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Aggregates
Published in M. Rashad Islam, Civil Engineering Materials, 2020
Alkali-aggregate reaction is the expansive reaction that occurs in PCC between alkali (available in cement) and silica (available in aggregates). In the presence of moisture, the alkalis found in cement break down the silica in the aggregate, producing an expansive gel. This expansion causes tensile forces in PCC, leading to loss of strength and resulting in map or pattern cracking, shown in Figure 2.13. This reaction can be controlled by: Avoiding susceptible aggregates, such as siliceous limestone, chert, shale, volcanic glass, synthetic glass, sandstone, opaline rocks, and quartzite. River rocks are sometimes susceptible.By using the pozzolanic admixture, as the silica contained in a pozzolan may react with the alkali in the cement, leaving less alkali available for the silica within the aggregate.Using low-alkali cementLowering the water–cement ratio, which limits the supply of water to the alkali–silica gel formation.
Constituent Materials
Published in Ganesh Babu Kodeboyina, High Performance Self-Consolidating Cementitious Composites, 2018
CEB-FIP recognizes the effect of smaller fractions in fine aggregates and states the following:Too high an amount of ultrafines will reduce the workability of fresh concrete unless superplasticizers are used.A replacement of the cement for ultrafines beyond limits (through mineral admixtures or supplementary cementitious materials) can adversely affect the concrete durability.If the addition of fines allows a reduction in the cement content without an increase in the water–cement ratio, shrinkage and creep of concrete are reduced.Aggregates constitute the bulk of the total volume of concrete. The characteristics of the coarse aggregate that influence the behavior of concrete are its type, size, shape, surface texture, and gradation (Ganesh Babu et al., 1992). The type of the aggregate is based on the nature of the parent rock, which influences its strength, porosity, and water absorption. Of particular interest in this regard are the chemical reactions between the aggregates and the cement paste, such as alkali–aggregate reaction. Most igneous aggregates do not show specific problems related to the alkali–aggregate reactivity. However, when other types of aggregates such as dolomite are used the increase in water requirement for the given workability has to be considered. Similarly, variation (possible increase) in strength is also an important factor.
Middle East & North Africa
Published in Ian Sims, Alan Poole, Alkali-Aggregate Reaction in Concrete: A World Review, 2017
Ted Kay, Alan B Poole, Ian Sims
The recognition of this potential for damage has led to extensive efforts by Iranian engineers in testing aggregates and concrete mixes prior to use and also in developing methods of avoiding the potential for alkali-aggregate reaction in new concrete structures. As already noted, a common preventative measure is to make use of mineral admixtures such as natural pozzolanas, which are widely available in the Kerman province, and ground granulated blastfurnace slag or silica fume. Although some Iranian produced cements tend towards high alkali contents, the use of a low alkali cement and the avoidance of potentially reactive aggregate types are also recognised as potential methods of avoiding potential damage associated with alkali-aggregate reaction.
Identification of Corroded Cracks in Reinforced Concrete Based on Deep Learning SCNet Model
Published in Research in Nondestructive Evaluation, 2022
Ying Xu, Xuelei Jiang, Tianrui Zhang, Gan Jin
In deep learning, several networks with high recognition rate benefit from their huge and real sample sets. Meanwhile, the accuracy and quality of image features directly affect the training and testing of the subsequent models. The data set of the collected images is divided into three categories: reinforced concrete corrosion cracks, other causes of concrete cracks, and complete concrete. Other causes of concrete cracks are mainly divided into plastic shrinkage cracks, structural cracks, temperature cracks, and cracks caused by alkali aggregate reaction. Plastic shrinkage cracks refer to the irregular crack or parallel crack of concrete perpendicular to the longitudinal reinforcement. The structural cracks or temperature cracks normally occur when the external load or the uneven force caused by temperature difference exceeds the bearing capacity of concrete, with irregular shape and no color change on the structure surface. Alkaline aggregate reaction crack is caused by expansion and compression around the aggregate, which is characterized by surface concrete mesh cracking, besides transparent or yellowish gel precipitation.