It is generally considered (Ohama, 1998) that the addition of polymer will reduce the stiffness of the PMC and will do so in proportion to the amount of polymer added. There is, however, very little data available to confirm this, and it has been suggested (Bureau, 2001) that the reduction is really only noticeable in the case of SBR modified mortars at p/c > 12.5%. No data is available on long-term behaviour.
A model describing the relationship between porosity, microstructure and strength of cement paste has been proposed by Kendal et al. (1983). The model differentiates between micro-cracks or elongated pores, which are long enough to initiate crack propagation but probably represent < 0.1 of the pore volume fraction, and the remaining pores which have a high volume fraction ~0.9 but are too small to initiate failure. This approach suggests that the tensile stress,
where E0 and R0 are the Young’s modulus and fracture energy at zero porosity
respectively, p is the porosity volume fraction, a is the crack length and k is a
constant. Measurement on a range of samples, including ones made with a water
soluble polymer, showed that the tensile and flexural strength of cement paste is
largely governed by the length of the largest (crack-like) pores, but there is also
an influence of the volume of porosity through reducing the elastic modulus and
Whilst this equation has not been applied to polymer modified mortars it represents one way of explaining available strength data and determining the way forward for improvements. Typical polymer modified mortars of aggregate : cement ratio 2 : 1, 15% polymer and w/c 0.38±0.45 have compressive and flexural strengths in the region of 30 MPa and 8MPa respectively, compared with values of around 20 MPa and 3MPa for the unmodified mortar (Ohama, 1998). The improvement in compressive strength can be attributed to the use of a lower w/c (for the same workability) thus reducing p in equation 10.4. This has been confirmed by (Barluenga, 2004; Wang, 2005) who showed that, at constant water cement ratio, the compressive strength of polymer modified mortars tends to decline with increase in p/c, this being attributed to increased porosity The improvement in flexural strength is considered practically advantageous and is one of the main reasons for modification. It is probable that the presence of a polymer film effectively reduces the value of a in equation 10.4. The mechanism may involve polymer bridging of cracks, a process that would be facilitated by the presence of the chemical anchors discussed in Section 10.2.3. When failure occurs, the fracture surface shows fibrils of polymer (Bureau, 2001). Whilst the strength values can be improved by, e.g. changing the type of cement or aggregate size, etc., changes in latex have little effect, although redispersible powder always seem to be somewhat inferior. This suggests that the presence of the polymer film cannot overcome the effect of flaws below some critical length, acrit . There may thus be little point in empirical attempts to enhance strengths of PMC by means of polymer development until the nature of acrit is more clearly understood.
An important aspect of PMC is their improved adhesion to substrates such as existing concrete, steel and many other materials. There are a number of test configurations used for assessing adhesion with the tensile (pull-off), flexural and slant-shear variations being the most popular. Values depend on factors such as test method, nature of substrate, e.g. porosity and environment, and often show considerable scatter. Wet curing does not provide a very good bond and failures tend to be adhesive (i.e., at the interface). Wet-dry curing gives a much improved bond which increases as p/c increases and bond strengths three times those of the unmodified materials can be obtained. Failure is mainly cohesive in nature. Subsequent prolonged re-wetting can reduce this improved bond with a return to adhesive type of failure. The reason for this is unclear but one possibility may be that hydration products, formed during further hydration, disrupt the interfacial polymer film rather than consolidating it. This could be compounded by water absorption-induced swelling of the polymer phase (Salbin, 1996).