1 INTRODUCTION From the literature it is known that concrete is, after water, the material most used by civilization. This finding raised by Brunauer and Copeland (apud Mehta and Monteiro, 2005), reflects the economic importance of this material for science and technology. It is common in civil construction situations in which the concrete is subjected to tensile stresses and, in these cases, the solution is include steel bars to absorb these kinds of efforts. However, for some particular situations, such as the execution of industrial floors, the inclusion of bars, though possible, is a practice that could be avoided if the concrete itself, produce an appropriate behavior with regard to absorption of tensile stresses. The lowest mechanical properties of concrete is known as transition zone (Mehta and Monteiro, 2005; Queiroga, 2000). This region, corresponding to the interface between the gravels and the cement slurry (Fig. 1), it has a low formation of C-S-H, which is the product of Portland cement hydration responsible for their mechanical properties and durability (Hewlett et al, 2004). Concrete with latex and biopolymer, part I: behaviour relative to the compressive strength and tensile strength Ulisses Targino Bezerra Federal Institute of Education, Science and Tech. of Paraíba, Civil Construction, João Pessoa, Brasil Rui Miguel de Jesus Ferreira University of Minho, Department of Civil Engineering, Guimarães, Portugal João Paulo de Castro Gomes University of Beira Interior, Department of Civil Engineering, Covilhã, Portugal ABSTRACT: The incorporation of polymers in concrete in an attempt to improve its performance dates from the beginning of the previous century, and seems distant from the exhaustion of possibilities. This work presents the results of the combination of two polymeric additives in concrete. The work is divided into two parts: evaluation of mechanical properties (compressive strength and tensile strength) and evaluation of phases formed by scanning electron microscopy and X-ray diffraction. Initially, the concretes were prepared with each of the polymers separately, and the results were ordinary. However, when combined, the polymers showed an interesting interaction improving the compressive strength and the tensile strength of the concretes. The expected effects were the coalescence of latex around the Portland cement hydrates and the chelation of calcium present in cement by the biopolymer in the first hours of hydration. Samples of concrete were prepared in the proportions of 0% to 4% for each of the polymers with 1% increments. Mechanical tests show that the compressive strength increases 21% for the percentages of 3% and 1% of biopolymer and latex, respectively, while the tensile strength increases 11% for the percentages of 1% and 3% of both polymers, both results for 28 days curing. There was an increase of 18% in tensile strength for the percentages of 2% of each polymer at 7 days. The two mechanical properties were shown to be sensitive to the incorporation of polymers. The effect of each one separately was not so interesting, because the increase in compressive strength, in the case of biopolymer, was only 8%, and for the case of latex, there was reduction of 14%. The latex should not be used in concrete to increase the compressive strength. The desired effect of the interaction between the biopolymer and the latex was observed, because the strength was increased when both were present, mainly to the combination of 2% of each polymer.