Embedded piezoelectric transducers based early-age hydration monitoring of cement concrete added with accelerator/retarder admixtures

Accelerator/retarder admixtures are often added into concrete to improve its early-age strength, which needs to be effectively monitored during its hardening process. The electromechanical impedance (EMI) technique has validated its effectiveness for concrete hydration monitoring, this study attempted to extend the EMI technique to monitor 28-day age of strength gain in concrete that added with accelerator/retarder admixtures. Two types of new piezoelectric (PZT) transducers namely cement/aluminum embedded PZT (CEP/AEP) were proposed for EMI monitoring. The feasibility of the CEP and AEP was first verified via finite element analysis, where hydration heat effect on the two types of transducers was comparatively evaluated by numerical modeling. In the experiment, CEP/AEP transducers were applied to monitor the strength gain in concrete cubes, where characteristics of EMI signature and its statistical indices including root mean square deviation (RMSD) and mean absolute percentage deviation (MAPD) were analyzed and correlated to strength development in concrete. Monitoring results demonstrated that concrete hydration triggered by retarder/accelerator were successfully captured by EMI signature. RMSD and MAPD indices further indicated that AEP had preferable performance than CEP transducer for monitoring early-age strength gain of concrete, as it could immune from hydration heat effect.

An embeddable spherical smart aggregate for monitoring concrete hydration in very early age based on electromechanical impedance method

In this paper, a new embeddable spherical smart aggregate (SSA) was utilized to monitor concrete curing in very early age. Overcoming the limitation of the existing PZT-patch-based transducers, the SSA provides vital changing information in all directions of host structure. To verify the advantage of SSA in structural health monitoring (SHM), the sensitivities of SSA and smart aggregate (SA) in monitoring concrete cube deformation and stiffness variation were analyzed and compared by numerical simulation. The feasibility of SSA in monitoring the concrete hydration process was studied by experiments utilizing electromechanical impedance (EMI) technique. At last, four SSAs were embedded in a concrete column to study the practicality of SSA in monitoring the concrete curing process in very early age. The EMI signatures and the root mean square deviation (RMSD) values of the collected information from SSAs were analyzed. The results illustrate that the SSA is more sensitive than SA in monitoring the concrete deformation and stiffness variation. The data measured by SSA in monitoring the concrete hydration process fluctuates more obviously than the data recorded by SA. The new spherical transducer can effectively and reliably monitor the concrete hydration process.

Evaluation of Mechanical and Shrinkage Behavior of Lowered Temperatures Cementitious Mortars Mixed with Nitrite–Nitrate Based Accelerator

Recently, calcium nitrite (Ca(NO2)2) and calcium nitrate (Ca(NO3)2) have been increasingly used as the main components of salt- and alkali-free anti-freezing agents, for promoting concrete hydration in cold-weather concreting. With an increase in the amount of nitrite-based accelerator, the hydration of C3A, C3S, and βC2S in the cement is accelerated, thereby improving its early strength and effectively preventing the initial frost damage. Meanwhile, with an increase in the amount of nitrite-based accelerator, the expansion and shrinkage of the concrete—and, therefore, the crack occurrence—are expected to increase. In this study, various experiments were conducted on shrinkage, crack initiation, and the development of mortar containing a considerable amount of a nitrite-based accelerator. The result confirmed that, as the amount of nitrite-based accelerator was increased, the shrinkage was increased, and cracking in early age was more likely to occur, compared to the cases without the addition of this accelerator.

Molecular dynamics and reaction kinetics analyses of gamma radiation impact on concrete hydration

MOPAC and LAMMPS molecular dynamics codes and reaction kinetics code based on multi-ionic continuum-based model are used to analyze the impact of gamma radiation on concrete hydration. The experimental studies showed that while cured with the low gamma dose concrete shows a statistically significant increase in its strength compared to conventionally cured concrete. The potential reason is the interactions of gamma rays with water causing concrete faster hydration. The question then to ask is would the higher gamma dose enhance the concrete curing further producing its higher strength. This paper provides in-depth numerical analyses of the high-dose gamma radiation effect on concrete based on molecular dynamics and reaction kinetics models. Under these conditions, it is assumed that gamma radiation interacting with water within the concrete induces water radiolysis. These numerical simulations show that the reactivity is generally increased in the presence of electrophiles. However, the early hydration models of tricalcium silicate (alite) and dicalcium silicate (belite) with H+, OH-, and H3O+ show that the hydration process is slowed down leading to a lower concrete strength. Additionally, the reaction kinetics model used to estimate the effect of [OH-] on tricalcium silicate hydration shows that an increase or decrease of [OH-] during tricalcium silicate hydration can respectively slow down or enhance its rate of hydration. The dose necessary to produce the water radiolysis resulting in varying [OH-] during tricalcium silicate hydration is required to be extremely high and therefore, will damage the concrete structure itself. This leads to the conclusion that increasing the gamma dose to concrete above that used in the experimental studies in order to induce water radiolysis will not improve concrete strength, therefore water radiolysis is not the required condition for improving concrete strength when cured under gamma radiation.