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International Concrete Abstracts Portal

Showing 1-5 of 44 Abstracts search results

Document: 

SP-338_02

Date: 

March 1, 2020

Author(s):

Kenneth C. Hover

Publication:

Symposium Papers

Volume:

338

Abstract:

PCA researchers interested in the problem of evaporation of bleed water from concrete surfaces borrowed an equation developed by hydrologists to predict evaporation from Lake Hefner in Oklahoma. PCA’s graphical representation of that equation, subsequently modified to its present form by NRMCA, was later incorporated into multiple ACI documents, and is known by concrete technologists world-wide as the “Evaporation Rate Nomograph.” The most appropriate use of this formulation in concrete construction is to estimate the evaporative potential of atmospheric conditions (known as “evaporativity”). Since the difference between actual and estimated evaporation rate can be in the range of ± 40% of the estimate, best use of the equation as routinely applied is as a semi-quantitative guide to estimate risk of early drying and inform decisions about timing and conduct of concrete placing and finishing operations. Use of the “Nomograph” and related “Apps” in specifications is more problematic, however, given: 1.) the inherent uncertainty in its underlying equation, 2.) the difficulty in obtaining input data that appropriately characterize jobsite microclimate, and 3.) establishing a mixture-specific criterion for tolerable evaporation rate.


Document: 

SP-338_06

Date: 

March 1, 2020

Author(s):

Oscar R. Antommattei

Publication:

Symposium Papers

Volume:

338

Abstract:

During hot weather concreting, contractors have several options for dealing with slump loss and rapid drying of concrete surfaces. Limiting slump loss requires cooperation between the concrete producer and contractor, especially with respect to reducing truck waiting time. Several options for minimizing surface drying are compared, based on effectiveness and cost. Finally, providing for adequate initial curing of concrete test cylinders can reduce the possibility of schedule delays and increased costs related to low strength-test results.


Document: 

SP326-119

Date: 

August 10, 2018

Author(s):

Caijun Shi, Bao Lu, Zhenjun Tu, and Xiaoying Pan

Publication:

Symposium Papers

Volume:

326

Abstract:

This presentation discusses the innovative uses of CO2 for the curing of concrete products, surface treatment of concrete and performance enhancement of recycled concrete aggregates (RCA). Using carbon dioxide for concrete curing is based on the chemical reactions between CO2 and the main silicate phases in the presence of water. This technology allows several advantages over traditional moisture curing in terms of decreasing the duration time of early curing and improving the mechanical properties and dimensional stability of concrete.

Concrete surface treatment is one of the effective protection methods to improve the durability of concrete. CO2 treatment produced a carbonated layer, but increased the compressive strength, and effectively reduced the water permeability, water-vapor transmission and chloride migration.

The cement paste attached on natural aggregates has a significant effect on the quality of RCA because it usually has higher porosity and lower strength than natural aggregates. This work attempted to improve the quality of RCAs through carbonation treatment. Carbonation increased the density and decreased the water absorption and crushing values of the RCA. Compared with the mortars made of uncarbonated RCAs, the mortars made with carbonated RCAs increased autogenous shrinkage, reduced drying shrinkage, water absorption, and chloride migration.


Document: 

SP326-28

Date: 

August 10, 2018

Author(s):

Alessandra Mobili, Alberto Belli, Antonio Telesca, Milena Marroccoli, and Francesca Tittarelli

Publication:

Symposium Papers

Volume:

326

Abstract:

This work presents calcium sulfoaluminate (CSA) cement and geopolymeric binder (GEO) as environment-friendly alternatives to ordinary Portland cement (OPC). Mortars based on these binders were tested and compared at the same non-structural strength class (R2 ≥ 15 MPa, according to EN 1504-3). Binder pastes were preliminarily prepared to study their hydration behaviour by means of differential thermal-thermogravimetric (DT-TG) and X-ray diffraction (XRD) analyses. Afterwards, the relative mortars were compared in terms of both fresh (workability) and hardened state properties (compressive strength, dynamic modulus of elasticity, adhesion to bricks, and water vapor permeability). Durability was also investigated in terms of capillary water absorption, drying and restrained shrinkage. Porosimetric analysis allowed to better correlate experimental results with microstructural features of the investigated mixtures. Results showed that GEO-based mortar exhibits the lowest modulus of elasticity, causing the lowest restrained shrinkage and the highest free drying shrinkage. Moreover, its highest porosity determines both the highest capillary water absorption and permeability to water vapor. On the contrary, the CSA-based mortar displays the lowest drying shrinkage, the greatest modulus of elasticity, and the lowest porosity which ensures the lowest capillary water absorption.


Document: 

SP310-14

Date: 

March 17, 2017

Author(s):

Tara Rahmani; Mehdi Bakhshi; Barzin Mobasher; Mohammad Shekarchi

Publication:

Symposium Papers

Volume:

310

Abstract:

Plastic shrinkage cracking due to a high rate of evaporation detrimentally affects durability and serviceability of concrete structures. The effect of different types of fibres to control these cracks, including steel, glass, polypropylene, and polyolefin fibres on the moisture loss and evaporation rates is investigated by performing ASTM C1579 tests. Using a dual stage methodology of constant drying rate period (stage I) and falling drying rate period (stage II), results are analysed. Moisture diffusivities are computed which in turn can be used for modelling the drying shrinkage and cracking under different environmental conditions. The formation of microcracks is documented using digital photography and processed by image analysis. The results show that moisture diffusivities at stage I drying are similar to each other between FRC and control samples. These magnitudes are approximately 50 times higher than diffusivities at stage II drying. The main difference is observed in stage II drying where the diffusivities in FRC are lower compared to plain concrete. Image analysis results indicate significant effects of fibres on controlling plastic shrinkage cracks.


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