tailieunhanh - Prevention of air void formation in polyme

The air voids that form when cement and aggregates are mixed with an aqueous polymer solution are not easily removed because they tend to be stabilized by the polymer. Antifoaming agents can be used to prevent air void formation, but they often cause problems of poor adhesion. An alternative approach is suggested for reducing air void formation in polymer-modified mortar, which is to pre-wet the cement and sand with water, and then add a concentrated polymer solution. The air content of a PVA-modified cement mortar prepared in this way was as low as 6%, which was lower than the 13% air content of the PVA-modified mortar with an antifoaming agent, and it was much lower than the 32% air content when a PVA solution was mixed with dry cement and sand. 0 1997 Elsevier Science Ltd. | Pergamon Cement and Concrete Research Vol. 27 No. 2 pp. 171-176 1997 1997 Elsevier Science Ltd Printed in the USA. All rights reserved 0008-8846 97 .00 PII S0008-8846 97 00001-X PREVENTION OF AIR VOID FORMATION IN POLYMER-MODIFIED CEMENT MORTAR BY PRE-WETTING Jae-Ho Kim and Richard E. Robertson Macromolecular Science and Engineering Center Department of Materials Science and Engineering The University of Michigan Ann Arbor MI 48109-2136 u. s. A. Communicated by s. Diamond Received December 12 1996 in final form December 31 1996 ABSTRACT The aữ voids that form when cement and aggregates are mixed with an aqueous polymer solution are not easily removed because they tend to be stabilized by the polymer. Antifoaming agents can be used to prevent air void formation but they often cause problems of poor adhesion. An alternative approach is suggested for reducing air void formation in polymer-modified mortar which is to pre-wet the cement and sand with water and then add a concentrated polymer solution. The air content of a PVA-modified cement mortar prepared in this way was as low as 6 which was lower than the 13 air content of the PVA-modified mortar with an antifoaming agent and it was much lower than the 32 air content when a PVA solution was mixed with dry cement and sand. 1997 Elsevier Science Ltd Introduction Water soluble polymers and aqueous polymer dispersions are often used to improve the properties of mortar concrete and cement-based composites 1-4 . But when the cement mixtures are mixed with these polymer admixtures a large volume of aữ voids often forms. To prevent this various antifoaming agents can be added to the mixtures 2 . These antifoaming agents however often cause a problem of their own viz. poor adhesion between the cement and the reinforcements 5 6 . The purpose of this report is to describe an alternative to the use of antifoaming agents for reducing the volume of air voids when water-soluble polymers or aqueous polymer dispersions are .