains in unreinforced concrete beams and cylinders to determine modulus of elasticity, the modulus of rapture and fracture energy of concrete. The second phase of this research is designed to investigate the creep and shrinkage using FBG sensors. Normal strength concrete (NC), High performance concrete (HPC) and ultra-high performance (UHPC) specimens’ are used to measure creep and shrinkage strains and to compare the values with typical prediction models. The measured creep and shrinkage strains are compared to four different models to determine which model is the most accurate.

Use of Fiber Brag Gating Sensors in civil engineering applications

This research presents an overview of development and application of Fiber Bragg Grating sensors (FBG) technology in civil engineering applications. The primary focus of this research is the use of FBGs to investigate two most important time-dependent properties of concrete namely: creep strain and shrinkage strain. The first phase of this investigation is focused on using FBG sensors to measure the concrete strains in unreinforced concrete beams and cylinders to determine modulus of elasticity, the modulus of rapture and fracture energy of concrete. The second phase of this research is designed to investigate the creep and shrinkage using FBG sensors. Normal strength concrete (NC), High performance concrete (HPC) and ultra-high performance (UHPC) specimens’ are used to measure creep and shrinkage strains and to compare the values with typical prediction models. The measured creep and shrinkage strains are compared to four different models to determine which model is the most accurate.

Temperature and shrinkage cracking in reinforced concrete walls

Concrete cracking due to restrained thermal and shrinkage strain is a widespread problem that could happen to any structural element including base restrained walls. This type of crack usually occurs in structures with rigidly interconnected parts cast after their adjacent parts are hardened. As concrete undergoes volumetric deformations right after casting, the developing strains due to temperature drop and moisture loss get restrained by neighboring parts which causes stress development and could lead to formation of cracks. Cracking could reduce the structure’s integrity and serviceability, cause deterioration which could also lead to esthetical concerns. Therefore, structures should be designed to limit cracks to an acceptable level depending on the functionality requirements of the structure and its exposure conditions. Although it has been proven that it is almost impossible to completely eliminate cracking, providing an adequate amount of appropriately positioned reinforcement can reduce the width of cracks significantly. This study aims to investigate the behavior of base restrained reinforced concrete (RC) walls under volumetric changes due to thermal and shrinkage strains and providing a procedure to determine the amount of reinforcement needed to control the width of cracks. The ABAQUS finite element (FE) program is used to simulate the structures used in this study. The models are verified by comparing the results with previous experimental studies. Based on the performed parametric study, a procedure is suggested to determine the amount of steel reinforcement required to satisfy the cracking limitations based on major parameters that affect the crack width.